According to the Arrhenius Equation, , the rate constant is exponentially dependent on the activation energy. By lowering , a catalyst significantly increases the value of , thereby increasing the reaction rate.
Catalysts can also influence the frequency factor (). They may assist in orienting reactant molecules correctly during a collision, ensuring that the geometry of the transition state is more easily achieved.
It is critical to note that catalysts do not change the thermodynamics of a reaction. The enthalpy change (), the energy of the reactants, the energy of the products, and the final position of equilibrium remain identical to the uncatalyzed reaction.
Adsorption: Reactant molecules diffuse to the solid surface and bind via weak forces (physisorption) or covalent bonds (chemisorption). This step is often the focus of the reaction as it prepares the molecules for transformation.
Reaction: The bonds within the adsorbed reactants are weakened or broken, and new bonds form between adjacent adsorbed species to create the product.
Desorption: The product molecules break their bonds with the catalyst surface and diffuse away into the surrounding phase, freeing up the active sites for new reactant molecules.
Surface Area: For heterogeneous catalysts, increasing the surface area (e.g., using a fine powder or a porous mesh) increases the number of available active sites and further enhances the reaction rate.
Enzymes are highly specific biological catalysts, usually large protein molecules, that facilitate complex biochemical reactions in living organisms.
The Lock-and-Key Model suggests that the enzyme's active site has a rigid, specific shape that perfectly fits a particular substrate molecule.
The Induced Fit Model provides a more modern view, suggesting that the active site is flexible and changes its conformation slightly to wrap around the substrate, creating a more effective transition state.
Identify the Catalyst: In a multi-step mechanism, look for a species that is a reactant in an early step and a product in a later step. If it is produced first and then consumed, it is an intermediate, not a catalyst.
Energy Profiles: Always check if the starting and ending energy levels are the same. If the peak is lower but the start/end points haven't moved, a catalyst was used.
Equilibrium Trap: Remember that catalysts speed up both the forward and reverse reactions equally. Therefore, they help a system reach equilibrium faster but do not change the equilibrium constant () or the yield.
Common Mistake: Do not assume catalysts increase the kinetic energy of molecules. Only temperature increases kinetic energy; catalysts simply lower the 'energy barrier' those molecules must cross.
