The total energy of a system is called its internal energy. The internal energy is, then, the sum of the rotational, vibrational, electronic, kinetic and potential energies. Hence, its units are joules again. Internal energy is a state function. This is a property of the system that is only dependent on the current state of the system – it doesn’t matter […]

## Introduction to Thermodynamics

We can break the universe into two parts, the system and the surroundings. The part we are looking at is the system, and everything else is the surroundings. There are a number of ways in which these two interact. An open system is one where both matter and energy can freely cross from the system to the surroundings and back. eg an […]

## Pressure Dependence of Equilibria

The simplest way to predict qualitatively the response of a system at equilibrium to any change in the external conditions is to make use of Le Chatelier’s principle. This states that a system in equilibrium adjusts so as to minimise the effect of an applied alteration in conditions. Thus, for example, compression of a gas that is in equilibrium with […]

## The Position of Equilibrium

The reaction Gibbs energy can always be written in the following way: where ΔGrº is the standard reaction Gibbs energy, given by the difference between the Gibbs energies of the products and the reactants, all weighted by the appropriate stoichiometric coefficient: The quantity Q is called the reaction quotient, and is given by the activities of the products divided by the activities of the […]

## Equilibrium and the Reaction Gibbs Energy

It is the tendency of chemical reactions to reach a condition of dynamic equilibrium, where the rate of the forward reaction (conversion of reactants into products) is equal to the rate of the reverse reaction (conversion of products back into reactants). The composition of a reaction mixture that has reached equilibrium thus does not change over time. […]

## The Third Law of Thermodynamics

At T = 0, there is no energy corresponding to thermal motion. Further, for a perfect crystal all the atoms or ions which make up the crystal are arranged in a regular, uniform fashion. The absence of spatial disorder and thermal motion may be used to argue that such a material, under these conditions, has zeroentropy. (This idea […]

## The Relationship between Entropy and Temperature

Our starting point for this discussion is the definition of a measurable entropy change: This definition may be used to calculate the entropy of a system at a temperature T2 from a knowledge of its entropy at a temperature T1 and the heat supplied to change the temperature from T1 to T2: If we consider the situation where the system is subjected to a […]

## Entropy of Phase Changes

We would expect that a phase change would be accompanied by a change in entropy. For example, when a liquid boils, a compact condensed phase is converted into a widely dispersed vapour phase. Clearly, the molecular disorder in a gas will be greater than that in a liquid, so there must be an entropy increase upon vapourisation. Likewise, […]

## The Clausius Relation

We now turn to verifying that the entropy is a signpost of spontaneous change, in the sense that the total entropy must increase for any spontaneous change. We consider a system at temperature T, in thermal and mechanical contact with its surroundings, which are also at temperature T. (Note that the system and surroundings are not necessarily in mechanical equilibrium – eg they may be […]

## Entropy and Volume

It is appropriate to illustrate the relationship between ΔS and q with a consideration of the relationship between entropy and volume. Our consideration will be the simple case of isothermal expansion of a perfect gas, but it is possible to apply the equations to more complicated situations. Our starting point is the following equation, the definition of entropy change: Given […]