Reactant concentration and rate of reaction
Aim: this experiment is to study if the concentration of reactants influence rates of reaction.
Hypothesis: the higher the concentration of Na2S2O3 is the faster the reaction will be.   
Equipment:
∙ Sodium thiosulfate solution
∙ Hydrochloric acid
∙ Distilled water
∙ Graduated cylinders
∙ Conical flasks
∙ Stop clock
Variables: Depended: The rate of the reaction
                Independed: The concentration of the reactant
                Controlled: The volume of the reactant
                                    The temperature
                                    The way to measure how soon the reactants take to fully ractant
                                    The same containers for the reaction.
Method:
1. Place 45 mL of 0.25 mol LNa2S2O3 in a 100 mL conical flask. Put the flask over a cross marked on a piece of white paper.
2. Add 5 mL of 2 mol L HCl and briefly agitate to ensure mixing of the reactants. Start a stop clock at the moment of the adding.
3. Note and record the time taken for the cross to ‘disappear’ when it is viewed through the
solution from directly overhead. The formation of solid sulfate causes the cross to be obscured.
4. Repeat the experiment using various sodium thiosulfate concentrations.
Results
Vol. of 0.25 mol L-1 Na2S2react to后面接什么O3 (mL)
Vol. of H2O added (mL)
Total volume after mixing (mL)
Concentration of Na2S2O3 on mixing (mol L-1)
Time for cross to disappear (s), 3 times taken;
Average time (s)
1Time (s)
45mL
0mL
50mL
0.225
21s, 17s, 18s,
18.66s
0.054
35mL
10mL
50mL
0.175
22s, 21s, 21s,
21.33s
0.046
25mL
20mL
50mL
0.125
32s, 31s, 30s,
31.00s
0.032
15mL
30mL
50ml
0.075
52s, 51s, 45s,
49.33s
0.020
5mL
40mL
50mL
0.025
364s, 360s, 360s,
361.33s
0.0027
In the graph, the five number underneath the black bar are the concentrations of sodium thiosulfate in the reaction and the numbers in the blue circles are the rate of the reaction.
Discussion:
This experiment is aim to find out the connection between concentration of the reactants and the rate of the reaction. As have been mentioned in the method and the variables, this experiment only have two variables which is the independed variable – concentration of the
reactants and the depended variable – the rates of the reactions. The experiment and the hypothesis are made base on the collision theory. The collision theory states that when suitable particles of the reactant hit each other a successful chemical reaction occur. Based on the collision theory, the hypothesis is that the higher the concentration of Na2S2O3 is the faster the reaction will be. We made this hypothesis because as in the collision theory says, if there are more collisions happened at the same time, the faster the reaction is, therefore if the number of the reactants’ Molecular in the solution increase which means the concentration of the two reactants increase the chance of the Molecular of the two reactants will surely increase which will increase the rate of the reaction. In this experiment we use water to control the concentration of the reactant. For example, if we want the concentration of the sodium thiosulfate be 0.225, we will add 45 mL of 0.25 mol L-1 sodium thiosulfate solution.  In my opinion this hypothesis is reasonable and logical and the method of this experiment is also possible and reliable.
The result of this experiment has strongly support the hypothesis. As the result table and the graph show, it is very easy to see that, as the concentration of two reactants and the rat
e of the reaction are the only two variables, concentration is certainly related to the rate of the reaction. From the result table we can see that as the concentration of the sodium thiosulfate decrease from the o.225 to 0.175 the rate of the reaction decrease from 0.054 to 0.046. When the concentration decrease from 0.175 to 0.125 the rate of the reaction decrease again which is from 0.046 to 0.032. The same situation happened to the following results, as the conclusion, the rate of the reaction will decrease. If we view this conclusion oppositely we can see it is exactly the same to the hypothesis. However as we study even more about the data we can find out that the concentration of the reactants and the rate of the reaction is not proportional although it seems to be. In the experiment results, as the concentration decrease from 0.225 to 0.075 which means the concentration decrease to one third and the rate of the reaction also decrease from 0.054 to 0.02. Many people think this pair of data can be used to prove the conclusion that the rate of the reaction and the concentration of the reactants are proportional. However if we review the experiment result table there are another pair of data that can prove their theory is wrong. When the concentration of reactants decrease from 0.075 to 0.025 which decrease to one third, the r
ate of the reaction doesn’t decrease as less, the rate of the reaction decrease to 0.0027 from 0.02 which is about one seventh. But fortunately, this can be also explain by the collision theory. As the collision theory says, the rate of reaction increase when the number of the collisions between reactants increase, therefore we can know the rate of reaction is proportional to the number of collision. From the experiment we are unable to tell that the concentration of the reactant and the collisions are proportional, therefore both the theory and the experiment results don’t support the conclusion that concentration of the reactants and the rate of the reaction is proportional.
Conclusion:
As the conclusion, I think our hypothesisi has be well support by the collision theory and the experiment data. In this experiment we have prove there is a connection between cincentration of the reactants and the rate of the reaction which is the rate of reaction will increase if the concentration of the reactants  increase and if the concentration of the reactants decrease the rate of the reaction will decrease as well but concetraction of the re
actants and the rate of reaction won’t increase or decrease in the same rate. As the result I think this experiment is a successdul experiment.

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