- Using a catalyst
A catalyst is a substance that alters the rate of reaction but remains chemically unchanged and is not used up at the end of the reaction. A catalyst speeds up the reaction. Using a catalyst lowers the activation energy for the reaction. More collisions have sufficient energy for reactions to occur.
Variable to be investigated
In this experiment we will investigate the effect of concentration on the rate of reaction.
B. The Experiment
1. Planning
- The reaction between Hydrochloric acid and Sodium thiosulphate
In order to observe the effects concentration of a chemical has on the rate of reaction when it is mixed with another chemical, an experiment has to be planned using two different chemicals, one of which will have concentration as the variable.
The experiment will be carried out using sodium thiosulphate, which will be the chemical that has to be subject to change of concentration. To vary the concentration of sodium thiosulphate different amounts must be used and mixed with equal amounts of hydrochloric acid.
The equation for this chemical reaction:
Sodium thiosulphate + Hydrochloric acid Sulphur + Sodium chloride + Sulphur dioxide + Water
Na2S2O3 (aq) + 2HCL (aq) S (s) + 2NaCl (aq) + SO2 + H2O
The sulphur created appears in the form of very small particles of solid. The particles do not settle; they remain in suspension. This leaves a white/yellow precipitate.
(b) Variables which are to be kept constant to ensure a fair test
To keep each reading and result fair and accurate it is imperative that no other factors that could affect the rate of reaction are allowed to take place. The following factors will be kept constant.
(i) Volume of the reactants
Can compromise the accuracy due to the fact that when the concentration of the sodium thiosulphate is altered different amounts of sodium thiosulphate will be used. When this happens the volume of the total reactants will change. To prevent this factor from compromising the accuracy of the experiment all reactants must react in the same volume of liquid, once the amount of sodium thiosulphate has decreased water will be added to substitute for the missing volume of the sodium thiosulphate.
(ii) Temperature of the reactants
Temperature alters the rate of reaction, so temperature must be kept constant; to fulfil this condition I will perform the experiment in the same location, so there will be no major variation in temperature. A thermometer may be used to ensure that the temperature remains constant through out. The solutions will be kept in the laboratory where the experiment is carried out so that there is no temperature variation caused by moving solutions from a colder or warmer room and then using them in the test.
(iii) The size of the cross on the paper
The same paper will be used with the same cross size so that variations in size do not affect the result. Also same person will time the reaction to account for variation between individuals.
The experiment will be reliable as, apart from the variables, the same conditions will be present throughout for every readings.
(c) The experimental procedure
To undertake the experiment a piece of paper with a cross, marked on it will be placed underneath a conical flask. The conical flask will contain the reactants and which is added to keep volume in the flask constant.
When the sodium thiosulphate and hydrochloric acid react they create a milky coloured solution which is due to the formation of sulphur, one of the products of the reaction. The time it takes for this solution to turn so milky that the cross placed on a white piece of paper becomes obscured gives the rate of reaction. The faster the reaction the quicker the cross will disappear because more sulphur will be produced.
5 readings, covering a large range must be taken to try and ensure accuracy for the experiment. The results were put into the form of a graph, as it was easier from the curve created to find a link between concentration of a reactant and the rate of reaction.
(d) Prediction
I predict that concentration and rate of reaction will be directly proportional. The higher the concentration of sodium thiosulphate, the faster the rate of reaction.
The concentration and time taken for the reaction to occur/cross to disappear, however, will be inversely proportional because as concentration increases, time taken for the reaction to occur/cross to disappear decreases.
A graph plotting concentration against time taken for the reaction to occur/cross to disappear will show a curve (y = a/x), this illustrates inverse proportionality. A graph plotting concentration against the rate of reaction will be a straight line (y=x), this shows direct proportionality.
(e) Risk Assessment.
Hydrochloric acid solution is corrosive and therefore any splashes on the skin or the eyes must be washed off with cold tap water immediately. It should be handled carefully.
Safety requirements- goggles must be worn to protect eyes.
Broken glass can cause injuries and all breakages of glass will be carefully removed by hand broom and a dustpan and the teacher informed.
Hands must be washed after the experiment to cleanse skin from chemicals.
2. Preliminary Experiment
Before the experiment was carried out a preliminary investigation was carried out by varying the hydrochloric concentration and keeping thiosulphate constant and varying thiosulphate but keeping hydrochloric constant. From this study it was possible to find out the best reactant to vary to give suitable results. Graphs of the results were drawn to show the best reactant to vary. The size of the cross was also varied to give a reliable indication of its presence. For the preliminary experiment the following results were obtained:
Soduim thiosulphate kept constant at 10ml
HCL kept constant at 10ml
It is clear from my preliminary results and graph that varying the concentration of sodium thiosulphate produces a better curve so it will produce more accurate results for my final experiment. The starting ratio of 25ml:10ml for the sodium thiosulphate produced accurate results in a good time, so I will stick to this starting ratio for my experiment. I have reduced 20% of the starting measurement of sodium thiosulphate (5ml) for every reading. In my main experiment I will use double the amount of reactants to reduce measurement errors, but will keep to the same ratio of sodium thiosulphate: HCL, and will still reduce 20% of the starting value of sodium thiosulphate for every reading. This will mean I will use a sodium thiosulphate: HCL ratio of 50ml:20ml and reduce 10ml for every reading.
3. Experimental Procedure
The procedure went according to plan.
4. Apparatus
The following apparatus was used in this experiment:
1 50 ml beaker
1 100 ml beaker
1 500 ml Conical flask
1 500 ml measuring cylinder
1 piece of paper marked with an x (cross)
1 stopwatch
Hydrochloric acid solution of concentration 2M or 2 moles per litre.
Sodium thiosulphate solution of concentration 40 grams per litre.
C. Observations
The concentration of hydrochloric acid was kept constant at 20ml.
Other observations: As the sodium thiosulpahte reacted with the HCL a yellow precipitate was left behind and sulphur could be smelt.
D. Analysis
In this experiment it was found that as the concentration of the thiosulphate increased, the time taken for the cross to disappear reduced. This indicated that the rate of reaction became faster. From graph 1, on the next page, it can seem that as the concentration doubles the time taken for the cross to disappear halves (from 10 to 2 ml of sodium thiosulphate the time fell from 134 seconds to 66 seconds which is approximately half).
The curve shows very fast reaction at the start, which slows down with time. This is because the amount of the reactants has become less as they have been changed to products and from the particle and collision theory it is because there are now fewer particles so fewer collisions take place.
Graph 2, shows a straight line, which passes through the origin. This confirms that the relationship between the concentration and the time is inverse (1/Time is proportional to concentration).
The graph shows that as the concentration increased the rate of reaction also increased in proportion. A doubling of the concentration almost doubled the rate of reaction. There is therefore a linear relationship between the concentration and the rate of reaction. This is because as the concentration increases the chances of the particles colliding also increases since there are more particles in unit volume. This type of reaction in which the rate increases in a linear way and in proportion with increase in concentration is called first order with respect to the reactant (in this case the reactant is sodium thiosulphate).
Rate of reaction is proportional to thiosulphate concentration and is first order with respect to thiosulphate.
The results support my hypothesis that the rate of reaction will increase with increasing concentration of the sodium thiosulphate. The particle and collision theory used to support my prediction has been proved to give the best explanation for this.
E. Evaluation
The procedure used was accurate enough to give the results obtained which prove the hypothesis. The points give the expected curve and a straight-line graph passing the through the origin.
The evidence obtained does not contain and anomalies ad all the readings are those expected but they could be improved to fall on a straighter line as some points do not fall on a straight line as expected.
The procedure is suitable for obtaining this type of relationship. It is simple and fairly accurate results can be obtained from it to give an idea of a relationship, which is present. It can not be modified to improve accuracy by:
- Using demineralised water instead of tap water so as to discount chemicals within the tap water from taking art in the reaction. But the tap water used was the same for each set of tests so this can be ignored.
- Carrying out the test with no sodium thiosulphate present to discount the possibility of hydrochloric acid and water reacting to form a cloudy solution. This could be a control.
- Checking the temperature of the solutions before adding them to ensure that they were all the same to discount any changes effecting results.
- Using a water bath, in which the conical flask containing the reactants is placed. A water bath will help to keep the temperature uniform throughout the experiment. A thermometer must also be kept in both the water bath and the solution, to ensure any temperature change can be recoded and accounted for.
- Measuring the solutions more accurately by using a pipette instead of a measuring cylinder.
- Taking more readings to give a better average result.
Repeating the experiment with varying the concentration of the hydrochloric acid should give similar results and prove whether the procedure is reliable or not. The same procedure would be used but this time acid concentration is changed and sodium thiosulphate is kept constant.
The experiment can also be repeated a very high or low concentrations of hydrochloric acid solution to prove that when one of the reactant is finished the reaction stops very quickly and when a lot of it present it carries on. This would give evidence than the particle and collision theory was correct.