Entropy
- Entropy is a measure of dispersal of energy through a system, which increases as disorder increases
- The terms feasible or spontaneous are used to describe reactions that can take place on their own.
- It is a fundamental observation that in any spontaneous process, the total entropy of the universe will always increase
- Reactions that produce gases result in an increase in entropy
- Entropy increases during changes in state that give the particle access to a greater number of configurations
solid < liquid < gas (in order of increasing entropy)
- The sign of an entropy change can be predicted when the reactants and products have different numbers of moles of gas molecules. If the products have fewer moles, then entropy decreases
Fewer moles of gas → Fewer particles → Fewer configurations → Lower entropy
Gibbs Free Energy
- Whether a reaction will happen spontaneously depends on temperature, enthalpy change and entropy changes
- The Gibbs Free-Energy Equation:
∆G = ∆H - T∆S
∆G = change in free energy of the system (kJ mol^-1)
∆H = change in enthalpy of the system (kJ mol^-1)
T = temperature of the system (K)
∆S = change in entropy of the system (J K^-1 mol^-1)
∆G = change in free energy of the system (kJ mol^-1)
∆H = change in enthalpy of the system (kJ mol^-1)
T = temperature of the system (K)
∆S = change in entropy of the system (J K^-1 mol^-1)
- ∆G allows us to predict whether a reaction is feasible
- Reactions are only feasible if ∆G is zero or negative
- ∆G depends on temperature – some reactions may be feasible at one temperature and not another
- To calculate the temperature at which a reaction becomes feasible, the Gibbs Free-Energy equation has to be rearranged
- Kinetics also influences whether a reaction can occur. If the activation energy is too high or the rate of reaction is very slow, the reaction may not happen even if the reaction is thermodynamically favourable.