KAR - Catalysts (Lesson)

Catalysts

The final lesson in this module will deal with the use of catalysts to increase the rate of reaction for reactions. Catalysts are substances that change the speed of a chemical reaction; however, they do not undergo a chemical change themselves. In other words, they are not consumed by the chemical reaction. An example of a chemical catalyst is shown below using the mechanism for the oxidation of SO₂ to SO₃:

Step #1: 2 NO₂ (g) + 2 SO₂ (g) $LaTeX: \longrightarrow$ $\longrightarrow$ 2 NO (g) + 2 SO₃ (g)

Step #2: 2 NO (g) + O₂ (g) $LaTeX: \longrightarrow$ $\longrightarrow$ 2 NO₂ (g)

Overall: 2 SO₂ (g) + O₂ (g) $LaTeX: \longrightarrow$ $\longrightarrow$ 2 SO₃ (g)

Notice a couple of things from this mechanism. First NO (g) is produced in step #1 and then consumed in step #2 making it an intermediate (shown in blue) in this process as discussed in a previous module dealing with reaction mechanisms. However, NO₂ (g) is slightly different. It also cancels out when steps 1 & 2 are added together, but it is not produced in the first step. Instead, it is consumed in an early step and then re-produced in a later step. Therefore, it can be shown that it did not undergo a chemical reaction itself and instead can be considered a catalyst (shown in red) for this process.

Recall from a previous module that reaction rates are primarily affected by the activation energy, E_A for that particular reaction. The higher the activation energy, the slower the reaction proceeds. It was shown that increasing the temperature of a reaction results in an increase in the number of particles with sufficient energy to overcome this barrier. The activation barrier remains the same, however. When a catalyst is introduced into a chemical reaction it also speeds up a chemical reaction, but it does so by lowering the activation energy. This is the primary mode of action of all catalysts: instead of causing more collisions to increase reaction rate, they lower the activation barrier to increase the rate of reaction. This is illustrated using the figure shown below. Notice that, once again, there is no change in the energy levels of either reactants or products, but rather a change in the activation energy only.

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