Gibbs Free Energy

Derivation of G

So far we have learned to determine whether or not a reaction is likely to be spontaneous using

Manipulating this equation yields another more useful equation that focuses only on the system, not the surroundings.

Since all quantities pertain to the system, this equation can be simplified to

This equation is true for constant pressure processes.

Gibbs free energy is a ______function and is defined as the maximum energy available to do ______.

If G is ______or ______it means that energy is released and available to do work. If G is ______or ______then G represents the minimum work that must be done to make a process occur.

Predicting Spontaneity Using G

G can be used to predict the spontaneity of a reaction. Use G = H –TS to think about the following situations.

H / S / G / Spontaneity
(-)
exothermic / (+)
entropy inc
(-)
exothermic / (-)
entropy dec
H / S / G / Spontaneity
(+)
endothermic / (-)
entropy dec
(+)
endothermic / (+)
entropy inc

Example: A reaction is non-spontaneous under standard conditions but becomes spontaneous at higher

temperatures. What conclusions may be drawn about the sign of H, S, and G under standard

conditions?

Standard Free Energy of Reaction (Gorxn)

Gorxn is the free-energy change for a reaction occurring under ______conditions. That is the both the reactants and products are in their standard states. The Gorxn for elements in their standard state is ______. Gorxn is measured in ______.

Gorxn can be calculated three different ways. The method used depends upon the information available to you and the information you are seeking. Because Go is ______dependent, calculation of Go for any reaction that does not occur at ______must be done using method 1. By contrast Ho and So are far less temperature dependent (as long as there is no ______change) and so we can use the tabulated values for reactions occurring at any temperature.

Method 1: Using Gorxn = Horxn –TSorxn

Example 1:Calculate Ho,So, and Gofor the following exothermic reaction at 25oC and 1 atm.

2SO2(g) + O2(g) ----> 2SO3(g)

Example 2:The production of quicklime (CaO) from limestone (CaCO3) is an endothermic equilibrium process that only becomes commercially viable at high temperatures. At what temperature will the production of quicklime favour products?

CaCO3(s)  CaO(s) + CO2(g)

Phase transitions are unique situations in which for a period of time while a substance changes phase the system is in ______between the two phases and the temperature remains ______. With this information the entropy change that occurs during a phase change can be calculated.

Example 3:Calculate the entropy change that accompanies the freezing of water.

Method 2: Using Gorxn = Gof products - Gof reactants

Because Gorxn is a state function, it can be calculated just like ______and ______using

Where Gof is ______

______

______

Example 1:Calculate the standard free energy changes at 1 atm and 25oC for:

CH4(g) + 2O2(g) ----> CO2(g) + 2H2O(l)

Method 3: Using Hess’ Law

Example:Calculate Gorxn for:2CO(g) + O2(g) ----> 2CO2(g) given:

2CH4(g) + 3O2(g) ----> 2CO(g) + 4H2O(g) ; Go = -1088 kJ/mol

CH4(g) + 2O2(g) ----> CO2(g) + 2H2O(g) ; Go = -801 kJ/mol

Silberberg: Rd p899-907 and do Q #20.46, 20.48, 20.50, 20.52, 20.54, 20.58, 20.60.