Energy, entropy and equilibrium

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  • Created by: J.E.C.
  • Created on: 02-05-14 15:01

Solids, liquids and gases - energy

Solids are rigid because they are held together in a lattice.

Liquids and gases are also known as 'fluids' because their molecules are moving around.

As a solid, liquid or gas is heated, its temperature increases, however this is not the case for;

> Solids at their meling point > Liquids at the boiling point = the temperature would stay constant

SHC units = Jg¯¹K¯¹  and symbol = Cp

The three forms of kinetic energy which molecules can possess:

> Translation: movement of the whole molecule from one place to another.

> Rotation: spinning around.

> Vibration: stretching and compressing bonds

Changes in electronic energy and bonding energy do not affect the motion of the molecules.

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Energy and entropy

Energy is quantised and the size of the quanta increases as folows:

Translation < Rotation < Vibration < Electronic

There are a large number of closely spaced levels for molecules to exist in.

Molecules are distributed among the energy levels in the way that gives the greatest entropy.

There are more ways of arranging the quanta of energy in molecules if they are more spread out in an energy stack.

First law of thermodynamics - energy cannot be created or destroyed

Closely spaced energy levels = smaller quantagreater number of quanta at a given temperature.

Energy levels more closely spaced in molecules with heavier atoms.

Greater entropy for  molecules with heavier atoms and larger number of atoms.

Entropy = measure of the number of ways of arranging molecules and distributing their quanta of energy.

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Entropy change and equilibrium

Entropy change of the reaction = enthalpy of products - enthalpy of reactants

The process of ice freezing = entropy change = negative because liquid water becomes a solid = exothermic - energy is transferred from the system to the surroundings

Entropy change of surroundings = enthalpy change in the surroundings / T

Total entropy change = entropy change of the system + entropy change in the surroundings

Enthalpy Change of the system = the change in disorder within the reaction system arising from the reactants changing to products.

Enthalpy change of the surroundings = change in disorder of the reaction's surrounds arising from the reaction taking in / giving out heat from / to the surroundings.

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Entropy change and equilibrium

Second law of thermodynamics = For a spontaneous chemical change to occur the total entropy change must be more than 0.

When sea water freezes, pure ice is produced. The energy is the same as when molecules of pure water form a lattice in an ice crystal. However the entropy change for the system will be different. The entropy for salt solution is higher than that of pure water. The entropy change is more negative when salt solution freezes.

A spontaneous change always takes place in the direction which corresponds to an increase in total entropy change.

When entropy change of solution is equal to 0 there is no net change = equilibrium

The total entropy change under actual experimental conditions must be zero at equilibrium. The standard entropy change does not however need to be zero at equilibrium.

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Spontaneous entropy change

Entropy change is spontaneous if:

 > No force is required

 > Products are at a lower energy level than reactants

 > Likely if reaction is exothermic

 > Some endothermic reactions

For an endothermic reaction to be spontaneous:

 > Something other than the enthalpy change must control the energetic feasibility.

At lower temperatures, there are a greater number of possible distributions of the energy quanta in the surrounds after receiving them from / giving them to the reaction system.

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Feasible reactions

1. Reaction is feasible if the total entropy change is positive

2. Endothermic reactions are more likely at a higher temperature because the entropy change of the surroundings is less negative.

3. Endothermic reactions are feasible provided  the entropy change of the system is positive enough

4. A reaction with a positive total entropy change may not occur if the activation enthalpy is too large. This causes an otherwise energetically feasible reaction to not occur because it is too slow ie it is 'kinetically stable / controlled'.

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