A-LevelChemistryChemical energetics9701/32 · May/June 2018 · Q215 Marks

2 In this experiment you will determine the enthalpy change, ∆H, for the decomposition of calcium hydroxide to calcium oxide. Ca(OH)2(s) → CaO(s) + H₂O(I) To do this, you will determine the enthalpy changes for the reactions of calcium hydroxide and calcium oxide with hydrochloric acid. Excess acid will be used for both experiments. You will then use Hess' Law to calculate the enthalpy change for the reaction above. FB 2 is 3.0 mol dm⁻³ hydrochloric acid, HCl. FB 3 is calcium hydroxide, Ca(OH)2. FB 4 is calcium oxide, CaO.

(a)Determination of the enthalpy change for the reaction of calcium hydroxide, FB 3, with hydrochloric acid, FB 2. (i) Method • Support a plastic cup in the 250 cm³ beaker. • Weigh the container with FB 3. Record the mass. • Use the measuring cylinder to transfer 30 cm³ of FB 2 into the 100 cm³ beaker. • Place the beaker on the tripod and gauze and heat FB 2 gently until its temperature is between 35 °C and 40°C. Turn off the Bunsen burner. • Carefully transfer all FB 2 from the 100 cm³ beaker into the plastic cup. • Measure and record the temperature of FB 2 in the plastic cup in the space below. • Immediately add all the FB 3 from the container to the FB 2 in the plastic cup. • Stir constantly until the maximum temperature is reached. • Measure and record the maximum temperature. • Weigh and record the mass of the container with any residual solid. • Calculate and record the mass of FB 3 used. • Calculate and record the temperature rise. Results [4][4]
(a)(ii)Calculations (ii) Calculate the energy produced during this reaction. [Assume that 4.2J of heat energy changes the temperature of 1.0 cm³ of solution by 1.0°C.] energy produced = .............................................. J [1][1]
(a)(iii)Calculate the number of moles of calcium hydroxide, FB 3, used in the experiment. moles of Ca(OH)2 = .............................................. mol [1][1]
(a)(iv)Calculate the enthalpy change, in kJ mol⁻¹, for reaction 1 below, ∆H₁. Ca(OH)2(s) + 2HCl(aq) → CaCl2(aq) + 2H2O(l) ∆H₁ = .............................................. kJ mol⁻¹ (sign) (value) [1][1]
(b)Determination of the enthalpy change for the reaction of calcium oxide, FB 4, with hydrochloric acid, FB 2. (i) Method • Support the second plastic cup in the 250 cm³ beaker. • Weigh the container with FB 4. Record the mass. • Use the measuring cylinder to transfer 30 cm³ of FB 2 into the 100 cm³ beaker. • Place the beaker on the tripod and gauze and heat FB 2 gently until its temperature is approximately 35 °C. • Carefully transfer all FB 2 from the 100 cm³ beaker into the plastic cup. • Measure and record the temperature of FB 2 in the plastic cup in the space below. • Immediately add all the FB 4 from the container to the FB 2 in the plastic cup. • Stir constantly until the maximum temperature is reached. • Measure and record the maximum temperature. • Weigh and record the mass of the container with any residual solid. • Calculate and record the mass of FB 4 used. • Calculate and record the temperature rise. Results [2][2]
(b)(ii)Calculation (ii) Calculate the enthalpy change, in kJ mol⁻¹, for reaction 2 below, ∆H₂. CaO(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) ∆H₂ = .............................................. kJ mol⁻¹ (sign) (value) [2][2]
(c)Use your values for ∆H₁ and ∆H₂ to calculate the enthalpy change for the decomposition of calcium hydroxide, ∆H. Show clearly how you obtained your answer by drawing a Hess' Law energy cycle. (If you were unable to calculate the enthalpy changes, assume that ∆H₁ is −129 kJ mol⁻¹ and ∆H₂ is −150 kJ mol⁻¹. Note: these are not the correct values.) Ca(OH)2(s) → CaO(s) + H₂O(l) ∆H = .............................................. kJ mol⁻¹ (sign) (value) [2][2]
(d)(i)Give a reason why FB 2 was heated before FB 3 or FB 4 were added to it. [1][1]
(d)(ii)The procedure in (b) was repeated using the same mass of calcium oxide, FB 4. However, 30 cm³ of 4.0 mol dm⁻³ HCl was used instead of 30 cm³ of 3.0 mol dm⁻³ HCl. How would the temperature rise compare with the one you obtained in the experiment in (b)? Explain your answer. [1][1]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[1] Unambiguous headings, units and six pieces of data listed. ∞ mass of container and FB 3 ∞ mass of container ∞ mass of FB 3 used ∞ first / start / initial temperature ∞ maximum temperature (allow final temp, temp after mixing) ∞ temperature rise / change
[1] Precision of readings shown in 2(a) and 2(b) ∞ all four thermometer readings are shown to 0.0 or 0.5 °C ∞ all four temperature readings > 30 °C
[2] Accuracy marks ∞ Check subtractions of supervisor and candidate temperature. ∞ Calculate the difference between the corrected candidate's and supervisor's temp rise (δ) If δ ≤ 3.0 °C award two marks If δ ≤ 5.0 °C award one mark
[1] Energy change = 30 × 4.2 × temp rise Answer must be expressed to 2, 3 or 4 sf
[1] Moles of FB 3 = mass used / 74.1 Answer must be expressed to 2, 3 or 4 sf.
[1] ∆H₁ = (answer (ii) / answer (iii)) × 1000 Negative sign must be shown on answer line. Answer should be expressed to 2, 3 or 4 sig fig
[1] Six pieces of data. ∞ mass of container and FB 4 ∞ mass of container ∞ mass of FB 4 used ∞ first / start / initial temperature ∞ final temperature ∞ temperature rise
[1] Accuracy mark ∞ Check subtractions of supervisor and candidate. ∞ Calculate the difference between corrected candidate's and supervisor's temp rise (δ) If δ < 5.0°C award one mark
[0.5] Energy released (30 × 4.2 × temp rise)
[0.5] No of moles used = mass of FB 4 used / 56.1
[0.5] ∆H₂ = (energy / no of moles) × 1000
[0.5] Negative sign
[1] Attempt at Hess Cycle diagram ∞ Downward arrow from Ca(OH)2, labelled with value and sign of ∆H₁ from (a)(iv) ∞ Downward arrow from CaO, labelled with value and sign of AH2 from (b)(ii)
[1] ∆H = ∆H₁ – ∆H₂
[1] To speed up the reaction / it is slow at room temperature / to supply the activation energy
[1] Acid is in excess / FB 4 is the limiting reagent (in both cases). Therefore temp rise would be the same

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