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

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

**Positive and Negative Entropy Changes**

**Chemists instead describe processes by saying they have a positive or negative entropy change **

### 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?

So far, we’ve talked lots about entropy, and how it increases and decreases.

But we haven’t looked at how chemists discuss and represent these changes on paper.

So let’s look at that now.

First up, chemists tend not to talk about entropy increasing or decreasing, but instead about positive or negative ‘entropy changes’

For example, when we melt ice into liquid water, instead of saying the “entropy increases”, chemists say “the entropy change is positive”

And if we refreeze that water, instead of saying the “entropy decreases”, chemists say “the entropy change is negative”

So now, using that language, how would we describe these processes?

For these processes the entropy change is positive

But for this process the entropy change is negative

Second, chemists also have a nice shorthand for representing entropy changes

For example, when we melt this ice into liquid water, we represent the positive entropy change like this

And when we refreeze that water we represent the negative entropy change like this

This ‘S’ means entropy, and this greek letter delta means ‘change in’.

So, now that we can represent how entropy changes during a reaction, we’re one step closer to figuring out whether that reaction can actually happen.

The next step is being able to predict how entropy changes during a reaction

And we’ll look at how to do that, next!

But first, to sum up…

Instead of describing processes by saying entropy increases or decreases, chemists instead say…

Chemists instead describe processes by saying they have a positive or negative entropy change

And they represent those entropy changes like this [ΔS = positive, ΔS = negative]