(3)Mechanisms of catalysis

oooh my ahh feel so smart cuh of these topic names

Intermediate formation theory

The theory that a catalyst works by temporarily forming an intermediate compound with a reactant. The intermediate then reacts further and the catalyst is regenerated.

Catalyst intermediate

A short-lived substance formed when a reactant combines with the catalyst. It may exist only briefly and can be difficult to detect.

Overall reaction in intermediate formation

The intermediate steps combine to give the original overall reaction, while the catalyst is regenerated and does not appear as a final product.

First step of intermediate formation

The reactant reacts with the catalyst to form an intermediate compound.

Second step of intermediate formation

The intermediate reacts with another reactant to form the products and regenerate the original catalyst.

Hydrogen peroxide decomposition

Hydrogen peroxide decomposes to form water and oxygen: 2H₂O₂ → 2H₂O + O₂.

Iodide ion catalyst

I⁻ ions can catalyse the decomposition of hydrogen peroxide. Potassium iodide (KI) can be used to provide the I⁻ ions.

Intermediate in hydrogen peroxide decomposition

When H₂O₂ reacts with I⁻, an intermediate containing iodine is formed. The intermediate then reacts with another H₂O₂ molecule and regenerates I⁻.

Surface adsorption theory

The theory that a catalyst speeds up a reaction by allowing reactant molecules to adsorb onto its surface, where they can react more easily.

Absorption

The process where one substance moves into another substance, such as a sponge absorbing water.

Adsorption

The accumulation of substances only at the surface of another substance, such as gas molecules collecting on the surface of a solid catalyst.

Platinum catalyst

Platinum can catalyse the reaction between hydrogen and oxygen to form water. Reactant molecules adsorb onto its surface.

Adsorption stage

Hydrogen and oxygen molecules settle onto the surface of the platinum catalyst and are held there by temporary bonds.

Vacant d orbitals

Platinum is a transition metal with vacant d orbitals. These allow reactant molecules to form temporary bonds with the catalyst surface.

Temporary bonds on a catalyst surface

Reactant molecules can be chemically attracted to the catalyst surface through temporary bonds, allowing them to remain close together and react.

Reaction on the catalyst surface

A high concentration of adsorbed reactant molecules makes collisions more likely, allowing bonds to break and new bonds to form.

Effect of adsorption on collisions

When reactants are concentrated on the catalyst surface, they are more likely to collide with each other and react.

Desorption stage

The products leave the surface of the catalyst after the reaction, freeing the surface for more reactants.

Repeating catalytic cycle

After products desorb, more reactants can adsorb onto the catalyst surface, allowing the process to repeat.

Nickel-catalysed hydrogenation of ethene

Hydrogen adsorbs onto a nickel surface, ethene adsorbs and its π bond breaks, new bonds form, and ethane is produced and desorbs from the surface.

Ethene adsorption on nickel

Ethene adsorbs onto the nickel surface, where the π bond of its C=C double bond breaks as part of the reaction.

Hydrogen adsorption on nickel

Hydrogen forms hydrogen atoms that adsorb onto the surface of the nickel catalyst.

Ethane formation on nickel

After the reactants have adsorbed and bonds have rearranged on the nickel surface, ethane forms as the product.

Ethane desorption

Once ethane has formed, it leaves the nickel catalyst surface, allowing the catalyst to be used again.