Gibbs Free Energy change can be calculated using the equation ΔG = ΔH -TΔS
Transcript
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
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.