Rates of Reaction

The factor we will investigate is the concentration of the reactants. The two substances will be sodium Thiosulphate solution (Na2S2O3) and Hydrochloric acid solution (HCl). In the experiment we will change the concentration of the acid to see if it has any effect. We are choosing this factor as apposed to the others because...

The surface area would be difficult to measure as both the reactants are in liquid form.

There are a limited amount of catalysts available for this reaction so we would not have a wide range of results.

Temperature would be a more difficult test to measure then concentration and I think it would give us less interesting results.

Prediction:

We are using two substances in their liquid form, and two things which are vital for the particles to react are

The particles must collide

The collisions must have enough energy

Because the two substances are liquid they will collide easily so what we are concerned with is how much energy the particles get in order to make successful collisions. If I increase the concentration of the hydrochloric acid by 50 % the speed of the reaction will increase by 50%. This is because the ions are closer together in a concentrated solution. The closer together they are, the more often the ions collide. The more often they collide, the more chance they have of reacting.

Method: I will create the correct concentrations of acid. We are given two concentrations of acid to start off with. 50g per litre and 60g per litre. We then need to mix the two together to create 3 other different concentrations. To measure the rate of the reaction we will put the two solutions together in a conical flask and by timing from the start to when the cross is no longer visable we can determine the speed of the reaction. The two solutions are translucent but when they are mixed sulphur if form and creates a sediment which covers the cross. I will have to take three repeat result to ensure accuracy.

This reaction is:

Na2S2O3 + 2HCl --> 2NaCl + H20 + S + SO2

The sulphur is the thing which shows us the rate.

Equipment: The equipment needed to carry out this experiment is:

HCl

Na2S2O3

Conical Flask

Stop Clock

Black cross

Diagram:

Fair test: First the stop clock will be set to 0 and the equipment will be set out and cleaned between each reading. The cross will have to be exactly the same for each reading. We want to ensure we keep all the other variables the same we will not want the temperature to change between readings. We will have to make sure the experiments aren't done near an open window or in the sunlight. The same person will have to look at the cross each time because eye sight will differ for different people.

Obtaining

Results:

Concentration of hydrochloric acid (grams per litre)

Time taken for sediment to form (seconds)

Test 1

Test 2

Test 3

5g/l

204

217

236

25g/l

50

40

46

30g/l

16

13

09

50g/l

54

55

59

60g/l

44

40

42

Analysing

Graphs:

Analysis of data:

I can see that as I increase the concentration the time it takes for a sediment to form is shorter. My results show that an increase in reactant concentration leads to an increase in reaction rate. This is because a higher concentration means a larger number of particles in a given volume. If there are more particles, there will be more collisions. More collisions means a larger number of successful collisions. Therefore the reaction rate will increase. As what is needed for a successful collision (The collision theory) is that particles must collide and the colliding particles between them must contain enough energy to cause bonds to break. Therefore my prediction was correct "If I increase the concentration of the acid the speed of the reaction will increase"

Evaluating

Evaluation:

My results are accurate the reason for the slight anomaly in the curve of the graph is the uneven interval of the concentrations. To improve the experiment I could have used a LDR (light dependant resistor) which is more accurate for measuring when the cross is covered by sediment then the human eye which is subject to error although the same LDR would have to be used each time as they have different tolerances to light. I could have used a 1/Time graph which would give me a straight line and show how 1/Time is directly proportional to the concentration of the acid.