A-LevelChemistryChemical energetics9701/33 · Oct/Nov 2023 · Q212 Marks
2 You will determine the enthalpy change for the reaction of magnesium oxide with water to form magnesium hydroxide. MgO(s) + H2O(l) → Mg(OH)2(s) The procedure will involve two experiments, one with magnesium oxide and the other with magnesium hydroxide. In each case you will react the solid with hydrochloric acid. FA 6 is magnesium oxide, MgO. FA 7 is 2.0 moldm⁻³ hydrochloric acid, HCl. FA 8 is magnesium hydroxide, Mg(OH)2.
- (a)(i)Method • Support a cup in the 250cm³ beaker. • Use the 50 cm³ measuring cylinder to transfer 30.0 cm³ of FA 7 into the cup. • Measure and record the temperature of the solution in the cup. • Weigh the container with FA 6. Record the mass. • Tip all of FA 6 into the cup containing FA 7. FA 7 is in excess. • Stir the mixture until the maximum temperature is obtained. Record the maximum temperature. • Weigh the container with any residual FA 6. Record the mass. • Calculate and record the mass of FA 6 used. • Calculate and record the temperature rise.[3]
- (a)(ii)Calculate the energy released in your experiment. energy released = ..................................................... J[1]
- (a)(iii)Calculate the enthalpy change of reaction, ∆Η₁, in kJ mol⁻¹ of MgO(s), for the reaction of magnesium oxide with hydrochloric acid. Show your working. ΔΗ₁ = ..................... sign ..................... value ..................... kJ mol⁻¹[2]
- (b)(i)Method • Place the other cup in the beaker. • Use the 50 cm³ measuring cylinder to transfer 30.0 cm³ of FA 7 into the cup. • Measure and record the temperature of the solution in the cup. • Weigh the container with magnesium hydroxide, FA 8. Record the mass. • Tip all of FA 8 into the cup containing FA 7. FA 8 is in excess. • Stir the mixture until the maximum temperature is obtained. Record the maximum temperature. • Weigh the container with any residual FA 8. Record the mass. • Calculate and record the mass of FA 8 used. • Calculate and record the temperature rise.[3]
- (b)(ii)Calculate the enthalpy change of reaction, ∆H₂, in kJ mol⁻¹ of Mg(OH)2(s), for the reaction of magnesium hydroxide with hydrochloric acid. Show your working. ΔΗ₂ = ..................... sign ..................... value ..................... kJ mol⁻¹[2]
- (c)Use your answers to (a)(iii) and (b)(ii) to calculate the enthalpy change, ∆Hᵣ, in kJ mol⁻¹, for the reaction between magnesium oxide and water. The equation for the reaction is shown. MgO(s) + H2O(l) → Mg(OH)2(s) Show your working. ΔΗᵣ = ..................... sign ..................... value ..................... kJ mol⁻¹[1]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.
- [3] I Six unambiguous headings recorded, with correctly displayed units and in the space provided.
- • Units: (°C), / g, in g, or °C or g against each value
- • (Mass of) container + FA 6 / MgO / contents / solid / g
- • (Mass of) (empty) container (+ residue) / g
- • (Mass of) FA 6 / MgO (used) / g
- • (Initial) temperature / °C
- • Maximum / final temperature / °C
- • Temperature rise / change / ∆T/ °C
- [3] II Precision of readings in both experiments, 2(a) and 2(b).
- • Four weighings to same number of decimal places (2 or 3)
- • Four thermometer readings to .0 or .5 °C
- [3] III Correct subtractions to calculate masses of FA 6 and FA 8 and for the temperature changes in both (a) and (b).
- [1] Correct calculation of energy change
- Energy change = 30 × 4.18 × temp rise (= 125.4 × ∆T)
- AND answer correctly rounded to 2–4 sf
- Do not penalise inappropriate sf twice in question 2.
- [2] Correct use of (a)(ii)
- M1: Correct display
- Amount of MgO = mass used/40.3 mol (no answer required)
- M2: Correctly uses
- Enthalpy change = (a)(ii)/mol MgO × 1/1000 kJ mol⁻¹
- and answer shows negative sign and is quoted to 2–4 sf
- [3] I Six pieces of data recorded in the space provided (same readings are required as in (a)(i))
- • Three temperatures
- • Three masses
- [3] Accuracy marks in 2(b)
- • Round thermometer readings to 0.5 °C if necessary
- • Check supervisor's and candidate's subtractions for temp rise
- • Calculate difference (δ) between supervisor and candidate (to 0.5 °C).
- [3] II + III: Accuracy marks in 2(b)
- ∆Tsup/°C <5.0 5.0-9.5 10.0-19.5 20.0-29.5 ≥30.0
- 1 mark ±1.0 ±1.5 2.0 2.5 3.0
- 2 marks ±0.5 ±1.0 1.0 1.5 2.0
- [2] Correct use of data in enthalpy calculation
- M1: Amount Mg(OH)2 reacting = 0.5 × mol HCl (since Mg(OH)2 in excess)
- = 0.5 × 0.030 × 2.00 = 0.030 mol
- M2: Correct use of data in remainder of calculation (all three bullets correct)
- • Energy released = 30 × 4.18 × temp rise in (b) (J)
- • Enthalpy change = heat released/1000 × 0.030 kJ mol⁻¹
- • Answer has negative sign and is quoted to 2–4 sig fig.
- [1] Enthalpy change (correctly calculated, with correct sign)
- ΔHᵣ = (a)(iii) – (b)(ii) kJ mol⁻¹
Write your answer and get it marked
On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.
Sign up free →