American Chemical Society (ACS) Chemistry Practice Exam

When the stoichiometry of a reaction is altered by multiplying all coefficients in a balanced chemical equation, the enthalpy change (ΔH) for the reaction is also affected proportionately. Specifically, if the coefficients are multiplied by a factor, the ΔH for the overall reaction must be multiplied by the same factor. This is based on the principle that enthalpy is a state function, which means its value depends on the initial and final states of the system, not on the path taken. When the amount of reactants or products is scaled, the total heat absorbed or released by the reaction changes in direct proportion to the change in moles of the substances involved. For instance, if a reaction has a ΔH of -100 kJ for the reaction as written, and we double all coefficients, indicating that we are doubling the amount of reactants and products involved, the new ΔH would be -200 kJ. Therefore, changing the coefficients by multiplying them directly adjusts ΔH accordingly, maintaining the relationship between the amounts of reactants and products and the heat exchanged during the reaction.