Gibb's Free Energy Change

• • Whether or not a reaction will proceed depends on a balance between entropy and enthalpy.
  • These two quantities combine to give a single term, known as free energy
  • The change in free energy of a reaction determines whether it will proceed.
• deltaG = deltaH - TdeltaS


  deltaG = change in free energy (kJ mol-1)

  delta H = change in enthalpy (kJ mol-1)

  deltaS = (J K-1 mole-1)

  T = temperature (K)
• Simply put, deltaG tells us about the spontaneity or feasability of a process.
• If deltaG is negative or zero a reaction can happen by itself

  (Note: even if a process has a negative deltaG value, the calculation does not give an indication about the rate at which it takes place)
• Finding the temperature at which a reaction becomes feasible

  To do this we need to work out the temperature at which deltaG = 0

  T = deltaH / deltaS
• Predicting how the feasibility of a reaction changes with temperature

  Because the free energy change deltaG depends on TdeltaS, the effect of entropy becomes more important at higher temperatures.
• If deltaH is positive and deltaS is positive, the reaction will be feasible at higher temperatures.
• If deltaH is negative and deltaS is positive, the reaction will be feasible at all temperatures.
• If deltaH is positive and deltaS is negative, the reaction will not be feasible at any temperature.
• If deltaH is negative and deltaS is negative, the reaction may be feasible at lower temperatures.
• Suggest one reason why a sample of magnesium appears to be stable in air at room temperature, despite its negative free energy value.

  A protective layer MgO forms, which prevents further reaction

  OR activation energy is very high

  OR reaction is very slow
• Explain why the standard entropy value for carbon dioxide is greater than that for carbon.

  carbon dioxide gas is more disordered than solid carbon
• State the temperature at which the standard entropy of aluminium is 0 J K-1 mol-1.

  0K
• Explain why the evaporation of water spontaneous even though this change is endothermic.

  The molecules become more disordered when water evaporates, therefore the entropy increases. TdeltaS is larger than deltaH, so deltaG is less than zero.