The aim of this coursework is to investigate the rate of reaction between sodium thiosulphate solutions and dilute hydrochloric acid.

GCSE CHEMISTRY COURSEWORK

Aim

Introduction

In this investigation, I am going to be looking at the reaction between sodium thiosulphate and hydrochloric acid. When they react, a yellow precipitate of sulphur is formed. The diagram below shows the different stages of the reaction taking place. This reaction can be timed by measuring how long it takes the precipitate to make the mixture opaque.

Solutions are mixed together Precipitate is formed Solution is now opaque

The word equation and formula for this reaction is:

Na2S2O3 (aq) + 2HCl (aq) → 2NaCl (aq) + S (s) + SO2 (g) + H2O (l)

sodium thiosulphate + hydrochloric acid → sodium chloride + sulphur + sulphur dioxide +

water

Rate of Reaction

The rate of reaction is the speed of the reaction between two particles. It is how quickly a reaction takes place. The rate of a reaction can be measured by measuring the rate that a reactant is used. Temperature, concentration, pressure of reacting gases, surface area of reacting solids and the use of catalysts are all factors which affect the rate of reaction.

Chemical reactions can only happen if the reactant particles collide with enough energy. The more frequently particles collide and the greater the proportion of collisions, the greater the rate of reaction.

There are two ways to measure the rate of a reaction:

Measure the rate that a reactant is used up
Measure the rate that a product is formed

The method chosen depends upon the reaction being studied. Sometimes it is easier to measure the change in the amount of a reactant that has been used up, and sometimes it is easier to measure the change in the amount of a product that has been produced.

The measurement itself depends upon the nature of the reactant or product:

The mass of a substance (solid, liquid or gas) is measure with a balance.
The volume of a gas is usually measured with a gas syringe, or sometimes with an upside down measuring cylinder or burette.
You could possibly record the mass or total volume at regular intervals and to plot graph.

The readings go on the vertical axis and the time goes on the horizontal axis.

Collision Theory

A chemical reaction can only occur between particles when they collide. These particles may be atoms, ions or even molecules.

There is a minimum amount of energy which colliding particles need in order to react with each other. If the colliding particles have less than this minimum energy, then they just bounce off each other and no reaction occurs. This minimum energy is called the activation energy.

The faster the particles are going, the more energy they have. Fast moving particles are more likely to react when they collide. There are several ways in which you can make the particle move quicker.

Increasing the temperature makes the particles move faster. This means that they will collide more frequently with each other and the rate of reaction will increase. Also, if the particles are moving faster, more of them will have the required activation energy for the reaction to occur. Generally, raising the temperature of a reaction by 10°C, will lead to the rate of reaction doubling.

When we increase the concentration of a substance in a solution, there will be more particles per cm3 of that particular substance. The more particles there are in the same volume, means that the particles will be closer together. This means that the particles will again collide more frequently with each other and the rate of reaction will increase.

A solid in a solution can only react when the particles collide with its surface. The bigger the area of the solid surface, the more the particles can collide with it, in a certain period of time. This results in an increase in the rate of reaction. You can increase the surface area of a solid by breaking it up into smaller pieces. A powder has the largest surface area and will have the fastest rate of reaction. This is why catalysts are often used as powders.

A catalyst will change the rate of reaction. In most cases, the catalyst is used to make a reaction go faster. The catalyst itself does not take part in the reaction as a reactant. It is a chemical substance which speeds up a chemical reaction, and remains chemically unchanged at the end of the reaction. A catalyst is usually a transition metal, a transition metal oxide or an enzyme in living cells.

It works by providing a convenient surface for the reaction to occur. The reacting particles gather on the catalyst surface and collide more frequently with ...

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It works by providing a convenient surface for the reaction to occur. The reacting particles gather on the catalyst surface and collide more frequently with each other. This results in a reaction between the particles. A catalyst also lowers the activation energy for the reaction.

Preliminary Work

Our first objective is to determine how we are going to measure the rate of reaction and what our independent variable is going to be. We decided that we would change the concentration of one solution and not investigate the effect of temperature on the rate of reaction

In the preliminary work, we took three conical flasks, and filled them with various amounts of sodium thiosulphate (Na2S2O3), hydrochloric acid (HCl) and water (H2O). Each conical flask was placed above a cross on a piece of paper. After adding the solutions, we timed how long it took for the solution to go opaque, thus making the cross invisible.

In the first conical flask, we filled it with 10 cm3 of hydrochloric acid and 10 cm3 of sodium thiosulphate. In the second conical flask, we decided to dilute the hydrochloric acid. Therefore we filled it with 10 cm3 of sodium thiosulphate, 5 cm3 of hydrochloric acid, 5 cm3 of water. We filled the third conical flask with 10 cm3 of hydrochloric acid, 5 cm3 of sodium thiosulphate and 5 cm3 of water. The purpose of this was to dilute the sodium thiosulphate.

After we had finished the preliminary experiment, we used our results to work out the rate of reaction for each conical flask. Here are the results of the preliminary experiment:

The first conical flask, with 10 cm3 of hydrochloric acid and 10 cm3 of sodium thiosulphate, is like the control. This is where there are equal amounts of both solutions. When we halved the concentration of hydrochloric acid, in the second conical flask, we can see that it has little effect on the rate of reaction, and that there is little change from the rate of reaction of the first conical flask. However, in the third conical flask, when we halved the concentration of sodium thiosulphate, we can see that the rate of reaction halves aswell.

This means that for the main experiment, we are going to change the concentration of sodium thiosulphate, and keep the concentration of hydrochloric acid constant. This will accurately show the effect of concentration on the rate of reaction.

In the main experiment, the total volume is going to be 60 cm3. In order to change the concentration of sodium thiosulphate, we will use various amounts of water, to dilute the solution. The amount of HCl will remain constant at 10 cm3. The table below shows the volumes of the different solutions and the concentration of sodium thiosulphate aswell.

The preliminary work is as important as the main investigation, as it allows you to set the parameters, necessary for the investigation. It has not only given me an idea of what method I will use, but more importantly it has helped me decide the range and extent of solutions, that are required for the experiment. I found that it was difficult to measure precise volumes of solutions using measuring cylinders; therefore I will try to find a way of improving this. The preliminary work also allows me to make a hypothesis of the main investigation.

Prediction

In the reaction between sodium thiosulphate solution and hydrochloric acid, a yellow precipitate of sulphur is formed. I predict that as you increase the concentration of sodium thiosulphate; the solid sulphur will be produced more quickly and there will be less time before the cross can be seen.

As the experiment continues, the more yellow sulphur precipitate is formed, thus causing the solution to become opaque.

Changing the concentration of a solution affects the rate of reaction. The concentration of a solution is how strong the solution is. For example, a more concentrated acid contains more acid particles and less water particles than a less concentrated acid.

Low Concentration High Concentration

In order for any reaction to happen, particles must first collide. Increasing the concentration will increase the number of particles colliding. By putting more particles into the reaction, the chance of them colliding increases and so the rate of reaction increases. I predict that by doubling the concentration of sodium thiosulphate, the rate of reaction will double, because there are twice as many particles in the solution, which means that there will be more collisions.

In a less concentrated acid, the number of collisions is low, so the rate of the reaction is slower. With a more concentrated acid, the number of acid particles is greater, so the number of collisions is greater and the rate of the reaction is faster.

Equipment List

250 ml Conical Flask – where the reaction will take place.

10ml Pipette – to measure the amount of HCl needed (10ml) precisely. It is more accurate compared to a beaker or measuring cylinder.

Pipette Filler – this is attached to the pipette.

50 cm3 Burette (x2) – this will accurately measure the various volumes of sodium thiosulphate and distilled water. It is more accurate compared to a beaker or measuring cylinder.

100ml Beaker (x2) – to leave at the bottom of the burette, so that the various solutions can be collected.

Paper

Marker Pen

Goggles

Stop Clock

630ml of 0.15M Sodium Thiosulphate Solution (Na2S2O3)

180ml of 0.1M Dilute Hydrochloric acid (HCl)

450ml of Distilled Water (H2O)

Method

Collect equipment and set it up as shown in the diagram.

Using the marker pen, draw a cross on the piece of paper. Place the paper on the bench and put the conical flask on top of it.

Collect some sodium thiosulphate solution and using the burette, measure 60ml of sodium thiosulphate and put it in the conical flask.

Take the pipette and measure 10ml of hydrochloric acid. Pour the acid into the flask and start the stop clock. Time how long it takes for the solution to go opaque, thus making the cross disappear.

Record the result and wash out the flask completely, making sure all the solution has been rinsed from the conical flask.

Repeat the experiment, using the other volumes of sodium thiosulphate and water, as shown in the blank results table. Make sure the volume of hydrochloric acid remains constant (10ml).

Once all the other volumes have been completed, repeat the experiment twice more and record all the results. Work out any averages and more importantly the rate of reaction.

Variables

The independent variable is going to be the concentration of sodium thiosulphate, as this is what we are going to change.

The dependent variable is the time it takes for the solution to go opaque (rate of reaction), as this is what we are going to be measuring.

The controlled variables are going to be the:

Volume of HCl
The cross on the paper
Size and dimensions of the conical flask
Total volume of Na2S2O3 and H2O

The uncontrolled variable is the room temperature.

Safety

Goggles must be worn, as we will be working with acid and solutions. Hydrochloric acid is an irritant and can be corrosive.

Amendment to Method

During our investigation, we found that when using the pipette, the hydrochloric acid, took too long to dispense into the solution. This meant that by the time, all the hydrochloric acid had been poured into the sodium thiosulphate solution, the reaction had already started. This would probably affect the rate of reaction. Therefore instead of using a pipette, we are going to use a measuring cylinder. Although it gives us a slight volume error, i.e. the volume may not exactly be 10ml, it doesn’t give us a huge timing error, and all the hydrochloric acid will be able to react.

Analysis

From the results, we can see that as the concentration of sodium thiosulphate decreases, the time it takes for the cross to disappear increases. When the concentration of sodium thiosulphate was 0.150M, the average time it took the cross to disappear was 27 seconds. At 0.125M, the average time for the cross to disappear was 33 seconds. At 0.100M, the average time was 42 seconds, whilst at 0.750M; the average time was 58 seconds. The average time it took for the solution to opaque, at 0.050M was 95 seconds. It took 179 seconds for the cross to disappear at 0.025M concentration of sodium thiosulphate. This proves that as the concentration decreases, the average time for the solution to go opaque increases. In the graph titled “concentration of sodium thiosulphate against average time”, the trend clearly seems to show that this is the case. This means that it takes less time for the cross to disappear, when the concentration of sodium thiosulphate is higher.

The rate of reaction was also affected. The results tell us that, as the concentration of sodium thiosulphate decreases, the rate of reaction decreases. This suggests that the concentration of sodium thiosulphate is directly proportional to the rate of reaction. The trend of the graph shows that rate of reaction is directly proportional to the concentration of sodium thiosulphate. The pattern clearly seems to be, that as the concentration of sodium thiosulphate increases by 0.025M, the rate of reaction increases by 0.006. This shows that not only are the results accurate, but it also justifies my prediction.

In my prediction, I stated that in a low concentrated solution, the rate of reaction will be much slower compared to the rate of reaction of a much higher concentrated solution. I also predicted that when the concentration of sodium thiosulphate doubles, the rate of reaction will also double.

I think that I fully justified my prediction. My results showed that as the concentration of sodium thiosulphate increases, the rate of reaction increases, thus causing the average time it takes for the solution to go opaque, decrease. In most cases, when the concentration of sodium thiosulphate doubled, the rate of reaction also seemed to double. For example, when the concentration was 0.050M, the rate of reaction was 0.011, and when the concentration was 0.100M (double), the rate of reaction was 0.024, which is almost double. Although it is not exact, we can generalise the trend, to say that as we increase the concentration of sodium thiosulphate by a certain amount (i.e. 0.025M), the rate of reaction also increases by a certain amount (i.e. 0.006).

I therefore conclude that the concentration of a solution affects the rate of reaction. In this case, when the concentration of the sodium thiosulphate increased, the rate of reaction also increased. The higher the concentration of sodium thiosulphate, results in less time for the solution to go opaque.

The rate of a reaction is the speed at which a reaction happens. The longer it takes, the lower the rate of reaction, and the quicker the reaction, means that the rate of reaction will be greater. The rate of reaction is linked with the collision theory. Collision theory states, that the more collisions there are in a reaction, the more chance the molecules will combine. If there are a higher number of collisions, the reaction will not only go faster, but the rate of reaction will be much higher.

There are several factors which affect the number of collisions in a certain reaction. Temperature, concentration, pressure, surface area and catalysts, all affect the number of collisions which occur in a certain reaction. In this experiment, we decided to investigate the effect of concentration on a reaction between sodium thiosulphate and hydrochloric acid.

When we talk about the concentration of a solution, we generally look at the amount of a specific substance in that solution. When the concentration of a solution is high, there is greater amount of particles in it, compared to the amount of particles in a lower concentrated solution. Therefore if we increase the concentration, we increase the number of particles in the solution. If we take the collision theory into account, we know that when there are more particles in a reaction, there are likely to be more collisions, which results in a greater rate of reaction.

In this investigation, by increasing the concentration of sodium thiosulphate, we in fact increased the number of particles in the solution. This meant that in the reaction, there were more collisions, which sped up the time it took for the solution to go opaque. This is why the rate of reaction increased, as the concentration of sodium thiosulphate increased.

Generally it is concluded, that the greater the concentration the greater the rate of reaction.

Looking at my results in general, I do not believe that there are any excessive anomalies; however I will be looking at this more closely in my evaluation.

Evaluation

In my opinion, I believe that the investigation was very successful. I justified my prediction and gained the results expected. This is a summary of the results gained:

When the concentration of sodium thiosulphate was low (i.e. 0.025M, 0.050M), the rate of reaction was low.
When the concentration of sodium thiosulphate was high (i.e. 0.125M, 0.150M), the rate of reaction was fairly high.
In general, when the concentration of sodium thiosulphate was increased, the rate of reaction also increased.

I think that the procedure that I used helped me achieve accurate and precise results. The fact that we repeated the experiment three times, made sure that our results were reliable and it helped identify any anomalies. Repeating the experiment always helps, as we can find an average and it eliminates any results that may not seem accurate. By using the burette, we were able to measure precise volumes of the various solutions. This particular apparatus was the most accurate and successful, as we could easily measure the required volumes. In general, I believe that the method used was successful; however we did encounter a few problems.

When we started our experiment, we found that when using the pipette, the hydrochloric acid took to long to dispense, thus causing an error in our results. Therefore we had to resort to using a measuring cylinder, which although it gave us a slight volume error, there was no timing error. This meant that we had to make an amendment to our method. The use of a measuring cylinder could have possibly affected out results, although it didn’t seem to be too drastic.

I think that we could possibly improve the experiment by:

Finding an alternative method to using a measuring cylinder and pipette
Finding a way of starting as soon as the reaction starts
Knowing exactly when the reaction has finished; as it was hard to judge precisely when to stop timing. We could possibly use a light sensor.
Varying the number of repeats, to get more accurate results.

On the whole, I was not able to find any anomalies, or results which seemed to be odd, although I think that we could look at any individual results to see if we can find any minor anomalies. In order to do this, I am going to plot a graph, which shows all three repeats. This will allow me to compare the results individually, and see if there are any results which stand out.

The circled points on the graph are the ones which stand out from the rest of the results. At 0.025M and 0.050M, there seems to be the greatest difference in the set of results, where one result in each case, seems to be separated from the rest. Although, this is the case, the average time isn’t affected that much. All the other set of results are more accurate, and the closer together they are, the more reliable they are.

Although this is the case, I believe that the results are still good enough to support my conclusion, as these are only minor anomalies.

Further experiment

I think that we could extend the experiment, by looking at how the other factors affect the rate of reaction, and see what happens to rate of reaction when we possibly have two independent variables, such as temperature and concentration. We could also what the effect of changing the hydrochloric acid is, in a full investigation, although according to the preliminary experiment, it shouldn’t make much of a difference.

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