An Investigation into the Factors that Effect the Rate of Reaction Between Hydrochloric Acid and Sodium Thiosulphate.

Tyrone Sinclair

Aim:

*- Investigating the diverse factors that affect the rate of a reaction, by monitoring the effect of varying concentrations on reactants. In this experiment I am going to find out how the rate of reaction between sodium thiosulphate and hydrochloric acid is affected by changing the concentration.

The equation to this is as follows:

Sodium + Hydrochloric = Sodium + Sulphur + Sulphur + Water

Thiosulphate Acid Chloride Dioxide

Na2S2O3 (aq) + 2HCL (aq) = 2NaCl(aq) + SO(g) + SO2(s) + H2O(l)

This reaction usually produces a yellow precipitate.

Preliminary Work:

*- Previous to my final experiment, I conducted a prior procedure to confirm a relevant range of readings. In order for me to provide myself with not only accurate, however also relevant information, I felt it appropriate to perform various personal assessments in order to provide myself with the best and also in-depth range of readings.

The speed of reaction can be observed either by how quickly the reactants move or how quickly the products are forming. It’s usually a lot easier to measure products forming. There are three different ways that the speed of the reaction can be measured:

*- Precipitation – this when the product of the reaction is a precipitate which clouds the solution how long it takes for it to disappear.

*- Change in Mass (usually gas given off) – Any reaction that produces a gas can be carried out on a mass balance and as the gas is released the mass disappearing is easily measured.

*- The Volume of Gas Given Off – this involves the use of a gas syringe to measure the volume of gas given off. But that’s about all there is to it.

From the above I felt precipitation was the most appropriate procedure applicable to me.

There mainly six factors which affect the rate of reaction:

*- Temperature – Temperature alters the rate of a reaction by supplying the reactant particles with more energy, the more energy the reactants have the faster they move around increasing their chance of bumping into one another and having a successful collision. Also the more energy they get from the heat the more particles have enough energy to react, this is known as activation energy. Generally the greater the temperature the faster the rate of a reaction, the rate often doubles every 10°c.

*- Surface Area – The greater the surface area of the reactants the more particles are available for a successful collision.

*- A Catalyst – A catalyst is a substance that changes the rate of a chemical reaction but remains chemically unchanged itself.

*- Concentration – Concentration effects the rate of a reaction because the higher the concentration of particles the greater the chance of a successful collision. The general trend for the concentration is the rate doubles as the concentration doubles (they’re directly proportionate).

*- Pressure (in gases) – If the pressure is increased the particles in the gas are pushed closer. This increases the concentration and thus the rate of reaction.

*- Also stirring and mixing can be seen as a factor, due to the fact that by stirring or mixing, you provide the particles with more kinetic energy, therefore altering the rate of reaction

I chose to use concentration as it was the most practical for my experiment.

I chose to use 25mls of the colourless solution, sodium thiosulphate (to be exact 10%=10g), as I felt it provided me with an appropriate volume, visible to the naked eye. I later discovered that this was also a suitable volume to use in my concluding experimentation. I also observed different molarity ...

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*- Also stirring and mixing can be seen as a factor, due to the fact that by stirring or mixing, you provide the particles with more kinetic energy, therefore altering the rate of reaction

I chose to use concentration as it was the most practical for my experiment.

*- 0.1M

*- 0.25M

*- 0.5M

*- 0.75M

*- 1M

*- 1.5M

*- 2M

For my preliminary experiment, I chose to use the 2molars of hydrochloric acid, in order to provide myself with the fastest rate of reaction, for a prelim, baring in mind that some concentrations may alter to how fast or slow the reactants reach their end-point. I didn’t want the reaction to be too quick or too slow. The reason for this was because I needed to evaluate the volume to which the hydrochloric acid was to be used, as it would have shown a steadily rate of reaction. The volume of the hydrochloric acid ranged from 1ml to 5mls, escalating in 1mls each time, providing me with a varied enough scope on which one to actually use for my final experiment.

From my findings, I decided to use the 4mls of dilute hydrochloric acid, as it would provide me with the best range of readings. I have also decided to carry out two experiments, in order for me to supply myself with more accurate results and also an average time.

Method:

Prepare the equipment/apparatus, which are going to be used in the experiment.
Draw a black cross on a white piece of paper to indicate when solution has precipitate.
Then, measure out 25mls of sodium thiosulphate and put into a conical flask.
Measure 4mls of hydrochloric acid and then add it to the sodium thisulphate solution.
Start with the highest concentration of the hydrochloric acid and work down to the smallest.
Stir the solution moderately.
Immediately when the two substances were combined start the stopwatch.
Place the solution on the piece of paper when you think the solution has precipitated. If the black cross is still visible when the conical flask is placed on it, continue to stir the conical flask.
When the cross can no longer be seen, stop the stopwatch.
Empty out the contents of the conical flask, and clean it out, and dried it.
Continue to repeat the experiment, altering the molarities, however keeping the hydrochloric acid and the sodium thiosulphate constant, in order to keep the experiment a fair test.
After obtaining each molarity result, collect all the information to form a table of results.

Apparatus:

Dilute Hydrochloric Acid
175mls of Sodium Thiosulphate
50ml measuring cylinder
10ml measuring cylinder
Stop Clock
Conical Flask
Piece of paper with a ‘X’ on it

Diagram:

Key Factors to Vary:

There mainly six factors which affect the rate of reaction:

*- Surface Area – The greater the surface area of the reactants the more particles are available for a successful collision.

*- A Catalyst – A catalyst is a substance that changes the rate of a chemical reaction but remains chemically unchanged itself.

*- Pressure (in gases) – The particles in a gas undergo random collisions in which energy is transferred between the colliding particles. As a result there will be particles with differing energies. If the pressure is increased the particles in the gas are pushed closer. This increases the concentration and thus the rate of reaction.

*- Also stirring and mixing can be seen as a factor, due to the fact that by stirring or mixing, you provide the particles with more kinetic energy, therefore altering the rate of reaction

The factor which I intent to change is the concentration. I intend to change the concentration of the hydrochloric acid.

Factors to Keep Constant:

As temperature is a one of the key factors which can alter the rate of a reaction, I have chosen to keep this constant in order me to get a accurate reading. In order for me to achieve an accurate reading, I will need to do all my experiments in the same room.

Fair test:

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. So, in this experiment we have chose to keep sodium thiosulphate at a constant concentration. We could have, however, used hydrochloric acid as a constant, but I was provided with the sodium thiosulphate.

*- Next we must make sure that the hydrochloric acid and the sodium thiosulphate 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 start the stop watch at the exact time as we put the hydrochloric acid 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.

*- I also had to ensure that all the other factors affecting the rate of a reaction were kept constant, as these obviously could alter the rate of reaction.

*- However there must only be one person who delegates when the watch starts and when its ends, for different people have different eyes. So in that case I made sure that I used the same eyes, stop and didn’t estimate.

*- I used a stop watch as it can provide me an accurate reading, therefore meaning I would not feel the need to estimate.

*- I have to do all of my experiments in the same room, in order for me to keep the temperature of the experiment constant.

*- 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.

Prediction:

*- I predict that as the concentration of the hydrochloric acid increases the rate of reaction will also increase. The reaction rate, however, should decrease as the experiment progresses because, as the reaction time increases, the number of hydrochloric acid molecules present will decrease as they have been reacted to form new bonds. The additional sodium thiosulphate present as the experiment progresses should decrease the rate of reaction, because of the decrease in concentration. This means that the graph drawn up in my analysis will have a positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the concentration is increased. This can be justified by relating to the collision theory.

When the concentration is increased the particles will have more kinetic energy and thus move faster. Thus the probability that they will collide is more apparent, as they should collide more often and with more energy. Particles with more kinetic 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 kinetic 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. 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. This can be thought of as like people in a refined space, if there are twice the number of people there will be twice the chance of people colliding. This situation may change over time, however, depending on the situation.

The activation energy for a given reaction can be marked on the distribution curve. Only particles with energy equal to this or greater than the activation energy can react when a collision occurs.

Secondary Sources Used:

*- Internet

*- Ms Amboule’s Chemistry Lessons

*- Heinemann Modular Science for AQA

*- CGP, The Revision Guide

Safety Precautions:

There are a lot of safety issues which must be abided by in this experiment.

Remember that the substances which are used in this experiment can be very harmful if used the wrong way.

When we doing 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.

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.

Also try not to spill any chemicals, and do not eat or drink in the lab while dealing with these harmful chemicals, as they can get on to hands.

Table of Results:

*- Results taken the from 1st experiment.

*- Results taken from the 2nd experiment.

*- Table Displaying the Average Results.

Analysis:

Trends and Patterns:

*- The average displays that all of the reactions gained as the concentration increased. In order for me to have got a more accurate reading, I could have carried on the experiment until I had a reading for all of my reactions, providing me not with an estimate however a more adequate number.

Conclusion:

*- From this a firm conclusion can be drawn which states that, if the concentration in a reaction is increased, the rate of reaction will increase, resulting in the fact that the higher the concentration, the faster the rate of reaction. This is in accordance with my prediction which clearly states that due to the collision theory, the concentration is a factor which can alter the rate of a reaction. In this case I have encountered that when the concentration is increased in a reaction, the particles will have more kinetic energy and thus move faster. It is clearly stated in my graph that there is a positive correlation which is coincidental with my prediction, however due to time, I was unable to discover whether or not the graph had a curve in it.

Evaluation:

*- I was pleased with the results that I obtained from my experiments; they proved to be accurate as my repeat readings were very close to each other. The results also undoubtedly showed a relationship between the increase in molarity and the increase in speed. There is however, always room for improvement. Firstly, I did not take into account that other scientific instruments such as light sensors which are connected to a computer can be used, and also the fact that if I had time, I could have repeated the experiment a few times more in order to provide myself with an even more accurate reading.

Strong evidence:

*- My evidence was reliable and strong enough to support my conclusion, in graph this was clearly exploited.

Anomalous results:

*- As far as I can tell, no anomalous results, (An anomalous result being a result that does not agree with the general trend of the other readings).

Further to provide additional relevant evidence:

*- There are a number of things which I could do to improve my results or even provide myself with a new, more accurate set of results. As I said, I could have used a light sensor which would have given me a more varied scope of results. The other thing I could have also done was to carry out the experiment more than twice.

Tyrone Sinclair