EnergeticsAQA A-Level Chemistry: Topic test

20 questions, 54 marks

AQA A-Level Chemistry

Energetics topic test

Total 54 marks

Name

Class

Date

1
A self-heating food can contains calcium oxide and water in separate compartments. When they are mixed the reaction is CaO(s) + H₂O(l) → Ca(OH)₂(s), for which ΔH = −65 kJ mol⁻¹.
(a)
Which statement about this reaction is correct?
[1 mark]
AThe reaction is exothermic, so the temperature of the surroundings rises
BThe reaction is endothermic because ΔH is negative
CThe products have more enthalpy than the reactants
DHeat energy is taken in from the surroundings
(b)
Which equation represents the standard enthalpy of formation of calcium hydroxide?
[1 mark]
ACaO(s) + H₂O(l) → Ca(OH)₂(s)
BCa(s) + O₂(g) + H₂(g) → Ca(OH)₂(s)
CCa(g) + O₂(g) + H₂(g) → Ca(OH)₂(s)
DCa(s) + O₂(g) + H₂(g) → Ca(OH)₂(aq)
(c)
State the conditions that define standard enthalpy changes, and explain why state symbols are essential in thermochemical equations.
[2 marks]
Total for question 1: 4 marks
2
A student measures the enthalpy change when solid sodium hydroxide dissolves in water. She dissolves 2.00 g of sodium hydroxide (relative formula mass 40.0) in 100 g of water in a polystyrene cup, and the temperature rises from 20.0 °C to 25.3 °C. Assume that the mass of the solution is 100 g and that its specific heat capacity is 4.18 J g⁻¹ K⁻¹.
(a)
What is the heat energy absorbed by the solution?
[1 mark]
A22.2 kJ
B2215 kJ
C2.22 kJ
D0.418 kJ
(b)
What is the enthalpy change of solution of sodium hydroxide, in kJ mol⁻¹?
[1 mark]
A+44.3
B−88.6
C−2.22
D−44.3
(c)
The true enthalpy of solution of sodium hydroxide is more exothermic than this experimental value. Give a reason for the difference and one change to the procedure that would improve the result.
[2 marks]
Total for question 2: 4 marks
3
A chemist uses data to find enthalpy changes that cannot be measured directly. Standard enthalpies of combustion, in kJ mol⁻¹: C(graphite) −394, H₂(g) −286 and C₂H₆(g) −1560. Standard enthalpies of formation, in kJ mol⁻¹: Fe₂O₃(s) −824, CO(g) −111 and CO₂(g) −394.
(a)
Calculate the standard enthalpy of formation of ethane, C₂H₆(g).
[3 marks]
(b)
Calculate the enthalpy change for the reaction Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g), and state the value of the standard enthalpy of formation of Fe(s) with a reason.
[4 marks]
Total for question 3: 7 marks
4
Methanol, CH₃OH, can be made from carbon monoxide and hydrogen: CO(g) + 2H₂(g) → CH₃OH(g). Mean bond enthalpies, in kJ mol⁻¹: C≡O in carbon monoxide 1077, H–H 436, C–H 412, C–O 360 and O–H 463. Standard enthalpies of formation, in kJ mol⁻¹: CO(g) −111 and CH₃OH(g) −201.
(a)
Calculate the enthalpy change for the reaction using (i) the mean bond enthalpies and (ii) the enthalpies of formation. Explain why the two values are different.
[6 marks]
(b)
The heat from burning liquid methanol is used to heat 200 g of water by 30.0 K. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹, the standard enthalpy of combustion of methanol is −726 kJ mol⁻¹ and its relative molecular mass is 32.0. Calculate the minimum mass of methanol needed. The student actually burned 1.60 g; calculate the percentage efficiency to two significant figures, give one reason it is below 100%, and define standard enthalpy of combustion.
[6 marks]
Total for question 4: 12 marks
5
Hydrogen iodide is made in the reaction H₂(g) + I₂(g) → 2HI(g). Mean bond enthalpies, in kJ mol⁻¹: H–H 436, I–I 151 and H–I 299. Under standard conditions iodine is a solid, and the measured enthalpy change for H₂(g) + I₂(s) → 2HI(g) is +53 kJ mol⁻¹.
(a)
Which statement defines mean bond enthalpy?
[1 mark]
AThe enthalpy change when one mole of bonds is formed, measured under standard conditions
BThe enthalpy change to break one mole of a specific bond in one particular compound
CThe energy needed to convert one mole of a molecule into atoms in their standard states
DThe average enthalpy change to break one mole of a given bond in gaseous molecules, averaged over a range of compounds
(b)
What is the enthalpy change for H₂(g) + I₂(g) → 2HI(g), calculated from mean bond enthalpies?
[1 mark]
A+11 kJ mol⁻¹
B−587 kJ mol⁻¹
C−11 kJ mol⁻¹
D+587 kJ mol⁻¹
(c)
The measured enthalpy change for H₂(g) + I₂(s) → 2HI(g) is more positive than the value calculated from the mean bond enthalpies for H₂(g) + I₂(g) → 2HI(g). Explain why.
[2 marks]
Total for question 5: 4 marks
6
The enthalpy change for the reaction C(graphite) + ½O₂(g) → CO(g) cannot be measured directly. Standard enthalpies of combustion, in kJ mol⁻¹: C(graphite) −394 and CO(g) −283.
(a)
Which statement is Hess's law?
[1 mark]
AThe enthalpy change of a reaction depends on the route taken
BThe total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same
CThe enthalpy change of a reaction equals the sum of the bond enthalpies of the reactants
DThe enthalpy change is greater when a catalyst provides an alternative route
(b)
Why can the enthalpy change for C(graphite) + ½O₂(g) → CO(g) not be measured directly?
[1 mark]
ACarbon also burns to form carbon dioxide, so the reaction cannot be stopped at carbon monoxide
BCarbon is not an element
CThe reaction is endothermic, so it does not occur
DCarbon monoxide has no enthalpy of combustion
(c)
Use the data to calculate the standard enthalpy of formation of carbon monoxide.
[2 marks]
Total for question 6: 4 marks
7
A student determines the enthalpy change for the reaction Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). She adds 0.243 g of magnesium (Ar = 24.3) to 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid, which is in excess, in a polystyrene cup. The temperature rises from 19.0 °C to 41.0 °C. Assume that the solution has a density of 1.00 g cm⁻³ and a specific heat capacity of 4.18 J g⁻¹ K⁻¹.
(a)
Calculate the enthalpy change for the reaction of magnesium with hydrochloric acid, in kJ mol⁻¹.
[3 marks]
(b)
The enthalpy change for MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l) is −146 kJ mol⁻¹, and the standard enthalpy of formation of H₂O(l) is −286 kJ mol⁻¹. Use these values and the result of −460 kJ mol⁻¹ for the reaction of magnesium with hydrochloric acid to calculate the standard enthalpy of formation of magnesium oxide.
[4 marks]
Total for question 7: 7 marks
8
A space-agency chemist compares propellants. Standard enthalpies of combustion, in kJ mol⁻¹: H₂(g) −286, forming H₂O(l), and CH₄(g) −890, forming CO₂(g) and H₂O(l). Relative formula masses: H₂ 2.0 and CH₄ 16.0.
(a)
Compare hydrogen and methane as fuels. In your answer, calculate the energy released per gram of each fuel and per mole of each gas, and use the products of combustion to reach a justified conclusion.
[6 marks]
(b)
Hydrazine, N₂H₄, can also be used as a propellant: N₂H₄(g) + O₂(g) → N₂(g) + 2H₂O(g). Mean bond enthalpies, in kJ mol⁻¹: N–N 158, N–H 391, O=O 498, N≡N 945 and O–H 463. The standard enthalpies of formation are N₂H₄(g) +95 kJ mol⁻¹ and H₂O(g) −242 kJ mol⁻¹. Calculate the enthalpy change for the reaction using the bond enthalpies and using the enthalpies of formation, and comment on the agreement between the two values.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).