Describe how the enthalpy of reaction remains unchanged when a catalyst is used in the reaction.

(N/A) When a reaction is carried out in the presence of a catalyst,the enthalpy of the reactant $H_{R}$ and the enthalpy of the product $H_{P}$ remain unchanged. The enthalpy of reaction,which is the change in enthalpy $\Delta_{r} H$,does not change.
$\Delta_{r} H = H_{P} - H_{R}$
$= \text{Enthalpy of product} - \text{Enthalpy of reactant}$
$= \text{Constant}$
Therefore,$\Delta H$ does not change in the presence of a catalyst.
In the presence of a catalyst,the height of the potential energy barrier decreases,but the energy levels of the reactants and products remain the same. Thus,$\Delta H$ remains constant.