Given that A → B has ΔH = +50 kJ/mol and B → C has ΔH = −80 kJ/mol, what is ΔH for the overall reaction A → C?
A+130 kJ/mol
B−130 kJ/mol
C−30 kJ/mol
D+30 kJ/mol
By Hess's Law, ΔH values add when reactions are combined in sequence. A → B → C gives ΔH = (+50) + (−80) = −30 kJ/mol. Because enthalpy is a state function, only the initial state (A) and final state (C) matter — the intermediate (B) and the path taken are irrelevant.
Question 2 True / False
In an exothermic reaction, the reacting system gains heat and increases in temperature.
TTrue
FFalse
Answer: False
This reverses what actually happens. In an exothermic reaction (ΔH < 0), the system releases heat to the surroundings. It is the surroundings — such as the water in a calorimeter — that increase in temperature. The system loses energy. A common confusion is equating 'the beaker gets hot' with 'the reaction is gaining energy,' when in fact the beaker is the surroundings absorbing the released heat.
Question 3 Short Answer
Write the formula for calculating ΔH°rxn from standard enthalpies of formation, and explain why elements in standard state do not appear in the final calculation.
Think about your answer, then reveal below.
Model answer: ΔH°rxn = ΣΔHf°(products) − ΣΔHf°(reactants). Elements in standard state have ΔHf° = 0 by definition, so they cancel out of the sum.
The formula is an application of Hess's Law: you conceptually decompose reactants into their elements (reversing formation reactions, changing signs) and then form products from those elements. The formation enthalpy of an element in its standard state is defined as zero because no reaction is needed to produce it. This cancellation means only compound formation enthalpies contribute to ΔH°rxn.