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

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

**Gibbs Free Energy: Why do some Feasible Reactions not happen?**

**Some reactions are feasible, but have a enough high activation energy that their rate is effectively zero.**

### 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 just because the entropy change of the universe means a reaction can happen, that doesn’t mean that it will.

For example, this reaction [C_{60}H_{120 }+ 90 O_{2}→ 60CO_{2} + 60H_{2}O] between a long chain hydrocarbon [black tarry substance] and oxygen gas [just show a translucent box] would increase the entropy of the universe.

But when you mix the two together, nothing happens

..even if we left it for years. [clock ticking]

On the other hand, this reaction [Ba(OH)_{2} + 2NH_{4}Cl → BaCl_{2} + 2NH_{3} + 2H_{2}O] between barium hydroxide and ammonium chloride also increases the entropy of the universe.

And it happens as soon as you mix them together

So what’s going on?

They’re both feasible, but only one of them actually happens.

Well, to find out, we need to consider their activation energies.

Now, this reaction is feasible, but it has a really high activation energy.

That means when particles collide…it’s incredibly unlikely that they will have enough kinetic energy to react with one another.

In fact, we could leave this for millions of years before even one successful collision happened, and trillions upon trillions of years before the reaction went to completion.

So technically, this reaction is happening, but the rate is so slow that we as humans would never ever notice.

On the other hand, this reaction has a much lower activation energy

This means when particles collide, it’s much more likely they’ll have enough kinetic energy to react with one another.

And therefore this reaction only takes minutes to go to completion.

So, for a reaction to actually happen it must…

For a reaction to actually happen, It must increase the entropy of the universe

…And occur at a reasonable rate.

So, to sum up…

Some reactions are feasible, but when scientists mix the reactants in a lab nothing happens.

That’s because the activation energy for the reaction is too high

And this means the reaction rate is exceptionally slow

So slow that, within the scientists entire lifetimes nothing would happen.