Investigate a factor which affects the rate of reaction in the reaction between sodium thiosulphate and dilute hydrochloric acid.

Rumel Ahmed 3873

Aim- To investigate a factor which affects the rate of reaction in the reaction between sodium thiosulphate and dilute hydrochloric acid.

When sodium thiosulphate and hydrochloric acid react they produce a cloudy Solution. The two chemicals are both clear solutions and will react together to form a yellow precipitate of sulphur.

As the solution will turn cloudy, we can observe the rate of reaction by placing a black cross underneath the beaker and seeing how long it takes for it to disappear by timing it.

The reaction happens because of the collision theory. Every atom vibrates and depending on the circumstances the strength of the vibrations might increase or decrease. When two substances are placed together, they will vibrate and often collide. When the two atoms collide a certain amount of energy is needed for the reaction to take place. The minimum amount of energy that is needed for a reaction to take place is called the activation energy (See Fig.1 and Fig.2).

Key

- Atom

- Direction atom is moving

As you can see in Fig.1 the atoms are about to collide, in Fig.2, you can see that the top pair of atoms did not react, that is because not enough energy was produced so they did not react instead they bounced off each other and moved along in another direction. Whereas the bottom two atoms reacted together because the activation energy was reached.

The activation energy will vary depending on the substance that is used.

These are some of the factors, which can affect the rate of reaction.

Temperature

The temperature of the solution will affect the rate of reaction. This happens because as the temperature is increased, the atoms in the solution will have gain energy. All this extra energy means that the speed at which the atoms are moving will increase rapidly, so the atoms will be more likely to react together, this is why the rate of

the reaction will increase. Increases of 10° of the temperature will often double the rate of reaction. The complete opposite happens when the temperature is decreased. As the temperature decreases the atoms lose their energy, which means they will move slower, this will decrease the chances of colliding with other atoms. This will slow down the reaction.

Concentration

The concentration also affects the reaction rate of the reaction. This happens because as the concentration is increased there will be more atoms in the solution, this means the atoms are more likely collide with each other, which will increase the rate of the reaction. The opposite happens when the concentration is decreased. When the concentration is decreased there are fewer atoms in the solution, less atoms means that the atoms are less likely to collide with each other. This will decrease the rate of the reaction.

Pressure

The pressure also affects the rate ...

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Concentration

Pressure

The pressure also affects the rate of the reaction. When a lot of pressure is applied the space the atoms have to move will decrease, this means that the atoms will be more likely to collide, which will increase the rate of the reaction. And also when the pressure applied decreases, there will be more room for the atoms to move around, this will decrease the rate of the reaction.

Surface area will not affect the rate of the reaction, as both of the chemicals will be liquids, the surface area does not apply to liquids.

For my investigation I will study the concentration of the sodium thiosulphate. I will keep the concentration and the volume of the hydrochloric acid the same throughout the whole experiment. I have chosen to do the investigation for the concentration because it is easy to observe and can easily be varied.

For the experiment I will keep the volume and the concentration of the hydrochloric acid constant, I will vary the concentration of the sodium thiosulphate. With the sodium thiosulphate I will add water to reduce the concentration of it. During the experiment the volume of the sodium thiosulphate and water solution will be 50ml. The amount of water that will be used will be determined by the amount of sodium thiosulphate. If the amount of sodium thiosulphate that is used is 20ml then the amount of water that will be used will be 30ml as the volume of the solution should always be 50ml.

1 mole of sodium thiosulphate will be used and 0.5 mole of hydrochloric acid will be used. I mole is the weight in grams of the relative atomic mass of the substance, for example, I mole of chlorine would be 37.5 grams, as the relative atomic mass of chlorine is 37.5.

Hypothesis

I think that the rate of reaction will decrease, as the concentration of the sodium thiosulphate solution with water becomes less concentrated.

Prediction

I predict that the higher the concentration of the sodium thiosulphate becomes and the amount of water goes down the rate of the reaction would increase.

Preliminary investigation

The reason that I have done the preliminary investigation is to determine what the interval between the tests should be.

Method for preliminary investigation

Set up the apparatus
Work out the suitable concentration for the sodium thiosulphate to start the experiment to start with.
Measure the amount of sodium thiosulphate and water
Measure the amount of hydrochloric acid.
Add them together
Start timing
Stop timing when the cross disappears.

After doing the preliminary investigation I have collected the correct information on how to make this experiment as fair as possible and how to get the most accurate results.

These are the following

The correct equipment that I will have to use.
The same cross will have to be used throughout the experiment to make it a fair test.
The same amount of hydrochloric acid must be used in all the tests to make it fair.
What the interval should be between the tests.
How many times the experiment should be done to get an accurate result.

The apparatus

This is a list of all the apparatus that I will use in the experiment-

1 x Ajar of hydrochloric acid
1 x A jar of sodium thiosulphate
1 x Conical flask
1 x beaker
2 x measuring cylinder
1 x Thermometer
1 x A paper with a cross on it
1 x Stop watch
1 x goggles (for safety)

I used two measuring cylinders because if I had used one for both the sodium thiosulphate and the hydrochloric acid then the reaction would have started in the measuring cylinders and I would have got wrong results. (See Fig.3 to see how to set up the apparatus). My initial thoughts was to have an interval of 10ml in the concentration of the sodium thiosulphate, but after the preliminary investigation I found out that if I used that there will be very few results and they wont be close enough to make accurate graphs.

Method

Set up the apparatus as above
There will be a 5ml interval in the concentration of the sodium thiosulphate
Measure 5ml of sodium thiosulphate
Measure 45ml of water and it to the sodium thiosulphate
Measure 5ml in the other measuring cylinder
Then add the sodium thiosulphate and the hydrochloric acid together in the conical flask and give it a gentle swirl
Start the timer instantly.
Stop the timer when the cross has disappeared
Repeat the experiment again with an interval of 5ml in the concentration of the sodium thiosulphate.
After the experiment has been done once repeat it three times to get an average.

This is the table of results

Analysis

From my graph I can see that most of my results were correct, but I did get some anomalous results.

The results agreed with my prediction that the higher the concentration of the sodium thiosulphate becomes and the amount of water goes down the rate of the reaction would increase.

My graph shows this because it curves downwards, indicating as the concentration of sodium thiosuphate increases, the time it takes for X to disappear reduces this means that the rate of the reaction increases.

This is because before two particles can react they must meet. In a low concentration, the particles will be few and widely spread. This means that the number of reactions will be limited because fewer particles will meet. At higher concentrations there are more particles and so the probability of them coming into contact with other particles is increased.

Therefore, as I increased the concentration of the sodium thiosulphate and decreased the amount of water, the reaction happened even faster.

Evaluation

For 50ml sodium thiosulphate, 5cm3 of hydrochloric acid, it took 61.09 seconds for the cross to disappear on my second try. In my first and third attempts I got 38.02 seconds and 45.6 seconds. This means that the second attempt was probably incorrect.

This could have been caused by any number of things such as:

Measuring- the actual measuring of the chemicals is difficult because of the meniscus of the liquid and the accurateness of the measuring cylinders. Also, the human eye causes errors such as this

The eye could also have made mistakes in judging whether or not the cross had actually completely disappeared

Again human error could have occurred. If the same person watching the reaction was working the timer, errors could occur in their co-ordination.

We could remove these errors by:

Using a burette or a to measure. This would remove the measuring errors associated with measuring cylinders, using a burettes are a far more accurate way of measuring the correct amounts.

Use light sensors to detect when the cross is no longer visible. A light sensor would be placed underneath the conic flask and would be connected to a timer. The light sensors will detect when there is no light shining through the substance. This would automatically stop the timer and therefore make the experiment fairer and more accurate.

To further investigate this experiment, I could use a wider range of times. I could have done the experiment at intervals of 5ml up to 100ml, this would give me more results, which would mean I could plot more results in my graph. This would be even more accurate.

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