Notes #11 16.5 Reaction spontaneity

Spontaneous or Nonspontaneous That is the Question ? Suppose Δ H is negative Δ Hf products < Δ Hf reactants It’s EXOTHERMIC When Δ Hr is negative reaction tends to be SPONTANEOUS Spontaneous means that it will occur without any outside influence How about Δ H positive?

Entropy (S) is the measure of the degree of disorder in a system. A spontaneous process is one that occurs in a system left to itself. No external action is needed to make it happen.

Entropy Increase in disorder; Decrease in disorder; ∆S > 0

Predicting changes in entropy pgs. 514-515 Entropy changes associated with changes in state H2O(l) → H2O(g) Dissolving of a gas in a solvent always results in decrease in entropy CO2(g) →CO2(aq) Entropy usually increases when the # of moles increases from reactants to products Increase in temp. always increases entropy.

GIBBS FREE ENERGY Gibbs free energy indicates whether a reaction will occur or not.

Exergonic reactions (spontaneous) Endergonic reactions (nonspontaneous)

Practice Determine if each of the following would be spontaneous or not ∆H= 15.6kJ T=415K ∆S=45 J/K spontaneous 2. ∆H= 35.6kJ T=415K ∆S=45 J/K nonspontaneous

Endothermic reactions occur spontaneously when T Δ S is large. The thermodynamic definition of a system in equilibrium is; when Δ H and Δ S have the same sign, and there is some temperature at which Δ H and T Δ S are numerically equal.

A spontaneous reaction proceeds toward equilibrium. Chemical potential energy, G, is least at equilibrium. Enthalpy, entropy, and free energy depend on temperature. ( We will only work at 298.15 K and 100.00 kPa-standard states)

GIBBS FREE ENERGY CALCULATIONS Δ H values are relative; free elements are considered to have change in enthalpy is found by;

entropy changes and Gibbs free energy is computes as follows;

1. Organize the data you will use from the appropriate table. 2. Multiply each Δ Gf0 by the number of moles from the balanced equation. (Always make sure the chemical equation is balanced). Substitute these values into the equation used to determine Δ Gf0 .

The Gibbs free energy decreases in a spontaneous reaction because the system is changing to a more stable state.

To find 1. Multiply each by the number of moles from the balanced equation. 2. Substitute these values into the equation.