The Effect of a Catalyst



 

Learning Objective

  • Evaluate the effect of a catalyst on a chemical equilibrium

Key Points

    • Catalysts are compounds that accelerate the rate of a reaction.
    • Catalysts accelerate reactions by reducing the energy of the rate-limiting transition state.
    • Catalysts do not affect the equilibrium state of a reaction.

Term

  • transition stateAn intermediate state in a chemical reaction that has a higher free energy than both the reactants the products.

The Effect of a Catalyst on Equilibrium

Reactions can be sped up by the addition of a catalyst, including reversible reactions involving a final equilibrium state. Recall that for a reversible reaction, the equilibrium state is one in which the forward and reverse reaction rates are equal. In the presence of a catalyst, both the forward and reverse reaction rates will speed up equally, thereby allowing the system to reach equilibrium faster. However, it is very important to keep in mind that the addition of a catalyst has no effect whatsoever on the final equilibrium position of the reaction. It simply gets it there faster.

Recall that catalysts are compounds that accelerate the progress of a reaction without being consumed. Common examples of catalysts include acid catalysts and enzymes. Catalysts allow reactions to proceed faster through a lower-energy transition state. By lowering the energy of the transition state, which is the rate-limiting step, catalysts reduce the required energy of activation to allow a reaction to proceed and, in the case of a reversible reaction, reach equilibrium more rapidly.

CatalysisA catalyst speeds up a reaction by lowering the activation energy required for the reaction to proceed.

To reiterate, catalysts do not affect the equilibrium state of a reaction. In the presence of a catalyst, the same amounts of reactants and products will be present at equilibrium as there would be in the uncatalyzed reaction. To state this in chemical terms, catalysts affect the kinetics, but not the thermodynamics, of a reaction. If the addition of catalysts could possibly alter the equilibrium state of the reaction, this would violate the second rule of thermodynamics; we would be getting “something for nothing,” which is physically impossible.

Interactive: CatalysisThe model contains reactants which will form the reaction: A₂ + B₂ –> 2 AB. In this case the model has been set so the activation energy is high. Try running the reaction with and without a catalyst to see the effect catalysts have on chemical reactions. 1. Run the model to observe what happens without a catalyst. 2. Pause the model. 3. Add a few (3 – 4) catalyst atoms to the container by clicking the button. 4. Run the model again, and observe how the catalyst affects the reaction.