The method and the results are on the enthalpy changes lab. Do the analysis yourself first, then check the answers below.
Quick answer: An enthalpy change ΔH is the heat transferred at constant pressure; it is negative for exothermic reactions. These answers go with the Chemistry 11-12 lab on enthalpy changes: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
The model answers use the sample results in the table. Check each calculation, the construction of the cycle and the discussion of uncertainty, and compare your own measurements with the sample data when you carry out the lab.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R1.1-R1.2
Mass of Mg: 0.01 / 0.30 = 3.3 %. ΔT₁: 0.2 / 13.6 = 1.5 %. ΔT₂: 0.2 / 8.4 = 2.4 %. The mass of magnesium (3.3 %) is the largest, so using 0.60 g would halve it.
Question 6 Heat loss
Heat lost to the surroundings makes ΔT too small, so each ΔH is less exothermic than the true value. Use a lid and insulation, and plot temperature against time and extrapolate the cooling line back to the time of mixing.
Question 7 Excess acid
Excess acid ensures that all the magnesium or magnesium oxide reacts, so the moles used to calculate ΔH are the moles of the solid weighed. The solid is the limiting reactant.
Question 8 Assumptions
The solution has density 1.00 g/cm³ and specific heat capacity 4.18 J/(g K), and the heat absorbed by the cup and thermometer is negligible. Both assumptions introduce small errors.
