In my investigation I am going to look at how concentration affects the rate of reaction, and the reaction I am going to look at is Sodium Thiosulphate (Na2 S2 O3) and Hydrochloric Acid (HCl).

I predict that more water added to the solution of Sodium Thiosulphate and Hydrochloric Acid, the longer “X” will take to be obscured.

Less concentration will mean a lower rate of reaction, as there are fewer particles in the same volume of solution. Fewer collisions will occur between the reacting particles which lead to fewer successful collisions in a given period of time, which decreases the rate of reaction.

But high concentration will mean a higher rate of reaction, as there are more particles in the same volume of solution. More collisions will occur between the reacting particles which lead to more successful collisions in a given period of time, which increases the rate of reaction.

 

All collisions will make a reaction in the experiment but not all collisions are successful, the rate of reaction depends on the number of successful collisions per second, which is directly proportional.

The two things that are colliding in the collision are the hydrogen ions from the acid and the thiosulphate ions.

In My Investigation:

Variables
The independent variable is the factor that I will change in my investigation.

Independent Variables -

• Concentration of the sodium thiosulphate solution.

The controlled variables will be the factors that must be kept the same during the investigation to make sure of a fair test.

Controlled Variables -  

• Volume of hydrochloric acid
• Concentration of hydrochloric acid
• Volume of sodium thiosulphate solution
• Temperature of hydrochloric acid and sodium thiosulphate solution
• Same apparatus

The dependent variable is the factor that is going to change naturally  

Dependent Variable -

• Time for the reaction between the hydrochloric acid and sodium thiosulphate to be completed

Apparatus

Sodium Thiosulphate        

Paper with “X” With Drawn On IT

Hydrochloric Acid

Pen/Marker

Water

Measuring Cylinder 50cm3

Measuring Cylinder 10cm3

Conical Flask

Large Beaker

Small Beaker

Stop Watch

Safety Glasses

Safety

There were a few safety procedures that needed to be taken while doing this experiment, such as wearing safety glasses to protect eyes from the acid being used in the experiment as this could potentially damage your eyes. Another important safety procedure to take was to wash your hands if any acid went on them and finally report any broken glass to the teacher if any glass was smashed.

Method

• Prepare a table to record the results of your findings

• Prepare a measuring tube of 10cm3 of hydrochloric acid and pour this into the conical flask

• Then measure out 50cm3 of sodium thiosulphate and add this to the beaker.

• Shake the beaker and start to stopwatch. (Remember to start the stop watch at the same point each time the experiment is carried out as this improves accuracy in the experiment.)

• Then stop the experiment when the “X” underneath the conical flask is obscured by the sodium thiosulphate and hydrochloric acid solution.

• Record results

• Again measure out 10cm3 of hydrochloric acid and pour it into the conical flask

• But this time measure out 45cm3 of sodium thiosulphate and now add 5cm3 of water, add this to the conical flask, shake and then time

• Stop the stop watch when the solution has obscured the “X” and again record the results

• Carry this same procedure out for the following measurements –

50cm3 of sodium thiosulphate and 0cm3 of water

45cm3 of sodium thiosulphate and 5cm3 of water

40cm3 of sodium thiosulphate and 10cm3 of water

35cm3 of sodium thiosulphate and 15cm3 of water

30cm3 of sodium thiosulphate and 20cm3 of water

25cm3 of sodium thiosulphate and 25cm3 of water

• Record the results of all these experiments

• Repeat your experiment twice more starting with 50cm3 of sodium thiosulphate and 0cm3 of water, this is to improve reliability and make sure when recording results it is measured to one decimal place to improve accuracy.

Strategy for results
I will find the average time taken for the “X” to be obscured by a solution of sodium thiosulphate and hydrochloric acid from 3 readings. I will record my results in a table form (as shown below) and then I will draw two graphs displaying my results; The Y-axis on my graph will be the rate of reaction or the average time and the X-axis on my graph will be the concentration measured in g/dm3

Results

Average Time = Original + Repeat 1 + Repeat 2

                                                 3

Concentration =    40g/dm3                              x          Volume of Na2S2O3

                            (Original concentration)                                  50

Interpretation and Evaluation

My graphs and table agree with my prediction which was that the higher the concentration the quicker the “X” would be obscured by the solution and therefore the higher the rate of reaction. The higher rate of reaction is cause by more particles present in the same volume of solution, which leads to more collisions between the reacting particles, leading to more successful collisions over a period of time which therefore increases the rate of reaction.

My first graph is negatively correlated which prove my prediction that as the concentration increases, the time taken for the “X” to be obscured will decrease, I think my results are very accurate as all my points on my graph are very close to my line of best fit and some of my points even lie on the line of best fit.

My second graph is positively correlated and also proves my prediction which is the higher concentration the greater the rate of reaction. This graph is again accurate as most points lie along the line of best fit and all the rest are close to the line of best fit.

A valid method was used so my results agreed with my theory was used so my results agreed with my theory. We repeated and took the average of three recordings of the experiment and, we also measured the volumes of the hydrochloric acid and sodium to eye level.

There were a few problems with the experiment, firstly the same flask was used each time, the flask had obtained a coating of sulphur on it from previous experiments and this was hard to was off. This may affect the results of our experiment as there would be more sulphur added to the solution, which would help to make the experiment go cloudy quicker.

Another factor that could of potentially affected results is that the experiment was done over two days which means the temperature of the sodium thiosulphate and the hydrochloric acid could have changed, meaning an increase/decrease in temperature would either speed up or slow down the rate of reaction.

Measuring the “X” being obscured by the solution by the eye is not the most scientific method: alternatively a colorimeter could be used. Also measuring cylinders aren’t the most accurate piece of equipment for measuring volume: a burette could be used as an alternative.