13B: Entropy
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Some endothermic reactions can occur at room temperature, state what this suggests.
Enthalpy changes alone do not control whether reactions occur.
Describe entropy
A measure of the disorder of a system. The natural direction of change is increasing total entropy.
Explain why entropy changes occur during changes of state.
Solid -> liquid -> gas Molecules have greater freedom of movement, so entropy increases
Explain why entropy changes occur during dissolving of a solid ionic lattice.
Molecules have greater freedom of movement, so entropy increases
Explain why entropy changes occur during reactions in which there is a change in the number of moles from reactants to products.
Number of moles increases, More disorder, Entropy increases.
State the equation to calculate the total entropy change of a reaction.
Total entropy change = Entropy change in the system + Entropy change in the surroundings
State the equation to calculate the entropy change in the surroundings
Entropy change in surroundings = - Enthalpy change (kJ/mol) / Temperature (K)
State what is determined by the balance between the entropy change and the enthalpy change
The feasibility of a reaction
State the equation for Gibb’s Free Energy change
ΔG = ΔH - TΔSsystem
State how the Gibb’s Free Energy change can be used to predict wether a reaction is feasible
If ΔG < 0, Reaction is feasible
State how the Gibb’s Free Energy change equation can be used to determine the temperature at which a reaction is feasible
Use ΔG = 0, Solve equation for T
State the equation that shows the relationship between Gibb’s Free Energy change and the equilibrium constant
ΔG = −RT ln K
Describe the relationship between Gibb’s Free Energy change and the equilibrium constant
Reactions which are feasible in terms of ΔG have large values for the equilibrium constant (ΔG = −RT ln K)
Explain why a reaction for which the ΔG value is negative may not occur in practice
May be inhibited by kinetic factors, e.g. large activation energy