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# free energy calculations Watch

1. I was looking through one of the worked examples and I'm really confused:

The thermal decomposition of Zinc carbonate, ZnCO3, is represented by
ZnCO3(s) => ZnO(s) + CO2(g) ΔH= +71 KJ mol-1
ΔS= 0.176 KJ K-1 mol-1

When ΔG = 0 (<= why is G taken as 0, for a reaction to be spontaneous doesn't this value have to be negative?)

ΔH -TΔS = 0

T = ΔH/ ΔS = 71/ 0.176 = 403K
403 - 271 = 130oC
2. (Original post by tazmaniac97)
I was looking through one of the worked examples and I'm really confused:

The thermal decomposition of Zinc carbonate, ZnCO3, is represented by
ZnCO3(s) => ZnO(s) + CO2(g) ΔH= +71 KJ mol-1
ΔS= 0.176 KJ K-1 mol-1

When ΔG = 0 (<= why is G taken as 0, for a reaction to be spontaneous doesn't this value have to be negative?)

ΔH -TΔS = 0

T = ΔH/ ΔS = 71/ 0.176 = 403K
403 - 271 = 130oC
When ΔG = 0 this is the 'tipping point' between spontaneity and non-spontaneity. i.e. the reaction can happen on one side of this temperature, but cannot on the other side.

At ΔG = 0 the system is in dynamic equilibrium with both forward and reverse processes equally likely.
The example shows you the temperature that must be attained before the compound can decompose ...
3. For the reaction to be theoretically feasible ΔG must either equal 0 or be negative. So usually questions will ask what is the lowest temperature for the reaction to be feasible? That is when you make ΔG=0
4. (Original post by paulpaulpauln)
For the reaction to be theoretically feasible ΔG must either equal 0 or be negative. So usually questions will ask what is the lowest temperature for the reaction to be feasible? That is when you make ΔG=0

(Original post by charco)
When ΔG = 0 this is the 'tipping point' between spontaneity and non-spontaneity. i.e. the reaction can happen on one side of this temperature, but cannot on the other side.

At ΔG = 0 the system is in dynamic equilibrium with both forward and reverse processes equally likely.
The example shows you the temperature that must be attained before the compound can decompose ...
Oh in the book it just says that G must be less than 0, I'll have to add that to my notes. Thanks
5. (Original post by paulpaulpauln)
For the reaction to be theoretically feasible ΔG must either equal 0 or be negative. So usually questions will ask what is the lowest temperature for the reaction to be feasible? That is when you make ΔG=0
Why did you neg my post above and then post virtually the same thing?

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Updated: April 19, 2013
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