Investigating the rate of reaction between sodium thiosulphate and hydrochloric acid
I am going to measure the rate of the reaction when hydrochloric acid
is added to sodium thiosulphate. I am going to investigate what effect
the concentration of the sodium thiosulphate has on the reaction rate.
When sodium thiosulphate and hydrochloric acid are mixed, a yellow
precipitate of sulphur is produced. The solution becomes increasingly
difficult to see through as more and more sulphur is formed. This is
how I plan to measure the rate of the reaction. I will place a
laminated cross underneath the solution and will stop the stop clock
when the cross can no longer be seen.
I aim to conduct a fair and safe investigation to determine what
affects the rate of the reaction.
Possible independent variables are temperature, amount of hydrochloric
acid, concentration of Hydrochloric acid, amount or concentration of
sodium thiosulphate and whether or not a catalyst is added.
* My independent variable is going to be the concentration of sodium
* My dependant variable will be the rate of the reaction, or time
taken for the reaction to take place. Watching the cross disappear as
precipitation occurs and sulphur is formed as a solid will be the
method used to realise the rate.
* My controlled variables will be the temperature, the concentration
of hydrochloric acid, the amount of sodium thiosulphate and the amount
of hydrochloric acid. I will try and control these as far as possible,
and be as accurate as I can when measuring out the chemicals.
I will not be using a catalyst.
I predict that the higher the concentration of sodium thiosulphate,
the quicker it will react with the hydrochloric acid. This prediction
is explained by the theory below: -
When two chemicals react, their molecules have to collide with each
other with sufficient energy for the reaction to take place. This is
collision theory. The two molecules will only react if they have
enough energy. This is known as activation energy. Increasing the
concentration of the reactants will increase the frequency of
collisions between the two reactants. The following diagram
illustrates this theory with the reaction between magnesium and
hydrochloric acid, whereas we are using sodium thiosulphate instead of
magnesium, but the theory is clearer if we use a solid as one of the
However, when collisions occur, they do not always result in a
reaction. Only if the two colliding molecules have sufficient energy
will they react. This is the kinetic theory. It is normally applied to
temperature, i.e., increasing temperature means the molecules move
faster, but it is also relevant here.
So, we can deduce from this that the temperature will also affect the
experiment and therefore must remain constant.
It is also relevant to note the following: -
For a chemical reaction to take place, some bonds in the reactants