Increasing the concentration of a reactant means more particles per unit volume.
Particles are closer together, so collisions occur more frequently.
More collisions per second → more effective collisions → faster reaction.
Pressure (for gases) behaves similarly:
Increasing pressure squeezes gas particles closer together.
Collision frequency increases, and so the reaction speeds up.
Surface Area (for Solids)
If one reactant is a solid:
Only particles at the surface can collide with particles in a gas or solution.
Breaking a solid into smaller pieces (or a powder) increases its surface area.
More surface area → more exposed particles → more collisions per second → faster reaction.
Example
A large lump of calcium carbonate reacts slowly with hydrochloric acid.
The same mass of calcium carbonate in powder form reacts much faster because many more particles are exposed at the surface.
Tip
Grinding solids into powders or using small chips is a common way to increase surface area and speed up reactions.
Why Are Some Reactions Fast and Others Slow? (Collision Theory View)
Collision theory explains reaction rates by combining:
Collision frequency (how often particles collide), and
Collision effectiveness (how many have enough energy and the right orientation).
Factor 1: Activation Energy
High $E_a$ → fewer particles have enough energy to react.
Fewer effective collisions → slower reaction.
Low $E_a$ → more particles have enough energy.
More effective collisions → faster reaction.
Explosive reactions (like combustion of gases such as hydrogen or petrol vapour) usually have low activation energies and are highly exothermic, so once started they proceed very rapidly.
Factor 2: Frequency of Collisions
Even if $E_a$ is not very high, reactions can still be slow if:
Concentrations are low → few collisions.
One reactant is in a large solid lump → small surface area → few collisions at a time.
By increasing concentration, pressure (for gases), or surface area (for solids), we can increase collision frequency and speed the reaction up.
Hint
Putting It Together
A reaction is fast when:
It has low activation energy, so many collisions have enough energy.
Reactant particles collide frequently (high concentration, high pressure, large surface area).
Temperature is relatively high, so many particles have energy ≥ Eₐ.
A reaction is slow when:
It has high activation energy.
Collisions are infrequent (low concentration, low surface area, lower temperature).
Or both.
Common Mistake
Students often think higher temperature only means “more collisions”.
In reality, higher temperature also means more energetic collisions, so a greater fraction of collisions are successful.
Active recall
According to collision theory, what three conditions must be met for a collision to lead to a reaction?
How does increasing temperature affect:
The frequency of collisions?
The energy of collisions?
Explain why a powdered solid reacts faster than a single large lump of the same substance.
Using collision theory, explain why the combustion of a fuel gas can be very fast, while the rusting of iron is very slow, even though both are reactions with oxygen.
How Can We Measure the Rate of a Chemical Reaction in the Lab?
When a chemical reaction occurs, reactants are transformed into products.
The rate of reaction tells us how fast this change happens.
Mathematical Definition
In simple terms:
Rate of reaction = how quickly the amount or concentration of a reactant or product changes with time.
Mathematically, we can write: $$\text { Rate }=\frac{\Delta \text { quantity }}{\Delta t}$$
where:
$\Delta \text { quantity }$ = change in amount, mass, volume, or concentration
$\Delta t$ = change in time
Note
Common Units of Reaction Rate
Depending on what you measure, the units can be:
mol dm⁻³ s⁻¹ → change in concentration per second
g s⁻¹ → change in mass per second
cm³ s⁻¹ → change in gas volume per second
For school-level kinetics, mol dm⁻³ s⁻¹ (concentration per time) is the most common.
Practical Methods to Measure Rate in the Lab
There are several ways to follow how fast a reaction occurs. The best method depends on what observable change happens.
Change in Mass (gas produced)
If a reaction produces a gas that escapes, the mass of the reaction mixture decreases over time.
You can place the reaction flask on a balance and record the mass every few seconds.
Example
Reaction of a metal carbonate with acid, producing CO₂.
Change in Volume (gas syringe)
If a gas is produced, you can collect it in a gas syringe and measure the volume at regular time intervals.
The steeper the volume–time curve, the faster the reaction.
Measure the volume of hydrogen gas produced every 10–20 seconds.