I am to investigate the factors that affect the rate at which Sodium
Thiosulfate (Na2S2O3) reacts with Hydrochloric Acid (HCl).
When sodium thiosulfate is mixed with hydrochloric acid, a reaction
takes place, leading to a precipitate of sulphur being formed. This
turns the solution cloudy. I shall use this clouding to investigate a
factor that affects the rate of reaction, the factor being
To do this, I shall react Sodium Thiosulfate with Hydrochloric Acid at
a range of temperatures, between 20°C and 40°C, at 5°C intervals,
leading to 5 results. The experiment shall be carried out twice, to
give accuracy, and allowing me to check for agreement between results.
I predict that as the temperature that the reaction takes place at
increases, the less time it will take for the reaction to take place
and for the solution to cloud. My prediction is based on the collision
theory, which says that
· Reacting species must hit together
· Species must hit together with more than the activation energy
Increasing the temperature of the atoms taking part makes them move
more and faster, enabling more collisions to take place, speeding up
the reaction. The added heat energy also allows more collisions to
have the required activation energy, again, speeding up the reaction.
In order to make this experiment a fair test, I changed only one input
variable, the temperature. I kept the amount and concentration of the
liquids the same (20ml of Sodium Thiosulfate, 5ml of HCl), so that
this did not affect the results. I used the same indicator of
clouding, a cross, drawn on a piece of paper underneath the conical
flash. This was protected from the liquids, which would have made the
ink run between experiments, making for inconsistent readings, by a
plastic wallet. The same sized conical flask was used for all of the
experiments as well, as different sizes or shapes may alter the speed
of the reaction.
20ml of Sodium Thiosulfate, measured using a measuring cylinder, was
added into a conical flask, and heated to the required temperature
using a water bath heated by a Bunsen burner. A thermometer was used
to monitor the Sodium Thiosulfate's temperature, so that the flask
could be removed from the water bath when the chemical was at the
correct temperature. 5ml of HCl was measured in a different measuring
cylinder, and was poured into the Sodium Thiosulfate when this was at
the required temperature. The stopwatch was started when the acid had
been completely poured in. Then, standing...