Calcium carbonate reacts with aqueous hydrochloric acid according to CaCO3(s)+2HCl(aq)→CaCl2(aq)+CO2(g)+H2O(l). A solution manual cites ΔH=−155kJ for this reaction. The manual claims the value was obtained by appropriately adding the following standard reactions: (1) CaO(s)+CO2(g)→CaCO3(s), ΔH=−178kJ; (2) CaO(s)+2HCl(aq)→CaCl2(aq)+H2O(l), ΔH=−204kJ. No work is shown, but the author asserts that reversing reaction (1) and adding reaction (2) gives the desired equation and enthalpy change.
(1) (a) Show explicitly the algebraic combination of reactions (1) and (2) performed by the manual's author, including any reversals, to verify whether the net equation matches the target reaction.
(2) (b) Determine the correct overall ΔH based on your verification in part (a). Compare it quantitatively with the value −155kJ given by the manual and compute the percent discrepancy.
(3) (c) Provide two possible reasons, rooted in thermochemical principles, that could explain how the manual's author arrived at the erroneous value even if the individual reaction data are accurate.