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.