What happens to the value of ΔH for a thermochemical reaction when the reaction is reversed?
The value of ΔH for a thermochemical reaction changes sign when the reaction is reversed. This is because the enthalpy change for the reverse reaction is equal in magnitude but opposite in sign to the forward reaction. For example, if the forward reaction has a ΔH of -100 kJ, the reverse reaction will have a ΔH of +100 kJ.
When a thermochemical reaction is reversed, the value of ΔH (the change in enthalpy) also changes sign. In other words, if the original reaction has a positive ΔH, indicating an endothermic reaction, the reversed reaction will have a negative ΔH, indicating an exothermic reaction. Similarly, if the original reaction has a negative ΔH, indicating an exothermic reaction, the reversed reaction will have a positive ΔH, indicating an endothermic reaction.
This change in sign occurs because when a reaction is reversed, the reactants become the products and the products become the reactants. As a result, the energetics of the reaction are also reversed. The ΔH value represents the change in enthalpy between the reactants and products, so when the reaction is reversed, the sign of ΔH must also be reversed to account for this change in energetics.