Entropy Changes in Chemical Reactions.

Because entropy is a state function, the property is what it is regardless of pathway, the entropy change for a given reaction can be calculated by taking the difference between the standard entropy values of products and those of the reactants.

DSoreaction = SnpDSo

products - SnrDSoreactants

Entropy Changes in Chemical Reactions.

Calculating DSo. Calculate DSo at 25o C for the reaction

2NiS(s) + 3O2(g) 2SO2(g) + 2NiO(s)

Entropy Changes IICalculate DSo for the reaction of aluminum oxide by hydrogen gas:

Al2O3(s) + 3H2(g) 2Al(s) + 3H2O(g)

Free Energy and Chemical Reactions

Standard free energy (DGo) is the change in the free energy that will occur if the reactants in their standard states are converted to the products in their standard states.

The value of DGo tells nothing about the rate of a reaction, only its eventual equilibrium position.

Free Energy and Chemical Reactions Calculating DGo as a State Function.

DGo = DHo - TDSo Consider the reaction 2 SO2(g) + O2(g) 2SO3(g) carried out at 25o C and 1 atm.

Calculate DHo and DSo, then calculate DGo.

Free Energy and Chemical Reactions

Calculating DGo as a State Function. Solving DGo Using Hess’s Law. Using the following data (at 25o C) Cdiamond(s) + O2(g) CO2(g) DGo = -

397 kJ Cgraphite(s) + O2(g) CO2(g) DGo = -

394 kJ Calculate DGo for the reaction

Cdiamond(s) Cgraphite(s)

Free Energy and Chemical Reactions Calculating DGo as a State Function. Standard Free Energy of Formation (DGf

o). DGo = SnpDGf

o products - SnrDGfo reactants

Methanol is a high-octane fuel used in high-performance racing engines. Calculate DGo for the reaction2CH3OH(g) + 3O2(g) 2CO2(g) + 4H2O(g)

Free Energy and Chemical Reactions

A chemical engineer wants to determine the feasibility of making ethanol (C2H5OH) by reacting water with the ethylene (C2H4) according to the equation

C2H4(g) + H2O(l) C2H5OH(l) Is the reaction spontaneous under standard conditions?

The Dependence of Free Energy on Pressure

The equilibrium position represents the lowest free energy value available to a particular reaction.

Free energy changes throughout the course of a reaction because it is pressure and concentration dependent.

For any 1 mole of a gas at a given temperature

S large V > S small V or S low P > S high P

The Dependence of Free Energy on Pressure

This leads to the equationDG = DGo + RT ln(Q) where Q is the reaction quotient, T is the Kelvin temperature, R is the ideal gas constant 8.31 J/mol K, DGo is the free energy at 1 atm, and DG is the free energy at a specified temperature.

This derivation of the equation has been removed from the equations sheet. There is, however, another derivation that is on the sheet.

The Dependence of Free Energy on Pressure

One method for synthesizing methanol (CH3OH) involves reacting carbon monoxide and hydrogen gases:

CO(g) + 2H2(g) CH3OH(l) Calculate DG at 25o C for this reaction

where carbon monoxide gas at 5.0 atm and hydrogen gas at 3.0 atm are converted to liquid methanol.