Although we can write any chemical reaction on paper, not all chemical
reactions will occur. A spontaneous reaction is a chemical
reaction that will occur because of the nature of the system, once it
is initiated. Note that just because a reaction is called spontaneous,
does not necessarily mean that the reaction will happen instantly or
quickly. It simply means that the chemical reaction will occur once it
is started (once it gets enough activation energy). The Gibbs Free Energy
Equation is used to determine whether or not a reaction is spontaneous.
The Gibbs Free Energy
Equation is:
 G
= H
- TS
H = the change in enthalpy (heat energy)
T = the Kelvin temperature (you MUST convert Celsius
to Kelvin, K = ºC + 273)
S
= the change in entropy (disorder)
G represents a quantity called free energy .
The letter G is used in honor of the American mathematician J. Willard Gibbs
(1839 – 1903)
who first showed that the effect of entropy is dependent upon temperature.
What you need to know is:
G
= - (negative), the reaction will occur spontaneously
G
= + (positive), the reaction
will NOT occur spontaneously
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For
example: Determine whether or not the following reaction
will occur at 25°C.
2Fe2O3  4Fe
+ 3O2 H
= 1644 kJ; S
= 542 J/K
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Before you begin
this problem, you have to make sure all of your units match. In
this problem I will convert the temperature from Celsius to Kelvin).
I will also change entropy from J/K to kJ/K so that S
and H
are both in kilojoules.
T = 25°C + 273 = 298 K
S
= 542 J/K ÷ 1000 = 0.542 kJ/K
G
= H
- TS
G
= (1644.4) - [(298)(0.542)]
G
= 1482.9 kJ Since G
is positive, this reaction is NOT SPONTANEOUS.
If you think about it, this makes
sense. According to the chemical equation, Fe2O3(rust)
would decompose in to Fe(solid iron) and O2(oxygen
gas). It would be great for rust to just disappear into solid iron, but unfortunately
this does not happen at 25°C. The reverse reaction 4Fe
+ 3O2 2Fe2O3
has values H
= -1644 kJ; S
= -542 J/K. The G
= -1482.9 kJ and is spontaneous. That is why iron rusts at 25°C. |
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