Thermodynamics – SS2 Chemistry Lesson Note (Second Term)

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

• First law: energy cannot be created or destroyed, only changed from one form to another.
• Second law: a spontaneous process occurs only if the total entropy of the system and its surroundings increases.
• Deciding factors: 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:

1. state the first and second laws of thermodynamics;
2. explain the terms system, surroundings, enthalpy, entropy and free energy;
3. use the relationships ΔU = q + w and ΔH = ΔU + PΔV;
4. explain why entropy increases from solid to liquid to gas;
5. 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.

• System: the part of the universe chosen for study, such as a reaction in a beaker.
• Surroundings: everything outside the system, where the process takes place and exchanges energy with the system.
• Spontaneous process: a change that happens on its own without continuous outside help.

What are the types of thermodynamic system?

There are three types of system: isolated, closed and open.

• Isolated system: exchanges neither matter nor energy with the surroundings, as in a sealed, well-insulated flask.
• Closed system: exchanges energy but not matter, as in a sealed bottle of water being heated.
• Open system: exchanges both matter and energy, as in an open pot of boiling water.

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.

• Heat (q): energy absorbed or evolved by the system.
• Work (w): any other form of energy transfer, such as the expansion of a gas.
• Internal energy (U): the total energy stored in the system. It changes when heat is exchanged, work is done, or both.

How is the change in internal energy calculated?

The change in internal energy is the heat absorbed by the system plus the work done on the system.

ΔU = q + w

• q is positive: heat is absorbed by the system.
• q is negative: heat is evolved by the system.
• w is positive: work is done on the system.
• w is negative: work is done by the system, so internal energy falls.

How are ΔU and ΔH related?

For a gas at constant pressure, ΔH = ΔU + PΔV.

1. Work done by the system is negative, so ΔU = q – w, where w is the size of the work done by the system.
2. For a gas, the work done by the system is w = PΔV, so ΔU = q – PΔV.
3. At constant pressure, the heat exchanged, q, equals the enthalpy change, ΔH.
4. Therefore ΔU = ΔH – PΔV.
5. Rearranging gives 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.

Three factors determine whether a process is spontaneous.

What is entropy (S)?

Entropy is the measure of the degree of disorderliness or randomness of a system.

• State function: entropy depends only on the initial and final states of the system.
• Standard entropy change: ΔS^θ = S^θ_products – S^θ_reactants
• SI unit: J K^-1 mol^-1.
• ΔS positive: entropy of the system increases.
• ΔS negative: entropy of the system decreases.

Why does entropy increase from solid to liquid to gas?

Entropy increases from solid to liquid to gas because the randomness of the particles increases in that order.

• Solid: particles are held in fixed, orderly positions, so entropy is lowest.
• Liquid: particles move past one another, so they are less ordered.
• Gas: particles move freely and randomly, so entropy is highest.
• Result: melting and boiling usually give a positive ΔS^θ.

How are entropy and heat related?

For a reversible change at constant temperature, the entropy change is the heat exchanged divided by the temperature.

• General form: ΔS = q/T, with T in kelvin.
• Phase change at constant pressure: q equals ΔH, so ΔS = ΔH/T, as in melting ice at its melting point.

What is Gibbs free energy (G)?

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

• State function: free energy depends only on the initial and final states of the system.
• Standard free energy change: ΔG^θ = G^θ_products – G^θ_reactants
• Definition of G: G = H – TS, which combines the enthalpy and entropy factors.
• At constant temperature: ΔG = ΔH – TΔS, with T in kelvin.

How does the sign of ΔG show whether a reaction is spontaneous?

A reaction is spontaneous when ΔG is negative.

• ΔG negative: the reaction is spontaneous or feasible.
• ΔG positive: the reaction is not spontaneous under those conditions. The reverse reaction is spontaneous.
• ΔG zero: the system is in equilibrium.

How do ΔH, ΔS and temperature affect spontaneity?

The signs of ΔH and ΔS decide whether ΔG is negative at a given temperature.

How do you calculate the free energy change? (Worked example)

The reaction C(s) + O₂(g) → CO₂(g) is carried out at 57°C. The enthalpy change is -5000 J and the entropy change is +15 J K^-1. Calculate the free energy change.

1. Write the equation: ΔG = ΔH – TΔS.
2. Convert the temperature to kelvin: T = 57 + 273 = 330 K.
3. Substitute: ΔG = -5000 – (330 × 15).
4. Multiply: 330 × 15 = 4950, so ΔG = -5000 – 4950.
5. Answer: ΔG = -9950 J or -9.95 kJ.
6. Conclude: ΔG is negative, so the reaction is spontaneous.

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

Answer both parts in your notebook.

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.