Factors Affecting the Rate of Reaction: MDCAT Chemistry notes
Factors Affecting the Rate of Reaction MDCAT notes: nature of reactants, concentration, surface area, temperature, light, catalysts and orientation.
Nature of reactants
- Ionic reactions in solution, such as neutralization and double decomposition (precipitation), are very fast because the ions only need to meet; no bonds must be broken.
- Reactions of covalent molecules, which need bonds to be broken and rearranged, are slower. Hydrogenation of alkenes, for example, is slow and needs a catalyst.
Concentration
Increasing the concentration of reactants increases the number of collisions per second, so the rate increases (except for zero-order reactions). The rate law is written in terms of reactant concentrations only, not product concentrations. It is wrong to say that concentration does not affect the rate.
Surface area
For reactions involving solids, a larger surface area exposes more particles to collision. Powdered zinc reacts much faster with 1 M HCl than a zinc rod, ribbon or pellets of the same mass. Powdered coal and fine flour dust can even explode.
Temperature
- Raising the temperature increases the kinetic energy of the molecules. Most importantly, it greatly increases the fraction of molecules with energy equal to or greater than $E_a$.
- As a rough rule, for many reactions a rise of 10 K roughly doubles the rate.
- Temperature affects the rate of practically every reaction. The dependence is described by the Arrhenius equation, $k = Ae^{-E_a/RT}$.
Light
Photochemical reactions (photosynthesis, $\mathrm{H_2 + Cl_2}$ in sunlight) are speeded up by light, which supplies the energy to break bonds.
Catalyst
A catalyst increases the rate of a reaction without being consumed. It provides an alternative path of lower activation energy.
- It lowers $E_a$ of both the forward and reverse reactions equally, so equilibrium is reached sooner but its position and yield are not changed.
- It does not change $\Delta H$ of the reaction.
- A catalyst does not increase the number of molecules possessing the original $E_a$; it lowers the barrier. Only raising the temperature increases the fraction of molecules above a given $E_a$.
| Type | Phase | Examples |
|---|---|---|
| Homogeneous | Same phase as reactants | NO in the lead chamber process; $\mathrm{H^+}$ in ester hydrolysis |
| Heterogeneous | Different phase from reactants | Fe in the Haber process; Ni in hydrogenation; $\mathrm{V_2O_5}$ in the contact process |
Enzymes are highly specific biological catalysts.
Orientation of molecules
For a collision to be effective the molecules must collide in the correct orientation. Orientation is especially important for complex and polyatomic molecules and matters more when collisions are frequent, as at high pressure. At low pressure collisions are rare, so orientation plays a smaller role.
Key formulas
- Arrhenius equation: $k = Ae^{-E_a/RT}$
- Rule of thumb: rate roughly doubles per 10 K rise.
Common MDCAT traps
- A catalyst increases rate, not yield.
- A catalyst lowers the activation energy of both forward and reverse reactions.
- Reaction rate depends on reactant concentration, not product concentration.
- Powder has the largest surface area of equal masses.
- Ionic reactions are fast; hydrogenation is slow.
Quick revision
- Homogeneous catalyst: same phase as the reactants.
- Heating increases the fraction of molecules with energy $\geq E_a$.
- 10 K rise roughly doubles the rate of many reactions.
- Larger surface area means faster reaction.
- Catalysts do not change $\Delta H$ or $K$.