Order and Spontaneity Entropy – a thermodynamic property is the measure of disorder in a system, the units are J/K and its symbol is S

Factors that determine if a reaction will occur easily If the enthalpy is negative or the reaction is exothermic it is more likely to occur If entropy is positive, there is greater disorder the reaction is more likely to occur.

Factors that Affect Entropy Diffusion causes an increase in entropy Entropy increases as solutions become more dilute Entropy increases when the pressure of a gas is reduced. Entropies increase with increases in temperature but it is only large if a phase change occurs.

Entropy change in reactions Entropy increases when the total number of moles of product is greater than the total number of moles of reactant. Entropy can increase if the total number of particles in a system increases Entropy increases when the reaction produces more gas particles, gases are more disordered than liquids or solids.

Hess’s Law and Entropy Molar entropies can be calculated from molar heat capacity data Entropies can be calculated by using Hess’s Law and entropy data for other reactions. When equations are added or subtracted the entropy changes must be added or subtracted. When equations are multiplied by a constant the entropy changes must be multiplied by the same constant. Atoms and molecules that appear on both sides of the equation can be canceled.

Standard Entropy Represented by the symbol S0 The entropy of 1 mole of a substance at a standard temperature of 298.15 K. Elements can have standard entropy values other than 0, unlike standard enthalpies of formation which are always 0 for elements. Most standard entropies are positive Entropy changes of a reaction can be calculated by using the equation: ΔSreaction = ΔSproducts - ΔSreactants

Gibbs Energy If a reaction has a negative ΔH and a positive ΔS the reaction will most likely happen. If a reaction has a positive ΔH and a negative ΔS the reaction will not happen. Gibbs Energy, another thermodynamic quantity, can be used to determine if a reaction will occur or not when the ΔH and ΔS are both positive or negative.

Gibbs Energy represented by G is defined by the equation: G = H – TS Also called free energy A spontaneous reaction is one that occurs without any outside assistance, such as input of energy. A nonspontaneous reaction will never occur without outside assistance. An avalanche is a good example of a spontaneous event. An avalanche, a spontaneous event, can always occur, however, the snow will never go from the bottom of the mountain to the top without assistance, a nonspontaneous event.

A reaction is spontaneous if the Gibbs energy change is negative. A reaction with a Gibbs energy change greater than 0 is nonspontaneous. A reaction with a Gibbs energy change exactly 0 is at equilibrium. ΔG = ΔH – TΔS is the equation used to fin d change in Gibbs Energy using enthalpy and entropy changes If Standard Gibbs Energies of Formation are known then the following equation can be used to calculated the Gibbs Energy and spontaneity ΔGreaction = ΔGproducts - ΔGreactants

Relating Enthalpy and Entropy Changes to Spontaneity Is the reaction spontaneous Negative Positive Yes, at all temperatures Either Neg. or Pos. Only if T < ΔH/ ΔS Either Pos. or Neg. T > ΔH/ ΔS never

Changes in temperature affect ΔH and ΔS Very little but temperature affects ΔG greatly. Increasing the temperature of a reaction can make a nonspontaneous reaction spontaneous. A nonspontaneous reaction cannot occur unless some form of energy is added to the system. Photosynthesis is a nonspontaneous reaction and can only occur if energy, light, is present.