Investigate factors that may affect the rate of reaction of sodium thiousulphate when added to hydrochloric acid

6. Light - Some chemical reactions take place faster when they absorb light.

To investigate this problem i will carry out an experiment looking at the effect of temperature on the above reaction.

Variables

Independent - I am going to change the temperature of the sodium thiosulphate solution.

Dependant - The time taken for the solution to go cloudy, when hydrochloric acid is added. This will be measured by timing how long it takes a cross marked on paper underneath the solution to disappear.

Controlled - Concentration of sodium thiosulphate, concentration of acid (which will be 2mol/dm3), temperature of acid, volume of sodium thiosulphate, volume of acid, pressure.

As temperature is the only that will be changed I will keep the other factors the same by doing the experiment in the same room, thus keeping the temperature and pressure etc. constant.

Prediction

I predict that as the temperature is increased the time taken for the reaction will decrease. Therefore the rate of reaction will increase. This means that the graph drawn up in my analysis will have positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the temperature is increased. This can be justified by relating to the collision theory. When the temperature is increased the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully. If solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur. All this can be understood better with full understanding of the collision theory itself:

For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions remains the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area.

Range

The temperature range I will use for the sodium thiosulphate solution will be from 30 to 70 (oC) at 10(oC) intervals. I have chosen to start at 30 because room temperature will be around 21 degrees and I is much more convenient to start a higher temperature rather than a lower one. From my own scientific knowledge I think that if the temperature is increased much higher than 70 the reaction will occur too rapidly.

The following apparatus will be used in my experiment:

thermometer

5 beakers (one for each temperature of sodium thiosulpahte solution)

burette

measuring cylinder

volumetric flask

tripod

gauze

heatproof mat

stop clock

Bunsen burner

Cross marked on paper

pair of tongs

pair of goggles

Method

* 5 grams of sodium thiosulphate is measured using a balance.

* The sodium thiosulphate is then added to 250cm3 of distilled water (distilled water is used incase any impurities in normal water would affect the results., in beaker, to form sodium thiosulphate solution. The beaker is stirred with a glass rod to help sodium thiosulphate dissolve. This solution is then poured into a volumetric flask and shaken at least 7 times to help the sodium thiosulphate dissolve.

* Rinse the solution through a burette then fill the burette. (A burette is used for accuracy in pouring off exact amounts of solution.) Pore off 40cm3 of solution into a beaker.

* Place the beaker on a tripod. Heat with Bunsen until desired temperature is reached. Check temperature using a thermometer.

* Remove beaker from heat using tongs and place on a cross marked on paper.

* Measure 10cm3 of 2mol/dm3 Hydrochloric acid in a measuring cylinder and then add it to the sodium thiosulphate solution in beaker.

* Using a stop clock record the time it takes for the cross to disappear.

* Record results in a table.

* The experiment is repeated for all temperatures and the entire temperature is repeated.

Repeat results and averages will be taken to improve the credibility of the findings and present solid grounding for the final conclusion. The repeat results will help iron out any anomalies and the average will give a good summary of the results of the experiment. However if one set of results is entirely different to the other, a third experiment will be produced to replace the anomalous set of results.

Two graphs will then be drawn so that my findings are clear; they will show temperature against average time for reaction and temperature against rate of reaction. (Which is proportional to 1/the time taken for the reaction to take place).

Safety

Standard laboratory safety regulations will be followed. A pair of goggles will be worn during the heating part of the experiment in order to protect the eyes. When handling hot beakers and measuring cylinders a pair of tongs will be used. A gauze and heatproof mat will be used while heating to avoid any damage to the equipment.

Fair Test

In order for my findings to be valid the experiment must be a fair one. I will use the same standard each time for judging when the X has disappeared. I will make sure that the measuring cylinders for the HCl and thiosulphate will not be mixed up. The amount of HCl will be 10cm3 each time, and the amount of thiosulphate will be fixed at 40cm3. During the heating stage of the experiment, a blue flame will be used throughout. Also the same Bunsen burner and gas tap will be used to maintain continuity. All of these precautions will make my final results more reliable and keep anomalies at a minimum so thus make the entire investigation more successful.

Results

Temp. (oC)

Time 1 (s)

Time 2 (s)

Time 3 (s)

Average Time (s)

Rate of reaction

30

39.1

39.2

39

39

0.03

40

26.41

25.66

26.11

26.06

0.04

50

9.2

8.8

9

9

0.05

60

1.85

0.47

1.45

1.39

0.09

70

7.47

7.89

8.05

7.8

0.13

Analysis of results

In this experiment I have found that as the temperature of the sodium thiosulphate solution is increased the time taken for the reaction to take place decreases. This means the rate of reaction increasers as it takes less time for a reaction to take place, so more take place per second. There is no real trend between the average time taken for the reaction at each temperature, although they decrease they do so by random amounts. Again there does not seem to be a trend between the rate of reaction at each temperature, although for 30, 40 and 50 (oC) the rate of reaction increases by 0.1 each time but at 60 and 70 (oC) the difference increases to 0.4

Using the graphs, with lines of best fit, I can draw a conclusion from my experiment. Firstly I can see that with the "time" graph (that plots temperature against time taken for the reaction to take place) the graph has negative correlation, meaning that as the temperature increased the time taken for the reaction to take place decreases. Secondly with the rate of reaction graph (that plots rate of reaction against temperature) it has a positive correlation, meaning that as the temperature is increased so is the rate of reaction,

This is because when the temperature is increased the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully.

My results prove my prediction to be correct

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Anomalies

For some reason the results for 50 (oC) seem to be incorrect. I think this is due to human error ie. It might not have been heated to exactly 50 (oC), incorrect volumes of solution may have been used, the stop clock may not have been started on time.

Evaluation

The procedure used was good and produced good results but it could have been improved. Improvements that could be made if the experiment was repeated:

. Try to measure out the quantities of all substances required more accurately as human error could be the cause for the anomalous result.

2. Measure the temperature of the sodium thiosulphate solution more accurately. Use a water bath heated to the required temperature and place the solution into it. The water bath will change the solution to the required temperature.

3. Gain a wider range of results by carrying out the experiment at more temperatures. This will help trends become even more visible.

4. Use a more accurate way of determining when the solution is completely opaque. A light sensor could be used and connected to a computer it could time each reaction to the same point, making the results more accurate.

All in all I think this was a good experiment and the best that could have been done with the time and resources available. The results supported my predictions and they seem to be fairly reliable results.