Free Chemical energetics questions for AS Level Chemistry (9701), part of Physical chemistry. Each question has the answer and the marking points an examiner would look for.
Generate a full Chemical Energetics worksheet →All Physical chemistry questionsStandard enthalpy changes of combustion in kJ mol−1: C(s) -393; H2(g) -286; ethane -1560.
What is the standard enthalpy change of formation of ethane, C2H6?
Answer: B
Which term is described by this statement?
“The enthalpy change when one mole of a compound is formed from its elements in their standard states.”
Answer: D
Hess’s law can be used to find enthalpy changes that cannot be measured directly.
State Hess’s law.
Use ΔHc(C) = −394, ΔHc(H₂) = −286 and ΔHc(CH₄) = −890 kJ mol−1 to calculate ΔHf(CH₄).
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | enthalpy change independent of route the enthalpy change of a reaction is independent of the route taken (if initial and final conditions are the same) B1 | 1 |
| (b) | −76 kJ mol−1 cycle drawn or ΔHf = −394 + 2(−286) − (−890) M1 correct arithmetic C1 −76 kJ mol−1 A1 | 3 |
This question is about the standard enthalpy change of formation of ethanol, C₂H₅OH.
Define standard enthalpy change of formation.
Write the equation that represents the standard enthalpy change of formation of the compound, with state symbols.
Use the standard enthalpy changes of combustion below to calculate the standard enthalpy change of formation of ethanol. ΔHc: C(s) −394, H₂(g) −286, C₂H₅OH −1367 kJ mol−1.
Explain why the enthalpy change of formation of the compound cannot be measured directly.
Define bond energy and explain why values calculated from mean bond energies differ from values found by Hess’s law.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | enthalpy change when 1 mol of compound formed from its elements in their standard states under standard conditions enthalpy change when one mole of a compound is formed B1 from its elements in their standard states B1 under standard conditions (298 K, 101 kPa) B1 | 3 |
| (b) | 2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l) 2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l) B1 | 1 |
| (c) | −279 kJ mol−1 Hess cycle with combustion products M1 2(−394) + 3(−286) − (−1367) M1 −279 kJ mol−1 A1 | 3 |
| (d) | elements do not react directly / other products form carbon and hydrogen do not react directly to give only this compound under standard conditions B1 | 1 |
| (e) | energy to break 1 mol of bonds in gaseous molecules; mean values averaged over compounds; states not gaseous energy needed to break one mole of a covalent bond in gaseous molecules B1 mean bond energies are averages over many compounds B1 and the bond energy method assumes all species are gaseous B1 | 3 |
The standard enthalpy change of combustion of a fuel can be found by calorimetry.
Define standard enthalpy change of combustion.
Explain why the experimental value is usually less exothermic than the data book value.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | enthalpy change when 1 mol burns completely in oxygen under standard conditions enthalpy change when one mole of a substance is burned completely in oxygen B1 under standard conditions (298 K, 101 kPa) B1 | 2 |
| (b) | heat loss; incomplete combustion heat lost to surroundings B1 incomplete combustion (or evaporation of fuel) B1 | 2 |
0.02 mol of magnesium oxide reacts with 50 cm3 of excess dilute hydrochloric acid in a polystyrene cup. The temperature rises by 7.3 °C. (c = 4.18 J g−1 K−1)
Calculate the enthalpy change for MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l).
The enthalpy change for Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) is −467 kJ mol−1, and ΔHf of H2O(l) is −286 kJ mol−1. Construct a Hess cycle and calculate ΔHf of MgO(s).
The data-book value is −602 kJ mol−1. Suggest two reasons for the difference.
Define standard enthalpy change of formation.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | −76.3 kJ mol−1 q = mcΔT C1 ÷ moles of MgO C1 −76.3 with negative sign A1 | 3 |
| (b) | −677 kJ mol−1 correct cycle M1 ΔHf = −467 + (−286) − ΔH(a) M1 correct signs A1 −677 A1 | 4 |
| (c) | heat losses to surroundings; heat capacity of cup ignored; incomplete reaction; density/c of solution not equal to water any two valid sources of error B2 | 2 |
| (d) | enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions one mole formed from its elements B1 standard states, standard conditions (298 K, 101 kPa) B1 | 2 |
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