Up Learn – A Level Chemistry (aqa) – Thermodynamics

Why are the units of entropy change ‘per mole’?

In this case, ‘per mole’ means ‘per the number of moles specified in the reaction equation’

More videos on Thermodynamics:

Entropy: Order and Disorder

What is Entropy?

Positive and Negative Entropy Changes

Predicting Entropy Changes of Reactions

Graphing Entropy

Entropy change formula: Calculating Entropy Changes

Why are the units of entropy change ‘per mole’?

Gibbs Free Energy: Why do some Feasible Reactions not happen?

Units of Gibbs Free Energy Change

Calculating Gibbs Free Energy Change

Gibbs Free Energy: Feasibility

Finding Entropy and Enthalpy from Gibbs vs. Temperature Graph

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Up Learn – A Level Chemistry (AQA)

Thermodynamics

1. Introduction to Entropy
2. Order and Disorder
3. What is Entropy?
4. How Does Temperature Affect Entropy?
5. How Does State Change Affect Entropy?
6. Comparing Entropy Between Substances?
6. How Does Dissolving a Substance Affect Entropy?
8. How Does the Number of Particles Affect Entropy?
9. Entropy Changes
10. Predicting the Entropy Change of a Reaction 1
11. Predicting the Entropy Change of a Reaction 2
1. Introduction to a Microscopic, Mathematical Definition of Entropy
2. A Simple System 1
3. A Simple System 2
4. Relating Configurations to Entropy
5. The Exact Mathematical Definition of Entropy
6. Relating Our Simple System to Atomic Systems
7. Why Does Temperature Affect Entropy?
8. Why Does Number of Particles Affect Entropy?
9. Why Does State Affect Entropy?
10. So Is Entropy Really a Measure of Disorder?
1. Introduction to Calculating Entropy Changes
2. Measuring Entropy for Larger Systems
3. Entropy at Absolute Zero
4. Explaining Entropy at 0 K Mathematically
5. Entropy at Non-Zero Temperatures
6. Graphing Entropy
7. Standard Molar Entropies
8. Investigating the Trends in the Table of Absolute Entropies
9. Calculating the Entropy Change of a Reaction
10. Why Did We Bother Predicting Entropy Changes in the First Place?
11. Why Are the Units of Entropy Change ‘Per Mole’?
1. The Surroundings
2. The Entropy Change of the Surroundings
3. Calculating the Entropy Change of the Surroundings
4. The Entropy Change of the Universe
5. What Reactions Can’t Happen?
6. Feasibility
7. Why Do Some Feasible Reactions Not Happen?
8. The 2nd Law of Thermodynamics
9. Gibbs Free Energy Change
10. The Units of Gibbs Free Energy Change
11. Calculating Gibbs Free Energy Change
12 .Assessing Feasibility
13. Assessing Feasibility – Making Ice
14. Assessing Feasibility – Thermal Decomposition of Calcium Carbonate
15. Exam Technique: Explaining Feasibility
16. Graphing Gibbs Free Energy Change
17. Using Graphs to Find Enthalpy and Entropy Changes
18. Assessing Feasiblility from Graphs
19. Finding the Temperature Where Reactions Become Feasible
20. The Limitations of Our Temperature-Finding Equation
21. Doesn’t Entropy Change…. Change With Temperature?
22. Calculating Gibbs Free Energy Change for Reverse Reactions
23. What About Reversible Reactions?
24. How Are Reversible Reactions Compatible With the Second Law of Thermodynamics?

Last time we saw how to calculate the entropy change of a reaction

And now that we know how to calculate entropy changes, we’re a big step closer to figuring out why some reactions happen and others don’t. 

But before we move onto that, there are a few loose ends we have to tie up…

And we’ll do that, now!

So, suppose we were asked to find the entropy change for this reaction. [4NH3 (g) + 5O2(g) → 4NO(g) + 6H2O(g)]

We used our equation, did all the calculations [don’t actually need to show] and got this answer [181 J K–1 mol-1]

That’s great, but, what does this answer actually represent? 

Well, it doesn’t represent the entropy change when one mole of ammonia reacts

And it doesn’t represent the entropy change when one mole of oxygen reacts 

Nor does it represent the entropy change when a total of 1 mole of reactants react

Or the entropy change when one mole of products form

So… what does it represent?

Well, the answer we worked out is actually the entropy change when exactly:

4 moles of ammonia react with 5 moles of oxygen 

To produce 4 moles of NO and 6 moles of

So this raises the question – what on earth is this [mol-1] doing in the units of our answer? 

Well, confusingly, this [“mol-1”] doesn’t mean ‘the entropy change per every one mole’ 

It really means ‘per the number of moles specified in the reaction equation’

So if the entropy change for this equation [2Al+3/2 O2(g) → Al2O3] was this [ΔS = -313.2 J K-1 mol-1], That means that when 2 moles of aluminium react with 1.5 moles of oxygen to produce 1 mole of aluminium oxide, the entropy change is -313.2 joules per kelvin per mole.

So, to sum up… 

The units we use for the entropy change of a reaction are joules per kelvin per mole. 


Where this ‘per mole’ [mol-1] means ‘per the number of moles specified in the reaction equation’.