Home 16.1 Rate expression and reaction mechanism

# 16.1 Rate expression and reaction mechanism

### Rate equation (Rate law)

- Consider reaction:
- Where x, y, p, q are stoichiometry coefficients

- The rate equation is:
- In general the rate law is expressed as such:
- Where m and n are the
**orders** with respect to their reactants
**Order of reaction** = m + n
- DO NOT USE STOICHIOMETRY COEFFICIENTS AS “m” AND “n” IN THIS EQUATION

- Rate equations can only be determined experimentally because orders can only be deduced empirically

### Molecularity and rate-determining step (slow-step) of a reaction

**Reaction mechanism**: The sequence of steps outlining the reaction pathway from reactants to products
**Elementary steps: **Any individual step in the reaction mechanism
**Molecularity: **the number of molecules or atoms involved as reactants in the elementary reaction
**Unimolecular: **Single molecule or atoms involved in an elementary step
**Bimolecular:** Two molecules or atoms involved in an elementary step

- Each elementary step has its own rate constant, k, and its own activation energy E
_{a}
- The rate of reaction depends on the slow step or the
**rate-determining step (RDS) **
- Catalysts alter reaction mechanism by introducing a step with a lower activation energy

### Deduction of a rate equation

- Decide on which step is the RDS
- Deduce rate equation for the RDS

### Graphical representations of reactions