Spontaneous Reactions
A reaction is said to be spontaneous if it occurs without
being driven by some outside force. There are two driving forces
for all chemical reactions. The first is enthalpy, and the second
is entropy. Entropy (DS) is
a measure of the disorder of a system, and systems tend to favor
a more disordered system: nature tends toward chaos. Spontaneous
reactions occur without outside intervention. They may occur quickly,
like the combustion of hydrogen, or slowly, like when graphite turns
to diamond.
Let’s try a problem that might come up on the SAT II Chemistry
test.
Example
The addition of 14.0 g solid potassium hydroxide pellets
to water causes the following reaction to take place:
-
Does the beaker get warmer or colder as the reaction
takes place?
- Is
the reaction endothermic or exothermic?
- What
is the enthalpy change for the dissolution of the 14.0 grams of
KOH?
Explanation
-
This is an exothermic reaction (heat is a product),
so heat is released to the surroundings and the beaker gets warmer.
- As
stated above, this is an exothermic reaction: Heat is a product
and the DH is positive.
- To
solve this problem, you would first convert the grams of KOH to
moles and then multiply that by the energy change seen when 1 mol
of KOH is added to water, in the following proportions:
14.0 g KOH
= 0.25 mol KOH
-43 kJ = -10.7 kJ
