Lesson: Chapter - 9

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:

1. Does the beaker get warmer or colder as the reaction takes place?
2. Is the reaction endothermic or exothermic?
3. What is the enthalpy change for the dissolution of the 14.0 grams of KOH?

Explanation

1. This is an exothermic reaction (heat is a product), so heat is released to the surroundings and the beaker gets warmer.
2. As stated above, this is an exothermic reaction: Heat is a product and the DH is positive.
3. 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 × 1 mole KOH/56gKOH = 0.25 mol KOH × -43 kJ = -10.7 kJ

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