1.4.2 Gibbs energy (HL)

chem whatdriveschemicalreactions

Reactivity 1.4.2 - change in Gibbs energy, , relates the energy that can be obtained from a chemical reaction to the change in enthalpy, , change in entropy, , and absolute temperature,

• basing the direction of change on enthalpy changes leaves endothermic reactions unexplained
• basing the direction of change on entropy changes leaves reactions that occur with a decrease of entropy unexplained.

to consider the total entropy change of a reaction, the entropy change of the surroundings must also be considered

this reaction increases the entropy of the universe by transferring heat (energy) to the surroundings, leading to a more general dispersal of energy

can be calculated from the enthalpy change in the system, , and the absolute temperature,

exothermic reactions, with a negative value, result in an increase in the entropy of the surroundings

• exothermic reactions are generally more common than endothermic reactions, not because of the decrease in energy of the system, but the associated increase in entropy of the surroundings

• the impact of a transfer of heat to the surroundings on its entropy depends on the current state of dispersal

  • if the surroundings have a high temperature, the addition of a little heat will make little difference
  • if the surroundings are cold, same amount of heat causes a more dramatic change in entropy

calculating total entropy changes and understanding endothermic reactions

the second law of thermodynamics says that

substitute for from the expression developed earlier:

• endothermic reactions occur if the change in the entropy of the system can compensate for the negative entropy change of the surroundings produced as the heat is transferred from the surroundings to the system
• if , then the reaction can occur spontaneously

Gibbs energy

this combination of entropy and enthalpy of a system gives a new function known as the Gibbs energy

must be negative for a spontaneous process

gives a measure of the quality of the energy available

is a measure of the quantity of heat change during a chemical reaction
is a measure of the quality of the energy available:

• a measure of how much energy that is free to do useful work rather than leaving the system as heat

• energy left over that can do work after compensating for the entropy decrease in the system

• spontaneous reactions have negative Gibbs energy changes because they can do useful work

the energy available to do work is

calculating values

calculating from

for reactions at can be calculated from in the same way that can be calculated from values.

NOTE

just as

using and values to calculate at all temperatures

since the standard values of refer to standard conditions, they can only be used to calculate at . can be calculated at other temperatures using

as and are effectively constant, and can be used in the equation