Archive for category Unit 15

Gibbs Free Energy in Exothermic and Endothermic Reactions

Gibbs free energy is a useful concept which lets us determine how likely a reaction is to take place spontaneously. It is defined using the following equation:

ΔG = ΔH – TΔS

  1. A reaction will take place if it creates a reduction in Gibbs free energy, ie ΔG < 0.
  2. Gibbs energy is reduced if H is reduced. H is the heat energy locked up in the system.
  3. Gibbs energy is reduced if S is increased. S is a measure of Entropy or Disorder.

A reaction is most likely to happen if ΔH < 0 and ΔS > 0, ie if the system enters a lower energy state and becomes more disordered.

Exothermic and Endothermic reactions

  1. In an exothermic reaction heat is given out so heat energy in the system is reduced. ΔH <0. This is favourable.
  2. In an endothermic reaction heat is taken in so heat energy in the system is increased. ΔH >0. This is unfavourable

An Endothermic reaction can still take place if it results in a large enough increase in entropy, ie ΔS > 0.

Water can melt even though this is an endothermic reaction since liquid water is less organised and therefore has a higher entropy than solid crystalline ice.

Polythene will shrink in an oven even though this is an endothermic reaction (it requires heat to take place). This is because the shrunken polythene is less ordered and has higher entropy.

For more information on answering these sorts of questions see the Bodner Research Web

Enthalpy

Enthalpy is heat

The thermodynamics song

Oh you can’t pass heat from a cooler to a hotter! You can try it if you you like but you’re far better notter!

Thermodynamics, entropy and Gibbs Free Energy introduction from the Chem Guy

Gibbs free energy and thermodynamics are topic which come up year on year. The Chemistry Guy gives a good introduction to Thermodynamics.

Mendelian Genetics