Increasing the concentration of a reactant increases the rate of reaction because particles are closer together and collide more frequently, as shown by the sodium thiosulfate and hydrochloric acid experiment.
Increasing the concentration of a reactant increases the rate of reaction. According to the collision theory, a higher concentration packs more reactant particles into the same volume, so they collide more frequently, and more frequent effective collisions mean a faster reaction. This is one of the core factors examined in the Rate of Reaction chapter of 4541, and it comes with a classic experiment.
The reaction and balanced equation
The standard experiment uses sodium thiosulfate solution and dilute hydrochloric acid, which react to form a fine precipitate of sulfur that makes the mixture turbid. With correct formulae and state symbols:
Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(g) + S(s) + H2O(l)
The pale yellow sulfur precipitate slowly hides a mark drawn under the flask, so timing how long the mark takes to disappear measures the rate. A second common example is marble chips (calcium carbonate) with dilute hydrochloric acid, where the gas released can be measured:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
Conditions and how the experiment is set up
To test concentration fairly, only the concentration of one reactant is changed; the volume of the solution, the temperature and the total amount of the other reactant are all kept constant. In the thiosulfate experiment, different concentrations of sodium thiosulfate are prepared by dilution, the same volume of acid is added, and a stopwatch records the time for the ink cross beneath the flask to be obscured by the sulfur.
What you observe
At a higher concentration of thiosulfate, the mixture turns cloudy more quickly and the cross disappears in a shorter time, so the rate is higher. At a lower concentration it takes longer. With marble chips and acid, a higher acid concentration gives faster effervescence, a steeper mass-loss curve, and a shorter time to collect a fixed volume of carbon dioxide. Plotting rate against concentration, or 1/time against concentration, gives a line that rises with concentration.
Where it appears in the SPM exam
In 4541/1 you predict how a change in concentration changes the rate and identify the collision-theory reason. In 4541/2 you explain the effect using collision theory, sketch or interpret rate graphs, and identify the manipulated, responding and controlled variables. In the practical paper 4541/3, the thiosulfate “disappearing cross” experiment is a standard task: you tabulate time against concentration, calculate 1/time as a measure of rate, and draw the graph.
How we teach it
Our teachers make sure your explanation always names the mechanism: more particles per unit volume, more frequent collisions, more frequent effective collisions, faster rate. Students most often lose marks by saying only “more collisions” without “per unit volume”, by muddling the variables, or by confusing the concentration effect with the temperature effect, which changes collision energy rather than frequency. Keeping the collision-theory sentence exact makes every rate question quick to answer.
Quick summary
A higher concentration means more particles in the same volume, more frequent effective collisions, and a faster rate. Learn the thiosulfate equation and the disappearing-cross method, the carbonate-and-acid alternative, the controlled variables, and the precise collision-theory explanation, and this dependable rate topic is fully covered. Remember too that 1/time is used as a measure of rate because a shorter time means a faster reaction, so the graph of 1/time against concentration should rise as a straight line.
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