IGCSE Official Exam Material
FREE Past Paper Questions by Topic
If you need the markscheme, including the examiners' reports for each question, and more exam and teaching materials on this topic, contact me through the contact page
Topic 3: Physical Chemistry
Subtopic 3a: Energetics
1) Iron can be formed by reacting aluminium powder with iron(III) oxide.
The diagram shows how this reaction can be demonstrated.
When the magnesium fuse is lit, a very exothermic reaction occurs.
(i) State the meaning of the term exothermic. (1) ............................................................................................................................................. .............................................................................................................................................
(Total for question = 1 mark )
2) The teacher uses this apparatus to heat 100 g of water.
He records the temperature of the water before and after heating. temperature of water before heating = 21.0 °C temperature of water after heating = 70.5 °C
(i) Calculate the heat energy change (Q) in joules.
[specific heat capacity of water is 4.2 J /g/°C] (3)
Q = ........................................................... J
(ii) The student burns 0.0200 mol of ethanol.
Use this information and your value for Q to calculate the molar enthalpy change (ΔH), in kJ/mol, for the combustion of ethanol. Include a sign in your answer. (2)
ΔH = ........................................................... kJ/mol
(Total for question = 5 marks)
3) A student uses this apparatus to investigate the heat energy released when a liquid fuel is burned.
This is the student's method.
• measure the mass of the spirit burner and fuel
• add 100 cm3 of water to the copper can
• record the temperature of the water
• use the spirit burner to heat the water until the temperature rises by 30 °C
• immediately measure the new mass of the spirit burner and fuel
(a) Suggest why the student measures the mass of the spirit burner and fuel immediately after heating the water. (1) ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(b) When the fuel is burned, the student notices that a black solid forms on the bottom of the copper can.
(i) Identify the black solid. (1)
.............................................................................................................................................
(ii) Explain why the black solid forms. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(c) (i) Show that the heat energy change, Q, to raise the temperature of 100 cm3 of water by 30 °C is approximately 13 kJ. [mass of 1.0 cm3 of water = 1.0 g]
[c for water = 4.2 J/g/ °C] (3)
(ii) The student burns 0.96 g of methanol, CH3OH.
Calculate the molar enthalpy change, ΔH, in kJ/mol, for the combustion of methanol. Include a sign in your answer. [Mr of methanol = 32] (3)
ΔH = ........................................................... kJ/mol
4) A student uses this apparatus to investigate the temperature change when magnesium powder reacts with hydrochloric acid.
The student uses this method.
• pour 100 cm3 of hydrochloric acid into a polystyrene cup
• measure the initial temperature of the acid
• add the magnesium powder and stir
• measure the highest temperature reached by the mixture
The table shows the student's results.
(i) Use the student's results to explain the type of reaction that occurs when magnesium is added to hydrochloric acid. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(ii) Give a reason why the student uses a polystyrene cup. (1) ............................................................................................................................................. .............................................................................................................................................
(iii) Use the student's results to calculate the heat energy change (Q), in joules, for this reaction.
[for 1.00 cm3 of the mixture, mass = 1.00 g]
[for the mixture, c = 4.2 J / g / °C] (3)
Q = ........................................................... J
(Total for question = 6 marks)
5) When ethanol is burned in air, complete combustion can occur.
The equation for this reaction is C2H5OH + 3O2 → 2CO2 + 3H2O
This equation can also be written using displayed formulae to show all the covalent bonds in the molecules.
The table gives the bond energies for these bonds.
(i) Use values from the table to calculate the energy needed to break all the bonds in the reactants. (2)
energy needed ........................................................... kJ
(ii) Use values from the table to calculate the energy released when all the bonds in the products are formed. (2) energy released ........................................................... kJ
(iii) Calculate the molar enthalpy change (ΔH) in kJ/mol, for the complete combustion of ethanol.
Include a sign in your answer. (1)
ΔH = ........................................................... kJ/mol
(Total for question = 5 marks)
6) Hydrogen peroxide solution decomposes slowly at room temperature to form water and oxygen.
The equation for the reaction is 2H2O2 → 2H2O + O2 (c)
The decomposition of hydrogen peroxide solution is exothermic.
On the diagram, draw and label the reaction profiles for the reaction
• without a catalyst
• with a catalyst
(i) Use this information to calculate the total amount of energy needed to break all the bonds in two moles of H2O2 (1) energy needed = ........................................................... kJ
(ii) Use this information to calculate the total amount of energy released when all the bonds in two moles of H2O are formed. (1)
energy released = ........................................................... kJ
(iii) Use the value of ΔH and your answers for (i) and (ii) to calculate the bond energy, in kJ/mol, for the bond. (2)
bond energy = ........................................................... kJ/mol
(Total for question = 6 marks)
Subtopic 3b: Rates of reaction
1) A student uses this apparatus to investigate the rate of reaction between marble chips and dilute hydrochloric acid.
The equation for the reaction is CaCO3 + 2HCl → CaCl2 + H2O + CO2 (a)
During the reaction the mass of the contents of the flask decreases.
(i) State why the mass of the contents of the flask decreases. (1) ............................................................................................................................................. .............................................................................................................................................
(ii) State the purpose of the cotton wool. (1) ............................................................................................................................................. .............................................................................................................................................
(iii) Explain why sulfuric acid is not a suitable acid to use in this investigation. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(i) In the investigation the marble chips are in excess.
Explain the shape of the graph. (4)
............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(ii) The student repeats the experiment using the same volume of hydrochloric acid but of half the concentration of the original acid.
All other conditions are kept the same. On the grid, draw the curve the student would obtain. (2)
(c) Explain, using particle collision theory, how increasing the temperature affects the rate of a reaction. (4) ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 14 marks)
2) A student uses this apparatus to investigate the rate of reaction between calcium carbonate chips and dilute hydrochloric acid.
Every 20 seconds the student records the reading on the balance. (a) Explain why using a cotton wool plug increases the accuracy of the student's results. (2)
............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(b) Complete the equation for the reaction by adding the state symbols. (2)
CaCO3(................) + 2HCl(................) → CaCl2(aq) + H2O(................) + CO2(................)
3) Hydrogen peroxide solution decomposes to give water and oxygen gas.
The equation for this reaction is 2H2O2(aq) → 2H2O(l) + O2(g)
(a) Three different solids are catalysts for the decomposition of hydrogen peroxide solution.
A student is given hydrogen peroxide solution and a sample of each of the solid catalysts.
The student has a timer, a measuring cylinder, a balance and the apparatus shown in the diagram.
Describe a method the student could use to find which of the three solids is the most effective catalyst for the decomposition of hydrogen peroxide solution. (5)
............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(b) The diagram shows the reaction profile for the decomposition of hydrogen peroxide without a catalyst.
(i) Label the diagram to show the activation energy (Ea) and the enthalpy change (ΔH) for this reaction. (2) (ii) On the diagram, draw a curve to show the reaction profile for the same reaction when a catalyst is used. (1) (Total for question = 8 marks)
4) Hydrogen peroxide solution decomposes slowly at room temperature to form water and oxygen. The equation for the reaction is 2H2O2 → 2H2O + O2 (a) A catalyst increases the rate of this reaction. State one other property of a catalyst. (1) ............................................................................................................................................. .............................................................................................................................................
5) In stage 2, sulfur dioxide is reacted with oxygen to form sulfur trioxide gas.
2SO2(g)+ O2(g) 2SO3(g)
The yield of sulfur trioxide is approximately 98%.
(i) A catalyst is used in this reaction.
Explain how a catalyst increases the rate of a reaction. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 2 marks)
6) This question is about nitrogen and some of its compounds.
(a) Nitrogen and oxygen do not react together at room temperature.
At the high temperatures in a car engine, nitrogen and oxygen react to form nitrogen monoxide, NO
(i) Give a chemical equation for this reaction. (1) .............................................................................................................................................
(ii) Give a reason why this reaction only occurs at high temperatures. (1) ............................................................................................................................................. .............................................................................................................................................
(iii) State why it is important that oxides of nitrogen are not released into the atmosphere. (1) ............................................................................................................................................. .............................................................................................................................................
(b) Nitrogen monoxide gas can be removed from car exhaust fumes when it reacts with carbon monoxide gas.
(i) The rate of the reaction is increased by passing the gases over a catalyst.
Explain how a catalyst increases the rate of a reaction. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(ii) Explain how increasing the pressure of gases increases the rate of reaction. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 8 marks)
7) This question is about reactions that form gases.
(a) Hydrogen peroxide decomposes to form water and oxygen.
The equation for the reaction is 2H2O2 → 2H2O + O2 25.0 cm3 of hydrogen peroxide solution are poured into a conical flask and 1.00 g of solid manganese(IV) oxide is added.
Bubbles of oxygen gas are formed.
(i) Give the test for oxygen gas. (1)
............................................................................................................................................. .............................................................................................................................................
(ii) Describe a method to show that solid manganese(IV) oxide is a catalyst in this reaction and not a reactant. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
Subtopic 3c: Reversible reactions and equilibria
1) Ethanol reacts with methanoic acid, HCOOH, in the presence of an acid catalyst to form an ester.
The equation for the reaction is C2H5OH + HCOOH ⇔ HCOOC2H5 + H2O
(iii) When this reaction takes place in a sealed container, the reaction can reach dynamic equilibrium. Give two characteristics of a reaction at dynamic equilibrium. (2)
1 .......................................................................................................................................... .............................................................................................................................................
2 .......................................................................................................................................... .............................................................................................................................................
(Total for question = 2 marks)
2) Dinitrogen tetraoxide, N2O4, is a colourless gas.
Nitrogen dioxide, NO2, is a brown gas.
The two gases can exist together in dynamic equilibrium according to the equation
(a) Explain what is meant by the term dynamic equilibrium. (2)
............................................................................................................................................. .............................................................................................................................................
(b) Some N2O4 and some NO2 are put into a sealed gas syringe and allowed to form an equilibrium mixture.
This equilibrium mixture is brown.
(i) The pressure of the gas in the syringe is increased by pushing in the piston.
The mixture is then allowed to reach a new equilibrium at the same temperature as before.
Explain why the new equilibrium mixture contains less NO2 than the original equilibrium mixture. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(ii) A student suggests that the new equilibrium mixture would be lighter in colour than the original equilibrium mixture, as there is now less NO2 present.
Suggest why the new equilibrium mixture is actually darker than the original. (1) ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
3) Carbon monoxide, CO, and oxides of nitrogen are produced in a car engine when petrol is burned.
These oxides can be partly removed by using a catalytic converter fitted to the car's exhaust system.
(i) State how oxides of nitrogen are produced in the car engine. (1) ............................................................................................................................................. .............................................................................................................................................
(ii) Give a disadvantage of allowing oxides of nitrogen to escape into the atmosphere. (1) ............................................................................................................................................. .............................................................................................................................................
(iii) Write a chemical equation for the reaction between nitrogen monoxide, NO, and carbon monoxide to form carbon dioxide and nitrogen. (1)
.............................................................................................................................................
(Total for question = 8 marks)
4) Carbon monoxide, CO, and oxides of nitrogen are produced in a car engine when petrol is burned.
These oxides can be partly removed by using a catalytic converter fitted to the car's exhaust system.
(i) State how oxides of nitrogen are produced in the car engine. (1)
............................................................................................................................................. .............................................................................................................................................
(ii) Give a disadvantage of allowing oxides of nitrogen to escape into the atmosphere. (1) ............................................................................................................................................. .............................................................................................................................................
(iii) Write a chemical equation for the reaction between nitrogen monoxide, NO, and carbon monoxide to form carbon dioxide and nitrogen. (1)
.............................................................................................................................................
(Total for question = 8 marks)
5) Hydrogen gas and iodine gas react together to form hydrogen iodide gas.
H2(g) + I2(g) ↔ 2HI(g)
(a) (i) The pressure of an equilibrium mixture of the three gases is increased.
Predict the effect of this change on the yield of hydrogen iodide at equilibrium, giving a reason for your answer. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(ii) A catalyst is added to an equilibrium mixture of the three gases.
Predict the effect of the catalyst on the yield of hydrogen iodide at equilibrium, giving a reason for your answer. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 4 marks)
5) This question is about three stages in the manufacture of sulfuric acid.
In stage 2, sulfur dioxide is reacted with oxygen to form sulfur trioxide gas.
2SO2(g)+ O2(g) 2SO3(g)
The yield of sulfur trioxide is approximately 98%.
(ii) The temperature is kept constant.
Give a reason why increasing the pressure would increase the yield of sulfur trioxide. (1) ............................................................................................................................................. .............................................................................................................................................
(iii) Suggest why it is not necessary to increase the pressure in stage 2. (1) ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 2 marks)
If you need the markscheme, including the examiners' reports for each question, and more exam and teaching materials on this topic, contact me through the contact page