Reactivity 1.4 – Entropy and spontaneity (HL)

1.4.4 – Ī”G and equilibrium

šŸ“Œ Equilibrium

  • Equilibrium can be written in terms of Gibbs as the following : Ī”G = 0
  • Entropy reaches a maximum when the amounts of products = the amount of reactants
  • At this time the mixture is in equilibrium and Q = 1 [see R2.3.5]
  • In this situation, the minimum Gibbs value is reached at Q = 1

šŸ“Œ Equilibrium constant K

  • The equilibrium constant K is the value of the reaction quotient Q at equilibrium
  • It can be given by the following expression :
  • When K>1 , the reactions tends to products and Gibbs is usually negative
  • When K<1, the reactions tends to reactants and Gibbs is usually positive

Equation : aA + bB ā†’ cC + dD

K = ([C]c[D]d) / ([A]a[B]b)

  • For Gibbs = 0, K = 1
  • When K>1 , the reactions tends to products and Gibbs is usually negative
  • When K<1, the reactions tends to reactants and Gibbs is usually positive

šŸ“Œ Relationship between Gibbs and K

Ī”GĀ°Equilibrium mixtureK
negativemainly productsK > 1
positivemainly reactantsK < 1
0both reactants and productsK = 1
  • This can be summarised by the following equation

Ī”G = -RTlnK

where R is the ideal gas constant (given in section 2 of the data booklet) and T is the temperature in Kelvin