Question

In thermochemical equation, if the reaction is exothermic then $\Delta H$ is:

• A. less than zero
• B. greater than zero
• C. equal to zero
• D. Both $A$ and $B$

Answer 1

The correct option is A less than zero

We are considering the enthalpic state of the system. Thus:

1. If the system has higher enthalpy at the end of the reaction, then it absorbed heat from the surroundings (endothermic reaction)
2. If the system has a lower enthalpy at the end of the reaction, then it gave off heat during the reaction (exothermic reaction)

Therefore:

1. For endothermic reactions $H_{final}$> $H_{initial}$ and $\Delta H$ is positive (+$\Delta H_{rxn}$)
2. For exothermic reactions $H_{final}$ < $H_{initial}$ and $\Delta H$ is negative (-$\Delta H_{rxn}$)