Standard level
The rate of a chemical reaction is defined as the change in concentration of a reactant or product with time.
Background
It is an observable fact that there is a large variation in the rate of chemical reactions from almost instantaneous to not at all.
To investigate the concepts that can affect reaction rate it is important to choose reactions that have a reactant or product that can be monitored in such a way as its concentration can be determined.
The reaction between thiosulfate ion and hydrogen ions in solution proceeds according to the following equation:
S2O32-(aq) + 2H+(aq) → S(s) + SO2(aq) + H2O(l)
As the solid sulfur forms, the liquid mixture slowly becomes cloudy. There reaches a point when the liquid mixture is completely opaque. This can be judged by the inability to perceive an object on the far side of the reaction test-tube.
It is reasonable to assume that the sulfur precipitate obscures light by a quantity proportional to the amount of sulfur precipitated. Hence, the point of opacity represents the same mass/moles of sulfur in every trial. In line with stoichiometric principles, the concentration change in the thiosulfate will also be the same in every trial.
Therefore the time taken to reach opacity may be used to measure relative rates for different acid concentration.
Chemicals
- Sodium thiosulfate, Na2S2O3.5H2O, 0.02 mol dm-3
- Hydrochloric acid, 2 mol dm-3
Apparatus
- Test-tubes
- Test-tube rack
- Black marker
- Chronometer
- Burette, 50ml (x2)
- Measuring cylinder, 10ml
- Fill one burette with the sodium thioslfate solution.
- Fill the other burette with the water.
- Note that the acid must be mixed with the water before adding the sodium thiosulfate solution.
- Transfer 10 cm3 of the hydrochloric acid into the test tube.
- Add 10 cm3 of the sodium thiosulfate and shake well as you start the stopwatch.
- When the cross is completely obscured stop the timer and record the time.
- Use the mixtures in the grid below for each part of the investigation.
- Hydrochloric acid is corrosive
- The products of the reaction should be collected in a disposal jar in the fume cupboard.