Unit 5: Topic 4 - Elementary reactions

Question 1

Define what an elementary reaction is and explain why they play an important role in kinetics.

Answer 1

An elementary reaction is a single-step reaction with one transition state and no intermediate molecules. Here is a diagram showing the energy of an elementary reaction. Elementary reactions are important because their rate laws are inferred from the stoichiometry of the reactants; for example, if an elementary reaction is of the form aA + bB –> cC + dD where A and B are elements and a and b are coefficients, the rate law is simply rate = k[A]^a[B]^b.

Question 2

In an elementary reaction, explain why stoichiometry directly relates to the rate law of the reaction.

Answer 2

In an elementary reaction, the reaction occurs in one step. Therefore, each reactant will play a part in the rate law of the equation because all reactants are part of this single step. Since the concentrations will be proportional to the rate, each reactant will contribute one factor of concentration. Therefore, for a bimolecular (two reactants) elementary reaction with A + B –> products, the rate law is rate = k[A][B].

Question 3

Explain why termolecular reactions, or elementary reactions involving three particles, are very rare.

Answer 3

In a termolecular reaction, three particles collide as part of a reaction. However, in order for the reaction to occur, these three reactants must simultaneously collide with correct orientation and sufficient energy. This is unlikely, as there are many possible orientations in a collision, and correct ones may not have sufficient energy.