Up Learn – A Level Chemistry (aqa) – Thermodynamics

Predicting Entropy Changes of Reactions

If a reaction has a larger number of products than reactants, it’s entropy change is likely to be positive.

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 said that if we can predict the entropy change of a chemical reaction, we’ll be one step closer to figuring out whether that reaction can actually happen. 

But so far we’ve not really discussed entropy changes for actual chemical reactions

We’ve seen how entropy changes when a substance’s state changes or if we increase its temperature….

…but those are physical changes, rather than chemical reactions. 

In fact the only chemical reactions we’ve investigated the entropy changes for are ones like this.. [S8 → 4S2]

…Where one substance breaks down into two or more substances.

Now we found that the entropy change for reactions like this was.

We found that the entropy change for reactions like this was positive…

…And we justified that by saying the number of products was higher than the number of reactants.

Well, as it turns out, that line of reasoning isn’t just helpful for reactions like this one…

It’s actually helpful for all reactions!

For example… 

In this reaction [2NH3 → N2 + 3H2 ] we’re going from 2 moles of reactants to 4 moles of products

So its entropy change is likely to be positive

In this reaction [C6H6 +3O2 → 6CO + 3H2] we’re going from 4 moles of reactants to 9 moles of products

So its entropy change is likely to be positive

And in this reaction [2H2 + CO  → CH4O ] ..

In this reaction, we’re going from 3 moles of reactants to 1 mole of products. 

And that means its entropy change is likely to be negative. 

So, during any chemical reaction, if the number of particles in the products is higher than the number of particles in the reactants, the entropy change for that reaction is likely to be positive 

Now, so far, we’ve been a bit non-committal with our language.

Rather than saying the entropy change will be positive, we’ve said it’s ‘likely’ to be positive.

That’s not just because we at Up Learn love sitting on the fence.

It’s actually because, as we’ve seen, the number of particles isn’t the only factor which determines the entropy change of a reaction

And we need to consider these other factors before we can be certain our prediction will be a good one.

And we’ll do that next!

But first to sum up…  

If a reaction has more moles of products than reactants, it’s entropy change is likely to be positive.