Unit 2: Equilibrium Lesson 3: Spontaneous Reactions

Spontaneous Reactions A spontaneous change is a change that occurs by itself, without outside assistance. We generally expect exothermic reactions to proceed spontaneously. We also expect endothermic reactions NOT to occur spontaneously, since they require the input of energy.

Enthalpy and Entropy Exothermic reactions are usually spontaneous because molecules tend to go to (or remain at) minimum energy (that is, minimum enthalpy). However, these general assumptions about exothermic and endothermic reactions are not always correct, because another factor must be considered – entropy.

Entropy is the amount of randomness in a system. There are more random states possible than ordered states, so random states are more likely to occur. Thus, entropy tends to a maximum value. In other words, things tend to become more random over time. Examples – parts of a watch, socks in a drawer, crystal structure

Brain Break!

Spontaneous or Not? Although this reaction is highly endothermic (requires a large energy input), the products are far more random than the reactants, so once they have formed, they will not convert back into reactants. Thus the reaction proceeds spontaneously due to entropy! Demo 12.II.4, predict/observe/explain – 10 min

Two drives exist in any reaction: The tendency for a reaction to go to the side with the maximum entropy. The tendency for a reaction to go to the side with the minimum energy.

Tendency toward Minimum Enthalpy Exothermic reactions tend to go to the product side - we say that the product side is favoured by enthalpy. In endothermic reactions, the reactant side is favoured by enthalpy.

Tendency toward Maximum Entropy The side containing the most particles of the most random phase is favoured. To determine this, ask yourself two questions: What is the most random phase present? How many particles of this phase are there?

Homework Pg. 48 #14 and 15 (part i and ii only) 5 min