Quick answer: An enthalpy change ΔH is the heat transferred at constant pressure; it is negative for exothermic reactions. This Chemistry 11-12 lab on enthalpy changes gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
This lab uses two simple calorimetry experiments and Hess’s law to find the enthalpy of formation of magnesium oxide. You will measure temperature rises, calculate two enthalpy changes, combine them in a cycle, and judge how accurate the final value is.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R1.1-R1.2
Aim
Use Hess’s law with two calorimetry experiments to find the enthalpy of formation of magnesium oxide, which cannot be measured directly.
Equipment
- two polystyrene cups with lids, one placed inside a beaker
- thermometer (0.1 °C divisions) and stirrer
- 100 cm³ measuring cylinder
- 1.0 mol/dm³ hydrochloric acid, in excess
- magnesium ribbon (0.30 g) and magnesium oxide powder (1.00 g)
- balance reading to 0.01 g, stopwatch, safety goggles
Safety
Wear safety goggles. Dilute hydrochloric acid is an irritant: wipe spills at once. Hydrogen gas is given off in experiment 1, so keep flames away and do not put a tight stopper on the cup. Magnesium oxide powder irritates the lungs, so do not breathe it in.
Method
- Measure 100 cm³ of hydrochloric acid into the cup, put the lid on and record the temperature every 30 seconds for 2 minutes.
- At 2 minutes add the weighed magnesium ribbon (experiment 1), replace the lid and stir gently.
- Keep recording every 30 seconds until the temperature has fallen steadily for 2 minutes. Note the highest temperature.
- Repeat steps 1 to 3 with 1.00 g of magnesium oxide (experiment 2).
- Use the data book value ΔHf(H₂O, l) = −285.8 kJ/mol.
Results
| Experiment 1: Mg | Experiment 2: MgO | |
|---|---|---|
| Mass of solid (g) | 0.30 | 1.00 |
| Volume of acid (cm³) | 100 | 100 |
| Initial temperature (°C) | 21.0 | 21.2 |
| Highest temperature (°C) | 34.6 | 29.6 |
| Temperature rise ΔT (°C) | 13.6 | 8.4 |
Analysis questions
Question 1 Experiment 1
Calculate the enthalpy change for Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) in kJ/mol (Ar of Mg = 24.3; assume 100 g of solution, c = 4.18 J/(g K)).Question 2 Experiment 2
Calculate the enthalpy change for MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l) (M of MgO = 40.3 g/mol).Question 3 Hess cycle
Draw the energy cycle (in words) and calculate ΔHf of MgO using ΔH₁, ΔH₂ and ΔHf(H₂O, l) = −285.8 kJ/mol.Question 4 Percentage difference
The data-book value is −601.6 kJ/mol. Calculate the percentage difference.Question 5 Uncertainty
The thermometer has an uncertainty of ±0.1 °C per reading and the balance ±0.01 g. Calculate the percentage uncertainty in the mass of Mg and in each ΔT, and identify the largest.Question 6 Heat loss
Explain how heat loss affects the results and describe two ways to reduce its effect.Question 7 Excess acid
Explain why the acid must be in excess in both experiments.Question 8 Assumptions
State two assumptions made in the calculations.
