2.2.2

# Rate Equations 2 (A2 Only)

Test yourself

## The Rate Determining Step (RDS)

Reactions don’t always happen in one step. They may need many different collisions. Fortunately, most reactions have one step much slower than the rest, and so rate equations only depend on this one step.

### RDS

• The rate determining step (RDS) is the slowest step in a reaction.
• It is the slowest because it has the highest activation energy for a successful collision.

### An analogy for the RDS

• You can think of it like the ticket barriers at a crowded tube station.
• People on the other side of the barriers can move very quickly, and people before the barriers can also move very quickly.
• But it takes ages to get on a tube in rush hour because you can’t get through the barriers very fast.
• Getting through the barriers is the rate determining step in your tube journey.

## Mechanistic Information

The rate equation for a reaction can give us some important details about the mechanism for the reaction.

### RDS and the reaction mechanism

• Any reactant appearing in the rate equation is involved in the rate determining step (RDS).
• If two reactants appear in the rate equation, we know they’re both involved in the RDS.
• This means the mechanism must involve them colliding at some point.

### Reactant orders

• If a reactant appears in a rate equation, it will have an order.
• If the order is one, then one molecule of the reactant is involved in the rate determining step.
• If the order is two, then two molecules are involved.
• This gives us more information about which molecules, and how many of them, are colliding at some point in the mechanism.

### Reaction - nucleophile and halogenoalkane

• There are two possible mechanisms for the reaction between a nucleophile (say, OH-) and a halogenoalkane (say, 2-bromo-2-methylbutane).
• You can have a one-step process where the nucleophile collides with the halogenoalkane and ejects the bromide ion.
• You could also have a two-step process where the bromide leaves first and then the nucleophile adds.
• For this reaction, it’s found that the rate equation is:
• Rate = k[CH3CH2CH(CH3)BrCH3]
• This implies that the nucleophile is not involved in the RDS, and so the mechanism does not involve the collision of the nucleophile with the halogenoalkane.