The method and the results are on the thermochemistry lab. Do the analysis yourself first, then check the answers below.
Quick answer: Thermochemistry measures heat changes in reactions. These answers go with the AP Chemistry lab on thermochemistry: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
The sample data give ΔH₃ of about −56 kJ/mol, within 2 % of the accepted value. Notice that both runs use the same experimental technique, so some systematic errors such as heat loss partly cancel when ΔH₁ is subtracted from ΔH₂.
The heat released depends on the amount of NaOH and the mass being heated. With equal mass and volume, ΔT depends only on the process, and the two runs can be compared through Hess’s law.
Cycle: NaOH(s) → NaOH(aq) (ΔH₁), then NaOH(aq) + HCl(aq) → products (ΔH₃); the direct route is ΔH₂. So ΔH₃ = ΔH₂ − ΔH₁ = −100.6 − (−44.3) = −56.3 kJ/mol.
Question 5 Comparison
Difference = (57.3 − 56.3)/57.3 × 100 = 1.7 %, a small discrepancy.
Question 6 Direction of errors
Heat lost to the room lowers the highest temperature, so each |ΔH| is too small. Other errors: the density and specific heat of the solution are not exactly those of water; NaOH pellets absorb moisture, so the real mass of NaOH is less than 2.00 g.
Question 7 Improvement
Plot temperature against time and extrapolate to the mixing time; use a lid and a second insulating cup; weigh NaOH quickly in a closed bottle; repeat and take a mean. Any two of these.
