Reaction Rates
Quick Notes
- Reaction rate = change in concentration of a reactant or product per unit time.
- Stoichiometry of the reaction affects how rates are related for each species.
- Key factors affecting rate:
- Concentration: more particles = more collisions = faster rate
- Temperature: higher temperature = more kinetic energy = more frequent and energetic collisions
- Surface area: more exposed particles = more collisions (relevant for solids)
- Catalysts: lower activation energy = faster reaction
- Nature of reactants: some substances react faster due to bond types or phase
Full Notes
What Is Reaction Rate?
The reaction rate refers to how quickly a chemical change occurs. It is usually expressed as the change in concentration of a reactant or product over time, typically in units such as mol·L⁻¹·s⁻¹.
Rate = Δ[product] / Δt
or
Rate = –Δ[reactant] / Δt
(negative because reactant concentration decreases)
Role of Stoichiometry
The balanced chemical equation provides the ratio of how quickly each species is used up or produced compared to one another.
Example For the reaction 2NO₂ → 2NO + O₂:
Rate of NO₂ disappearance = Rate of NO formation
Rate of O₂ formation = ½ × Rate of NO formation
These ratios allow us to relate the rates of change for all substances involved.
Factors That Affect Reaction Rate
The factors affecting reaction rates are covered in more detail in specific sub-topics (see menu) however below is an introductory outline.
When asked why a reaction is faster or slower, focus on particle collisions and activation energy (see 5.5 - Collision Theory). Always link your explanation to:
1. How often particles collide, and
2. How many of those collisions are successful (i.e., have enough energy and the right orientation).
Temperature
C–H and O=O bonds broken, C=O and O-H bonds formed
- Increases average kinetic energy of particles
- More collisions have sufficient energy to overcome activation energy
- Reaction rate increases exponentially with temperature
Concentration of Reactants
- Higher concentration = more particles per unit volume
- More collisions = increased rate
- Especially important in reactions involving gases or solutions
Surface Area (for solids)
- Finely divided solids expose more particles to react
- More contact points = higher reaction rate
- Example powdered magnesium reacts faster than a solid strip
Catalysts
- Provide an alternative pathway with lower activation energy
- Not consumed in the reaction
- Increase the number of effective collisions
Nature of Reactants
- Reactions involving ions in solution are often very fast
- Reactions involving breaking strong covalent bonds are usually slower
- Phase (solid, liquid, gas) also plays a role in how easily particles can interact
Summary
The rate of a chemical reaction depends on the nature of the reactants and several experimental parameters: concentration, temperature, surface area, and the presence of a catalyst. By understanding how each factor influences collision frequency and energy, we can explain and control how fast chemical reactions occur in the lab and in real-world processes.