Up Learn – A Level Chemistry (aqa) – Thermodynamics
Calculating Gibbs Free Energy Change
Gibbs Free Energy change can be calculated using the equation ΔG = ΔH -TΔS
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 before we plug numbers into this equation, we need to convert entropy into the correct units.
And now that we know what values to plug in, we can finally do some calculations
So, for this reaction, with these enthalpy and entropy changes at this temperature, the Gibbs free energy change is given by this expression
| CH4 + H2O → CO + 3H2 | |
| ΔH = | 206 kJ mol-1 |
| ΔS= | 216 |
| T = | 300 |
ΔG = 206 – 300(216/1000)]
Which evaluates to give this
ΔG = 206 – 300(216/1000)
And it’s a positive value, so that means the reaction isn’t feasible at this temperature
Next, for this reaction, the Gibbs free energy change is…
For this reaction, the Gibbs free energy change is given by this expression
Which evaluates to give this
And since it’s a negative value, that means the reaction is feasible at this temperature
Lastly for this reaction, the Gibbs free energy change is …
For this reaction, the Gibbs free energy change is given by this expression
Which evaluates to give this
And since it’s a negative value, that means the reaction is feasible at this temperature
Now, for all the calculations we’ve done so far, all the data we need has been given to us on a platter [ΔS, ΔH, T on a silver platter]
But sometimes, in exams, they don’t give you all the data straight away, and instead they get you to work it out yourself.
Like this question, for instance…
Here, instead of knowing the entropy change straight away, the question gives you the standard molar entropies.
So, using this data, what’s the entropy change for this reaction…
The entropy change for this reaction is given by this equation, which evaluates to give this..
So, now that we know the entropy change, we can calculate that the delta G for this reaction is…
The delta G for this reaction is this
And, since it’s a negative value, that means this reaction is feasible at this temperature
So, sometimes you won’t be given the entropy change, and you’ll have to calculate it using standard molar entropy values before you can calculate delta G
Similarly, sometimes you won’t be given the enthalpy change, and you’ll have to calculate it using a hess cycle before you can calculate delta G
Or sometimes you’ll be given temperature in degrees celsius, and you’ll have to convert it into kelvin before you can calculate delta G
And sometimes examiners are extra mean, and ask you to do all three
But don’t worry, we’ll give you plen-tee or prac-tees calculating delta gees like these in the next queez.