Thermodynamics – SS2 Chemistry Lesson Note (Second Term)

Thermodynamics is the study of the relationship between heat and other forms of energy. The first law says energy cannot be created or destroyed, only changed from one form to another. The second law says a spontaneous process needs an increase in entropy. Enthalpy, entropy and free energy together decide whether a reaction is spontaneous.

Behavioural objectives

By the end of the lesson, students should be able to:

• state the first and second laws of thermodynamics;
• explain the terms system, surroundings, enthalpy, entropy and free energy;
• use the relationships ΔU = q + w and ΔH = ΔU + PΔV;
• explain why entropy increases from solid to liquid to gas;
• use ΔG = ΔH – TΔS to decide whether a reaction is spontaneous and to calculate the free energy change.

What is thermodynamics?

Thermodynamics is the study of the relationship between heat and other forms of energy.

A system in thermodynamics is any part of the universe chosen for thermodynamic consideration, that is, the physical and chemical phenomenon or process occurring in a given environment. A system can be isolated, closed or open. The surroundings are the environment in which a phenomenon or a process occurs.

What is the first law of thermodynamics?

The first law of thermodynamics states that energy can neither be created nor destroyed but may be converted from one form to another.

In thermodynamics, heat is represented by q, and other forms of energy are referred to as work, denoted by w. The condition or state of a chemical system is changed when:

1. heat is evolved or absorbed, and/or
2. work is done on or by the system.

In either case, the internal energy, U, of the system is affected and it changes.

From the first law, heat is changed into internal energy of the system. The change in internal energy is the heat absorbed by the system plus the work done on the system:

ΔU = q + w

Work done by the system is negative, since this leads to a decrease in internal energy. Therefore:

ΔU = q – w

For a gaseous system, w = PΔV, so:

ΔU = q – PΔV

ΔU = ΔH – PΔV

ΔH = ΔU + PΔV

What is the second law of thermodynamics?

The second law of thermodynamics states that a spontaneous process occurs only if there is an increase in the entropy of a system and its surroundings.

The factors which determine the spontaneity of a process are:

1. Enthalpy, H: the heat content of the substances involved.
2. Entropy, S: the measure of the degree of disorderliness or randomness of a substance.
3. Free energy, G: the energy which is available for doing work.

What is entropy (S)?

Entropy is the measure of the degree of disorderliness or randomness of a system. The standard entropy change (ΔS^θ) is a state function because it depends on the initial and final states of the system. That is:

ΔS^θ = S^θ_products – S^θ_reactants

The SI unit is J K^-1 mol^-1.

Entropy increases from solid to liquid to gaseous state because randomness increases in that order, so ΔS^θ tends to be positive.

For a reversible process at a constant temperature:

ΔS = ΔH/T

When ΔS is positive, there is an increase in the entropy of a system. When ΔS is negative, there is a decrease in the entropy of a system.

What is Gibbs free energy (G)?

The free energy of a system is the energy which is available for doing work in the system, that is, the driving force that brings about a chemical change.

The standard free energy change (ΔG^θ) is a state function because it depends on the initial and final states of the system. That is:

ΔG^θ = G^θ_products – G^θ_reactants

Free energy takes into account the effect of the enthalpy and entropy factors, as shown in the equation below:

G = H – TS

For a change at a constant temperature:

ΔG = ΔH – TΔS

Note:

1. When ΔG is negative, the reaction is spontaneous or feasible.
2. When ΔG is positive, the reaction is not spontaneous, unless the resultant effect of both ΔH and ΔS leads to a net decrease in G.
3. When ΔG is zero, the system is in equilibrium.

Worked example: calculating the free energy change

The reaction C(s) + O₂(g) → CO₂(g) is carried out at a temperature of 57°C. If the enthalpy change is -5000 J and the entropy change is +15 J, calculate the free energy change.

Solution

ΔG = ΔH – TΔS

= -5000 – (57 + 273) × (+15)

= -5000 – 330 × 15

= -5000 – (+4950)

= -5000 – 4950

= -9950 J or -9.950 kJ

Evaluation

1. State the first law of thermodynamics.
2. State the second law of thermodynamics.
3. Define entropy and state its SI unit.
4. What does it mean when ΔG is negative, positive or zero?
5. Write the relationship between ΔH, ΔU, P and ΔV for a gaseous system.

Assignment

1. Explain why entropy increases from solid to liquid to gaseous state.
2. Write the equation that links free energy, enthalpy and entropy, and state what each term represents.