The reaction between Sodium Thiosulphate and Hydrochloric Acid.

Phil Scott 10V June 24th, 2003

Chemistry Coursework

The reaction between Sodium Thiosulphate and Hydrochloric Acid

Aim:

In this investigation, we are trying to determine the effect that changing the concentration of the hydrochloric acid has on the rate of reaction between sodium thiosulphate and hydrochloric acid. We have chosen to measure how long it takes before the solution becomes cloudy and the light beam is no longer detected through it. We can do this by using light gates attached to a computer that will record the results on a graph.

Prediction:

I believe that the greater the concentration of hydrochloric acid, the less time it will take before the light beam is no longer detected through the solution. When the acid is pure and isn’t diluted at all then the reaction should be the quickest. When the acid is the weakest then the reaction will be slower because the concentration of the molecules isn’t as much and so there is less chance of a successful collision to cause the reaction.

Scientific Information:

The rate of a reaction is the amount of a substance that takes part in a chemical reaction in a given time. Chemical reactions happen at widely different rates. The weathering of buildings and statues caused by acid rain is very slow, but the reactions that take place when a firework explodes are very fast. Most reactions take place at a rate somewhere between that of a firework explosion, which is almost instantaneous, and that of weathering stone. Speeding up useful reactions and slowing down harmful ones can be important in industrial production processes, and in activities such as preserving buildings or foodstuffs.

The rate of reaction can be calculated by using one of two formulae:

Rate of reaction = amount of product forming

time taken

Rate of reaction = amount of reagent reacting

time taken

There are four factors that change reaction rates: -

Temperature: -

All chemical reactions go faster at a higher temperature and if reacting substances are heated, the rate of the reaction usually rises. Temperature has a very noticeable effect, and can sometimes double the reaction rate. Many industrial processes use high temperatures to make reactions go fast. The faster the reaction, the quicker you get products, which make the process more profitable. However, there are limits because high temperatures are expensive because it means using a lot of energy. In practice, most industrial processes use moderately high temperatures rather than extremely high temperatures. Changing the temperature works backwards as well, so if reacting substances are cooled, the reaction slows down. In order to react, the particles in the substances must collide with each other. Heat gives them more energy to move around and so increases the chances of a collision. Also, when particles do collide, they are more likely to react, rather than just bounce off each other, if they are moving faster. Cooling has the opposite effects, and the particles slow down and are less likely to collide with each other and start a chemical reaction

Concentration: -

In any reaction involving solutions, a concentrated solution always reacts faster than a dilute one. ‘Concentrated’ means that the solution has a lot of solute dissolved in a particular volume. Increasing the concentration of reactants can have an effect similar to heating them, because the more particles present, the more likely a collision, and so the higher the reaction rate. Also like the temperature, it works backwards as well. If you reduce the concentration by diluting the solution, then it will take a lot longer to react because there aren’t as many reactants to collide and so there is less chance of a successful collision.

Surface Area: -

Surface area also affects the rate of a reaction. The larger the surface area the faster the reaction rate. If something has a very small surface area then it will take a long time to react because the particles are less likely to collide because there is a smaller area to collide with. If you were to chop up a large potato into very small pieces then the smaller pieces would react a lot faster because there is more area for the particles to collide and react with.

Catalysts: -

The final factor that affects the rate of a reaction is the presence of a catalyst. Catalysts are substances that alter the rate of a chemical reaction without getting used up. When a reaction has finished, the catalyst is still there. This makes catalysts very useful because they can be used again and again. Different reactions require different catalysts so you can’t just buy one and use it for any reaction. You have to buy a different one for each different reaction. Catalysts work by absorbing the molecules of reacting substances onto the surface of the catalyst. This brings the molecules of reactants closer together and weakens the bonds so that they break more easily. The reactants can then form new bonds to give the products of the reaction. Catalysts work best with large surface areas because they have to absorb lots of reactants onto their surface.

Catalysts don’t speed up the chemical reaction directly, but they lower the activation energy that is required to start the reaction. This means that more of the colliding particles will overcome the activation energy barrier and react, thus meaning the overall reaction is quicker. Activation energy is the minimum amount of energy with which two reacting particles must collide for a reaction to take place. Chemical reactions take place when reacting particles collide with an amount of energy greater than or equal to the activation energy.

The next diagram shows what the activation energy of a reaction looks like when represented on a graph. As you can see, the graph goes up before it starts to come down. This is because the reaction needs a certain amount of energy to break the bonds between the reactants before the reaction can occur and new bonds can be formed. The energy is absorbed, usually in the form of heat, to break the bonds, and then is given off again when the new bonds are made. This is an exothermic reaction because more energy is given off when making new bonds than is taken in when breaking old bonds.

To explain reaction rates, collision theory is used. Collision theory states that chemical reactions will occur when particles of the reactants collide. They must collide with a certain minimum energy, the activation energy. Collision theory is all about the chances of a successful collision taking place between reactants. If one particle collides with another one in the right place at the right speed with the right amount of energy then the reaction will occur. If it doesn’t, then nothing will happen and the particles will bounce off each other. That may seem like a lot to happen for ...

This is a preview of the whole essay

When particles collide they slow down, stop, and then fly apart again. This occurs regardless of whether they have enough energy to react or not. In an unsuccessful collision the particles bounce off each other and are unchanged, but in a successful collision, the activation energy barrier is crossed and the particles that separate from each other are chemically different from those that collided.

Variables:

My independent variable is the concentration of hydrochloric acid because it is the variable that I’m changing. By diluting the hydrochloric acid, the reaction will occur slower because there will be less hydrogen and chlorine molecules for the sodium thiosulphate to react with and so there will be less chance of a successful collision that will spark off the reaction.

My dependent variable is the time taken before the light is undetectable because it is what I’m measuring. As the hydrochloric acid reacts with the sodium thiosulphate, the solution will become cloudy and the light will no longer be visible through it. This will happen quicker if the hydrochloric acid is pure.

The volume of sodium thiosulphate used could affect how quickly the reaction occurs. If there is a bigger volume, then there will be more molecules in the solution and so a bigger chance of a successful collision. To keep the volume of sodium thiosulphate the same every time, I’m going to use a measuring cylinder so that I can make sure the volume is always 25cm3. I’m using 25cm3 of sodium thiosulphate so that I have the same amount of sodium thiosulphate as I do hydrochloric acid.

The concentration of sodium thiosulphate used could affect how quickly the reaction occurs. If there is a greater concentration of sodium thiosulphate then there will be more molecules in the given volume and so there will be a greater chance that the molecules will successfully collide with hydrogen and chloride molecules.

Reactions always happen faster at higher temperatures because the molecules are moving around faster and colliding with each other more often and so there is a bigger chance of a successful collision. In order to make my investigation and results accurate, I must make sure that all my experiments are conducted at the same temperature. To do this, I have decided to use a thermometer and water bath at 40ºC so that the solutions are always kept the same. I have chosen 40ºC because the molecules will be moving quickly enough to make the reaction happen at a fast rate, but they won’t be moving too quickly so that the reaction happens too fast.

The very low concentrations of hydrochloric acid may take a very long time to complete the reaction because there isn’t much reactant, but there are lots of water molecules. This means that the sodium thiosulphate particles are more likely to collide with the water and not start the reaction.

Safety:

Hydrochloric acid:

Hydrochloric acid is a corrosive liquid that may cause burns if it touches bare skin. The vapour from it is very irritating to the respiratory system. Solutions greater than 6.5 moles are corrosive and those greater than 2 moles are irritant. When hydrochloric acid is mixed with aluminium, magnesium, calcium, sodium, phosphoric (V) acid and sulphuric acid, a vigorous reaction occurs and hydrogen chloride gas is released. If mixed with potassium manganate (VII) then explosions can occur. If spilt then the area should be ventilated and the spillage should be covered with a mineral absorbent. To neutralise hydrochloric acid, sodium carbonate should be used.

Sodium Thiosulphate:

Sodium thiosulphate has minimal hazards but is harmful if ingested. If swallowed then you should drink plenty of water and seek medical attention. If spilt, add mineral absorbent and rinse area of spill thoroughly. To dispose of properly, sodium thiosulphate should be dissolved in five litres of water and washed away.

Sodium Chloride (salt):

Same as Sodium Thiosulphate

Sulphur Dioxide:

Sulphur dioxide is toxic by inhalation and mat cause burns if it comes in contact with the skin. It is irritating to the eyes and respiratory system. It can produce a choking gas that has serious affects on the lungs and could lead to bronchitis. Sulphur dioxide may trigger and asthmatic attack and people who suffer from asthma should avoid using it. If released, open a few outside windows and let the gas disperse into the outside air. To dispose of sulphur dioxide, use an efficient fume cupboard to vent the gas and make sure that the gas doesn’t return to other parts of the building.

Sulphur:

Sulphur is flammable and burns to form sulphur dioxide, which is explained above. Sulphur may be irritating to eyes and the respiratory system if inhaled as a dust. When sulphur is mixed with zinc, magnesium, aluminium, and alkali metals, very reactive mixtures are formed which can be very dangerous. If mixed with oxidising agents or metal oxides explosive mixtures are formed. If spilt, scoop up as much solid as possible and wash the area of spillage thoroughly. To dispose of sulphur effectively, mix with an equal volume of sand and dispose through normal refuse.

Method:

Equipment List:

Heat Proof Mat
Bunsen Burner
Tripod
Gauze
Test Tube Rack
Beaker filled with 100cm3 of water
Boiling tubes
Thermometer
25cm3 of Sodium Thiosulphate
25cm3 of Hydrochloric Acid (and water)
Computer
Light Sensor
Bulb
Battery
Wooden Box with computer

Instructions:

Set up the Bunsen Burner on the heatproof mat and position the tripod above the Bunsen Burner as shown in the diagram.
Place the gauze on top of the tripod and rest the beaker with 100cm3 of water in on top of the gauze. Use 100cm3 of water so that the contents of the boiling tube will be completely submerged in the water, but it won’t take too long to heat up.
Light the Bunsen Burner and heat the water to 40ºC, thus creating a water bath for the boiling tubes. Test the temperature using a thermometer to make sure it is at 40ºC.
While the water is heating, collect the 25cm3 of hydrochloric acid (and water) and the 25cm3 of sodium thiosulphate. Pour the sodium thiosulphate into one boiling tube, and the hydrochloric acid (and water) into another and place them in the test tube rack to keep them safe from spilling.
When the water is at 40ºC, place the two test tubes into the water and wait until they are both at 40ºC. Turn off the Bunsen Burner so that they don’t heat up anymore.
Turn on the computer and check that the light sensor is plugged in and working.
Place the bulb, battery, light sensor, and an empty boiling tube in the wooden box in the correct places. Turn on the bulb and line it up with the light sensor.
When both the solutions are at 40ºC, pour them into the boiling tube in the wooden box simultaneously and start the computer logger.
When the graph stops descending, or the total time you allow for the reaction is reached, stop the logger.
Save and print the graph so it can be used later.
Repeat steps 1-10 for the other concentrations of hydrochloric acid (and water).
Repeat the whole experiment to get more reliable results, but make sure they are still accurate by keeping everything the same as the first time you did the experiment.

Diagram:

Range and Repeats:

I’m conducting the experiment with five different intervals of the concentration of hydrochloric acid. I have decided that 25cm3 is the best volume of hydrochloric acid to use because it means that you can easily do five equal intervals without having to measure out difficult volumes. It is also a good volume because even when the hydrochloric acid is 100% pure, the reaction won’t go really fast, because the volume won’t big enough. My first experiment will be with 0.5 moles of hydrochloric acid and the 25cm3 of sodium thiosulphate. I will then conduct four further experiments using 0.4, 0.3, 0.2, and 0.1 moles of hydrochloric acid, whilst still using 25cm3 of sodium thiosulphate each time.

The ratios of sodium thiosulphate, hydrochloric acid, and water that I’m going to use are as follows:

I plan to time how long it takes before the light is no longer visible through the solution. The computer sensor will plot a graph for me as the light decreases.

I also plan to repeat each experiment three times and take the average result of the three different experiments. This will make my results more reliable, providing that the three results I get are quite close together. If the results for the experiments are far apart, then my data will not be very reliable and so my findings will not be as conclusive.

My results will show me how changing the concentration of hydrochloric acid affects the rate of reaction between it and sodium thiosulphate. They will tell me how long it took for the reaction to complete and the cross to become invisible through the solution. This will enable me to draw conclusions from my results and compare them to see if my prediction was correct.

Pretest:

In my pretest, I didn’t use the computer because it wasn’t set up, and so I conducted the experiment by drawing a cross on a piece of paper and timing how long it took before the cross became invisible. Although this wasn’t the way I conducted my real experiment, it gave me the idea of how much of each chemical to use and what concentrations to use so it did help me in that respect. In my pretest, I used 20cm3 of hydrochloric acid and 20cm3 of sodium thiosulphate, instead of using 25cm3 of both. This difference in the procedure was critical in the results that I obtained.

The results I obtained were:

Because I only used 20cm3 of hydrochloric acid, it meant that I had to measure out complicated volumes in the measuring cylinders, and I may not have measured them accurately enough to get accurate results. You can see this because the numbers of moles are nearly all two-digit numbers. The reason I chose to change my procedure is because I found it difficult to measure out the complicated volumes. I made the volumes 25cm3 instead of 20cm3 so that it is easier for me to measure the amounts. Apart from that one change, everything else in my procedure is exactly the same.

Results:

Unfortunately, the computer doesn’t tell you how long it takes before the light is no longer visible; all it does is plot a graph of the decreasing light intensity. The graphs that the computer produces are all I have as results, so I have included them. However, I have also tried to determine, by using the time scale, how long it was before the light intensity stopped decreasing. I have put these results into tables, and then plotted a graph of the final results.

Graph:

See graph paper at back of write up

Conclusion:

From my graph, you can see that there is a strong negative correlation between the time taken for the reaction to complete and the concentration of hydrochloric acid. This means that as the concentration of the hydrochloric acid is decreased, so is the rate of reaction. The graph also shows that the concentration of hydrochloric acid is inversely proportional to the time taken for the reaction to complete. This means that as the concentration of the hydrochloric acid is increased the time taken for the reaction to complete is decreased.

This ties in with the scientific explanation that I described in my experiment plan. Because the concentration is greater, it means that there are more particles and so the chances of collision are increased. If there is a greater chance of a collision then there is also a greater chance that the collision will be successful and that the reaction will occur. This means that the time taken for the reaction to complete is decreased. So, if you just cut out the middle bit, then it comes down to an increase in the concentration of the reactant means that the time taken for the reaction to complete will be less.

The scientific reason behind my results is known as collision theory. Collision theory states that if there are more reactants (a higher concentration) then there is a bigger chance of a successful collision, which will cause the reaction to occur. If there are less reactants (a smaller concentration) then there is a smaller chance of a successful collision because there are less particles for the other reactants to collide with and so the reaction is less likely to happen.

The results that I obtained and the conclusions that I have made from those results fully support the prediction that I made. In my prediction I suggested that the higher the concentration of the hydrochloric acid, the quicker the reaction between the hydrochloric acid and the sodium thiosulphate would be. My graph shows exactly those results. When the concentration of the hydrochloric acid was 0.5 moles, the reaction only took 43 seconds. Whereas, when the concentration of the hydrochloric acid was 0.1 moles, the reaction took 69 seconds. This shows that the concentration affects the rate of reaction by making the reactions take longer to complete when the concentrations are small.

Evaluation:

Overall, I think that my results are quite accurate. However, there is one anomalous result. The 2nd time that I conducted the experiment, my results for 0.2 moles of hydrochloric acid was incredibly different from the result I got the first time I did the experiment. I knew it was the 2nd result that was anomalous because the first result followed the trend of the other results I got the first time I did the experiment. The result from the 2nd time I did the experiment was 75 seconds for 0.2 moles of hydrochloric acid. I could tell this was anomalous because the result before it for 0.3 moles was 58 seconds, and the result after it, for 0.1 moles, was 69 seconds. There was a big jump up and a big jump back down again so I knew that the result was anomalous. If the result had continued the trend, then it would have been at 64 seconds and not 69 like it was. Although the difference is only 5 seconds, when conducting a scientific experiment, there should only be a difference of 1 second between the results. Anything more than a one-second difference, and the result is classed as anomalous.

I think that this anomalous result occurred because of a human error and not because of a fault with the computer. All the computer did was plot a graph of the light intensity as it decreased, which isn’t enough work for there to be a problem with it. I think that I made a mistake when setting up the experiment, and that is what caused the anomalous result. I don’t know exactly what I did wrong, but it could be one of several things. I may have started the computer logger to early and so the reaction would have started until about 10 seconds into the logging. This would mean that the reaction would go on for 10 seconds longer at the end, and would thus explain the anomalous result. I may have got the concentrations wrong and put too much water in or not enough hydrochloric acid in, which would mean the concentration was less than 0.2 moles and so the reaction would take longer to complete. It may have been that the particles weren’t heated to 40ºC like they should have been and so they wouldn’t have had as much energy and therefore the reaction would have taken longer because it would have taken the particles more time to collide with each other and cause a successful collision. I don’t know the real reason why this result is anomalous, but it could be one of these I mentioned or it could be something completely different.

Limitations in method:

Most of my procedure worked really well, but there were two main areas were I thought I really needed to change it. The first is the computer and the way in which it records the results. All it does is plot a graph as the light intensity decreases, and it doesn’t show you any times along the graph, so you have to guess by looking at the axis what the time was when the light stopped decreasing. I would like to change the program used so that it still draws the graph, but so that it also allows you to move along the graph and find out what the exact times were when the graph reached certain points. The second major problem with my procedure was the water bath. I just created a water bath using a beaker full of water and a Bunsen burner. This isn’t a problem if you want boiling water, but because the heat of a Bunsen burner is very difficult to control, it is almost impossible to get the water at the right temperature. When the water is at the right temperature, it has to be removed from the Bunsen flame so that it doesn’t heat up anymore. This means that the beaker is being placed in the cool air and so the temperature starts to drop. Keeping the solutions that I was heating at 40ºC was even harder because the temperature of the water bath kept dropping below 40ºC, so I ended up having my solutions heated to about 35ºC, this could account for the anomalous result. If the solutions weren’t heated up enough then the particles wouldn’t have had lots of energy and wouldn’t have been able to move around very quickly and so would have taken longer to collide successfully and cause a reaction. To fix this, I would use a water bath that allowed me to keep my solutions at the same temperature permanently. The water bath would need a thermistor so that if the temperature did drop, the resistance could be decreased and more heat allowed to flow into the water.

These were the two main problems that I encountered with my procedure and I would certainly change my method to eliminate them. If I used a water bath that allowed me to keep the solutions at the same temperature throughout the reaction, then I would have no need for the Bunsen burner, or any of the associated equipment (tripod, gauze etc.), and I could therefore remove it from my method. Another thing that I would change from my original method would be the wooden box that holds the light sensor, battery and bulb. When you place the reactants between the sensor and the bulb, some of the light still refracts around the glass tube and is picked up by the sensor. This means that the results won’t be entirely accurate because some light is still getting to the sensor, when none should. To fix this problem, a black piece of paper would need to be placed around the solution to stop any light getting to the sensor in any other way except through the solution. The final adjustment to my procedure that I would make would be what I mixed my reactants in. When I conducted the experiment I used a boiling tube, but I was finding that putting 50cm3 of liquid into the boiling tube was nearly making it overflow. To overcome this, I would use a larger beaker that could easily hold 50cm3 of liquid without there being a danger of it overflowing.

Reliability:

My results were very reliable, except for the result that I obtained for 0.2 moles of hydrochloric acid. All of my other results were within one or two seconds of each other, but the results for 0.2 moles of hydrochloric acid were 13 seconds apart. This is most probably a manual slip that I only made once. Because it is such a large difference and it only happened on this one result, it is unlikely to be to do with any of the equipment used. If it had been then there would probably have been other results that were so unreliable. Despite this one unreliable result that moved the average away from the curve of best fit, I still think that my other results are extremely reliable and are sufficient enough to support the conclusions that I made, and thus prove my prediction to be correct.

Further Experiments:

If I had the time and the equipment to do further research into the factors that affect rates of reaction, then I would probably conduct a very similar experiment, except that I would change it slightly by changing my independent variable to either temperature, or the concentration of sodium thiosulphate. After doing this, I would be able to draw a conclusion about which factor has the most effect on the rate of reaction between sodium thiosulphate and hydrochloric acid.

I would try to determine the effect of changing the temperature and the effect of changing the concentration of sodium thiosulphate on the rate of reaction between hydrochloric acid and sodium thiosulphate. Again, I would time how long it took for the reaction to complete by using light gates with a sensor attached to a computer. I would then see how it changed when I altered the temperature or the concentration of hydrochloric acid.

My prediction would be very similar to that of the experiment that I did. I would predict that the more concentrated the sodium thiosulphate was or the higher the temperature was, then the less time it would take before the light beam was no longer detected through the solution. When the sodium thiosulphate was pure and isn’t diluted at all or when the temperature is at its optimum level, then the reaction should be the quickest. When the sodium thiosulphate is the weakest or when the temperature is very low, then the reaction will be slower because the concentration of the molecules isn’t as much or the molecules aren’t moving around as quickly and so there is less chance of a successful collision to cause the reaction.

If I were really going to conduct the experiment, then my equipment list and method would be very similar to that of the experiment that I actually did.

My equipment list would include:

Heat Proof Mat
Bunsen Burner
Tripod
Gauze
Test Tube Rack
Beaker filled with 100cm3 of water
Boiling tubes
Thermometer
25cm3 of Sodium Thiosulphate
25cm3 of Hydrochloric Acid (and water)
Computer
Light Sensor
Bulb
Battery
Wooden Box with computer

I said I would change my method and get rid of the Bunsen burner and use a better water bath that allowed me to keep the temperature the same throughout the reaction. Unfortunately, to keep it a fair test, I would need to keep everything the same; otherwise, I wouldn’t be able to draw valid conclusions from my end results.

The method would also be very similar, but it would obviously have slight variations, depending on whether I was changing the concentration of sodium thiosulphate or the temperature. This method is for changing the concentration of sodium thiosulphate, but if it were for changing the temperature then all that would have to be changed would be the temperatures and volumes of chemicals.

Set up the Bunsen Burner on the heatproof mat and position the tripod above the Bunsen Burner as shown in the diagram.
Place the gauze on top of the tripod and rest the beaker with 100cm3 of water in on top of the gauze. Use 100cm3 of water so that the contents of the boiling tube will be completely submerged in the water, but it won’t take too long to heat up.
Light the Bunsen Burner and heat the water to 40ºC, thus creating a water bath for the boiling tubes. Test the temperature using a thermometer to make sure it is at 40ºC.
While the water is heating, collect the 25cm3 of sodium thiosulphate (and water) and the 25cm3 of hydrochloric acid. Pour the sodium thiosulphate (and water) into one boiling tube and the hydrochloric acid into another and place them in the test tube rack to keep them safe from spilling.
When the water is at 40ºC, place the two test tubes into the water and wait until they are both at 40ºC. Turn off the Bunsen Burner so that they don’t heat up anymore.
Turn on the computer and check that the light sensor is plugged in and working.
Place the bulb, battery, light sensor, and an empty boiling tube in the wooden box in the correct places. Turn on the bulb and line it up with the light sensor.
When both the solutions are at 40ºC, pour them into the boiling tube in the wooden box simultaneously and start the computer logger.
When the graph stops descending, or the total time you allow for the reaction is reached, stop the logger.
Save and print the graph so it can be used later.
Repeat steps 1-10 for the other concentrations of sodium thiosulphate (and water).

Repeat the whole experiment to get more reliable results, but make sure they are still accurate by keeping everything the same as the first time you did the experiment.

This method is almost identical to that of the experiment that I actually conducted. This is because all of the equipment would be exactly the same and the set up would be the same. The only things that are changed are the concentrations of sodium thiosulphate and hydrochloric acid and the temperature that the reaction is conducted at.

Page of