Effect of Concentration and Pressure on Rate

In this lesson you will learn how changing the concentration of a solution or the pressure of a gas affects the rate of a chemical reaction. You will also study the core practical investigation for this topic and learn how to interpret the graphs produced. These concepts are tested regularly in the Edexcel GCSE Chemistry (1CH0) exams.

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Effect of Concentration on Rate

When the concentration of a reactant in solution is increased, there are more particles of that reactant in a given volume. This leads to:

1. More frequent collisions between the reactant particles.
2. Therefore more successful collisions per second.
3. So the rate of reaction increases.

The total amount of product formed remains the same (provided the same total amount of reactant is used), but it is produced more quickly.

Concentration	Number of particles in a given volume	Collision frequency	Rate of reaction
Low	Fewer particles	Less frequent collisions	Slower
High	More particles	More frequent collisions	Faster

Exam tip: Increasing concentration increases the frequency of collisions. It does not change the proportion of particles with energy ≥ Eₐ (that is a temperature effect). Make sure you do not confuse the two explanations.

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Effect of Pressure on Rate (Gases Only)

For reactions involving gases, increasing the pressure has the same effect as increasing concentration:

• At higher pressure, the gas particles are pushed closer together into a smaller volume.
• There are effectively more particles per unit volume.
• This means more frequent collisions.
• Therefore there are more successful collisions per second and the rate increases.

Pressure	Particle spacing	Collision frequency	Rate
Low pressure	Particles far apart	Less frequent	Slower
High pressure	Particles closer together	More frequent	Faster

Exam tip: Pressure only affects reactions involving gases. If a question asks about a reaction between two solutions, pressure is not relevant — only concentration matters.

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Core Practical: Investigating the Effect of Concentration on Rate

Marble Chips and Hydrochloric Acid Experiment

This practical is specified in the Edexcel course and you need to know it in detail.

The Reaction

Calcium carbonate (marble chips) reacts with hydrochloric acid:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Carbon dioxide gas is produced and can be collected and measured.

Method

1. Place a known mass of marble chips (e.g. 5 g of medium-sized chips) into a conical flask.
2. Set up a gas syringe connected to the flask (or use a delivery tube and an inverted measuring cylinder filled with water).
3. Measure a fixed volume (e.g. 50 cm³) of hydrochloric acid at the desired concentration (e.g. 0.5 mol/dm³, 1.0 mol/dm³, 1.5 mol/dm³, 2.0 mol/dm³).
4. Add the acid to the flask and immediately start a stopwatch.
5. Record the volume of gas collected at regular intervals (e.g. every 10 seconds) for a set period (e.g. 3 minutes).
6. Repeat for each concentration of acid.
7. Plot graphs of volume of gas (y-axis) against time (x-axis) for each concentration.

Variables

Variable	Details
Independent variable	Concentration of hydrochloric acid
Dependent variable	Volume of gas collected (measured at regular time intervals)
Control variables	Mass and size of marble chips, volume of acid, temperature

Why Control the Size of Marble Chips?

Using chips of the same approximate size ensures the surface area is kept constant. If you used powder for one experiment and large lumps for another, the change in surface area would affect the rate and you would not be able to draw valid conclusions about concentration alone.

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Interpreting the Graphs

When you plot volume of gas against time for different concentrations on the same axes, you see several important features:

Steepness of the Initial Gradient

• A higher concentration produces a steeper initial gradient — the reaction is faster at the start.
• A lower concentration produces a shallower initial gradient — the reaction is slower at the start.

The Plateau (Flat Region)

• All curves eventually level off when the reaction is complete.
• If the same mass of marble chips is used each time and the acid is in excess, the curves level off at the same final volume of gas — the same total amount of CO₂ is produced.
• If the acid is the limiting reactant, a lower concentration of acid will produce less total gas, so the curve levels off at a lower volume.

Comparing Two Curves

Feature	Higher concentration	Lower concentration
Initial gradient	Steeper	Shallower
Time to reach plateau	Shorter	Longer
Final volume of gas (acid in excess)	Same	Same
Final volume of gas (acid is limiting)	Higher	Lower

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Worked Example: Reading a Graph

Imagine two experiments are plotted on the same axes:

• Experiment A: 2.0 mol/dm³ HCl
• Experiment B: 1.0 mol/dm³ HCl

Both use 5 g of marble chips and 50 cm³ of acid.

Time (s)	Volume A (cm³)	Volume B (cm³)
0	0	0
10	28	15
20	48	27
30	60	36
40	68	43
50	72	48
60	74	52
90	76	58
120	76	62
180	76	65

Observations:

• Experiment A has a steeper gradient — the reaction is faster.
• Experiment A levels off sooner — the reaction finishes more quickly.
• Experiment A produces more total gas (76 cm³ vs 65 cm³) because more acid was available to react (higher concentration × same volume = more moles of acid).

Mean rate for Experiment A in first 30 seconds:

rate = 60 ÷ 30 = 2.0 cm³/s

Mean rate for Experiment B in first 30 seconds: