martinotundo@gmail.com: Chemistry teaching materials. Martin Otundo is an educationalist and a PhD student at JKUAT Kenya AIR AND COMBUSTION A. THE ATMOSPHERE 1. The atmosphere is made up of air. Air is a mixture of colourless , odourless gases which is felt as wind(air in motion).All living things breath in air for respiration . Plants use air for respiration and photosynthesis. 2. The main gases present in the atmosphere/air: Gas Nitrogen Oxygen Carbon(IV)oxide Noble gases Water vapour 3. Approximate % composition by volume 78.0 21.0 0.03 1.0 Vary from region The following experiments below shows the presence and composition of the gases in air/atmosphere (a) To find the composition of air supporting combustion using a candle stick Procedure Measure the length of and empty gas jar M1. Place a candle stick on a petri dish. Float it on water in basin/trough. Cover it with the gas jar. Mark the level of the water in the gas jar M2. Remove the gas jar. Light the candle sick. Carefully cover it with the gas jar. Observe for two minutes. Mark the new level of the water M3. Set up of apparatus Novels,Updated ICT,KASNEB,College,High School,Primary Softcopy Notes +254721246744 (whatsapp) 1 Sample observations Candle continues to burn then extinguished/goes off Level of water in the gas jar rises after igniting the candle Length of empty gas jar = M1= 14cm Length of gas jar without water before igniting candle = M2= 10 cm Length of gas jar with water before igniting candle = M1 - M2= 14- 10 = 4 cm Length of gas jar with water after igniting candle = M3 = 8 cm Length of gas jar without water after igniting candle = M1 - M3 = 10 -8 = 2 cm Explanation Candle burns in air. In a closed system(vessel),the candle continues to burn using the part of air that support burning/combustion. This is called the active part of air.The candle goes off/extinguished when all the active part of air is used up.The level of the water rises to occupy the space /volume occupied by the used active part of air. The experiment is better when very dilute sodium/potassium hydroxide is used instead of water . Dilute Potassium/ sodium hydroxide absorb Carbon(IV)oxide gas that come out from burning/combustion of candle stick. From the experiment above the % composition of the: (i) active part of air can be calculated: M2 - M3 x 100% => 10- 8 x 100% = 20% M2 10cm (ii) inactive part of air can be calculated: 100% -20% = 80% // M3 => 8 x 100% = 80% M2 10cm (b) To find the composition of active part of air using heated copper turnings. Procedure Clamp a completely packed/filled open ended glass tube with copper turnings. Seal the ends with glass/cotton wool. Label two graduated syringes as “A” and “B” Push out air from syringe “A”. Pull in air into syringe “B”. Attach both syringe “A” and “B” on opposite ends of the glass tube. Determine and record the volume of air in syringe “B” V1. Heat the glass tube strongly for about three minutes. Push all the air slowly from syringe “B” to syringe “A” as heating continues. Push all the air slowly from syringe “A” back to syringe “B” and repeatedly back and forth. After about ten minutes, determine the new volume of air in syringe “B” V2 Set up of apparatus Sample observations Colour change from brown to black Volume of air in syringe “B” before heating V1 = 158.0cm3 Volume of air in syringe “B” after heating V2 = 127.2cm3 Volume of air in syringe “B” used by copper V1 - V2 = 30.8cm3 Sample questions 1.What is the purpose of (i) glass/cotton wool To prevent/stop copper turnings from being blown into the syringe/out of the glass tube (ii) passing air through the glass tube repeatedly To ensure all the active part of air is used up (iii) passing air through the glass tube slowly To allow enough time of contact beteewn the active part of and the heated copper turnings. 2. State and explain the observations made in the glass tube. Colour change from brown to black Brown copper metal reacts with the active part of air/oxygen to form black copper(II)oxide. Chemical equation Copper + Oxygen -> Copper(II)oxide O2(g) 2Cu(s) + -> 2CuO(s) The reaction reduces the amount/volume of oxygen in syringe “B” leaving the inactive part of air. Copper only react with oxygen when heated. 3. Calculate the % of (i)active part of air % active part of air = V1 - V2 x 100% => 30.8cm3 x 100% = 19.493% V1 158.0cm3 (ii) inactive part of air Method 1 % inactive part of air = V2 x 100% =>127.2cm3 x 100% = 80.506% V1 158.0cm3 Method 2 % inactive part of air = 100% -% active part of air => 100 % - 19.493 % = 80.507% 4. The % of active part of air is theoretically higher than the above while % of inactive part of air is theoretically lower than the above. Explain. Not all the active part of air reacted with copper 5. State the main gases that constitute: (a)active part of air. Oxygen (b) inactive part of air Nitrogen, carbon(IV)oxide and noble gases 6. If the copper turnings are replaced with magnesium shavings the % of active part of air obtained is extraordinary very high. Explain. Magnesium is more reactive than copper. The reaction is highly exothermic. It generates enough heat for magnesium to react with both oxygen and nitrogen in the air. A white solid/ash mixture of Magnesium oxide and Magnesium nitride is formed. This considerably reduces the volume of air left after the experiment. Chemical equation Magnesium + Oxygen -> magnesium (II)oxide O2(g) 2Mg(s) + -> 2MgO(s) Magnesium + Nitrogen -> magnesium (II)nitride N2(g) -> Mg3N2 (s) 3Mg(s) + (c) To find the composition of active part of air using alkaline pyrogallol. Procedure Measure about 2cm3 of dilute sodium hydroxide into a graduated gas jar. Record the volume of the graduated cylinder V1. Place about two spatula end full of pyrogallol/1,2,3-trihydroxobenzene into the gas jar. Immediately place a cover slip firmly on the mouth of the gas jar. Swirl thoroughly for about two minutes. Invert the gas jar in a trough/basin containing water. Measure the volume of air in the gas jar V2 Sample observations Colour of pyrogallol/1,2,3-trihydroxobenzene change to brown. Level of water in gas jar rises when inverted in basin/trough. Volume of gas jar /air in gas jar V1= 800cm3 Volume of gas jar /air in gas jar after shaking with alkaline pyrogallol/1,2,3trihydroxobenzene V2= 640 cm3 Sample questions 1. Which gas is absorbed by alkaline pyrogallol/1,2,3-trihydroxobenzene Oxygen 2. Calculate the (i) % of active part of air V1-V2 x 100% => (800cm3 - 640 cm3) x 100% V1 800cm3 (ii) % of inactive part of air V2 x 100% => V1 640 cm3 x 100% 800cm3 = 20% = 80% (d) To establish the presence of carbon(IV)oxide in air using lime water Pass tap water slowly into an empty flask as in the set up below Sample observation questions 1.What is the purpose of paper cover? To ensure no air enters into the lime water. 2. What happens when water enters the flask? It forces the air from the flask into the lime water. 3. What is observed when the air is bubbled in the lime water A white precipitate is formed. The white precipitate dissolves on prolonged bubbling of air. 4. (a) Identify the compound that form: (i)lime water Calcium hydroxide / Ca(OH)2 (ii)white precipitate Calcium carbonate/ CaCO 3 (iii)when the white precipitate dissolves Calcium hydrogen carbonate/ CaHCO 3 (b) Write the chemical equation for the reaction that tale place when: (i) white precipitate is formed Calcium hydroxide + carbon(IV)oxide Ca(OH)2(aq) + CO2 (g) -> Calcium carbonate + water -> CaCO3(s) + H2O(l) (ii) white precipitate dissolves Calcium carbonate + water+ carbon(IV)oxide -> Calcium hydrogen carbonate CaCO3(s) + H2O(l) + CO2 (g) -> CaHCO3(aq) 5. State the chemical test for the presence of carbon (IV)oxide gas based on 4(a) and (b)above: Carbon(IV)oxide forms a white precipitate with lime water that dissolves in excess of the gas. 6. State the composition of carbon(IV)oxide gas by volume in the air. About 0.03% by volume B.OXYGEN. a) Occurrence. 1. Fifty 50% of the earths crust consist of Oxygen combined with other elements e.g.oxides of metals 2. About 70% of the earth is water made up of Hydrogen and Oxygen. 3. About 20% by volume of the atmospheric gases is Oxygen that form the active part of air. b) School laboratory preparation. Oxygen was first prepared in 1772 by Karl Scheele and later in 1774 by Joseph Priestly.It was Antony Lavoisier who gave it the name “Oxygen” Procedure Method 1: Using Hydrogen peroxide Half fill a trough/basin with tap water. Place a bee hive shelf/stand into the water. Completely fill the a gas jar with water and invert in onto the bee hive shelf/stand. Clamp a round bottomed flask and set up the apparatus as below. Collect several gas jars of Oxygen covering each sample. Sample observation questions 1. What is observed when the hydrogen peroxide is added into the flask Rapid effervescence/bubbling/fizzing 2. Describe the colour and smell of the gas Colourless and odourless. 3.(a)Name the method of gas collection used. -Over water -Upward delivery -Down ward displacement of water (b) What property of Oxygen make it to be collected using the method above -Slightly soluble in water 4. What is the purpose of manganese(IV)oxide? Manganese(IV)oxide is catalyst. A catalyst is a substance that speeds up the rate of a chemical reaction but remain chemically unchanged at the end of the reaction. Hydrogen peroxide decomposes slowly to form water and Oxygen gas. A little Manganese(IV)oxide speeds up the rate of decomposition by reducing the time taken for a given volume of Oxygen to be produced. 5. Write the equation for the reaction. Hydrogen peroxide 2H2O2 (aq) -> Water + Oxygen -> 2H2O (l) + O2 (g) 6. Lower a glowing splint slowly into a gas jar containing Oxygen gas. State what is observed. The glowing splint relights/rekindles Oxygen relights/rekindles a glowing splint. This is the confirmatory test for the presence of Oxygen gas Method 1: Using Sodium peroxide Half fill a trough/basin with tap water. Add four drops of phenolphthalein indicator. Place a bee hive shelf/stand into the water. Completely fill a gas jar with water and invert in onto the bee hive shelf/stand. Clamp a round bottomed flask and set up the apparatus as below. Collect several gas jars of Oxygen covering each sample. Sample observation questions 1. What is observed when water is added (i) into the flask containing sodium peroxide Rapid effervescence/bubbling/fizzing (ii) phenolphththalein Remains colourless /Phenolphthalein indicator is colourless in neutral solution 2. Describe the colour and smell of the gas Colourless and odourless. 3.(a)Name the method of gas collection used. -Over water.Oxygen is slightly soluble in water. 4. Test the gas by lowering a glowing splint slowly into a gas jar containingthe prepared sample. The glowing splint relights/rekindles. This confirms the presence of Oxygen gas 5. Write the equation for the reaction. Sodium peroxide + Water -> Sodium hydroxide + Oxygen + 2H2O (l) -> 4NaOH(aq) + O2 (g) 2Na2O2 (aq) 1. Test the gas by lowering a glowing splint slowly into a gas jar containing the prepared sample. The glowing splint relights/rekindles. This confirms the presence of Oxygen gas 2. Write the equation for the reaction. Potassium Chlorate(V) -> Potassium Chloride + Oxygen 2KClO3 (aq) -> 2KCl(aq) + 3O2 (g) 3. What is the purpose of manganese(IV)oxide? Manganese(IV)oxide is catalyst. A catalyst is a substance that speeds up the rate of a chemical reaction but remain chemically unchanged at the end of the reaction. Potassium Chlorate(V) decomposes slowly to form potassium chloride and Oxygen gas. A little Manganese(IV)oxide speeds up the rate of decomposition by reducing the time taken for a given volume of Oxygen to be produced. (c) Uses of Oxygen 1. Oxygen is put in cylinders for use where natural supply is not sufficiently enough. This is mainly in: (i) Mountain climbing/Mountaineering-at high altitudes, the concentration of air/oxygen is low. Mountain climbers must therefore carry their own supply of oxygen for breathing. (ii) Deep sea diving-Deep sea divers carry their own supply of Oxygen. (iii) Saving life in hospitals for patients with breathing problems and during anaethesia. 2. A mixture of oxygen and some other gases produces a flame that is very hot. (i) Oxy-acetyline/ethyne flame is produced when Ethyne/acetylene gas is burnt in pure oxygen. The flame has a temperature of about 3000oC.It is used for welding /cutting metals. (ii) Oxy-hydrogen flame is produced when Hydrogen is burn in pure oxygen. The flame has a temperature of about 2000 oC.It is used also for welding /cutting metals. 3. Oxy-hydrogen mixture is used as rocket fuel 4. A mixture of charcoal , petrol and liquid Oxygen is an explosive. (d) Chemical properties of Oxygen /combustion. Oxygen is a very reactive non metal. Many elements react with oxygen through burning to form a group of compounds called Oxides. Burning/combustion is the reaction of Oxygen with an element/substances. Reaction in which a substance is added oxygen is called Oxidation reaction. Burning/combustion is an example of an oxidation reaction. Most non metals burns in Oxygen/air to form an Oxide which in solution / dissolved in water is acidic in nature. They turn blue litmus red.e.g. Carbon(IV)oxide/CO2 , Nitrogen(IV)oxide/ NO 2 , Sulphur(IV)oxide/ SO 2 Some non metals burns in Oxygen/air to form an Oxide which in solution / dissolved in water is neutral in nature. They don’t turn blue or red litmus. e.g. Carbon(II)oxide/CO, Water/ H2O. All metals burns in Oxygen/air to form an Oxide which in solution/dissolved in water is basic/alkaline in nature. They turn red litmus blue.e.g. Magnesium oxide/MgO, Sodium Oxide/ Na2O ,Copper(II)oxide/CuO Elements/substances burn faster in pure Oxygen than in air. Air contains the inactive part of air that slows the rate of burning of substances/elements. (i) Reaction of metals with Oxygen/air The following experiments show the reaction of metals with Oxygen and air. I. Burning Magnesium Procedure (a) Cut a 2cm length piece of magnesium ribbon. Using a pair of tongs introduce it to a Bunsen flame. Remove it when it catches fire. Observe. Place the products in a beaker containing about 5cm3 of water. Test the solution/mixture using litmus papers (b) Cut another 2cm length piece of magnesium ribbon. Using a pair of tongs introduce it to a Bunsen flame. When it catches fire, lower it slowly into a gas jar containing Oxygen. Place about 5cm3 of water into the gas jar. Test the solution/mixture using litmus papers. Test the solution/mixture using litmus papers Observations (a)In air Magnesium burns with a bright blindening flame in air forming white solid/ash /powder. Effervescence/bubbles/ fizzing Pungent smell of urine. Blue litmus paper remains blue. Red litmus paper turns blue (b) In pure Oxygen Magnesium burns faster with a very bright blindening flame pure oxygen forming white solid/ash /powder. No effervescence/bubbles/ fizzing. No pungent smell of urine. Blue litmus paper remains blue. Red litmus paper turns blue Explanation Magnesium burns in air producing enough heat energy to react with both Oxygen and Nitrogen to form Magnesium Oxide and Magnesium nitride. Both Magnesium Oxide and Magnesium nitride are white solid/ash /powder. Chemical equations Magnesium + 2Mg(s) + Magnesium 3Mg(s) + + Oxygen O2(g) -> -> Nitrogen -> N2(g) -> Magnesium Oxide 2MgO(s) Magnesium Nitride Mg3N 2 (s) Magnesium Oxide dissolves in water to form a basic/alkaline solution of Magnesium hydroxide Chemical equations Magnesium Oxide + Water -> Magnesium hydroxide O2(l) 2Mg(s) + -> 2MgO(s) Magnesium Nitride dissolves in water to form a basic/alkaline solution of Magnesium hydroxide and producing Ammonia gas. Ammonia is also an alkaline/basic gas that has a pungent smell of urine. Chemical equations Magnesium Nitride + Water -> Magnesium hydroxide + Ammonia gas Mg3N2 (s) + 6H2O (l) -> 3Mg (OH)2 (aq) + 2NH3(g) II. Burning Sodium Procedure (a) Carefully cut a very small piece of sodium . Using a deflagrating spoon introduce it to a Bunsen flame. Remove it when it catches fire. Observe. Place the products in a beaker containing about 20cm3 of water. Test the solution/mixture using litmus papers (b) Carefully cut another very small piece of sodium. Using a deflagrating spoon introduce it to a Bunsen flame. When it catches fire, lower it slowly into a gas jar containing Oxygen. Place about 20 cm3 of water into the gas jar. Test the solution/mixture using litmus papers. Test the solution/mixture using litmus papers Observations (a)In air Sodium burns with a yellow flame in air forming a black solid. Blue litmus paper remains blue. Red litmus paper turns blue (b) In pure Oxygen Sodium burns faster with a golden yellow flame in pure oxygen forming a yellow solid. Effervescence/bubbles/ fizzing. Gas produced relights glowing splint.Blue litmus paper remains blue. Red litmus paper turns blue. Explanation (a) Sodium burns in air forming black Sodium Oxide Chemical equations -> -> Sodium Oxide dissolves in water to form a basic/alkaline solution of Sodium hydroxide Chemical equations Sodium Oxide + Water -> Sodium hydroxide + H2O (l) -> 2NaOH(aq) Na2O(s) (b) Sodium burns in pure oxygen forming yellow Sodium peroxide Chemical equations Sodium + 2Na(s) + Oxygen O2(g) -> -> Sodium peroxide Na2O2 (s) Sodium peroxide dissolves in water to form a basic/alkaline solution of Sodium hydroxide. Oxygen is produced. Chemical equations Sodium Oxide + Water -> Sodium hydroxide + Oxygen 2Na2O2 (s) 2H2O (l) -> + 4NaOH(aq) + O2 (l) III. Burning Calcium Procedure (a) Using a pair of tongs hold the piece of calcium on a Bunsen flame. Observe. Place the products in a beaker containing about 2cm3 of water. Test the solution/mixture using litmus papers (b) Using a pair of tongs hold another piece of calcium on a Bunsen flame. Quickly lower it into a gas jar containing Oxygen gas .Observe. Place about 2cm3 of water. Swirl. Test the solution/mixture using litmus papers Observations (a) In air Calcium burns with difficulty producing a faint red flame in air forming a white solid. Blue litmus paper remains blue. Red litmus paper turns blue (b) In pure Oxygen Calcium burns with difficulty producing a less faint red flame Oxygen forming a white solid. Blue litmus paper remains blue. Red litmus paper turns blue Explanation (a)Calcium burns in air forming white calcium Oxide. Calcium Oxide coat/cover the calcium preventing further burning. Chemical equations Calcium + Oxygen/air -> calcium Oxide O2(g) 2Ca(s) + -> 2CaO(s) Small amount of Calcium Oxide dissolves in water to form a basic/alkaline solution of Calcium hydroxide. The common name of Calcium hydroxide is lime water. Chemical equations Calcium Oxide + Water -> Calcium hydroxide CaO(s) + H2O (l) -> Ca(OH) 2 (aq) IV. Burning Iron Procedure (a) Using a pair of tongs hold the piece of Iron wool/steel wire on a Bunsen flame. Observe. Place the products in a beaker containing about 2cm3 of water. Test the solution/mixture using litmus papers (b) Using a pair of tongs hold another piece of Iron wool/steel wire on a Bunsen flame. Quickly lower it into a gas jar containing Oxygen gas .Observe. Place about 2cm3 of water. Swirl. Test the solution/mixture using litmus papers Observations (a)In air Iron wool/steel wire burns producing a Orange flame in air forming a brown solid. Blue litmus paper remains blue. Red litmus paper turns faint blue (b) In pure Oxygen Iron wool/steel wire burns producing a golden Orange flame in Oxygen forming a Brown solid. Blue litmus paper remains blue. Red litmus paper turns faint blue Explanation (a)Iron burns in air forming brown Iron(III) Oxide Chemical equations Iron + Oxygen/air -> Iron(III) Oxide 4Fe(s) + 3O2(g) -> 2Fe2O3(s) Very small amount of Iron(III)Oxide dissolves in water to form a weakly basic/alkaline brown solution of Iron(III) hydroxide. Chemical equations Calcium Oxide + Water -> Iron(III) hydroxide Fe2O3(s) 3H2O (l) 2Fe(OH) 3 (s) + -> V. Burning Copper Procedure (a) Using a pair of tongs hold the piece of copper turnings/shavings on a Bunsen flame. Observe. Place the products in a beaker containing about 2cm3 of water. Test the solution/mixture using litmus papers (b) Using a pair of tongs hold another piece of Copper turnings/shavings on a Bunsen flame. Quickly lower it into a gas jar containing Oxygen gas .Observe. Place about 2cm3 of water. Swirl. Test the solution/mixture using litmus papers Observations (a) In air Copper turnings/shavings burns with difficulty producing a green flame in air forming a black solid. Blue litmus paper remains blue. Red litmus paper turns faint blue (b) In pure Oxygen Copper turnings/shavings burns less difficulty producing a green flame in Oxygen forming a Brown solid. Blue litmus paper remains blue. Red litmus paper turns faint blue Explanation (a) Copper burns in air forming black Copper(II) Oxide Chemical equations Copper + Oxygen/air -> Copper(II) Oxide 2 Cu(s) + O2(g) -> 2CuO(s) Very small amount of Copper(II)Oxide dissolves in water to form a weakly basic/alkaline blue solution of Copper(II) hydroxide. Chemical equations Copper(II) Oxide + Water -> Copper(II) hydroxide H2O (l) Cu(OH) 2 (s) CuO(s) + -> (i) Reaction of non metals with Oxygen/air The following experiments show the reaction of non metals with Oxygen and air. I. Burning Carbon Procedure (a) Using a pair of tongs hold a dry piece of charcoal on a Bunsen flame. Observe. Place the products in a beaker containing about 2cm3 of water. Test the solution/mixture using litmus papers (b) Using a pair of tongs hold another piece of dry charcoal on a Bunsen flame. Quickly lower it into a gas jar containing Oxygen gas .Observe. Place about 2cm3 of water. Swirl. Test the solution/mixture using litmus papers Observations -Carbon chars then burns with a blue flame -Colourless and odourless gas produced -Solution formed turn blue litmus paper faint red. Red litmus paper remains red. Explanation Carbon burns in air and faster in Oxygen with a blue non-sooty/non-smoky flame forming Carbon (IV) oxide gas. Carbon burns in limited supply of air with a blue non-sooty/non-smoky flame forming Carbon (IV) oxide gas. Carbon (IV) oxide gas dissolve in water to form weak acidic solution of Carbonic (IV)acid. Chemical Equation Carbon + Oxygen -> Carbon(IV)oxide (excess air/oxygen) C(s) Carbon + O2(g) -> CO2(g) + Oxygen -> (limited air/oxygen) (in excess air) Carbon(II)oxide 2C(s) + O2(g) -> 2CO(g) Carbon(IV)oxide + Water -> CO2(g) + H2O (l) -> (in limited air) Carbonic(IV)acid H2CO3 (aq) (very weak acid) II. Burning Sulphur Procedure (a) Using a deflagrating spoon place sulphur powder on a Bunsen flame. Observe. Place the products in a beaker containing about 3cm3 of water. Test the solution/mixture using litmus papers (b) Using a deflagrating spoon place sulphur powder on a Bunsen flame. Slowly lower it into a gas jar containing Oxygen gas. Observe. Place about 5cm3 of water. Swirl. Test the solution/mixture using litmus papers. Observations -Sulphur burns with a blue flame -Gas produced that has pungent choking smell -Solution formed turn blue litmus paper faint red. Red litmus paper remains red. Explanation Sulphur burns in air and faster in Oxygen with a blue non-sooty/non-smoky flame forming Sulphur (IV) oxide gas. Sulphur (IV) oxide gas dissolve in water to form weak acidic solution of Sulphuric (IV)acid. Chemical Equation Sulphur + Oxygen -> Sulphur(IV)oxide S(s) + O2(g) -> SO2(g) (in excess air) Sulphur(IV)oxide + Water -> Sulphuric(IV)acid SO2(g) + H2O (l) -> H2SO3 (aq) (very weak acid) III. Burning Phosphorus Procedure (a) Remove a small piece of phosphorus from water and using a deflagrating spoon (with a lid cover)place it on a Bunsen flame. Observe. Carefully put the burning phosphorus to cover gas jar containing about 3cm3 of water. Test the solution/mixture using litmus papers (b) Remove another small piece of phosphorus from water and using a deflagrating spoon (with a lid cover) place it on a Bunsen flame. Slowly lower it into a gas jar containing Oxygen gas with about 5 cm3 of water. Observe. Swirl. Test the solution/mixture using litmus papers. Observations -Phosphorus catches fire before heating on Bunsen flame -Dense white fumes of a gas produced that has pungent choking poisonous smell -Solution formed turn blue litmus paper faint red. Red litmus paper remains red. Explanation Phosphorus is stored in water.On exposure to air it instantaneously fumes then catch fire to burn in air and faster in Oxygen with a yellow flame producing dense white acidic fumes of Phosphorus(V) oxide gas. Phosphoric(V) oxide gas dissolve in water to form weak acidic solution of Phosphoric (V)acid. Chemical Equation Phosphorus + Oxygen -> Phosphorous(V)oxide 4P(s) + 5O2(g) Phosphorous(V)oxide + Water + 3H2O (l) -> P2O5(s) -> -> 2P2O5(s) Phosphoric(V)acid 2H3PO 4 (aq) (very weak acid) (e) Reactivity series/competition for combined Oxygen. The reactivity series is a list of elements/metals according to their affinity for oxygen. Some metals have higher affinity for Oxygen than others. A metal/element with higher affinity for oxygen is placed higher/on top of the one less affinity. The complete reactivity series of metals/elements Element/Metal Most Carbon Zinc Iron Tin Lead Hydrogen Copper Mercury Silver Gold Platinum Zn Cu Hg Au Pt Least reactive Metals compete for combined Oxygen. A metal/element with higher affinity for oxygen removes Oxygen from a metal lower in the reactivity series/less affinity for Oxygen. When a metal/element gains/acquire Oxygen, the process is called Oxidation. When a metal/element donate/lose Oxygen, the process is called Reduction. An element/metal/compound that undergo Oxidation is called Reducing agent. An element/metal/compound that undergo Reduction is called Oxidizing agent. A reaction in which both Oxidation and Reduction take place is called a Redox reaction. Redox reaction between Magnesium and copper(II)Oxide Procedure Place about 2g of copper (II)oxide in a crucible with a lid. Place another 2g of Magnesium powder into the crucible. Mix thoroughly. Cover the crucible with lid. Heat strongly for five minutes. Allow the mixture to cool. Open the lid. Observe. Observation Colour change from black to brown. White solid power formed. Explanation Magnesium is higher in the reactivity series than Copper. It has therefore higher affinity for Oxygen than copper. When a mixture of copper(II)oxide and Magnesium is heated, Magnesium reduces copper(II)oxide to brown copper metal and itself oxidized to Magnesium oxide. Magnesium is the reducing agent because it undergoes oxidation process. Copper(II)oxide is the oxidizing agent because it undergo redox reduction process. The mixture should be cooled before opening the lid to prevent hot brown copper from being reoxidized back to black copper(II)oxide. The reaction of Magnesium and Copper(II)oxide is a reaction Chemical equation Reduction process Oxidation 1. Copper (II)oxide + Magnesium -> Magnesium oxide + Copper (black) (white ash/solid) (brown) CuO(s) + Mg(s) -> MgO(s) + Cu(s) (Oxidizing Agent) (Reducing Agent) 2. Zinc (II)oxide + Magnesium -> Magnesium oxide + Zinc (yellow when hot) (white ash/solid) (grey) ZnO(s) + Mg(s) -> MgO(s) + Zn(s) (Oxidizing agent) (Reducing agent) 3. Zinc (II)oxide + Carbon -> Carbon(IV) oxide gas + Zinc (yellow when hot) (colourless gas) (grey) ZnO(s) + C(s) -> CO2(g) + Zn(s) (Oxidizing agent) (Reducing agent) The reactivity series is used during extraction of metals from their ore.An ore is a rock containing mineral element which can be extracted for commercial purposes. Most metallic ores occur naturally as: (i) oxides combined with Oxygen (ii)sulphides combined with Sulphur (iii)carbonates combined with carbon and Oxygen. Metallic ores that naturally occur as metallic sulphides are first roasted in air to form the corresponding oxide. Sulphur(IV)oxide gas is produced. e.g. Copper(I) sulphide + Oxygen -> Copper(I)Oxide + Sulphur(IV)oxide Cu2S(s) + O2(g) -> 2Cu(s) + SO2(g) Zinc(II) sulphide ZnS(s) + Oxygen -> Zinc(II)Oxide + O2(g) -> Zn(s) + Sulphur(IV)oxide + SO2(g) Lead(II) sulphide PbS(s) + Oxygen -> Lead(II)Oxide + Sulphur(IV)oxide + O2(g) -> Pb(s) + SO2(g) Iron(II) sulphide + Oxygen + O2(g) FeS(s) -> Iron(II)Oxide -> Fe(s) + + Sulphur(IV)oxide SO2(g) Metallic ores that naturally occur as metallic carbonates are first heated in air. They decompose/split to form the corresponding oxide and produce Carbon (IV) oxide gas. e.g. Copper (II)carbonate -> Copper(II)oxide + Carbon(IV)oxide -> CuO(s) + CO2(g) CuCO3(s) Zinc (II)carbonate -> Zinc(II)oxide + Carbon(IV)oxide -> ZnO(s) + CO2(g) ZnCO3(s) Lead (II)carbonate -> Lead(II)oxide + Carbon(IV)oxide -> PbO(s) + CO2(g) PbCO3(s) Iron(II)carbonate FeCO3(s) Metallic ores -> Iron(II)oxide -> FeO(s) + Carbon(IV)oxide + CO2(g) TOPICAL QUESTIONS ON THE TOPIC BASED ON PAST KCSE PAPERS 1. 2. Study the experiment set up represented by the diagram below and answer the question that follows. a) Explain what would be observed if red and blue litmus papers were dipped into the water at the end of experiment. (2mks) b) Write an expansion in terms of X and Y to show the (%) percentage of gas used by the burning candle. (1mk) The diagram below represents two iron nails with some parts wrapped tightly with zinc and copper strips respectively. What observations would be made at the exposed points A and B if the wrapped nails are left in the open for several months? Explain. (3mks) 3. In an experiment, rods of metals P, Q and R were cleaned with a sand paper and placed in a beaker containing water. Another set of rods was also cleaned and placed in a beaker containing dilute acid. After placing the rods in the two liquids bubbles of gas were seen around some of the rods as shown in the diagram below. 23 a) Why was it necessary to clean the rods with sand paper before dipping them into the liquids? (1mk) b) Arrange the three metals in order of their reactivity starting with the most reactive. (1mk) 4. When magnesium is burnt in air it reacts with oxygen and nitrogen gas giving a white ash. Write two equations for the two reactions that take place. (2mks) 5. Oygen reacts with the elements phosphorous, sulphur and chlorine to form oxides in which the elements is in its highest oxidation number. The table below gives the oxide of sulphur and its highest oxidation number. Complete the table for phosphorous and chlorine. (Atomic number p=15, s=16, Cl= 17) (2mks) Elements Oxides Highest oxidation number P S SO 3 +6 Cl 6. Write an equation for the reaction that takes place when carbon (II) Oxide gas is passed over heated Lead (II) Oxide. (1mk) 7. The set up below was used to study some properties of air State and explain two observation that would be made t the end of the experiment. (3mks) 8. Give the formula of an oxide which reacts both dilute Hydrochloric acid and hot concentrated sodium hydroxide. 9. In an experiment a certain volume of air was passed repeatedly from syringe over heated excess zinc powder as shown in the diagram below. The experiment was repeated using excess magnesium powder. In which of the experiments was the change in volume of air greatest? Give reasons. (3mks) 10. 11. State and explain the change in mass that occurs when the following substances are separately heated in open crucibles. i) Copper metal ii) Copper (II) Nitrate (3mks) The diagram below shows an iron bar, which supports a bridge. The iron is connected to Iron bar a piece of magnesium metal. Connecting wire Soil Magnesium metal Explain why it is necessary to connect the piece of magnesium metal to the iron bar. 12. Explain why magnesium continue to burn in a gas jar full of Sdulphur (IV) Oxude while burning splint would be extinguished. 13. The diagram below is a set up for the laboratory preparation of oxygen gas. a) Name solid R. (1mk) b) Write an equation for the reaction that takes place in the flask. (1mk) c) Give one commercial use of oxygen. 14. Nitrogen (II) Oxide and nitrogen (IV) Oxide are some of the gases released from car exhaust pipes. State these gases affect the environment. (2mks) 15. The set up below was used to abtain a sample of iron. Write two equations which occur in the combustion tube. 16. (2mks) The low chart below outlines some of the process involved during extraction of copper from pyrites. Study it and answer the questions that follow. a) b) i) Name gas K. (1mk) ii) Write an equation for the reaction that takes place in 1 st roasting furnance. (1mk) iii) Write the formula of the cations present in the slag M (1mk) iv) Identify gas P. v) What name is given to the reaction that takes placein chamber N? Give a reason for your answer. (2mks) (1mk) Copper obtained from chamber N is not pure. Draw a labelled diagram to show the set up you would use to refine the copper by electrolysis. (2mks) 17. c) Given that the mass of copper obtained from the above extraction was 210 kg, determine the percentage purity of the ore (copper pyrite) if 810 kg of it was fed to 1st roasting furnance. Cu= 63.5, Fe= 56.0, S=32.0 (3mks) d) Give two effects that this process could have on the environment. (2mks) The table below gives the information about the major constituents of crude oil. Study it and answer the questions that follow. Constituents Boiling point in 0 C Gases Below 40 Petrol 49-175 Kerosene 175-250 Diesel oil 259-350 Lubricating oil 350-400 Bitumen Above 400 i) Which one of the constituent of crude oil has molecules with the highest number of carbon atoms? (2mks) ii) Name the process you would use to separate a mixture of petrol and diesel and explain how the separation takes place. (2mks) iii) Explain why constituents of crude oil do not have sharp boiling points. iv) a) Name one gas that is likely to be a constituent of crude oil and write its formula. 18. (2mks) (2mks) b) What conditions could cause a poisonous gas to be formed when kerosene is burnt. Explain. (2mks) c) Give one use of bitumen. (1mk) The diagram below shows a set up used by a student in an attempt to prepare collect oxygen gas a) b) i) Complete the diagram by collecting the mistakes in it. (2mks) ii) Identify solid w. (1mk) A piece of phosphorous was burnt in excess air. The amount of hot water to make a solution. i) Write an equation for the burning of phosphorous in excess air. (1mk) ii) The solution obtained in (b) above was found to have a PH of 2.0. Give reasons for this observation. (2mks) c) Explain why cooking pots made of aluminium do not corrode easily when exposed to air. (1mk) d) The reaction between sulphure (IV) Oxide and oxygen to form Sulphur (VI) Oxide per day (condition for the reaction a catalyst, 2 atmospheric pressure and temperature between 400 0 5000 C) 2SO(aq) + O 2(g) 2SO 3(g) Factory manufacturing sulphuric acid by contact process produces 350kg of sulphur trioxide per day (conditions) for the reaction catalyst. 2 atmospheres pressure and temperatures between 400 – 500 o C. i) What is meant by an exothermic reaction? (1mk) ii) How would the yield per day of sulphur trioxide be affected lower than 400oC are used? Explain. iii) All the sulphur (VI) Oxide produced was absorbed in concentrated sulphuric acid to form oleum. if temperatures (1mk) SO3(g) + H4 SO4(l) → H2 S2 O7(l) Calculate the mass of oleum that was produced per day. (S+ 32.0, O= 16: H 1.0) 19. a) (3mks) Fractional distillation of liquid air usually produces nitrogen and oxygen as the major by-product. i) Name one substance that is used to remove carbon (IV) Oxide from air before it is changed into liquid. (1mk) ii) Describe how liquid Nitrogen gas is obtained from liquid air. Boiling points; Nitrogen = -1960C; Oxygen = -1830C. b) (1mk) Study the flow chart below and answer the questions that follows i) Name element M. (1mk) c) ii) State and explain the change in mass that is likely to occur in tube N by the end of the experiment. (2mks) iii) Name two gases that come out through tube M. (1mk) iv) Write an equation for the reaction in stem 7. (1mk) v) Give one use of Ammonium –Nitrate. (1mk) State and explain the observations that would be made if a sample of sulphur is heated with concentrated Nitric acid. (Nitric (V) acid. 20. a) Candle wax is mainly a compound consisting of two elements. Name the two elements b) The up below was used to investigate the burning of candle. Study it and answer the questions that follow. i) What would happen to the burning candle if the pump were turned off? Give reasons. (3mks) ii) State and explain the change in mass that is likely to occur in tube N by the end of the experiment. (2mks) iii) Name another substance that would be used in place of calcium oxide. (1mk) 21. Why is iron not used to make steam boilers? 22. Study the arrangement below and answer the questions that follows. (1mk) Explain what happens to the lime water after some time. (1mk) 23. When air is bubble through pure water (Ph 7.0). The PH drops to 6.0. Explain why. (1mk) 24. A white compound was moistened with a little concentrated Hydrochloric acid and placed over a flame. A yellow flame was observed. Identify the metallic ions in the compound. (1mk) 25. Magnesium ribbon was burned in a gas jar of Nitrogen. A few drops of water were then added to the jar. Write equation for the reactions in the jar. (2mks) 26. The diagram below shows an experiment to compare the heating effect of luminous and non luminous flame. 27. a) What was observed at the bottom of each beaker at the end of the experiment? (1mk) b) Which sample of water boils first? Give a reason for your answer. c) Besides the amount of heat produced by the two flames, state other differences. (2mks) a) Study the equation below and answer the questions that follow. CO 3-2(aq) + H2 O (l) → HCO (aq) + OH (aq) Which substance is an oxidizing agent? Give reasons. b) (2mks) Identify the reducing agent in the equation below Fe 28. (2mks) 2+ (aq) + Cl2(g) → Fe3+(aq) + 2CL –(aq) A candle was burnt using the apparatus shown below. The initial volume of measuring cylinder was 90cm3. The apparatus was allowed to cool and the volume of air in the measuring cylinder had dropped to 70cm 3. 29. a) Why was the volume recorded when the air was cooled? (1mk) b) What was the purpose of sodium Hydroxide? (1mk) c) Use the results given to calculate the percentage of oxygen in air. (2mks) The graph below shows the changes that occur when a pure and an impure substance are heated. a) Which curve represents pure substance? Explain. (2mks) b) Name one factor which affects the melting point of a solid and state effects. (2mks) ANSWERS 1. 2. 3. (a) The blue litmus paper would turn pink/ red. Red litmus paper remains red. The carbon (IV) oxide produced when the candle burns dissolves in water to form a solution of weak carbonic acid. (b) x- y x 100% x Observation: At No rusting takes place Explanation: Zinc is more reactive than iron. It reacts with oxygen in presence to iron hence preventing it from rusting. It acts as a sacrificial metal Observation at B The nail is covered by reddish brown substance/coating/rust Explanation: Copper is less reactive than iron. Iron combines first with oxygen in presence of moisture and rust. (a) (b) To remove the layer of oxide on their surfaces which could inhibit the reaction Q, R,P 4. 2Mg(s) + O¬ 2(g) → 2MgO(s) 3 Mg(s) + N 2(g) →Mg3 N2(s) 5. Oxide: Highest oxidation number P2O5 (+5) Cl2 O 7 (+7) 6. CO(g) + PbO(s) →Pb(s) + CO2 (g) 7. Observations -Iron will be covered by a reddish brown substance/coating/rust -Water in test tube rise and water in a beaker drops Explanation: Iron Combines with oxygen in a presence of moisture to form hydrated Iron (III) oxide / rust water rises up to occupy the space which was occupied by oxygen in the tube. 8. Al2O3 (Aluminium Oxide) 9. Change was greatest with Magnesium. Both react with oxygen gas to form oxides, but magnesium also reacts with nitrogen to form magnesium nitrate (Mg3N2) (i) Mass increase: Oxygen combines with copper metal to form copper (II) Oxide. 10. (ii) Mass decrease: copper Nitrate decomposes to give gases that escape leaving behind copper (II) oxide. 11. Magnesium is above iron in the reactivity series. It supply electrons to the iron bar hence prevent it from rusting/ cathode protection. 12. Magnesium produces a lot of heat/ energy when burning. This splint sulphur (IV) oxide into sulphur and Oxygen. Magnesium burns in the oxygen produced. Burning splint produces less energy which is not enough to break sulphur (IV) oxide. 13. (a) Manganese (IV) Oxide/ MnO2(s) (b) 2 H2 O2 (aq) MnO 2 2 H2 ¬O(l) + O2 (c) - Respiratory aids from patients suffering from respiratory diseases / during surgery. - High mountain climbers and deep see divers - Helps in combustion of rocket fuel - Welding together with other gases such as hydrogen/ oxygen (hydrogen flame) acetylene/ oxyacetylene flame. 14. Nitrogen (II) Oxide is oxidized by oxygen in air to form nitrogen (IV) oxide. This gas is acidic when dissolved in water. May cause acidic rain. If inhaled by animals/ man may corrode respiratory surfaces exposing them to disease causing agents. 15. 2C(s) + O2 (g) →2CO(g) Fe2 O3 + 3CO(g) →2 Fe(s) + 3 Co2(g) 16. (i) (ii) (iii) (iv) (v) (b) SO2/ sulphur(IV) Oxide 2CuFeS 2 + 402 (g) → 2FeO(s) + 3S)2 (g) + Cu2S(s) Fe2+ Carbon (IV) Oxide or carbon (II) Oxide Reduction/ oxidation = Redox since Cu2O is reduced to Cu and CO oxidized to Co2 (c) Mole ratio of CU in CuFeS 2 = 1.1 Moles of Cu produced = 210 = 3.3 moles 63.5 RFM of CuFeS2 = 63.5 + 56 + 64 = 183.5 Mass of Cu in CuFeS 2 = 3.3 x 183. 5 = 605. 6 kg % purity = 605 x 100 = 74. 76% 810 17. (d) - Formation of acidic rain due to presence of sulphur (IV) oxide - Sulphur (IV) oxide is poisonous - Carbon (II) is poisonous - Global warming due to presence of carbon (IV) oxide - Dumping of wastes like slag prevents growth of vegetation - Soil erosion due to the excavation of the ores (i) (ii) (iii) Bitumen: It has the highest boiling point Fractional distillation: they have different boiling points, petrol boils out first Each component is a mixture of hydrocarbons/ impure or there is presence of isomes in each component. Methane → CH4 all alkane gases up to C = 4 Burning in limited air will produce carbon (II) oxide which is poisonous - Manufacture of tar used in tarmac road/ surface of roads - Amending leaking roofs. (i) (iv) (b) (c) 18. (a) (b) (c) (d) (ii) Sodium peroxide Na 2 O 2 (i) 4P(s) + 5O2 (g) → 2P2 O5 (g) (ii) Phosphorous (V) oxide dissolves in water to form an acid (Phosphoric acid) A firm oxide (aluminium Oxide) is formed on the surface of the metal. This oxide protect aluminium from further attack (i) A reaction which proceeds by production of heat i.e heat is lost to the surroundings. 19. (a) (b) (c) (ii) The yield be lowered: through by Le- Chateliers principle, the yield is expected to increase. But lower temperatures will result into fewer particles attaining activation energy. (iii) RMM of SO 3 = 32 + 48 = 80 Moles of SO 3 used = 350 = 4.38 moles 80 Moles of H2S2O7 = 4.38 moles RMM of H2 S2 O7 = 2 + 64 + 112 = 178 Mass of H 2 S2 O7 = 4.38 x 178 = 779.6 kg (i) (ii) Potassium Hydroxide or sodium hydroxide Air allowed to expand and warm up. Nitrogen gas vaporizes first since it is more volatile. On further heating- oxygen vaporizes. (i) Hydrogen gas (ii) - For the complete oxidation of ammonia gas To increase the yield of nitrogen (II) Oxide To reduce the cost (iii) Nitrogen gas NH3(g) + HNO3(aq) →NH4 NO3(aq) (iv) Brown gas (Nitrogen (IV) Oxide gas) and an acidic gas (sulphur (IV) oxide) formed Nitric acid reduced into nitrogen (IV) oxide, water and oxygen. Sulphur is oxidized into sulphur (IV) oxide which dissolves in water forming sulphuric acid. 20. (a) (b) Carbon and hydrogen (i) The candle will go off/ extinguished since carbon (IV) oxide and water vapour accumulate around the candle carbon (IV) oxide does not support burning. OR The supply of oxygen will be supported and candle goes off (ii) Mass increase Water combines with calcium oxide to form calcium hydroxide solution. This combine with carbon (IV) oxide to form calcium carbonate. (iii) - Carbon (IV) oxide - Carbon (II) oxide (iv) Protect calcium from obtaining water from the atmosphere (v) -Concentrated sulphuric acid -Calcium chloride 21. 22. Iron metal is corroded by rust in presence of water and oxygen There will be formation of a white precipitate. Candle burns producing carbon (IV) oxide. 23. 24. 25. Air contains carbon (IV) Oxide which dissolve in water producing a weak carbonic acid Na + ions 3Mg(s) + N 2 (g) →Mg3 N2 (s) Mg3 N2(s) + 6H 2 O(l) → 3Mg(OH)2(aq) + 2NH3 26. (a) (b) (c) Beaker A: No soot at the bottom Beaker B: A lot of black soot at the pattern Sample A: Non luminous flame produces a lot of heat. Luminous - Produce a lot of light - Very sooty - Large and wary - Burns quietly 27. (a) (b) 28. (a) (b) (c) 29. (a) (b) Non Luminous - Produces less light - Not Sooty - Short and steady - Burns with roaring noise CO -2 is an oxidizing agent. It removes hydrogen from water (H2O) and oxidizes it to OH. Fe2+ is a reducing agent. It adds electrons to Cl2 and reduces it to 2CLTo allow all oxygen to be used up and also to allow the gas to contract/ cater for any expansion of gases To absorb carbon (IV) oxide which was produced by the burning candle % of oxygen 90 – 70 x 100 = 22.2% 90 Curve B: Pure substances has sharp/ fixed constant melting and boiling points Impurities rises the boiling point pressure rises the boiling point i.e when pressure is high b.p is very high.