• Gibbs free energy provides an effective way of focusing on a reaction system at constant temperature and pressure to determine its spontaneity
• For a reaction to be spontaneous, Gibbs free energy must be have a?negative?value (ΔG^??≤?0)
• We can use the Gibbs equation to calculate whether a reaction is?spontaneous?/?feasible?or not

ΔG^??= ΔH_reaction^??– TΔS_system^?

• • When ΔG^??is?negative, the reaction is?spontaneous / feasible?and likely to occur
  • When ΔG^??is?positive, the reaction is?not spontaneous / feasible?and unlikely to occur
• We can also look at the the values for enthalpy change, ΔH, and entropy change, ΔS
  • Depending on the value for ΔH?and ΔS?we can determine whether the reaction is spontaneous at a given temperature (T)

Factors affecting ΔG and the spontaneity / feasibility of a reaction?

Answers:

Answer 1:

Step1:?Calculate ΔS_system^?

• • ΔS_system^??= ΣΔS_products^??– ΣΔS_reactants^?

ΔS_system^??= (2 x ΔS^??[CaO(s)]) –? (2 x ΔS^??[Ca(s)] + ΔS^??[O₂(g)])

= (2 x 40.00) – (2 x 41.00 + 205.0)

= -207.0 J K^-1?mol^-1

Step 2:Convert ΔS^??to kJ K^-1?mol^-1

• • ΔS_system^??= -207.0 J K^-1?mol^-1?÷ 1000 = -0.207 kJ mol^-1

Step 3:?Calculate ΔG^?

• • ΔG^??= ΔH_reaction^??– TΔS_system^?

ΔG^?? = -635.5 – (298 x -0.207)

= –573.8 kJ mol^-1

Answer 2:?

• • Since ΔG^??is?negative, the reaction is?feasible

Factors affecting ΔG?and the spontaneity / feasibility of a reaction

• We can also look at the the values for ΔH?and ΔS?to determine whether the reaction is spontaneous / feasible at a given temperature (T)
• The Gibbs equation will be used to explain what will affect the spontaneity / feasibility of a reaction for exothermic and endothermic reactions

Exothermic reactions

• In exothermic reactions, ΔH_reaction^??is?negative
• If the ΔS_system^??is?positive:
  • Both the first and second term will be?negative
  • Resulting in a?negative?ΔG^??so the reaction is?feasible
  • Therefore, regardless of the temperature, an exothermic reaction with a positive ΔS_system^??will?always be feasible
• If the ΔS_system^??is?negative:
  • The first term is?negative?and the second term is?positive
  • At very high temperatures, the –TΔS_system^??will be very?large?and?positive?and will overcome ΔH_reaction^?
  • Therefore, at high temperatures ΔG^??is?positive?and the reaction is?not feasible
• Since the relative size of an entropy change is much smaller than an enthalpy change, it is unlikely that TΔS >?ΔH?as temperature increases
• These reactions are therefore usually spontaneous at normal conditions

The diagram shows under which conditions exothermic reactions are feasible

?Summary of factors affecting Gibbs free energy?