Rates of Reaction Virtual Lab: Acid Concentration and Gas

This rates of reaction virtual lab matches the AQA GCSE Chemistry required practical on rates, using the gas-collection method. Students drop calcium carbonate lumps into hydrochloric acid, collect the carbon dioxide in a gas syringe and plot volume against time. Then they repeat the run at different acid concentrations. They calculate mean rates, compare times with rate ∝ 1/t, and explain the results with collision theory. It also supports the AP Chemistry kinetics lab with marble and acid, where students compare initial rates to find the order of reaction.

Curriculum links

• AQA GCSE Chemistry 4.6.1.2 (factors that affect the rates of chemical reactions) and its required practical on how concentration affects rate, by measuring the volume of gas produced. In Combined Science: Trilogy the same content is 5.6.1.2. The second method in that practical, the color or turbidity change ("disappearing cross"), is not in this simulation.
• AP Chemistry Unit 5, Topics 5.1 Reaction Rates and 5.2 Introduction to Rate Law: see the Extension.
• NGSS HS-PS1-5: explain the effect of changing concentration or temperature on the rate of a reaction.

Simulic is not affiliated with or endorsed by the College Board or AQA.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"You double the acid concentration from 1.0 M to 2.0 M and keep everything else the same. What happens to the total volume of gas and to how fast it is made?"

Many students expect "twice as much gas". Don't correct them yet.

Method in the simulation

The flask holds 50 mL of HCl. The run plays at ten times real speed, so 300 seconds of reaction take 30 seconds. Each run stays on the graph, and the results table lists every run.

1. Keep the starting values: CaCO₃, Lumps, m = 0.40 g, C(HCl) = 1.0 M, temperature 25 °C. Press Run and wait until the stopwatch reaches 05:00.
2. The orange markers t₁ and t₂ start at 20 s and 60 s. Drag them to 30 s and 60 s. The formula line shows the gas volume at both times.
3. Read Final V and t to collect ½ final V from the table.
4. Press New run, change only C(HCl) to 0.5, 1.5 and 2.0 M, and repeat steps 1–3 for each.
5. Calculate the mean rate over the first 60 s (volume at 60 s ÷ 60) and the rate as 1/t½.

C(HCl) (M)	V at 30 s (mL)	V at 60 s (mL)	Final V (mL)	t½ (s)	Mean rate 0–60 s (mL/s)
0.5
1.0
1.5
2.0

Expected results

All readings come from the simulation with 0.40 g of CaCO₃ lumps at 25 °C:

C(HCl) (M)	0.5	1.0	1.5	2.0
V at 30 s (mL)	17.9	33.6	47.1	58.7
V at 60 s (mL)	32.7	57.5	75.4	87.3
Final V (mL)	99.2	99.2	99.2	99.2
t½ (s)	103	49	32	24
Mean rate 0–60 s (mL/s)	0.55	0.96	1.26	1.46

• Every run ends at 99.2 mL. The 0.40 g of CaCO₃ (4.00 mmol) is the limiting reactant, and even 0.5 M acid is in excess.
• The curves are steepest at the start and level off as the lumps are used up.
• Doubling the concentration halves t½ (49 s to 24 s), so the rate, measured as 1/t½, roughly doubles.
• The mean rate over 60 s does not double (0.96 to 1.46 mL/s): at 2.0 M the reaction is nearly over by 60 s. That is why rates are compared at the start, or as the time to collect a fixed volume.

Questions for students

1. (Prediction, asked again after the lab) If you double the acid concentration, what happens to the final volume and the speed of the reaction?
2. Which variable must you keep the same in every run?
3. At 1.0 M, what is the mean rate of reaction over the first 60 s?
4. Using rate ∝ 1/t½, how many times faster is the 2.0 M reaction than the 1.0 M reaction?
5. Why does the 2.0 M acid react faster but not make more gas?

Answers for teachers: (1) The same final volume, collected faster. (2) The mass and form of the CaCO₃ and the temperature. (3) 57.5 mL ÷ 60 s = 0.96 mL/s (accept 0.94–0.98). (4) 49 ÷ 24 ≈ 2.0 (accept 1.85–2.15). (5) More acid particles per unit volume give more frequent collisions with the CaCO₃ surface, so the rate rises. The CaCO₃ is the limiting reactant, so the amount of CO₂ is fixed at 4.00 mmol (99.2 mL).

Common misconceptions

• "Stronger acid makes more gas." The limiting reactant sets the amount of product. Concentration only changes how fast it forms.
• "Higher concentration makes the particles move faster." Speed depends on temperature. Concentration raises the number of collisions per second, not their energy.
• "The rate stays constant during a run." The gradient of V(t) is greatest at the start and falls to zero.

Extension

• AP: order of reaction. Drag t₁ to 0 s and read "v at t₁" in the table: 0.657, 1.314, 1.971 and 2.628 mL/s for 0.5, 1.0, 1.5 and 2.0 M. The initial rate is proportional to the concentration, so the reaction is first order in H⁺ in this model.
• Surface area and temperature: at 1.0 M, switch to Powder: t½ falls from 49 s to 10 s. Back to lumps at 35 °C: t½ is 24 s, half the 25 °C value.

FAQ

Can students do the practical without a fume cupboard or gas syringe?

Yes. They get a full data set, graphs and calculations from the simulation. Use it to teach the method before the wet lab, or instead of it.

Why is the acid in excess in every run?

With 0.40 g of CaCO₃ and 50 mL of acid, even 0.2 M HCl (10 mmol) is more than the 8 mmol needed. That keeps the final volume the same, so only the rate changes.

Does the simulation cover the "disappearing cross" method?

No. It only uses gas collection. Run the sodium thiosulfate method as a wet lab, or use the collision model linked below for the particle picture.

Related simulations and guides

Reaction rate – collision model (concentration, temperature, catalyst, surface area)

For more chemistry activities, see interactive chemistry lesson ideas. To run the prediction as a full cycle, see predict, observe, explain with simulations.