Experiment: Sodium thiosulfate and acid rate of reaction

The reaction between sodium thiosulfate solution and dilute acid is the classic Form 4 rate experiment, because it produces a slowly forming sulfur precipitate that you can time with a simple cross-and-stopwatch method. It is a favourite Paper 3 practical, since it lets you vary concentration cleanly, measure a clear end point, and calculate a rate. This guide sets out the aim, method, observations and the Paper 3 skills the practical assessment rewards.

Aim

To investigate how the concentration of sodium thiosulfate solution affects the rate of its reaction with dilute hydrochloric acid, measured by the time taken for a fine sulfur precipitate to hide a cross marked under the flask.

Apparatus and materials

• Conical flask
• White paper with a cross (X) drawn on it
• Measuring cylinders (for example, 50 cm³ for the thiosulfate and 10 cm³ for the acid)
• Stopwatch
• Thermometer
• Sodium thiosulfate solution of a known concentration
• Distilled water (to dilute the thiosulfate to different concentrations)
• Dilute hydrochloric acid of one fixed concentration

Procedure

1. Draw a cross on a piece of white paper and place the conical flask on top of it.
2. Measure a fixed volume of sodium thiosulfate solution into the flask, then add distilled water to make the first chosen concentration, keeping the total volume the same each time.
3. Record the temperature of the mixture with the thermometer.
4. Measure a fixed volume of dilute hydrochloric acid, add it to the flask, and start the stopwatch at once.
5. Look down through the mixture at the cross and stop the stopwatch the instant the cross can no longer be seen.
6. Record the time taken for the cross to disappear.
7. Wash out the flask and repeat steps 2 to 6 for several different concentrations of thiosulfate, keeping the total volume, the volume and concentration of the acid, and the temperature the same.
8. For each concentration, calculate the rate as one divided by the time taken, and plot rate against concentration.

Expected observations

When the acid is added, the colourless mixture slowly turns cloudy and pale yellow as a fine precipitate of sulfur forms. Looking down through the flask, the cross becomes harder to see and then disappears. With a more concentrated thiosulfate solution, the cloudiness forms sooner and the cross disappears in a shorter time. As the concentration falls, the time taken increases, so the calculated rate (one divided by time) decreases. The graph of rate against concentration rises.

Inference and conclusion

The shorter the time for the cross to disappear, the faster the rate of reaction. A more concentrated thiosulfate solution gives a shorter time and therefore a faster rate. Collision theory explains this: a more concentrated solution has more thiosulfate particles in the same volume, so they collide with the acid particles more frequently, giving more effective collisions per second. The conclusion is that the rate of reaction increases as the concentration of sodium thiosulfate increases. The same method can be used to study temperature by warming the mixture instead of changing the concentration.

Science process skills (Paper 3 style)

Stating a hypothesis. When the concentration of sodium thiosulfate increases, the rate of reaction increases. The hypothesis links the manipulated variable to the responding variable.

Identifying variables. The manipulated variable is the concentration of the sodium thiosulfate solution. The responding variable is the rate of reaction, found from the time for the cross to disappear. The controlled variables are the total volume of the mixture, the volume and concentration of the acid, and the temperature.

Tabulating data. Draw a table with columns for the concentration of thiosulfate, the time for the cross to disappear, and the rate (one divided by time), recording times to the precision of the stopwatch.

Plotting a graph. Plot rate (y-axis) against concentration (x-axis). The upward trend shows that the rate rises with concentration. A graph of one divided by time against concentration gives a straight line through the origin, a useful check.

Making an operational definition. The rate of reaction is defined operationally as one divided by the time taken for the cross to become invisible through the mixture.

Making an inference. The reaction is faster at a higher concentration because the thiosulfate particles are more crowded and collide with the acid more often, the collision-theory reasoning the examiner rewards.

Safety precautions

• Wear safety goggles, because dilute hydrochloric acid is irritant and can damage the eyes if it splashes.
• Carry out the experiment in a well-ventilated area or fume cupboard, because the reaction gives off sulfur dioxide gas, which is choking and irritating to the lungs.
• Do not lean directly over the flask to smell the gas; keep your face to the side, so sulfur dioxide is not inhaled.
• Wash any acid splashes off the skin at once with plenty of water, because the acid is irritant.

Common errors

• Changing the total volume between runs. If the total volume of thiosulfate plus water differs, the concentration is not controlled fairly; always make the total volume the same by adding the right amount of water.
• Judging the end point inconsistently. Different people decide the cross has vanished at different moments; use the same observer and the same viewing position each time.
• A different depth of liquid. A deeper or shallower layer changes how soon the cross is hidden; keep the total volume, and so the depth, the same.
• Letting the temperature vary. Warmer mixtures react faster; keep every run at the same temperature so only concentration changes.
• Confusing time with rate. A shorter time means a faster rate; plot rate (one divided by time), not time, when the question asks for rate against concentration.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use the disappearing-cross experiment to teach the whole rate toolkit at once: a fair test with one variable, a clear operational end point, turning time into rate, and the collision-theory explanation in the exact words the marking scheme wants. We also stress the sulfur dioxide safety point, which is easy to forget. Because concentration and rate are examined across SPM Chemistry and this is a classic Paper 3 practical (Paper 3 is a practical test assessing science process skills), a clean method and precise reasoning here protect marks in both the theory papers and the practical test.