A catalyst is a substance which affects the rate of reaction but is chemically unchanged at the end of the reaction. Higher temperature and greater surface area make them more effective. It is possible that the catalysts speed up reactions by facilitating a different reaction with lower activation energy than the uncatalysed reactions. This results in a higher proportion of collisions being successful, leading to a higher reaction rate. Many catalysts work by providing a surface on which other molecules or atoms can react. However, others work in more complex ways.
When one or more of the reactants is a solid, the more finely powdered the solid, the greater is the rate of reaction. This is because reactions involving solids take place on the surface of the solids. A solid has a much larger surface area when it is powdered than when it is in larger pieces. For reactions involving two solids, grinding the reactants means that they can be better mixed. The mixed powders are then in greater contact with each other and more likely to react. If a solid is being reacted with a liquid (or solution), the greater the surface area, the more the solid is exposed to the liquid.
Before I start doing the real experiment I did a short experiment to find out what happen to the concentration of Hydrochloric acid and sodium thiosulphate solution in the experiment and also to find out what concentration of Hydrochloric acid should I use for my investigation.
The result shows that 0.1 M and 0.2 M concentration of Hydrochloric acid with 0.3 M of sodium thiosulphate, there were no reaction as this take too long for cross to disappear. However, in 0.3 M of Hydrochloric acid with 0.3 M of sodium thiosulphate, they react but it took too long for cross to disappear. So, I increase the concentration of sodium thiosulphate to 0.5 M.
This investigation was useful to me because instead of doing the same experiment I would be able to change the concentration for the real experiment rather than doing the same thing. However, I would not have much time to do the same experiment. Consequently, this experiment helped me chose the right concentration of Hydrochloric acid for my real experiment which I will the investigation so this won’t be time consuming.
Prediction
My prediction is if I increased the concentration of hydrochloric acid the rate of reaction increases because not all particles react they collide. They must have a certain minimum energy to react when they collide. At a higher temperature, the particles have more energy so the react more often when they collide. Also, if I double the concentration then I will expect to see the rate of the reaction double. This is because there are twice as many particles in 1M hydrochloric acid than in 0.5M hydrochloric acid, so there are twice the amount of collisions per seconds and because there are twice as many successful collision per second, increasing the rate of reaction. For the mean time graph will be curve, this will show the rate of reaction will increase and the time taken for the cross to disappear will be short. The reaction speeds up when the concentration is higher. A graph of 1/time against concentration would show how the rate increases with a rise in concentration. If I plotted such a graph, the curve obtain will be a straight line.
How make test reliable?
I will be able to vary the concentration of hydrochloric acid and I will keep the concentration of sodium thiosulphate same. In this experiment and this is the only factor I will change so I can keep this experiment fair. I will not vary the experiment using the catalysts because there are none available to. Furthermore, it’s very hard to control it in the experiment and measure it. I will not vary the temperature as my result would inaccurate because i will have to heat the solution to certain temperature for every time I do the experiment in my time limit and I will have to make the solution goes down or if the room temperature is different hence changing of the equipment, for these reasons the result might not be reliable. Additionally, I will not use surface area because one of the reactant should be solid. Also I will not use pressure because to increase the pressure you have to use gases.
Safety
The things that I will need to do to keep my experiment safe for myself and other students around me are as follows:
- I will wear goggles to protect my eyes
- I will wear a lab coat to protect my clothes and skin
- I will tuck my stool out of the way, so no one can trip over it.
- I will stand up whilst doing my experiment, so that just in case if something spills, I will be able to get away fast
- I will not leave the apparatus at the side of the table, so it cannot get knocked over.
Plan
The reaction between sodium thiosulphate and hydrochloric acid is well known and has been used to study the rates of reaction. When these chemical are combined they form a sulphur precipitate in the form of a suspension which turns the transparent opaque.
Hydrochloric + Sodium thiosulphate sodium + water + sulphur + sulphur acid chloride dioxide
HCl (aq) + Na2S2O3 (aq) NaCl (aq) + SO2 (g) + S (s) + H2O (l)
A simple set of experiments may be performed to determine the effect of concentration on the rate of reaction. By using a conical flask with a cross marked piece of paper positioned underneath it, then add 25cm3 of sodium thiosulphate and then add 25 cm3 of
0.3 mole of hydrochloric acid then start the stop watch and time the duration of the cross to disappear. The pattern will disappear after a specific quantity of sulphur has formed
The faster the reaction, the shorter the length of time during which the cross is invisible. The experiment is carried out several times with solution of hydrochloric acid to different concentrations. The solution of sodium thiosulphate and conditions of the experiments remains the same, only the concentrations of hydrochloric acid are change. A graph can then be plotted of the time taken for the cross to disappear against concentration
The factor that could affect the rate of reaction of my experiment is concentration of hydrochloric acid. This could affect the rate of reaction because the higher the concentration of hydrochloric acid then the more acid particles per 25 cm3 so more collisions per second and then there will be more successful collisions per second.
Apparatus
- Goggles
- stop clock
- Conical flask
- 25 cm3 measuring cylinder
- 0.5M sodium thiosulphate solution
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Paper and marker (to draw a cross)
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Five different concentrations of hydrochloric acid
(0.3, 0.5, 0.7, 0.9, & 1.00M)
Methods
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Wear a goggle and a lab coat. Draw a dark cross on a piece of paper.
- Measure 25 cm3 of 0.5M sodium thiosulphate put it into conical flask.
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Measure 25cm3 of 0.3M of hydrochloric acid. Pour the acid into flask,
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Start the stopwatch. Time how long it takes for the cross to disappear.
- Wash out the flask thoroughly.
- Do each experiment three times
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Repeat the experiment using the same concentration of sodium thiosulphate but different concentration of hydrochloric acid
- Finally, calculate the rate of reaction
1/mean time
After that I will plot a graph for mean time and the rate of reaction (1/mean time)
This diagram shows the experiment between sodium thiosulphate and hydrochloric acid.
This can be studied by following the appearance of the precipitate. The cross drawn on the paper grows fainter with time. Time how long it takes for the cross to disappear.
Result
Error
- There is an outlier in 0.7 M and 0.1M of hydrochloric acid which is shaded in grey; it is so different from the others, something must have gone wrong. Maybe because inaccurate measuring, reading or recording of the results.
Conclusion
The rate of reaction is directly proportional to the concentration of the reacting substances.
Increasing the number of concentration will cause more frequent collisions and hence increase the rate of the reaction.
The first graph shows two important points:
- The cross disappears quicker at higher concentration. The shorter the time needed for the cross to disappear, the faster the reaction.
- The curve is not a straight line
These results show that:
The rate of reaction increases when the concentration of the reaction is increased.
My results table and graph show that when I increase the concentration of hydrochloric acid, the initial rate of reaction also increases. For instance, the result in the table shows 0.3 M of hydrochloric acid took 84 second for the cross to disappear and 0.1 M of hydrochloric acid took 308 second for cross to disappear. Altogether, I tested 5 different concentration of hydrochloric acid 0.1M, which was the lowest concentration of acid that I used, produced the slow rate of reaction. 1M hydrochloric acid, which was the highest concentration that I used, produced the faster rate of reaction. I repeated all 5 concentration three times to be sure that they were reliable results and in all cases the higher the concentration the higher the rate of reaction. I stated this in my prediction.
In the first graph I put the range bar of each concentration like for example: the range of 0.1 M is between 303 to 311 second. However, the longest range bar is 0.3 M which is from 72 to 94 second and the shortest range bar is 1.0 M which from 18 to 19 seconds.
Furthermore, I also stated in my prediction that if I doubled the concentration from 0.5M to 1M hydrochloric acid then the rate of reaction will also be doubled.
As you can see, the greater the concentration of the acid used in a reaction the steeper the curve and the shorter the reaction time. Hence, these results show that an increase in concentration increases the rate of a reaction.
The second graph plot graph shows that the time taken for the completion is inversely proportional to the concentration of the hydrochloric acid.
Rate of reaction x 1/time
This means that as the hydrochloric acid concentration is increased the reaction time decreases because the rate increases.
Therefore I know that:
- The reaction rate is inversely proportional to the time taken.
- The reaction time is inversely proportional to the concentration.
Therefore I can conclude that the reaction rate is proportional to concentration.
Moreover, the graph shows a positive correlation as the concentration of hydrochloric acid increases the rate of reaction increases. However, there is some point that are away from straight line as it shown in the second graph which 0.7 M and 1.0 M.
The reaction between sodium thiosulphate solution and hydrochloric acid. When a dilute acid is added to sodium thiosulphate solution, a pale yellow precipitate of sulphur is formed.
One molecule of sodium thiosulphate and 2 molecule of hydrochloric acid produce two molecule of sodium chloride, one molecule of water, one molecule of sulphur and one molecule of sulphur dioxide.
Evaluation
My method helped me make my experiment successful. Without it I would be confused and lost, it helped me make sure my experiment was carried out to my best ability. My experiment that I carried out was fair. All my measurement were accurate, as I doubled-checked them. However, I have done a preliminary work to know which concentration of hydrochloric acid and sodium thiosulphate to use for reaction to occur to make the cross to disappear, 0.1 and 0.2 M concentration of Hydrochloric acid with 0.3 M of sodium thiosulphate there Was a reaction them but it took long to react. So as I increase the concentration the time gets shorter.
There was an outlier in 0.1 and 0.7 M concentration of Hydrochloric acid which is different from the other maybe because inaccurate measuring, reading or recording of the results. So I have done the experiment fourth times to improve my results.
If I could do this experiment again, this should hopefully give me an extra set of results that prove my results are reliable and my prediction is right. This all depends if I have more time to do so.
I believe that the experiment was designed well but there were a few problems. Although the initial rate of reaction ( which is what I am concerned with in this investigation) seemed to fit a trend, the rate of reaction curves of all concentration on the graphs crossed only 0.6 M concentration acid didn’t crossed. The reason is that these anomalities could occurred, is that I could not measured the acid or sodium thiosulphate out correctly from the measuring cylinder. Also, I could have started the stop watch slightly after or before I actually should. In the second graph the only concentration that I could make straight line is 1.0 M concentration of hydrochloric acid. However, it is a strong positive correlation which mean that as I increase the concentration the rate of reaction increases.
The result on the first graph which is a curve graph there was only one problem which on 0.6 M, it is not going with the curve line. And on the second graph there is two point which are o.7 M and 1.0 M.
I conducted all experiment for each concentration at the same time to save time. An error in my graphs (plotting, drawing curves) could have also affected the calculated rates of reaction.
The information that I collected could be adequate to support my conclusion fully. However, I could collect more evidence by doing more repeats and maybe doing more different concentration of hydrochloric acid. Additional work, which could be carried out, is to repeat the experiment using, a wider range of concentrations.