Question

In the exothermic reaction, the enthalpy of reaction is always:
A. Zero
B. Positive
C. Negative
D. None of these

Answer 1

Exothermic Reactions. Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light. In an exothermic reaction, energy is released because the total energy of the products is less than the total energy of the reactants.

Complete step by step answer:
The standard enthalpy of reaction (denoted$\vartriangle {{H}_{r}}^{o}$) is the enthalpy change that happens in a system when matter is transformed by a given reaction, when all reactants and products are in their standard states.
The formula for calculating the enthalpy of the reaction is as follows:
$\vartriangle {{H}_{r}}^{o}\,=\,\sum\limits_{{}}{{{v}_{B}}\vartriangle {{H}_{f}}^{o}(B)}$
$\vartriangle {{H}_{r}}^{o}\,$= standard enthalpy of reaction
B = entity B
${{v}_{B}}$ = stoichiometric number
${{H}_{f}}^{o}$ = standard enthalpy of formation
Exothermic and endothermic reactions cause energy state differences and thus differences in enthalpy ($\Delta H$), the sum of all potential and kinetic energies. $\Delta {{H}_{r}}^{o}$ is decided by the system, not the encompassing environment during a reaction. A system that releases heat to the environment, and chemical reaction, features a negative $\Delta H$ by convention, because the enthalpy of the products is less than the enthalpy of the reactants of the system.

So, the correct answer is Option C.

Additional information:
Endothermic reactions are chemical reactions during which the reactants absorb heat from the surroundings to make products. These reactions lower the temperature of their surrounding area, thereby creating a cooling effect. A reaction that absorbs energy is an endothermic reaction; its enthalpy change ($\Delta H$) is positive. The enthalpy of the products of the reaction is bigger than that of the reactants. Energy is absorbed from the surroundings.

Note: The enthalpy of a chemical system is actually its energy. The enthalpy change ΔH for a reaction is adequate to the heat q transferred out of (or into) a closed system at constant pressure without in- or output of electrical energy. Heat production or absorption during a reaction is measured using calorimetry, e.g. with a bomb calorimeter. One common laboratory instrument is the reaction calorimeter, where the heat flow from or into the reaction vessel is monitored. The heat release and corresponding energy change, $\Delta H$, of a combustion reaction can be measured particularly accurately.