Considering the definition of enthalpy of a chemical reaction and endothermic and exothermic reaction, the correct answer is option C: the change in energy in a system during an exothermic reaction.
The enthalpy of a chemical reaction as the heat absorbed or released in a chemical reaction when it occurs at constant pressure. That is, the heat of reaction is the energy that is released or absorbed when chemicals are transformed into a chemical reaction.
On the other side, an endothermic reaction requires energy while an exothermic reaction releases energy.
In other words, a chemical reaction is endothermic when it absorbs energy from the environment. In this case, the heat is transferred from the outside to the inside of the system.
On the other hand, an exothermic reaction is one where energy flows out of the system. This energy is released in the form of heat.
So, if the enthalpy of reaction is negative, it means that during the course of the reaction, the system loses or releases heat to the environment and is exothermic, while if the enthalpy of reaction is positive, it means that during the course of the reaction, the system absorbs heat from the environment and is endothermic.
So, the correct answer is option C: the change in energy in a system during an exothermic reaction.
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