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 looked at tables of standard molar entropies

And, now that we’ve seen what they are and how to use them, that means we can finally 

move from predicting entropy changes to actually calculating them!

For example, say we wanted to find the entropy change of this reaction [N2 + 3H2 → 2NH3

We would use this table of values to find the total entropy of the products, which is…

And then use the same table to find the total entropy of the reactants, which is… 

And subtract one from the other, to give this. 

And this approach doesn’t just work for this reaction

In fact, to find the entropy change of any reaction, we just need to find the sum of entropies of the products

which we represent like this [ΣS products]

…and subtract the sum of entropies of the reactants

which we represent like this [ΣS reactants]

So, using this equation and this table , what’s the entropy change of this reaction? [H2(g) + Cl2(g) → 2HCl (g)]

The total entropy of the products is this [pause]

The total entropy of the reactants is this [pause]

And so the entropy change of this reaction is this [pause]

And what about this reaction? [CH4(g) + 2O2(g) → CO2(g)+ 2H2O (g) ]

The total entropy of the products is this [pause]

The total entropy of the reactants is this [pause]

And so the entropy change of this reaction is this [pause]

So, to sum up…

To calculate the entropy change of any reaction, we can use this formula

Where this [ΣSproducts]represents the sum of the entropies of the products 


And this [ΣSreactants] represents the sum of the entropies of the reactants