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The thiosulphate and hydrochloric acid reaction is the classic "disappearing cross" experiment. It looks simple: mix, start the stopwatch, stop it when the cross vanishes. But the viva asks why 1/t measures the rate, what your graph shows and how temperature fits in. This post explains the reasoning, works through sample readings and shows how to get an activation energy from two temperatures.
Where does this experiment appear?
The CBSE Chemistry curriculum for 2026-27 lists "Effect of concentration and temperature on the rate of reaction between Sodium Thiosulphate and Hydrochloric acid" under Chemical Kinetics in the Class 12 practical syllabus. The JEE (Main) 2026 syllabus and the NEET (UG)-2026 syllabus list a different kinetics experiment (iodide with hydrogen peroxide) in their practical units, but the rate ideas are the same ones in the NCERT Class 12 chapter Chemical Kinetics. Check the curriculum for your own year.
What happens in the flask?
Na2S2O3 + 2HCl → 2NaCl + S + SO2 + H2O
Sulphur forms as a fine solid that makes the solution cloudy. You place the flask on paper marked with a cross and time how long it takes for the cross to disappear when viewed from above. Sulphur dioxide is also released, so work in a ventilated room and do not breathe in over the flask.
Why does 1/t measure the rate?
The cross disappears when a certain amount of sulphur has formed. If the same flask, the same volume and the same cross are used each time, that amount is the same in every run. Rate is roughly amount ÷ time, so
rate ∝ 1/t.
This only works if the total volume and the viewing set-up stay the same. Change the volume or the flask and you change the amount of sulphur needed to hide the cross.
How do you show the effect of concentration?
Keep the total volume at 50 mL by mixing thiosulphate solution with water, then add the same volume of acid each time. Here are sample readings, not real data, with 0.10 M thiosulphate:
50 mL thiosulphate + 0 mL water: 0.10 M, t = 25 s, 1/t = 0.0400 s⁻¹
40 mL + 10 mL: 0.08 M, t = 31 s, 1/t = 0.0323 s⁻¹
30 mL + 20 mL: 0.06 M, t = 42 s, 1/t = 0.0238 s⁻¹
20 mL + 30 mL: 0.04 M, t = 63 s, 1/t = 0.0159 s⁻¹
10 mL + 40 mL: 0.02 M, t = 125 s, 1/t = 0.0080 s⁻¹
Now divide 1/t by concentration: 0.400, 0.404, 0.397, 0.397 and 0.400. The ratio stays the same, so a graph of 1/t against concentration is a straight line through the origin. With these sample numbers, the rate is directly proportional to thiosulphate concentration, which means first order in thiosulphate under these conditions. A graph of t against concentration would be a curve, which is why you plot 1/t.
How do you find the effect of temperature?
Warm the thiosulphate solution in a water bath, check its temperature, add the acid and time as before. Keep the concentrations the same as in one of your earlier runs. NCERT notes that for many reactions a 10° rise roughly doubles the rate constant.
Worked example with sample readings. At 30 °C (303 K) the cross disappears in 40 s; at 40 °C (313 K) it disappears in 20 s. With the same concentrations, k2/k1 = t1/t2 = 40/20 = 2.
Use the Arrhenius equation in the form NCERT gives:
log (k2/k1) = [Ea / (2.303R)] × [(T2 − T1) / (T1 T2)]
log 2 = 0.3010.
(T2 − T1)/(T1 T2) = 10 / (303 × 313) = 10 / 94 839 = 1.054 × 10⁻⁴ K⁻¹.
Ea = 2.303 × 8.314 × 0.3010 / (1.054 × 10⁻⁴) ≈ 54 700 J/mol, or about 55 kJ/mol.
This is the number behind the "doubles every 10°" rule: near room temperature, a reaction with Ea around 55 kJ/mol roughly doubles its rate for a 10 K rise.
Why do students lose marks here?
Lab work often stays at the level of following steps. Abrahams and Millar (2008) watched 25 typical science practical lessons in English secondary schools. The work was generally effective at getting students to do the intended things with the equipment, but much less effective at getting them to use scientific ideas to guide their actions and make sense of the data they collected. The viva tests exactly that link. Typical slips:
Changing the total volume between runs. Then 1/t no longer compares like with like.
Different observers or crosses. One person should judge every run, using the same mark.
Not checking the temperature after mixing. Cold acid lowers the temperature of a warmed solution.
Plotting t against concentration and calling it linear.
Leaving out the units of 1/t. They are s⁻¹.
For the theory of rate laws and half-life, read our post on rate laws, order and half-life, and for order from data, the initial rates method.
A practice routine before the practical
Write the equation and explain in two sentences why 1/t is proportional to the rate.
Make your own table of five dilutions that keep the total volume fixed.
Calculate 1/t and (1/t)/concentration for the sample readings above, then check them.
Redo the Ea calculation with t = 48 s at 30 °C and 24 s at 40 °C. You should get the same answer, because the ratio is still 2.
If the Arrhenius equation still feels like a formula without meaning, revise it as an idea first. The Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise activation energy and logarithms before the lab.
Next step: draw your results table, with columns for volumes, concentration, t and 1/t, before the practical day.
