Entropy

Entropy is a common concept tested in A-level Chemistry. Below are some key facts, definitions, and concepts about entropy. Our full set of Revision Notes gives you all the specifics that you need to know, broken down and tailored to your specification, so you can master what matters for your exams! 

What is entropy?

• Entropy is a key term in thermodynamics
• In simple terms, it is a measure of how disordered or chaotic a system is 
• When a substance changes state, its entropy changes 
• Different states of matter have different amounts of disorder, and therefore different entropy 
• The more ordered a system is, like in a solid for example, the lower the entropy 
• The more disordered a system is, the higher its entropy 
• Systems with a higher entropy are more energetically favourable 
• In equations, you use the letter S to show the entropy 

When does entropy increase and decrease?

Entropy changes when a substance changes state

When does entropy increase?

• Entropy increases when a solid melts and turns into a liquid
  • When you raise the temperature to melt a solid, you give the particles more energy 
  • As the particles begin to vibrate more, they break apart from their fixed, regular lattice structure 
  • The structure now becomes an irregular arrangement - although still close together, the particles are able to move around, rotate and slide over each other 
  • This means the disorder of the system has increased 
  • Therefore, the entropy of the system has increased 
• Entropy also increases when a liquid boil and turns into a gas 
  • Similar to with a solid, the particles now have more energy 
  • The particles can now move around even more freely, and are even more disordered 
  • This means the entropy of the system has increased 

The diagram below shows how entropy changes as states change 

When does entropy decrease?

• Entropy decreases when states change
• If a gas is condensed and becomes liquid, the particles are bought closer together and the arrangement becomes more regular 
• This means the disorder decreases
• Therefore, the entropy decreases
• If a liquid is frozen and changed into a solid, the particles are bought very close together forming a regular, lattice structure 
• The ability of the particles to move decreases, as they are now in a regular arrangement and only able to vibrate 
• There are far fewer ways of arranging the energy, meaning the disorder has decreased 
• Therefore, the entropy decreases 

When else does entropy change?

• Other examples of entropy increasing include 
• Reactions that involve the dissolving of solids 
• Reactions that increase the number of gas molecules 
• Reactions that result in more products than there were reactants 
• Other examples of entropy decreasing include 
• Reactions that combine molecules into one (for example, addition reactions)
• Reactions where the number of gas molecules is reduced 

What are examples of entropy changing? 

The decomposition of calcium carbonate 

• During the decomposition of a substance, for example, calcium carbonate, the entropy of the system increases 
• This is because one reactant, calcium carbonate, forms two products, one of which is a gas 

CaCO₃ (s) → CaO (s) + CO₂ (g)

• In this decomposition reaction, a gas molecule (CO₂) is formed
• The CO₂ gas molecule is more disordered than the solid reactant (CaCO₃), as it is constantly moving around
• As a result, the system has become more disordered and there is an increase in entropy

How do you calculate entropy change? 

Entropy calculations are common in A-level chemistry, so you need to make sure you know how to complete these calculations 

• Entropy changes are an order of magnitude smaller than enthalpy changes, so entropy is measured in joules rather than kilojoules
• The full unit for entropy is J K^-1 mol^-1
• The standard entropy change (ΔS^Θ_system) for a given reaction can be calculated using the standard entropies (S^Θ) of the reactants and products
• The equation to calculate the standard entropy change of a system is:

ΔS^Θ_system = ΣS^Θ_products - ΣS^Θ_reactants

(where Σ = sum of)

• The values you need will be provided in the exam question
• The values of entropy for elements and compounds can be found in data books 

Entropy Exam Tip

A key thing to remember is that the enthalpy of formation values of elements will be zero. But, entropy values for elements will not be!