A technician collects calorimetric data for four different combustion reactions carried out at constant pressure. For each reaction, exactly 1.00 mol of compound is burned. The technician summarizes the magnitude of the measured heat release (|q|) in a bar-chart format. The bars are reported relative to an arbitrary unit such that:
• Reaction W: reference bar length set to 1.00 unit
• Reaction X: bar length 0.75 unit
• Reaction Y: bar length 2.20 units
• Reaction Z: bar length 1.40 units
Subsequent analysis reveals that Reaction Y corresponds to the combustion of C2H2(g) with ΔH_comb = –1 300 kJ mol⁻¹.
(a) Using the information above, determine the enthalpy change (ΔH_comb, in kJ mol⁻¹) for each of Reactions W, X, and Z. State the sign of each value.
(b) Rank all four reactions from the one that would produce the greatest temperature rise in an identical mass of water to the one that would produce the least, assuming complete combustion of 1.00 mol in each case. Justify your answer quantitatively.
(c) The technician now combusts 0.500 mol of the compound used in Reaction Z in the same calorimeter. Predict the expected heat released (in kJ) and the relative bar length that would appear on the original chart scale. Show all reasoning.
(d) Discuss two experimental factors, other than the amount of substance burned, that could cause the observed bar lengths to deviate from the values predicted purely from enthalpy change calculations.