gibbs free energy

• both entropy and enthalpy must be taken into consideration for a reaction to be feasible
• reaction feasibility depends on:

  • temperature
  • enthalpy change
  • entropy change
• gibbs standard free energy

  a measure of the potential work that may be done by a system at a constant temperature and pressure
• standard free energy (delta G) is defined by ∆G = ΔH + ΔS
• for a reaction to be feasible delta G must be equal to or less than 0
• the 2nd law of thermodynamics shows that the reaction is more likely to happen spontaneously
• when the value of delta G is greater than 0 is shows the reaction is more unlikely to occur unless external energy is provided (by heating)
• theres a difference between a reaction being feasible and happening spontaneously
• a reaction may be feasible at a particular temperature but may not proceed because the activation energy is too high
• for a spontaneous reaction to occur, delta G must be less than 0 and the activation energy must be low at standard conditions
• T= delta H / delta S

  is the equation used to determine the temperature at which a reaction occurred spontaneously, assuming delta G was 0
• standard free energy can also be determined from the formation of reactants and products, and by the equation:

  delta G = {delta G (products) - {delta G (reactants)
• delta G is the Gibbs free energy change under standard conditions, comparing the free energies of pure reactants and products
• the standard free energy of formation of an element is the most stable form, so is always 0
• equilibrium is reached when the free energy of the reactants has fallen, by using them up, to the same value as the products
• at equilibrium, the free energy of the system has reached is minimum value under the given conditions

  delta G = 0
• once the reaction has started, delta G will be lower then delta G degrees, which is a result of the change from standard conditions and the increased entropy due to mixing the products and reactants
• equilibrium is established when the free energy of the system drops to the lowest point, where delta G = 0
• thermodynamic calculations don't provide information about reaction pathway or rate, how fast or which route it takes, it only shows how far a reaction will proceed
• delta G degrees is negative

  • reaction is feasible
  • equilibrium favours products
  • equilibrium constant will be greater than 1
• delta G degrees is positive

  • reaction isnt feasible
  • equilibrium favours reactants
  • equilibrium constant is less than 1
• delta G degrees is 0

  • neither reactants or products is favoured
  • equilibrium position is in the middle of reaction and is called balance point
  • equilibrium constant is 1
• if a reaction is feasible but does not occur spontaneously, the activation energy is too high
• the fact that standard free energy change is greater than 0 does not mean that no reaction takes place, it means the products are less favoured than the reactants