Gibbs free energy

• Free energy change (AG) is a balance between enthalpy, entropy and temperature
• AG - is a measure used to predict whether a reaction is feasible
• Feasible reaction: a reaction that, once started, will carry on to completion without any energy being supplied to it
• If AG negative or equal to zero: reaction is feasible

  If AG positive: a reaction will never take place
• Equation to calculate Gibbs free energy change

  AG = AH - TAS
• Units for AG

  • KJ mol-1
• • When AG = 0 a reaction is just feasible
  • Temperature when AG = 0 can be calculated
• Effect of temperature on feasibility

  A) negative

  B) always feasible

  C) Depends on temperature

  D) Not feasible at high temperatures

  E) Depends on temperature

  F) Not feasible if temperatures are low

  G) Positive

  H) Not feasible at any temperatures

  
• Explain why the following reaction does not occur at very high temperatures even though the reaction is exothermic.

  2So2(g) + o2(g) ⇌ 2So3(g)

  • AS is negative
  • The number of moles has decreased from 3 moles to 2 moles (disorder decreases hence negative AS)
  • At high temperatures TAS is larger than AH and AG is positive
• Explain why some reactions despite having AG value of less than or equal to zero may not occur spontaneously

  • Rate of reaction is too slow
  • Activation energy is too high
• What each value is in y = mx + c

  • AG = y axis
  • AH = y-intercept
  • -AS = m
  • T = x axis
• Deductions that can be made from the gradient:

  • If the gradient is positive = AS is negative
  • If the gradient is negative = AS is positive
• What is the significance of the temperatures shown by the arrow in the graph ? (arrow points to where graph crosses x-axis)

  • Boundary between feasible and non-feasible reaction
• State the units for gradient

  • KJ K-1 mol-1
• Explain with the aid of a thermodynamic equation why this line obeys y = mx + c

  • AG = AH - TAS
  • AH = C
  • -AS = m