Question

Which of the following will lower the enthalpy (ΔH) for a reaction?

Increasing the concentration of the reactants.

Raising the temperature of the reaction.

Adding a catalyst for the reaction.

Increasing the pressure of the reaction.

None of the above.

Answer 1

The factors affecting the change in enthalpy of a reaction are :

(1) Physical state of reactants & products : The enthalpy of reaction changes with change in physical state. This is because as the physical state changes, extent heat is evolved.

(2) Quantity of reactants : The change in enthalpy of reaction depends upon the quantity of reactants used. When the number of moles of reactants are doubled, the enthalpy change also becomes double.

(3) Allotropic modification :For elements existing in different allotropic modifications, the heat of reaction is different if different allotropic form is involved in reaction.

(4) Temperature & Pressure : The enthalpy of reaction depends upon the temperature and pressure of reaction. Therefore, the values are generally expressed under standard conditions of temperature (298K) and pressure(1 atm)

So Increasing the pressure of the reaction will lower the enthalpy (ΔH) for a reaction.