Chemical EnergeticsCambridge IGCSE Chemistry: Topic test

20 questions, 54 marks

Cambridge IGCSE Chemistry

Chemical Energetics topic test

Total 54 marks

Name

Class

Date

1
A student places 5 g of solid ammonium chloride into 50 cm3 of water in an insulated plastic cup and stirs with a thermometer. The temperature of the water falls from 21°C to 14°C as the solid dissolves.
(a)
What term describes this reaction, and what happens to thermal energy?
[1 mark]
AEndothermic; thermal energy is taken in from the surroundings
BExothermic; thermal energy is transferred to the surroundings
CEndothermic; thermal energy is transferred to the surroundings
DExothermic; thermal energy is taken in from the surroundings
(b)
Which statement about the enthalpy change, ΔH, of this reaction is correct?
[1 mark]
AΔH is negative, because this is an exothermic reaction
BΔH is positive, because this is an endothermic reaction
CΔH is negative, because this is an endothermic reaction
DΔH is positive, because this is an exothermic reaction
(c)
Using the idea of bond breaking and bond making, explain why dissolving ammonium chloride in water in this experiment is endothermic.
[2 marks]
Total for question 1: 4 marks
2
A technician adds a strip of magnesium ribbon to excess dilute hydrochloric acid in an insulated cup and measures the temperature every 10 seconds. The temperature rises rapidly from 20°C to a maximum of 38°C before slowly falling back towards room temperature.
(a)
In terms of energy, which statement about the reactants and products of this reaction is correct?
[1 mark]
AThe products have more energy than the reactants, and the difference is taken in from the surroundings
BThe products and reactants have the same energy, and no thermal energy is transferred
CThe products have less energy than the reactants, and the difference in energy is transferred to the surroundings
DThe products have less energy than the reactants, and the difference in energy is taken in from the surroundings
(b)
What is meant by the activation energy, Ea, of this reaction?
[1 mark]
AThe total energy released by the reaction
BThe energy difference between the reactants and the products
CThe energy needed to melt the magnesium ribbon
DThe minimum energy that colliding particles must have in order to react
(c)
State two pieces of evidence from the results that show this reaction is exothermic.
[2 marks]
Total for question 2: 4 marks
3
A chemist calculates the enthalpy change for the reaction between hydrogen and oxygen to form water vapour: 2H2 + O2 -> 2H2O. She uses the following average bond energies, in kJ per mole of bonds: H-H = 436, O=O = 498, O-H = 463.
(a)
Use the bond energies given to calculate the total energy required to break all the bonds in the reactants, showing your working.
[3 marks]
(b)
The reaction forms 2 mol of water, each molecule containing 2 O-H bonds. Calculate the total energy released when the bonds in the products form, and hence calculate the overall enthalpy change, ΔH, for the reaction. State whether the reaction is exothermic or endothermic.
[4 marks]
Total for question 3: 7 marks
4
An engineer studies the reaction profile for the industrial combustion of a fuel. The reaction is exothermic and has a measurable activation energy, and a platinum catalyst can be added to speed up the reaction without being used up.
(a)
Sketch in words a reaction pathway (energy profile) for this exothermic reaction, describing how the energy of the reactants, the energy of the products, the activation energy Ea and the overall enthalpy change ΔH would appear relative to each other, and explain, in terms of the reaction pathway, why the reaction is described as exothermic.
[6 marks]
(b)
The engineer adds a platinum catalyst to the reaction. Explain, in terms of the reaction pathway and activation energy, the effect that adding the catalyst has on the rate of the reaction and on the overall enthalpy change, ΔH, of the reaction, and explain why the platinum itself is unchanged at the end of the reaction.
[6 marks]
Total for question 4: 12 marks
5
A student dissolves 4 g of solid potassium nitrate in 50 cm3 of water in an insulated cup and stirs with a thermometer. The temperature falls from 22°C to 16°C as the solid dissolves.
(a)
What term describes this reaction, and what happens to thermal energy?
[1 mark]
AExothermic; thermal energy is transferred to the surroundings
BEndothermic; thermal energy is taken in from the surroundings
CExothermic; thermal energy is taken in from the surroundings
DEndothermic; thermal energy is transferred to the surroundings
(b)
Which statement about the enthalpy change, ΔH, of this reaction is correct?
[1 mark]
AΔH is positive, because this is an endothermic reaction
BΔH is negative, because this is an exothermic reaction
CΔH is positive, because this is an exothermic reaction
DΔH is negative, because this is an endothermic reaction
(c)
Using the idea of bond breaking and bond making, explain why dissolving potassium nitrate in water in this experiment is endothermic.
[2 marks]
Total for question 5: 4 marks
6
A student adds 25 cm3 of dilute hydrochloric acid to 25 cm3 of sodium hydroxide solution in an insulated cup and measures the temperature every 10 seconds. The temperature rises rapidly from 19°C to a maximum of 26°C before slowly falling back towards room temperature.
(a)
In terms of energy, which statement about the reactants and products of this reaction is correct?
[1 mark]
AThe products have more energy than the reactants, and the difference is taken in from the surroundings
BThe products and reactants have the same energy, and no thermal energy is transferred
CThe products have less energy than the reactants, and the difference in energy is taken in from the surroundings
DThe products have less energy than the reactants, and the difference in energy is transferred to the surroundings
(b)
What is meant by the activation energy, Ea, of this reaction?
[1 mark]
AThe total energy released by the reaction
BThe energy difference between the reactants and the products
CThe minimum energy that colliding particles must have in order to react
DThe energy needed to heat the acid to room temperature
(c)
State two pieces of evidence from the results that show this neutralisation reaction is exothermic.
[2 marks]
Total for question 6: 4 marks
7
A student calculates the enthalpy change for the reaction between nitrogen and oxygen to form nitrogen monoxide inside a car engine: N2 + O2 -> 2NO. She uses the following average bond energies, in kJ per mole of bonds: N≡N = 945, O=O = 498, N=O = 630.
(a)
Use the bond energies given to calculate the total energy required to break all the bonds in the reactants, showing your working.
[3 marks]
(b)
The reaction forms 2 mol of nitrogen monoxide, each molecule containing 1 N=O bond. Calculate the total energy released when the bonds in the products form, and hence calculate the overall enthalpy change, ΔH, for the reaction. State whether the reaction is exothermic or endothermic.
[4 marks]
Total for question 7: 7 marks
8
A chemist studies the reaction profile for the endothermic thermal decomposition of a metal carbonate in a furnace. The reaction has a measurable activation energy, and the furnace operator considers adding a catalyst to speed up the process.
(a)
Sketch in words a reaction pathway (energy profile) for this endothermic reaction, describing how the energy of the reactants, the energy of the products, the activation energy Ea and the overall enthalpy change ΔH would appear relative to each other, and explain, in terms of the reaction pathway, why the reaction is described as endothermic.
[6 marks]
(b)
The furnace operator considers adding a catalyst to this decomposition reaction. Explain, in terms of the reaction pathway and activation energy, the effect that adding a catalyst would have on the rate of the reaction and on the overall enthalpy change, ΔH, of the reaction, and explain why a catalyst is described as unchanged at the end of a reaction even though it takes part in it.
[6 marks]
Total for question 8: 12 marks

End of questions