Entropy, Enthalpy and Spontaneous Reactions

Spontaneous Reaction A reaction that occurs without outside intervention. Once it occurs it can be fast or slow. It can go to completion. (Recall the three products produced : Precipitate (table F), gas, or water.) Or be in a state of equilibrium.

Two factors determine if a reaction is spontaneous Enthalpy: (heat of reaction ) Entropy : (randomness)

Enthalpy: Heat of Reaction Exothermic Reaction Endothermic Exothermic Heat is absorbed H P > H R ▲H = H P - H R ▲H is positive Example from table I Heat is released H R > H P ▲H = H P - H R ▲H is negative Example from table I

Heat of Formation Heat absorbed or releases when a mole of a compound is formed from its elements. Standard heat of formation is at 25 degrees celcius and 1 atm Stable compounds require a large input of energy to decompose a compound. Therefore more stable the compound the larger the – heat of formation. Unstable compounds require little or no energy to decompose. They have small –heat of formation or even a + heat of formation.

Hess’s Law of Constant Heat Summation

Enthalpy and Spontaneous Reactions At constant temperature and pressure a system tends to undergo a reaction so that in its final state it has a lower energy than its initial state. Exothermic reactions in nature are favored (-delta H)

Entropy S Measures the disorder, randomness or lack of organization of a system. Order (-s) Disorder (+S) Solid liquidaqueous ionsgas Higher entropy is favored by higher temperatures and more moles of gases.

Probability of Disorder Is there a higher probability your room will be messy or neat as time goes on?

Example: Which has more entropy in its system? H2O (s) or H2O (g)

Entropy and Spontaneous Reactions At constant temperature a system tends to undergo a reaction so that in its final state it has a higher entropy (greater randomness) than its initial state.

Summary Case 1: Exothermic and disorder will always occur Case 2: Endothermic and order will never occur Case 3: Exothermic and order will sometimes occur Case 4: Endothermic and disorder will sometimes occurs. What determines if it will sometimes occur?

Gibbs Free Energy Change Free energy of a system can be defined as the measure of energy available (free) to do work at a constant pressure and temperature. In a chemical reaction it measures the extent to which the substances can react.

Gibbs Free Energy G 0 (positive value) reaction will not occur. Favors the reactants. G  0 (negative values) reaction will occur be spontaneous, Go to completion and favors the products. G = 0 reaction will be in a state of equilibrium.

Summary of Gibbs Free energy

Examples: Physical change: When doe ice melt? Enthalpy_______ Entropy________ Always/never/sometimes Chemical Change: When does ammonia form? Enthalpy________ Entropy_________ Always/never/ sometimes