Reaction Spontaneity and Entropy

1. Reaction Spontaneity and Entropy Thermochemical Reactions Lecture 3

2. Spontaneous Processes • A spontaneous process is a physical or chemical change that occurs with no outside intervention. • However, for many spontaneous processes, some energy must be supplied to get the process started. • Many exothermic reactions are spontaneous processes, while a few endothermic reactions are spontaneous processes.

3. Entropy • Entropy (S) is a measure of the disorder or randomness of the particles that make up a system. • The law of disorder states that spontaneous processes always proceed in such a way that the entropy of the universe increases (becomes more disorganized). • Change in entropy = ΔS • ΔSsystem = Sproducts – Sreactants • If S products > S reactants, then ΔS is positive • If S products < S reactants, then ΔS is negative

4. Predicting Entropy • Entropy changes associated with changes in state will be positive (ΔS is positive). • Dissolving a gas in a solvent will produce a negative entropy (ΔS is negative). • An increase in the temperature of a substance increases entropy (ΔS is positive).

5. Entropy, the Universe, and Free Energy • ΔSuniverse > 0 • ΔSuniverse = ΔSsystem + Δssurroundings • ΔSuniverse is positive when the reaction is exothermic or the ΔH is negative, or • The entropy of a system increases. • Free energy (Gsystem) is the amount of energy available to do work at constant pressure and temperature. • When a reaction occurs under standard conditions the standard free energy change is expressed as ΔGsystem = ΔHsystem – TΔSsystemT = Kelvin temperature

6. Calculating Free Energy • Convert ΔHsystem to joules (multiply kJ by 1000) • Substitute known values into the equation ΔGsystem = ΔHsystem – TΔSsystem

7. Example Problem • ΔGsystem = ΔHsystem – TΔSsystem ΔHsystem = 145 kJ ΔSsystem= 322 J/K T = 382 K • ΔGsystem = 145,000 J – 382 K (322 J/K) ΔGsystem = 145,000 J – 123,004 J ΔGsystem= 21,996 J ΔGsystem is positive, so the process is nonspontaneous.