A2-Level Enthalpy and Entropy

  • The feasibility of a reaction refers to how likely it is to happen (high feasibility means a reaction is likely to happen, low feasibility means a reaction is unlikely to happen).

  • For a reaction to be feasible, energy must be released overall.

  • The overall energy change of a reaction is determined by both the change in enthalpy and the change in entropy that would occur.

  • Gibbs free energy is a value calculated that shows how energy changes overall during a reaction. The equation is:       ∆G = ∆H - T∆S
    where ∆H = enthalpy change, T = temperature (in Kelvin) and ∆S = entropy change.

  • If Gibbs free energy is negative – reaction is feasible (overall increase in stability).

  • If Gibbs free energy is positive – reaction is not feasibility (overall decrease in stability).

QUICK NOTES

Free Energy

 

The changes in enthalpy and entropy in a reaction influence how likely it is to happen - both need to be considered.

 

A reaction that can occur is called feasible. A reaction that happens by itself (needs no ‘input’ of energy) is called spontaneous.

 

Reactions are favoured if they lead to an increase in stability (lower energy overall than before the reaction occurred). In an exothermic reaction, the enthalpy change is negative – because heat energy is released from the reactants during the reaction. It is easy to see how exothermic reactions can proceed as the products are at a lower energy (more stable) than the reactants, meaning overall stability has increased.

 

It seems strange that endothermic reactions can occur at all. The enthalpy change in an endothermic reaction shows us that the products have gained heat energy and are less stable than the reactants. It is not strange, however, if the entropy change is also considered when we are determining how feasible a reaction is. Remember entropy is also a unit of energy, but not heat energy!


If the products of a reaction have a higher entropy than the reactants, they are more stable. This suggests that all solids should spontaneously react to form gases (gases have a higher entropy value than solids) and be ‘more stable’. The fact they don’t is because the enthalpy change of a reaction also determines how feasible it is. If the enthalpy change of a reaction is positive (endothermic), then the change in entropy would have to be very large – to make up for the heat energy gained by the products.

When determining the feasibility (how to likely it is to happen) of a reaction, we must consider the balance between the enthalpy change and entropy change of a reaction. To do this, we must also include the temperature the reaction is occurring at, as entropy changes with temperature. This is why some reactions are more likely to happen at higher temperatures – as the entropy change would be greater.

 

The result of balancing these factors is called free energy (sometimes referred to as Gibbs free energy, named after Willard Gibbs who pioneered early calculations using entropy).

 

Free energy, ∆G, can be calculated using:

 

∆G = ∆H - T∆S

 

where ∆H = enthalpy change, T = temperature (in Kelvin) and ∆S = entropy change.

 

For a reaction to be feasible there must be an overall release of energy by the reaction and the free energy must be less than zero. A negative free energy (∆G) shows us that the reactants have more ‘free energy’ than the products.

 

If a reaction has a negative enthalpy change (exothermic) and a positive entropy change, the reaction will always be feasible, as the free energy value will always be negative.

 

If a reaction has a negative enthalpy change (exothermic) and a negative entropy change, the reaction can be feasible but only at low temperatures.

 

If a reaction has a positive enthalpy change (endothermic) and a positive entropy change, the reaction can be feasible but only at high temperatures.

 

If a reaction has a positive enthalpy change (endothermic) and a negative entropy change, the reaction is never feasible, as the free energy value will always be greater than zero.

 

When calculating free energy it is important to remember units. Entropy is given in J K mol and enthalpy in kJ mol .  For calculations, convert entropy into kJ before finding the free energy change.

-1

-1

-1