Up Learn – A Level Chemistry (aqa) – Thermodynamics
Units of Gibbs Free Energy Change
Gibbs Free Energy change is usually reported in kJ mol-1
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, in chemistry, we use the Gibbs free energy change to decide whether a reaction is feasible or not
And that reaction is feasible when…
And that a reaction is feasible when the Gibbs free energy change is negative
Also, we saw that we use this equation to calculate Gibbs
But, so far, we haven’t actually calculated anything.
Now, we’ll get to calculating shortly, but first we need to talk about units
So what units do we tend to use for each of these components [dS, dH, T]?
In calculations, we tend to use Kelvin for temperature, kilojoules per mole for enthalpy change, and joules per mole per kelvin for entropy change
So, that means the units of this part of the equation [dH] are kJ mol-1
And the units of this part of the equation [TdS] are…
The units of this part of the equation are joules per mole
Now these units pose a problem
For instance, say we’re given this information.
[enthalpy change = -203kJ mol-1 entropy change = 123 J K-1 mol-1 temp = 298K]
It’s tempting to just plug all of these values straight into the equation like this…
[dG = (-203) – (298)(123)]
…Expand brackets…
[dG = -203 – 36654]
…And evaluate to get our answer
[dG = -36857]
But actually we can’t do that.
The units of these two terms are different
[arrow to (-203) saying ‘kJ mol-1 and arrow to -36654 saying J mol-1]
…so we can’t just add them together to get our answer.
That’d be like saying 3 metres plus 24 kilometers equals 27… [3m + 24km = 27 ???]
…It just doesn’t work
So, before we do any addition, we need to make sure these terms have the same units
And, since it’s convention to give Gibbs free energy change in kilojoules per mole, that means we need to change the units of this term.
To do that, instead of plugging in this value for entropy [dG =dH -T(123)] , we plug in this value for entropy [dG =dH -T(1231000)]
Or if we’re given this data [dS = -223 J K-1 mol-1 dG =dH -T(dS)], we plug this into our equation [dG =dH -T(-2231000)],
Or If we’re given this data, we plug…
If we’re given this data, we should plug this into our equation
In fact, whenever we’re given entropy data with these units [J K-1 mol-1], we need to divide it by 1000 before we plug it into this equation.
This changes the units from joules per kelvin per mole into kilojoules per kelvin per mole
And, with that change, the units of these two terms are both the same, so we can now add them together and find delta G.
And we’ll look at that, next
But first, to sum up…
If we’re given this data for a reaction, we can’t just plug it in and evaluate to find Gibbs free energy change, because the units don’t match up.
So, before we plug in this value for entropy we first need to convert it from these units to these units
And to do that, we divide by a thousand.