_{Up Learn – A Level Chemistry (aqa) – Thermodynamics}

_{Up Learn – A Level Chemistry (aqa) – Thermodynamics}

**Graphing Entropy**

**How to interpret a graph which shows how the entropy of a substance changes with temperature.**

### 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

**Want to see the whole course?**

^{No payment info required!}

## 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 chemists can calculate the entropy of any substance at any temperature

For instance, the entropy of a mole of water at * this *temperature [263K] is

*[39.9 J K*

__this__^{–1}mol

^{-1}].

At * this *temperature, it’s

*[298K – 69.9 J K*

__this__^{–1}mol

^{-1}]

And at * this *temperature [373K] it’s

*[202.5 J K*

__this__^{–1}mol

^{-1}]

Now, if we continued charting the entropy of water at different temperatures we’d end up with a graph like this

This graph shows exactly how the entropy of water varies with temperature.

And in this lesson, we’re going to investigate its unique shape using what we already know about entropy.

First, the graph starts at the origin…

And that makes sense because, as we’ve seen before, the entropy of any substance at zero kelvin is zero

Next, as the temperature rises from 0 to 273K entropy increases steadily.

And that makes sense because, as we’ve seen before, when temperature increases, entropy increases.

Next we get to 273K, or 0^{o}C, and the entropy jumps up sharply and significantly

And that makes sense because 273K is the point at which water turns from solid to liquid.

And, as we’ve seen before, liquids are much more disordered than solids.

So that means, at water’s melting point, a *tiny* increase in temperature results in a large jump in entropy.

Next up, from 273K to 373K entropy increases steadily again

And that makes sense because when temperature increases, entropy increases.

Then we reach 373K, or 100^{o}C, and entropy shoots up again…

And that makes sense because 373K is the point at which water turns from a liquid into a gas.

And, as we’ve seen before, gases are much more disordered than liquids.

So that means, at water’s boiling point, a *tiny* increase in temperature results in a huge jump in entropy.

Even larger than the jump here…

And that makes sense because there’s a larger increase in disorder when a substance turns from liquid to gas… than there is when that substance turns from solid to liquid.

Lastly, from 373K onwards, entropy increases steadily again.

So, we’ve now analysed every aspect of this graph which charts water’s entropy at different temperatures.

…But water isn’t the only substance with a graph like this.

For example, here’s the graph for diatomic nitrogen, and here’s the graph for aluminium

And we can interpret these just like we did the one for water

So, using * this *graph, what’s the boiling point of aluminium?

The boiling point of aluminium is 2743 kelvin, since that’s where the second large jump takes place

And, using * this *graph, what’s the entropy of a mole of nitrogen at 298K?

The entropy of a mole of nitrogen at 298K is __191 __joules per kelvin

So, to sum up…

For any substance, chemists can make a graph of how its entropy changes with temperature.

And these graphs.

These graphs start at the origin, show that entropy increases with temperature, and have large jumps at the melting and boiling points of the substance where the boiling point jump is larger than the melting point jump.