Enthalpy and Entropy

  • Created by: Samantha
  • Created on: 02-05-14 11:40

Entropy, S

Entropy is the qualitative measure of the degree of disorder in a system

Always positive

Nature tends to move from order to disorder in isolated systems. Gas molecules will spread out to fill a space, increasing their entropy.

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Standard Entropy Change of Reaction

The entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states

At 0K perfect crystals have 0 entropy
Entropy increases with changes in state that result in more disorder
When water changes state from a liquid to a gas, its entropy rises from +70 to +189 JK^-1mol^-1
Entropy increases when a solid lattice dissolves
Entropy increases when there is an increase in the number of gaseous molecules ie when a gas is produced in a reaction
Entropy is higher for molecules with more atoms

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Standard Entropy Change for a Reaction

Change in Entropy = Sum of the Entropy of the Products - Sum of the Entropy of the Reactants

A more chaotic system has a higher entropy

If a change makes a system more random the entropy change is positive

If a change makes a system more ordered the entropy change is negative

Increase in moles usually means a +ve entropy change

Decrease in moles usually means a -ve entropy change

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Spontaneous Processes

Spontaneous processes occur on their own and lead to lower energy in the system and increased stability.

Most exothermic reactions occur spontaneously at room temperature. The enthalpy decreases and excess energy is released to the surroundings. This increases stability.

Some endothermic reactions occur spontaneously at room temperature. The enthalpy of the chemical system increases, energy is taken in from the surroundings.

A process is spontaneous if a chemical system beomes more stable and its overall energy decreases. The overall energy decrease is due to enthalpy and entropy. The contribution of entropy depends on the temperature. Energy derived from entropy is T x delta S (where T is measured in Kelvin)

As T increases, the energy derived from entropy becomes more significant

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Spontaneous Reactions

Three factors determine whether a reaction is spontaneous:

  • temperature, T, in Kelvin
  • entropy change in the system, delta S
  • enthalpy change, delta H, with the surroundings

The relationship between the three factors is the free energy change, delta G:

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Spontaneous Reactions and delta G

A process can take place spontaneously when delta G is less than 0
If delta G is positive at room temperature, there may still be a higher temperature where the reaction becomes feasible
If delta S is positive, there must be a point at which T x delta S is big enough to out weigh the enthalpy

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