Introduction to Entropy | A-Level Chemistry | AQA, OCR, Edexcel

Entropy can be defined as the randomness or dispersal of energy of a system. The more disordered a system is, the higher (the more positive) the value of entropy.

Entropy is affected by:

1.State of Matter - Generally, solids have lower entropy than liquids, which have lower entropy than gases. This is because the arrangement of particles is more random in gases than solids.

2.Dissolving - When a solid dissolves in a solvent, its entropy increases because of the free movement of the dissolved particles

3.Number of particles - For an increased number of particles there is and increased number of ways in which these can be arranged and therefore an increased entropy. When in a reaction the number of moles of product formed is greater than the number of moles of reactants, there is an increase in entropy.

The more disordered a system is, the higher its entropy and the more stable it is. Therefore, substance will naturally move in order to increase their entropy.

Entropy Changes

In a chemical reactions, products and reactants have different entropies - there is an entropy change.

For reactions with a negative entropy change, reactants will have a higher entropy than products.

For reactions with a positive entropy change, products will have a higher entropy than reactants.

Calculating Entropy Changes

Entropy values have been determined and can be found in databases. They are typically found at standard conditions.

The formula for calculating ΔS is as follows:

ΔS = ∑ΔS(products) - ∑ΔS(reactants)

This is just the difference in entropy between the sum of the products and the sum of the reactants.

Example: Calculate the entropy change for this reaction:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)

CH4 (g) O2 (g) CO2 (g) H2O (l)
S J K-1 mol-1 186 205 214 69.9

1.Calculate the entropy of the products. You can work out the entropy of the products by adding up the entropy values for each product in a reaction

S (CO2) + 2 x S (H2O) = (214) + (2 x 69.9) = 353.8 J K-1 mol-1

2.Calculate the entropy of the reactants. You can work out the entropy of the reactants by adding up the entropy values for each reactant in a reaction

S (CH4) + 2 x S (O2) = (186) + (2 x 205) = 596 J K-1 mol-1

3.Calculate the entropy change for the reaction using the equation. Notice the negative sign, this indicates that entropy has decreased in the reaction. This is as we would expect since we have gone from two gases to a gas and a liquid. A liquid is more ordered than a gas, therefore entropy has decreased.

ΔS =∑Δ S (products) - ∑ΔS (reactants)

ΔS = 353.8 - 596 = -242.2 J K-1 mol-1

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