Rates of Reaction: Disappearing Cross Experiment

Aim

To investigate the effect of concentration (or temperature) on the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid.

Apparatus

• Conical flask (100 cm3 or 250 cm3)
• Measuring cylinders (10 cm3 and 50 cm3)
• Stopwatch
• White paper with a bold black cross (X) drawn on it
• Sodium thiosulfate solution (Na2S2O3, 0.15 mol/dm3)
• Dilute hydrochloric acid (HCl, approximately 1 mol/dm3)
• Distilled water
• Thermometer (if investigating temperature)
• Water bath or Bunsen burner, tripod, gauze, and heatproof mat (if investigating temperature)

Method

Investigating concentration

1. Place the conical flask on top of the paper cross so the cross is visible through the bottom of the flask.
2. Using a measuring cylinder, add 50 cm3 of sodium thiosulfate solution to the flask.
3. Using a separate measuring cylinder, add 5 cm3 of dilute hydrochloric acid to the flask and immediately start the stopwatch.
4. Swirl the flask gently once to mix, then observe the cross from above.
5. Stop the stopwatch when the cross is no longer visible through the cloudy solution. Record the time.
6. Rinse and dry the flask. Repeat with different concentrations of sodium thiosulfate by diluting with distilled water (e.g., 40 cm3 thiosulfate + 10 cm3 water, 30 cm3 + 20 cm3, and so on), keeping the total volume and acid volume constant.
7. Repeat each concentration at least twice and calculate a mean time.

Investigating temperature

1. Use the same concentration of sodium thiosulfate throughout (e.g., 50 cm3 of 0.15 mol/dm3).
2. Heat the sodium thiosulfate in a water bath to the desired temperature before adding it to the flask on the cross.
3. Add 5 cm3 of dilute HCl, start the stopwatch, and time until the cross disappears.
4. Repeat at five different temperatures (e.g., 20, 30, 40, 50, 60 degrees C).

Expected results

As concentration of sodium thiosulfate increases, the time for the cross to disappear decreases. Rate is inversely proportional to time (rate = 1/time), so the rate increases as concentration increases.

As temperature increases, the time decreases and the rate increases. Roughly, a 10 degree C rise in temperature approximately doubles the rate.

The reaction equation is: Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO2(g) + H2O(l). The sulfur precipitate causes the cloudiness.

Analysis

Plot time (y-axis) against concentration or temperature (x-axis). For concentration, the graph is a curve showing an inverse relationship. For a clearer picture, plot 1/time (y-axis) against concentration — this should give an approximately straight line through the origin, showing that rate is directly proportional to concentration.

For temperature, plot time against temperature. The curve shows a decreasing relationship. Plotting 1/time against temperature gives an increasing curve (not a straight line), showing that the relationship between rate and temperature is not linear.

Sources of error

• Subjective endpoint: Different people may judge the disappearing cross at different times. The same person should judge all readings, and they should look from the same angle and distance each time.
• Temperature control: When investigating concentration, the temperature of the room may vary between experiments. Carry out all experiments on the same day in the same conditions.
• Mixing time: The time between adding acid and starting the stopwatch introduces a delay. Practise the technique to minimise this.
• Measuring volumes: Use the same measuring cylinder for the same solution throughout to maintain consistency. Read from the bottom of the meniscus.

Exam tips

Examiners frequently ask you to explain why the cross disappears (sulfur precipitate makes the solution cloudy/opaque). They also ask why this method is not perfectly accurate (subjective endpoint, not a true rate measurement). If asked to improve reliability, suggest repeating and averaging, or using a light sensor and datalogger to detect the cloudiness objectively instead of using the human eye.

When describing the graph, state the relationship clearly: “as concentration increases, time decreases” and “rate increases with concentration.” Always define rate as 1/time in your answer. If a question gives you time data and asks for rate, calculate 1/time for each result before plotting.