Core Practical: Investigating Concentration and Rate

This lesson covers the core practical on investigating how concentration affects the rate of reaction, as required by the Edexcel GCSE Combined Science specification (1SC0). You will learn the method for carrying out this investigation, how to collect and analyse data, and how to evaluate the experiment.

---

Background Theory

The rate of a chemical reaction is how fast reactants are converted into products. One factor that affects rate is the concentration of a reactant in solution.

Increasing the concentration of a reactant increases the rate of reaction because there are more particles per unit volume, so collisions between reactant particles occur more frequently, leading to more successful collisions per second.

This links to collision theory — for a reaction to occur, particles must collide with sufficient energy (the activation energy).

flowchart TD
    A[Increase concentration of HCl] --> B[More acid particles per dm cubed]
    B --> C[More frequent collisions per second]
    C --> D[More successful collisions per second]
    D --> E[Faster rate of reaction]
    E --> F[Steeper initial gradient on volume vs time graph]

---

The Investigation

Aim

To investigate the effect of changing the concentration of hydrochloric acid on the rate of reaction with marble chips (calcium carbonate).

Reaction

$\text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$CaCO3​(s)+2HCl(aq)→CaCl2​(aq)+H2​O(l)+CO2​(g)

Carbon dioxide gas is produced, so you can follow the reaction by:

• Method A: Measuring the volume of gas collected over time (using a gas syringe).
• Method B: Measuring the decrease in mass over time (using a balance) as $\text{CO}_2$CO2​ escapes.

---

Equipment List

Item	Purpose
Conical flask	Reaction vessel
Marble chips (calcium carbonate)	Reactant (excess)
Hydrochloric acid (various concentrations)	Reactant (variable)
Gas syringe or inverted measuring cylinder over water	Collect $\text{CO}_2$CO2​ gas
Top-pan balance (if using mass loss method)	Measure mass decrease
Stopwatch	Time the reaction
Measuring cylinder	Measure acid volume
Cotton wool plug (for mass loss method)	Prevent splashing while allowing gas to escape

---

Method (Gas Syringe Method)

1. Measure 50 cm³ of hydrochloric acid of the first concentration using a measuring cylinder. Pour into a conical flask.
2. Weigh out 5.0 g of marble chips (medium-sized, roughly the same size each time).
3. Set up the gas syringe connected to the conical flask via a delivery tube.
4. Add the marble chips to the acid, quickly connect the gas syringe, and start the stopwatch immediately.
5. Record the volume of gas every 30 seconds for 5 minutes (or until the reaction stops).
6. Repeat with the same volume of acid but at a different concentration (e.g. 0.5, 1.0, 1.5, 2.0 mol/dm³).
7. Keep all other variables the same (control variables).

---

Variables

Variable Type	Variable	Details
Independent	Concentration of HCl	Changed deliberately: e.g. 0.5, 1.0, 1.5, 2.0 mol/dm³
Dependent	Volume of CO₂ produced (or mass lost)	Measured at regular time intervals
Control	Volume of acid	Same each time (e.g. 50 cm³)
Control	Mass of marble chips	Same each time (e.g. 5.0 g)
Control	Size of marble chips	Same approximate size each time
Control	Temperature	Room temperature — not heated

Exam Tip: When describing control variables in the exam, always state both what you are controlling and how (e.g. "keep the volume of acid the same at 50 cm³ by measuring with a measuring cylinder").

---

Recording Results

Example Results Table

Time (s)	Volume of CO₂ (cm³) — 0.5 M	Volume of CO₂ (cm³) — 1.0 M	Volume of CO₂ (cm³) — 2.0 M
0	0	0	0
30	8	18	40
60	15	32	65
90	20	44	82
120	24	52	92
150	27	58	96
180	29	62	98
210	30	64	98
240	30	65	98

---

Plotting Graphs

Plot volume of gas (y-axis) against time (x-axis) for each concentration on the same axes.

What the Graph Shows

• Steeper initial gradient = faster rate of reaction.
• The higher the concentration, the steeper the initial slope.
• All curves level off at the end — the reaction is complete.
• If the marble chips are in excess, all concentrations eventually produce the same total volume of gas (because the acid is fully used up).
• If the acid is in excess, higher concentrations produce more gas because more CaCO₃ reacts.

Volume of CO₂ (cm³)
│
│    ╭──────── 2.0 M (levels off first — faster)
│   ╱  ╭───── 1.0 M
│  ╱  ╱  ╭── 0.5 M (levels off last — slowest)
│ ╱  ╱  ╱
│╱  ╱  ╱
├─────────────── Time (s)

---

Calculating Rate from the Graph

Method 1 — Average Rate

$\text{average rate} = \frac{\text{total volume of gas}}{\text{total time taken}}$average rate=total time takentotal volume of gas​

Units: cm³/s

Method 2 — Initial Rate (Higher Tier)

Draw a tangent to the curve at time = 0 and calculate its gradient:

$\text{initial rate} = \frac{\Delta y}{\Delta x} = \frac{\text{change in volume}}{\text{change in time}}$initial rate=ΔxΔy​=change in timechange in volume​

The initial rate is more useful because it reflects the rate when concentration is at its highest.

Exam Tip: When drawing a tangent, use a ruler and make it touch the curve at only one point. Extend the tangent line as far as possible before reading off values — a longer line gives a more accurate gradient.

---

Mass Loss Method (Alternative)

If using a balance instead of a gas syringe:

1. Place the conical flask + acid + cotton wool on the balance.
2. Note the initial mass.
3. Add the marble chips, replace the cotton wool, and start the stopwatch.
4. Record the mass every 30 seconds.
5. The mass decreases as $\text{CO}_2$CO2​ escapes.

Plot mass loss (y-axis) against time (x-axis).

The interpretation is the same: steeper initial gradient = faster rate.

---

Evaluating the Experiment

Sources of Error

Error	Effect	Improvement
Gas escapes before syringe is connected	Volume recorded is too low	Practise connecting quickly, or use a bung
Marble chips vary in size	Surface area varies between repeats	Use chips of similar size, or crush to powder
Temperature changes	Affects rate — confounding variable	Carry out all experiments at the same room temperature
Reading gas syringe at wrong angle	Parallax error	Read at eye level

Improving Reliability

• Repeat each concentration at least three times and calculate a mean.
• Use excess marble chips so that the acid is the limiting reactant — this ensures a fair comparison.

Improving Accuracy

• Use a more precise measuring instrument (e.g. burette instead of measuring cylinder).
• Use a data logger connected to a gas pressure sensor for more frequent, automatic readings.

Exam Tip: "Reliability" means getting consistent, repeatable results. "Accuracy" means getting results close to the true value. Know the difference — examiners award marks for using these terms correctly.

---

Linking to the Specification

This core practical assesses your ability to:

• Plan a method with identified variables.
• Collect, record and analyse data.
• Plot and interpret graphs.
• Evaluate the experiment and suggest improvements.
• Explain results using collision theory.

---

Summary