How does the presence of a catalyst affect the enthalpy change (ΔH) of a chemical reaction?

• A. It reverses the sign of the enthalpy change.
• B. It increases the enthalpy change of the reaction.
• C. It does not change the enthalpy; ΔH remains the same.
• D. It decreases the enthalpy change of the reaction.

Answer: (C)

Verified step by step guidance

1. Understand that the enthalpy change (\$\Delta H\$) of a reaction is a state function, meaning it depends only on the initial and final states of the system, not on the path taken.
2. Recognize that a catalyst works by providing an alternative reaction pathway with a lower activation energy, which speeds up the reaction rate but does not alter the energies of the reactants or products.
3. Since the catalyst does not change the energy levels of reactants or products, the overall enthalpy change (\$\Delta H\$) remains the same regardless of whether a catalyst is present.
4. Therefore, the presence of a catalyst does not reverse, increase, or decrease the enthalpy change; it only affects the kinetics, not the thermodynamics, of the reaction.
5. In summary, the correct understanding is that \$\Delta H\$ is unchanged by a catalyst because it is determined solely by the difference in enthalpy between products and reactants.