Question

Question: Considering each of the following values and neglecting entropy, tell whether the starting material or product is favored at equilibrium:

(a) ${\mathbf{\Delta H}}^{\mathbf{°}}\mathbf{=}\mathbf{80}\mathbf{}\mathbf{kJ}\mathbf{/}\mathbf{mol}$

(b) ${\mathbf{\Delta H}}^{\mathbf{°}}\mathbf{=}\mathbf{-}\mathbf{40}\mathbf{kJ}\mathbf{/}\mathbf{mol}$

Short answer

Answer

(a) Starting material is favored at equilibrium.

(b) Products are favored at equilibrium.

Step-by-step solution

Step-by-Step SolutionStep 1: Relation between ΔG°and ΔH°  

${\mathrm{\Delta G}}^{°}$corresponds to the change in free energy, ${\mathrm{\Delta H}}^{°}$corresponds to the enthalpy change, and ${\mathrm{\Delta S}}^{°}$ represents the change in entropy.

width="134" height="22" role="math">ΔG°=ΔH°+TΔS

Estimating the sign of ΔG°

The free energy change of a reaction can be estimated/approximated using the change in the bonding energy i.e., the change in enthalpy.

${\mathrm{\Delta G}}^{°}\approx {\mathrm{\Delta H}}^{°}$

The free energy change for favorable reactions (the reactions that favors product formation) is always negative.

ΔH° for chemical reactions

Since ${\mathrm{\Delta H}}^{°}$is directly related to ${\mathrm{\Delta G}}^{°}$, a negative value of ${\mathrm{\Delta H}}^{°}$indicates that the reaction favors products, and a positive value of ${\mathrm{\Delta H}}^{°}$indicates that the reaction favors the reactants (starting material).

Predicting the direction of a reaction using ΔH° values

In the given case the reaction favors starting material as the value of ${\mathrm{\Delta H}}^{°}$is positive.

The negative value of indicates that the reaction favors products at equilibrium.