-------CHEMISTRY------------

Question

Explain the mechanism of catalyst in increasing and decreasing the rate of chemical reaction.

Answer 1

The catalyst functions to change the pathway in order to change the energy of activation. Typically, it is used to increase the rate of reaction by providing an alternate pathway with a LOWER energy of activation. I guess it would be possible for it to also provide an alternate pathway with a greater/higher energy of activation, and this would slow the rate of reaction.

Answer 2

The addition of a catalyst causes the chemical reaction to go through a different mechanism (a sequence of elementary steps), in which the rate determining step (the slowest one - the one with the highest activation energy) has an activation energy lower than that for the rate determining step of the un-catalyzed reaction.

What does the above explanation mean?

In every chemical reaction, reactants become products following a specific sequence of one or more steps, called elementary reactions. These steps are collectively known as the reaction mechanism. The step with the highest activation energy is the slowest one, which consequently determines the overall rate of the reaction.

Example 1:Un-catalyzed Reaction

The un-catalyzed reaction of nitryl chloride (NO₂Cl) decomposes to nitrogen dioxide (NO₂) and chlorine gas (Cl₂):

2NO₂Cl(g) ⟶ 2NO₂(g) + Cl₂(g)

By looking at the balanced equation, you might think that two molecules of NO₂Cl collide and, presto, you have made products but actually the reaction follows this mechanism (these specific series of steps):

Step 1: 2NO₂Cl(g) ⇋ ClO₂(g) + N₂O(g) +ClO(g) Fast step

Step 2: N₂O(g)+ClO₂(g )⇋ NO₂(g) + NOCl(g) Fast step

Step 3: NOCl(g) +ClO(g) ⟶ NO₂(g) + Cl₂(g) Slow step

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Addition of steps: 2NO₂Cl(g) ⟶ 2NO₂(g) + Cl₂(g) Addition must match target equation

Step 3 happens to be the slowest (the one with the highest activation energy) and therefore it is the rate determining step.

Adding a catalyst to this reaction would speed up the reaction by causing the reaction to follow a mechanism different from the one shown above, in which the slowest step would have a lower activation energy than the slow step above.

Example 2: Catalyzed Reaction (Hypothetical Mechanism)

For the sake of explaining how a catalyst works to speed up the rate of the reaction, let's now assume that a catalyst, such as palladium, has been found to catalyze the reaction.

Now, the reaction follows a different mechanism:

Step 1: 2NO₂Cl(g) ⟶ Cl₂(g) + 2NO(g) +O₂(g) Fast step (In the presence of Pd catalyst)

Step 2: 2NO(g) +O₂(g) ⟶ 2NO₂(g) Slow step

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Addition of steps: 2NO₂Cl(g) ⟶ 2NO₂(g) + Cl₂(g)

The reaction now proceeds at a faster rate because step 2, the rate determining step has a lower activation energy than the slow step in the un-catayzed reaction.