Up Learn – A Level Chemistry (aqa) – Thermodynamics
Finding Entropy and Enthalpy from Gibbs vs. Temperature Graph
A graph of ΔG vs T has a gradient of -ΔS and an intercept of ΔH
More videos on Thermodynamics:
Positive and Negative Entropy Changes
Predicting Entropy Changes of Reactions
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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Thermodynamics
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
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?
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’?
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 that all graphs of delta g against temperature will have a gradient of this, and an intercept of this.
So, here’s the graph for the reaction between methane and water.
What’s the enthalpy change for this reaction?
The enthalpy change for this reaction is this, because that’s the intercept of this graph.
And when we compare our answer to the data book value, they match up well!
Next, what’s the entropy change for this reaction?
The entropy change for this reaction is related to the gradient of this graph by this equation
We find the gradient by drawing a triangle like this, finding the change in y and the change in x, dividing the change in y by the change in x, and evaluating to give this.
And so, since the entropy change for the reaction is the negative of the gradient of this line, that means it must be this.
However, this time, when we compare our answer to the data book value, they match up poorly.
The value we calculated is exactly a thousand times smaller than the data book value.
So why are they different? Have we made a mistake?
Nope, they’re different thanks to units.
This value is in joules per kelvin per mole
But this value is in kilo joules per kelvin per mole
…which explains why it’s a thousand times smaller.
So, since it’s convention to give entropy in joules per kelvin per mole, whenever we work out the entropy change from the gradient of a graph, we need to multiply our answer by one thousand.
So now, what’s the entropy change for this reaction?
The gradient of the graph is this
And that means the entropy change for the reaction is this in these units [negative of that value calculated in kJ K-1 mol-1],
But we need our answer in these units [last answer multiplied by 1000 with units of J K-1 mol-1]
And, to find that, we multiply this answer by a thousand..
So, to sum up…
If we’re given a graph of delta g against temperature for a reaction, we can find the enthalpy change – in units of kilojoules per mole – by simply reading off the intercept
However to find the entropy change – in units of joules per kelvin per mole – we need to find the gradient…
Reverse its sign
And multiply it by a thousand.