MrChemCoach
Concentration & Rate of Reaction Gas Syringe Method · IGCSE
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MrChemCoach · rates of reaction

Does concentration change the rate or the amount?

Marble chips react with hydrochloric acid and the carbon dioxide pushes the gas syringe out. Watch the apparatus and the graph build at the same time, then run a second concentration without clearing.

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g) chips 0.30 g acid 50 cm³ temp 22 °C

Gas syringe apparatus A conical flask of colourless hydrochloric acid with white marble chips stands on a bench, sealed with a rubber bung and joined by a glass delivery tube to a gas syringe held in a clamp stand. Bubbles of carbon dioxide rise through the acid and push the plunger outwards. Gas syringe · 100 cm³ 0.30 g marble chips + 50 cm³ HCl clamp stand
volume CO₂0.0
stopwatch0.0
rate now0.00
acid
sound

Volume of CO₂ against time

Volume against time graph Curves of carbon dioxide volume against time for each acid concentration. Every curve levels off at seventy-two cubic centimetres, but steeper curves reach it sooner. 72 cm³ maximum - same for every concentration 0 20 40 60 Volume of CO₂ (cm³) 0 30 60 90 120 150 180 Time (s)

Results for the current run

t (s)0102030405060708090100110120
V (cm³)

Readings appear as the stopwatch passes each ten-second mark, the way you would read a real syringe. Changing concentration starts a fresh table but keeps the finished curve on the graph.

MrChemCoach · simulated with V = 72(1 − e−kt), k ∝ [HCl] acid in excess, marble chips limiting.

What this experiment shows

This bench isolates one variable: concentration. Everything else - temperature, volume, surface area, the same flask, the same cross - is held still, so any change in the time taken can only be the concentration. That is what makes it a fair test, and being able to say which variable is independent, which is dependent and which are controlled is worth marks on its own.

rate ∝ 1 / time taken

What you need

  • Sodium thiosulfate solution of known concentration, and distilled water to dilute it
  • Dilute hydrochloric acid
  • Conical flask, printed cross, stopclock
  • Measuring cylinders, one for each solution

How it is done

  1. Make up five thiosulfate concentrations by diluting the stock with measured water.
  2. Keep the total volume the same every time, or the depth of liquid changes.
  3. Add the same volume of acid to each and time how long the cross takes to disappear.
  4. Repeat each concentration and take a mean.
  5. Plot 1/time on the y-axis against concentration on the x-axis.

What you should see

  • Higher concentration, shorter time.
  • 1/time rises in proportion to concentration - a straight line through the origin.
  • Repeats that are far apart usually mean the end point was judged differently.

Where marks are lost

  • Letting the total volume change between runs, so the depth of liquid changes too.
  • Using a different person to judge the end point on different runs.
  • Drawing a line through every point instead of a single best-fit line.
  • Not stating which variables were controlled.

Questions students ask

What is the independent variable here?
Concentration of sodium thiosulfate. The dependent variable is the time for the cross to disappear, from which the rate is found.
Why keep the total volume the same?
The cross is hidden when enough sulfur has formed in the light path. Change the volume and you change the depth of liquid, so you are no longer comparing like with like.
How many repeats?
At least two at each concentration, three if there is time, then take a mean and ignore any clear anomaly.

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Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.