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4.4 Catalysts

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    32228
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    When solid potassium chlorate is heated, potassium chloride and oxygen are produced. The reaction is not particularly fast.

    \(\ce{2KCLO_3(s) \rightarrow 2KCL(s) + 3O_2(g)}\)

    This reaction can take place much more rapidly, and at lower temperatures, if solid manganese dioxide \(\ce{MnO_2}\) is added. After the reaction is complete, no more potassium chlorate remains, but all of the manganese dioxide remains.

    Catalysts are very specific regarding which reactions they work with - the same catalyst will not necessarily work with many other (or any other) reactions. Finding a catalyst for a specific equation is one of the important jobs of an industrial chemist.

    Catalysts most likely work by helping to promote a proper orientation between reacting particles. In doing so, it provides an alternate reaction pathway with a lower activation energy.

    In the potential energy diagram shown here, the regular, uncatalyzed pathway is shown as a solid line, and the catalyzed pathway is shown as the dotted line:

    ΔH for the both the catalyzed and uncatalyzed reaction is -15 kJ.

    Since ΔH is independent of the pathway, it is not changed by the presence of a catalyst.

    Activation energy, Ea, does have different values for the catalyzed and uncatalyzed reactions.

    Ea for the uncatalyzed reaction = +25 kJ

    Ea for the catalyzed reaction = +10 kJ

    Because more particles will possess the new 10 kJ energy minimum for a successful collision, the rate of the reaction will increase.

    It is important to note that the original pathway is still present, and reacting particles will still follow that route.

    Think of a catalyst as a new shortcut in your walk to school, or work, or whatever. The longer road still exists and can be used, but the shortcut makes for a faster trip.

    Here's another example:

    \(\ce{N_2O \rightarrow 2N_2 + O_2}\)​

    Ea for the uncatalyzed reaction is 250 kJ, but when a gold metal catalyst is used, Ea is lowered to 120 kJ.

    The \(\ce{N_2O}\) is chemically adsorbed on the metal surface. A bond is formed between the O of the \(\ce{N_2O}\) and an Au atom. This weakens the bond joining the O to the N, thus making it easier for the molecule to break apart.

    In addition to kinetic energy there is another basic form of energy -potential energy. Potential energy is energy of position; we often refer to it as stored energy.

    Kinetic Energy
    Energy of motion.

    A rock rolling down a hill has kinetic energy.

    Atoms and molecules have kinetic energy as they are always in motion

    Potential Energy
    Stored energy.

    A rock sitting on top of a hill has potential energy

    Inhibitors

    Some substances, known as inhibitors, slow down chemical reactions. These work by tying the reactants up in "side reactions".


    4.4 Catalysts is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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