Investigate the rate of reaction between sodium thiosulphate and hydrochloric acid, when you change the concentration

Antoine Allen

Aim

The aim of this experiment is to investigate what affects the rate of reaction between Sodium Thiosulphate and Hydrochloric Acid. By using the bariable of sodium thiosulphate concentration

Equations

Na2S2O3 + 2HCl = 2NaCl +SO2 H2O + S

Sodium thiosulphate + hydrochloric acid = Sodium Chloride + sulpur dioxide + water + sulpur

Background research

What is the most accurate way of obtaining the reaction rate between sodium thiosulphate and hydrochloric acid?

The average rate of a chemical reaction, over a certain time, is equal to the change in concentration of the reactant or the product divided by the time taken. Which method you use depends on the type of reaction it is. One way of finding this out is by carrying the reaction out in a thermostatically controlled bath - i.e. one that you can control the temperature of. A sample of the reacting mixture is withdrawn with a pipette and the reaction within this bit of the mixture is stopped. This can be done in a number of ways like removing one of the reagents or suddenly cooling the mixture. You then perform a titration to find the concentration of one of the reactants or one of the products. If you do this at regular intervals, you can determine the rate at which the reaction is happening. In a reaction where a gas is formed, you have to use a different method. The volume of gas must be recorded at various times.

The rate of a reaction is defined as the speed at which the reaction takes place.

Most molecular collisions do not result in chemical change. Before any change takes place on collision, the colliding molecules must have a minimum kinetic energy called the Activation Energy. This is shown on the energy level diagrams below. It does not matter whether the reaction is an exothermic or an endothermic energy change. Now when heated molecules have a greater kinetic energy, greater proportions of them have the required activation energy to react. The increased chance of higher energy collisions greatly increases the speed of the reaction.

The Collision Theory
In order for a reaction to take place, the reacting substances must collide and energy, called the activation energy, must be reached. If the collision between the particles can produce a lot of energy, then a reaction can take place. For the collision to take place, the particles must collide fast enough and in the right direction. The higher the number of collisions, the faster the rate of reaction. Increasing the temperature of the substrate can increase the number of collisions.

Kinetic Theory
The kinetic theory is a theory, which emphasises on pressure, volume, and temperature. It states that gases contain particles that are always moving. Gases then exert pressure because their particles are colliding with the wall of the container. If the volume of the gas is decreased, then the particles would hit the wall more often and the pressure would increase.
When a lump of dough, which contains air and carbon dioxide, is heated in the oven, the volumes of the gases increase, resulting in the bread rising. This process can be shown by this word equation:

Increase in temperature Volume increases (Gas expands)

This is because as the temperature of the gas increases, the particles get more energy. This causes the particles to move faster and collide with each other and the walls of the container. If the gas cannot expand, the pressure would increase. If the pressure is kept constant while the temperature is increased, then the gas will expand

Plan

A reaction will only occur where the particles of the reactants meet and combine. This is called the collision theory. Therefore it stands to reason that to increase the rate of reaction it is necessary to cause more particles to collide harder and make it happen more often. There are several ways to do this and these make up the variables for this experiment. They are listed below along with predictions as to their affect on the reaction.

Increasing the pressure. By reducing, the volume in which the same amount of particles exists the ...

This is a preview of the whole essay

Plan

Increasing the pressure. By reducing, the volume in which the same amount of particles exists the pressure is increased. Once the same number of particles are in a smaller area there is less space in which to move and so the particles are more likely to hit each other. It is therefore possible to predict that increasing the pressure will result in an increase in the rate of reaction. I will not test this variable because the school doesn't have the facilities to test it. However pressure is a continuous variable. To make it a fair test I will use the same size test tube each time. Heat energy. By giving the particles extra energy, they will move faster. This means that they cover more ground and are therefore more likely to hit each other, which in turn makes the reaction faster. The best way to give energy to a particle is as heat and so I can predict that raising temperature will increase the rate of reaction. This is a continuous, independent variable. I predict that temperature is proportional to rate of reaction.
Concentration. Just as increasing the pressure will increase the number of particles colliding, so will the concentration. By putting more particles into the reaction, the chance of them colliding increases and so the rate increases. This variable is continuous and independent. I shall test this variable. I predict that by doubling the concentration of the acid, the rate of reaction will double.
Surface area. Particles can only collide when the two sorts can meet. Therefore, a reaction can only occur on the surface of the material. Therefore, by increasing the area of the material that is available to collide the speed of the reaction will increase. I predict that doubling the surface area will double the speed of the reaction. This variable is continuous but I shall not test it because it is hard to control the exact surface area of the two reactants as they both come in an aqueous solution.

I am going to test the two variable concentrations. This is an independent, continuous variable. I think that concentration will have the biggest affect because the reaction is exothermic. Therefore, even while I am testing concentration, heat will be given out by the reaction, which will give more energy to the particles and so cause them to reach their activation energy sooner. I think that concentration is proportional because:

· Doubling the number of particles doubles the probability that they will collide
and

· Doubling the speed at which these particles travel will double the distance they can travel in a set time and so double the probability of them colliding?

This proportionality can be expressed using algebra thus: X' = XY' / Y

Preliminary Test

In the preliminary I worked out to make it a fair test I would place a black cross underneath the experiment. To no when to the solutions has become completely cloudy and the reaction has reached the point I am measuring to. In addition, I decided the concentrations I would use. I choose between 0.2-0.05

These are the results

Diagram

Prediction

I predict that the greater the concentration of Sodium Thiosulphate solution the faster the chemical reaction will take place. Therefore, the cross will disappear more quickly due to the cloudiness of the solution.

I think that the concentration of a solution effects the rate of reaction because ‘the rate of reaction depends on how frequently the molecules of the reacting substances collide. A more concentrated substance has more molecules for a given volume than a more dilute substance. Because there are more molecules about, the frequency of successful collisions is greater, and the reactions happen faster.’

My scientific reason for this is

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. The higher the concentration the more collision per second. This will also increase the rate of reaction.
Fair test

To make this experiment a fair test, we need to make sure we do a number of things.

In this experiment we are trying to find the rate of reaction using concentration as a factor, so there is a number of things we need to make sure we do to keep it a fair test.

Firstly, we need to keep a chemical at a constant concentration. Therefore, in this experiment we have chose to keep hydrochloric acid at a constant concentration. We could have, however, used sodium thiosulphate as a constant, but we had chose to use hydrochloric acid.

Next, we must make sure that the solution is kept at a constant volume throughout the experiment. If the volume is different, then it could give different results to if it was at a constant volume.

We must also make sure that we add both the water and the sodium thiosulphate at exactly the same time (into the conical flask with the hydrochloric acid in it), or it could effect the results of the experiment.

We must start the stopwatch at the exact time as we put the water and the sodium thiosulphate into the conical flask. To do this it is a lot easier if there are two people doing the experiment, so one person can put the two substances in the HCL, and one person can start the stop watch.

Another thing we must do is to make sure that the conical flask is completely clean and free of any water or any other substance before we attempt to start the next experiment.

Safety

We must abide by in this experiment also many safety issues.

We must remember that the substances, which we use in this experiment, can be very harmful if used the wrong way.

When we do this experiment, it may be necessary to wear safety goggles, as things are very unpredictable, and even though it is very unlikely that the solution would come out of the conical flask during the experiment, one must still be cautious of spills.

We must make sure that coats and bags are all out of the way while doing the experiment. Ties and hair should be tucked out of the way, so they do not make contact with any of the chemicals. It would also be preferable to wear a scientific apron, however this is not essential.

We should also try our best not to spill any chemicals, and we must not eat or drink in the lab while dealing with these harmful chemicals, as they can get on to our hands.

Accuracy

In order to make my investigation go to plan I will be as accurate as I can be so I will measure to the correct measuring size.

· Measure the volume in cm³

· Do the experiment twice to ensure that there isn’t an odd result. Two is a good number to use as you can see if there is one odd one where if you just done the experiment once then you wouldn’t know.

· Also to average out the results.

Controls

I will pour sodium thiosulphate into the beaker and check if it is creating a reaction.

Secondly, I will pour hydrochloric acid into the beaker to see if there is a reaction and then finally I will do the same with water. I do this to make sure it is a fair test.

Method

First, I will collect my equipment. Then I will get the concentrations of sodium thiosulphate I am going to use which are 0.2, 0.175, 0.15, 0.1, 0.05. Then put the hydrochloric acid into the beaker. Place a black cross underneath, put the sodium thiosulphate in to the solution, and time the reaction.

Equipment list

Beaker

Pipette

Hydrochloric acid

Sodium thiosulphate

Water

Meauring beaker

Black cross

Stopwatch

I used the same diagram that was in my prelimenary.

Results

Analysis

Conclude that the more concentrated a reactant is, the quicker the rate of reaction time will be.

I have come to this conclusion because of several reasons. Firstly, my results give pretty conclusive evidence that as the amount of Sodium Thiosulphate decreases, and the amount of water in the solution increases there are less atoms to collide and therefore less successful collisions causing chemical change so the reaction rate is slower. In a more concentrated solution, there are more atoms to collide so the reaction time is quicker.

My results support the prediction I made because I said ‘the greater the concentration of Sodium Thiosulphate the faster the rate of reaction time.’ I believe I was correct and a secondary source states that ‘the reaction time will be faster with a more concentrated solution because, the more molecules there are, the frequency of successful collisions is greater and therefore the reaction rate is speeded up’. – Secondary source Britannia Interactive Encyclopaedia 1998.

By looking at my graph, I can see that when the concentration of the Hydrochloric acid is increased, the time taken for the cross to disappear decreases. This means that the concentration is inversely proportional to the time taken for the cross to disappear.

The time taken for the cross to disappear relies on the concentration of the Hydrochloric acid. This means that the acid to water ratio is bigger. If there is more acid to water in the solution then this means that there are more hydrochloric molecules in it.

The fact that there are more HCl molecules knocking about between the water molecules means that collisions between the Sodium Thiosulphate molecules are more likely to occur.

In addition, the graph has a straight line of best fit. This means my prediction was right again.

Anomalies

I had no anomalous results; this was because I worked with precision and skill. If I had had anomlisious results, it might have been because when I was estimate the time when the cross-disappeared I could not be 100 percent sure. I could solve this by using a light sensor next time.

Evaluation

I am pleased with my results, as they are accurate and well produced with precision and skill. I know they are produced with precision and skill because there are no anomalous results. As I predicted it is a straight-line graph.

From my results, I have found that the higher the concentration of sodium thiosulphate, the quicker the reaction rates.
I have succeeded in what I planned to do, which was to find out how the concentration of sodium thiosulphate affects the reaction bettween hydrochloric acid. The results I got were what I had expected and predicted and I did not get any anomalous results. The results I got were what I wanted so I was happy with them.
The experiment could have been made more accurate by using other ways of doing things that were important to the experiment. Measurements that are more accurate could have been used as the measuring cylinders used were only to either every 0.5cm2 or 1cm2. This is not very accurate. To help make this experiment more accurate, I repeated it two times and then used the average of all the results to plot a graph with a line of best fit. I tried to keep all the variables except for the concentration of sodium thiosulphate the same for all the experiments. However, in reality it is impossible to keep all the variables precisely the same. For example:

b) It is also impossible to precisely measure out the amounts of Hydrochloric acid, sodium thiosulphate, and Water each time. As the scale on the pipettes shows the volume to the nearest mm3 the volume of the solutions that I used should be correct to the nearest mm3. The volume of gas in the test tube to start with is slightly affected by the amount which the bung is pushed down each time, if the bung is pushed down further then the volume in the tube will be less and the gas will reach faster.

c) Due to the slow speed of our reactions, it is only possible to measure the time of the reaction to the nearest 0.1-second although the stopwatch shows the measurements to the nearest 0.01 second

Improvements

I have stated many the improvements in my evaluation. You could use pipettes to a high degree of accuracy

Alternatively, you could change the layout of the experiment as it shows in his diagram.

You could have a light bulb and a light sensor then you would no exactly when the reaction had got to the point where you can no longer see through it.

Extend

I could extend the experiment by having bigger quantities of the solution. On the other hand, I could change the variable. Alternatively, I could put the experiment into a water bath and see if keeping the temperature the same changes the out come of the experiment.