In this experiment you will determine the enthalpy change, ΔH, for the reaction shown. MgO(s) + CO2(g) → MgCO3(s) To do this, you will determine the enthalpy changes for the reactions of magnesium oxide and magnesium carbonate with hydrochloric acid. Excess hydrochloric acid will be used in each reaction. You will then use Hess' Law to calculate the enthalpy change for the reaction. FA 2 is hydrochloric acid, HCl. FA 4 is magnesium oxide, MgO. FA 5 is magnesium carbonate, MgCO3.

1. (a)Determination of the enthalpy change for the reaction of magnesium oxide, FA 4, with hydrochloric acid, FA 2 Method • Support a plastic cup in the 250 cm³ beaker. • Use the measuring cylinder to transfer 40 cm³ of FA 2 into the plastic cup. • Measure and record the initial temperature of the solution. • Weigh the container with FA 4. Record the mass. • Add all the FA 4 from the container to the FA 2 in the plastic cup. • Stir constantly until the maximum temperature is reached. • Measure and record the maximum temperature. • Weigh the container with any FA 4 remaining. Record the mass. • Calculate and record the mass of FA 4 used. • Calculate and record the temperature rise. I II III IV[4]
2. (b)(i)Calculate the energy produced during this reaction. (Assume that 4.2 J are needed to raise the temperature of 1.0 cm³ of solution by 1.0°C.) energy produced = ...................................... J [1][1]
3. (b)(ii)Calculate the number of moles of MgO used. moles of MgO = ...................................... mol [1][1]
4. (b)(iii)Calculate the enthalpy change, in kJ mol⁻¹, for the reaction below. MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) enthalpy change = ...................................... kJ mol⁻¹ [1] sign value[1]
5. (c)By means of a calculation, use your answer to 1(c)(iv) to show that the hydrochloric acid, FA 2, was in excess for the reaction in 2(a). (If you were unable to carry out the calculation in 1(c)(iv), you should assume that the concentration of HCl in FA 2 is 3.75 mol dm⁻³. This may not be the correct value.) [1][1]
6. (d)(i)Method • Support the second plastic cup in the 250 cm³ beaker. • Use the measuring cylinder to transfer 40 cm³ of FA 2 into the plastic cup. • Measure and record the initial temperature of the solution. • Weigh the container with FA 5. Record the mass. • Add approximately half of the FA 5 from the container to the FA 2 in the plastic cup. • Stir constantly for approximately 30 seconds. • Then add the remainder of the FA 5. • Stir constantly until the maximum temperature is reached. • Measure and record the maximum temperature. • Weigh the container with any FA 5 remaining. Record the mass. • Calculate and record the mass of FA 5 used. • Calculate and record the temperature rise. [2][2]
7. (d)(ii)Apart from the change in temperature, what observations did you make during the reaction? [1][1]
8. (d)(iii)Calculation Calculate the enthalpy change for this reaction. MgCO3(s) + 2HCl(aq) → MgCl2(aq) + CO2(g) + H2O(l) enthalpy change = ...................................... kJ mol⁻¹ [1] sign value[1]
9. (e)Use your values for the enthalpy changes calculated in (b)(iii) and (d)(iii) to calculate the enthalpy change for the reaction below. (If you were unable to calculate the enthalpy changes, assume that the magnitude of the enthalpy change in (b)(iii) is 110.3 kJ mol⁻¹ and the magnitude of the enthalpy change in (d)(iii) is 65.9 kJ mol⁻¹. Note: these may not be the correct magnitudes and the signs have been deliberately omitted.) MgO(s) + CO2(g) → MgCO3(s) enthalpy change = ...................................... kJ mol⁻¹ [1] sign value[1]
10. (f)Outline one improvement to the method that would lead to more accurate values for the enthalpy changes. Do not include a change to the apparatus used, such as the use of a lid. [1][1]
11. (g)Give the error in a single balance reading. error = ± ...................................... g Which is greater, the percentage error in the mass of magnesium oxide used or the percentage error in the mass of magnesium carbonate used? Percentage error is greater in ...................................... Calculate the greater percentage error. greater % error = ...................................... % [2][2]