Reaction Feasibility

• The standard enthalpy of formation, ∆ ° H f , is the enthalpy change when one mole of a substance is formed from its elements in their standard states.
• The standard state of a substance is its most stable state at a pressure of 1 atmosphere and at a specified temperature, usually taken as 298 K.
• The standard enthalpy of a reaction can be calculated from the standard enthalpies of formation of the reactants and products:

  ∆H° = = Σ∆ ° f(products) - Σ∆ ° f(reactants)
• The entropy (S) of a system is a measure of the degree of disorder of the system.
• The greater the degree of disorder, the greater the entropy.
• Solids have low disorder and gases have high disorder.
• Entropy increases as temperature increases.
• There is a rapid increase in entropy at the melting point of a substance and an even more rapid and larger change in entropy at the boiling point.
• The second law of thermodynamics states that the total entropy of a reaction system and its surroundings always increases for a spontaneous process.
• Heat energy released by the reaction system into the surroundings increases the entropy of the surroundings.
• Heat energy absorbed by the reaction system from the surroundings decreases the entropy of the surroundings.
• The third law of thermodynamics states that the entropy of a perfect crystal at 0 K is zero.
• The standard entropy of a substance is the entropy value for the substance in its standard state.
• The change in standard entropy for a reaction system can be calculated from the standard entropies of the reactants and products:

  ∆S ° = ΣS °(products)- ΣS °(reactants)
• The change in free energy for a reaction is related to the enthalpy and entropy changes:

  ∆G °=∆H °− T∆S °
• If the change in free energy (∆G °) between reactants and products is negative, a reaction may occur and the reaction is said to be feasible. A feasible reaction is one that tends towards the products rather than the reactants. This does not give any indication of the rate of the reaction.
• The standard free energy change for a reaction can be calculated from the standard free energies of formation of the reactants and products using the relationship:

  ∆G ° = Σ∆G °(products) − Σ∆G °(reactants)
• The feasibility of a chemical reaction under standard conditions can be predicted from the calculated value of the change in standard free energy (∆G °).
• The temperatures at which a reaction may be feasible can be estimated by considering the range of values of T for which ∆G °<0
• Under non-standard conditions any reaction is feasible if ∆G is negative.
• At equilibrium, ∆G = 0.
• A reversible reaction will proceed spontaneously until the composition is reached where ∆G = 0.