MrChemCoach

Concentration & rate of reaction

Gas syringe method · IGCSE
Gas made0.0 cm³
Stopwatch0.0 s
Rate now0.00
t (s)0102030405060708090100110120
V (cm³)

Study this experiment

Six short ideas, in the order you meet them on the bench. The buttons inside set the lab up so you can watch each idea happen.

Graph of gas volume 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)
For revisionTheory, method and exam tips

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.

Try these next

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.