Enzymes

Catalysts

Catalysis General Information

• A catalyst is a species which is not consumed in the reaction but changes the RATE of the reaction
• A catalyst can increase the rate of a reaction by lowering the activation energy
• Activation Energy (Ea): Is the minimum amount of energy that the reacting species must possess to undergo the specified reaction.

• Since the catalyst lowers Ea for a reaction, does it speed up just the forward, backward, or forward and backward reaction?
• Both!

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• Speeds up the forward and reverse reaction for that step equally.

• Will the equilibrium constant change when we add a catalyst?
• No!

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• Would the change in enthalpy be affected by a catalyst? If so, how? (Draw where we could see deltaH or deltaG on the above plot)
• No!

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• **This is a reminder that catalysts have to do with kinetics/thermodynamics:

  kinetics

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  so they don't affect equilibrium constants, Gibb's Free Energy, reaction enthalpy, equilibrium concentrations, etc, which have to do with kinetics/thermodynamics:

  thermodynamics

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Catalysts Cntd.

• Simply put, a catalyst can affect kinetic parameters of a reaction such as rate and rate expression but cannot affect thermodynamic parameters such as ΔG, ΔH, ΔS, ΔE, and K.
• Catalysts are classified based on phase
• If the catalyst is the same phase as the reactants the catalyst is said to be homogenous
• If the catalyst is of a different phase than the reactants the catalyst is said to be heterogeneous
• main advantage of using a heterogeneous catalyst is that it can be easily separated from the reaction mixture
• A huge advantage of catalytic reactions is that we can use very small amounts (loadings) of catalyst in a reaction because it is not consumed.

• Since the catalyst is not consumed in the reaction would you expect to see it in the overall balanced equation?

  No!

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• Which elementary step in a mechanism would the catalyst be a part of (slow/fast)?

  Slow!

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• If we were writing the rate of the overall reaction then, would the catalyst be written in that expression as well or not?

  Yes!

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Identifying A Catalyst Vs An Intermediate In a Mechanism

S2O82- + 2I- -> 2SO42- + I2

1) S2O82- + 2Fe2+ -> 2SO42- + 2Fe3+

2) 2Fe3+ + 2I- -> 2Fe2+ + I2

Is there an intermediate in the above mechanism? If yes, what is it?

• Fe3+ is produced in the first step and is then used in the second step

If there a catalyst in the above mechanism? If yes, what is it?

• Fe2+ was present in the beginning of the 1st step and is seen at the end of the reaction in the 2nd step