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

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

**Gibbs Free Energy: Feasibility**

**Reactions with a positive enthalpy change and negative entropy change are never feasible. Reactions with a negative enthalpy change and a positive entropy change are always feasible.**

### 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 how to take values for enthalpy change, entropy change and temperature, and use them to calculate dG

And, from that, we decided whether a reaction was feasible or not

But actually, in some cases, we don’t need to do all of that work to decide whether a reaction is feasible

For example, suppose we know that a reaction has a negative enthalpy change and a positive entropy change.

And so, looking at this equation ΔG = ΔH – TΔS , we can say that…

We can say that, because both of these numbers are positive [T and ΔS], this term [TΔS] is always positive

And so, overall, we’ve got some negative number…subtract some positive number

Which, in any case, means we’ll end up with a negative number overall

In other words, for reactions like this [ΔH = negative, ΔS = positive] delta g is always negative, and therefore they’re always feasible, at any temperature.

On the other hand, suppose we now know that a reaction has a positive enthalpy change and a negative entropy change.

We can say that this term [TΔS] is always negative

And so, overall, we’ve got some positive number…subtract some negative number

Which, in any case, means we’ll end up with a positive number overall

In other words, for all reactions like this [ΔH = positive , ΔS = negative] delta g is always positive, and therefore they’re never feasible, at any temperature.

dH/dS | positive | negative |

positive | Never feasible | |

negative | Always feasible |

So, using this information, which of these reactions are always feasible?

This reaction has a positive enthalpy change and a negative entropy change, so it’s never feasible.

But this reaction has a negative enthalpy change and a positive entropy change, so it’s always feasible

Take a second to let that sink in.

We only needed two pieces of information to decide that this reaction has never and will never happen… at any temperature, anywhere in the universe

And not only that, sometimes we don’t even need to be told whether the entropy change is positive or negative, we can predict it ourselves!

For example, we can predict that the entropy change for this reaction will be…

Will be positive, because there are more moles of products than reactants.

And so, since the enthalpy change is negative, this reaction should always be feasible.

This time…we only needed one value, plus the reaction equation, to determine that this reaction will always be feasible, at any temperature, anywhere in the universe.

It’s absolutely mind boggling…

So, to sum up…

Any reaction with a negative enthalpy change and a positive entropy change will always be feasible.

But any reaction with a positive enthalpy change and a negative entropy change will never be feasible.