To study the effect of temperature on reaction rate using sodium thiosulfate and hydrochloric acid

Theory

Sodium thiosulfate and hydrochloric acid react to produce a precipitate of sulfur as follows:

Na₂S₂O₃ (Limiting) + 2HCl (excess) → 2NaCl + SO₂ + S + H₂O

• A solution of sodium thiosulfate are prepared and reacted against a dilute hydrochloric acid solution at different temperatures

• For each concentration, the time taken for a certain mass of the sulfur precipitate to form is recorded and the rate of reaction measured

Procedure

➢ Using a graduated cylinder, add a fixed volume of 0.5 M sodium thiosulfate is added to a conical flask

➢ The conical flask has a cross (‘X’) drawn on the bottom with a marker

➢ The conical flask is placed in a water bath on a hot plate and using the thermometer set to a specific temperature

➢ Using another graduated cylinder, a fixed volume of dilute hydrochloric acid is quickly added to the sodium thiosulfate solution in the conical flask

➢ The conical flask is swirled, and the stopwatch is immediately started

➢ While looking down through the solution, the time for it takes for the yellow sulfur precipitate to obscure the cross is recorded

➢ Using a hot plate, water bath and thermometer, the experiment is repeated five times at DIFFERENT TEMPERATURES

➢ The results are entered in a table and a graph of rate (1/time) Vs temperature is plotted

Conclusion

As temperature increases, rate of reaction increases exponentially

Notice: As the graph is not a straight-line graph it is not a directly proportional relationship- rate increases exponentially with temperature

Explain the relationship between rate of reaction and temperature.

• As temperature increases, rate of reaction increases exponentially

1.

- Increasing the temperature increases the energy of the particles

- Higher energy means more effective collisions - more collisions reach the required activation energy

2.

- Increasing the temperature increases the velocity of the particles

- Higher velocity means greater frequency of collisions will occur

- Greater frequency of collisions means more effective collisions - more collisions reach the required activation energy

Note: Exponential means: For every X˚C rise in temperature, rate of reaction increases by a constant factor

(Typically for many reactions – For every 10˚C rise in temperature, rate of reaction doubles)

Why is a water bath preferable to a bunsen burner/directly heating with a hot plate?

It is easier to obtain a desired temperature using a water bath - allows for more gentle heating

Why is it essential the same volume and concentration of hydrochloric acid are used during each reaction?

• Only one variable can be changed in order for the experiment to be a fair test i.e. the concentration of sodium thiosulfate

• All other possible variables must remain fixed

Identify the precipitate in this reaction? Describe its appearance

Sulfur – precipitates as a fine pale-yellow powder