22.5 Free energy

• A reaction can happen if the products have a lower overall energy than the reactants. The term feasibility describes wether a reaction is able to happen and is energetically feasible.
• The term feasibility us used to describe wether a reaction is able to happen and is energetically feasible.
• the overall chnage in energy during a chemical reaction is called the free energy change (delta G) and is made of two types of energy

  1. the enthalpy change (delta H). The heat transfer between the chemical system and the surroundings
  2. the entropy change at the temperature of the reaction (T delta S). This is the dispersal of energy within the chemical system
• The relationship between the two types of energy is shown in the Gibbs equation.

  
• the feasibility of a reaction depends on the balance between delta H and T delta H. for a reaction to be feasible, there must be a decrease in free energy.

  • delta G < 0
• the value for delta H is usually much larger than delta S. delta H values are shown in the units kJ mol^-1 whereas the much smaller delta S values are shown in the units JK^-1mol^-1.
• In the Gibbs equation the value of and units of delta S must be changed to kJ^-1mol^-1 by dividing by 1000. This matches the units for delta H.
• for reactions at room temperature delta H has a much larger magnitude of T delta S and so delta G is largely dependant on delta H. As temperature , the T delta S becomes more important.
• the feasibility of a reaction depends on the balance between delta H and T delta S.

  
• the worked example below shows how the minimum temperature for feasibility can be calculated

  
• some ionic compounds dissolve on water at room temperature in an endothermic process, cooling down the water. to find out why endothermic process can take place at room temperature, you need to use entropy and free energy change.
• free energy change is useful for predicting freasability, but many reactions have a negative delta G and don’t take place. although signs of delta G indicates the thermodynamic feasability, it takes no account of the kinetics or rate of reaction.