SME
Grade 10 Science · Chemistry
Entropy, free energy and spontaneity
Use this for “ΔH = 50 kJ/mol and ΔS = 200 J/(mol·K). Find ΔG at 300 K. Is the reaction spontaneous?”
Chart
f(300) = −10
This is an example. Type your own values to start a new problem.
ΔG (kJ/mol)
=
ΔH (kJ/mol)
−
T (K)
×
ΔS (J/(mol·K))
1,000
Formulas
ΔG
=
ΔH
−
T
×
ΔS/1,000
−10
=
50
−
300
×
200
1,000
Assumptions
•
A reaction is spontaneous, able to go on its own, when ΔG is below 0.
•
ΔG = ΔH − TΔS, with T in kelvins; ΔS is in J/(mol·K), so divide it by 1,000 to match ΔH’s kJ.
•
Spontaneous says nothing about speed: diamond turning to graphite is spontaneous but far too slow to see.
Step-by-step
WE KNOW
ΔH = 50 kJ/mol, ΔS = 200 J/(mol·K), T = 300 K
FIND
free energy change (ΔG)
Step 1 · Find free energy change (ΔG)
Use ΔG = ΔH − T × ΔS/1,000
A
negative
ΔH
and
a
positive
ΔS
both
make
ΔG
smaller;
the
TΔS
term
counts
more
as
T
rises.
ΔG
=
50
−
300
×
200
1,000
ΔG = 50 − 60
ΔG = −10 kJ/mol (below 0: spontaneous at 300 K)
Check
Put the numbers back in. Both sides should match.
−10
=
50
−
300
×
200
1,000
✓
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