A student measures three reactions with a coffee-cup calorimeter to verify Hess's law for the dissolution of NaOH and reaction with HCl: (1) NaOH(s) -> NaOH(aq), delta H1 = -44.5 kJ/mol (2) NaOH(s) + HCl(aq) -> NaCl(aq) + H2O(l), delta H2 = -100.0 kJ/mol (3) NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l), delta H3 = ? Reactions (1) and (2) used 2.00 g of NaOH each; (3) used 50.0 mL of 1.00 M NaOH and 50.0 mL of 1.00 M HCl. Combine results to predict delta H3.
The student measures delta H3 directly and gets -54.0 kJ/mol. The discrepancy from the Hess's-law prediction is best attributed to:
- A
phenolphthalein interfering
- B
rounding errors in molar masses only
- C
Hess's law not applying to aqueous reactions
- Dcheck_circle
heat loss to surroundings during the most exothermic run causing magnitudes to be slightly underestimated
Explanation
Heat loss to the surroundings systematically lowers measured deltaT and so |delta H|, especially in the more exothermic run (2). The 1.5 kJ discrepancy is consistent with such loss; Hess's law itself is exact.