The rate of reactions and what affects them - hydrochloric acid and sodium thiosulphate.

January 2001

∙Aim:

Investigating the rate of reactions and what affects them. In my case I am going to look at hydrochloric acid and sodium thiosulphate. I am going to investigate how the concentration of hydrochloric acid will affect the reaction.

∙Background Information:

In the investigation I am going to do I am looking at the rate of reactions. In Microsoft Encarta it is defined as the “amount of a substance that takes part in a chemical reaction”. Factors can change reaction rates. If substance that are wanted for reaction are heated, the rate of reaction usually rises as well, this also works the other way, if reacting substances are cooled the reaction will slow down. This is due to the fact that in order for a reaction to take part the particles in the substance must collide with each other. This is called the colliding theory. When the substance is heated it gives the particles more energy to move around and so increases the chance of a collision. Also when the particles are moving faster and collide they are more likely to react, rather than just to bounce off each other.

When a substance is cooled it has to opposite effect due to the fact that the particles in the substances do not have a lot of energy causing the to move around slower. This then causes fewer collisions and a slower reaction.

The concentration of reactants, the amount dissolved in a given volume of solution, is another factor that can change the rate of reaction. Concentration has a similar effect to heating reacting substances, this is because the more particles present, the more likely a collision, and so the higher reaction rate.

Reaction rate can also be affected by particle size. Smaller particles have a much larger amount of surface than a larger amount of surface than a large block of the same material. Chemicals react together by colliding. If there is more surface to collide with, the rate at which the chemicals react will be greater. So we usually get faster rates of reaction if the solid particles are smaller.

Using a catalyst is another method I could use to effect t the rate of reaction. A catalyst is a separate substance that speeds up a reaction but without being chemically changed or used up by the end of the reaction.

The rate of reactions can be obtained by following some measurable property that alters as the reaction occurs. These measurable properties are gas volumes, concentration of reaction mixture, and electrical conductivity.

In the case of the investigation I am going to look at I am using the chemicals hydrochloric acid and sodium thiosulphate which when put together is a precipitation reaction. The reaction is illustrated in the equation below.

Sodium thiosulphate + hydrochloric acid = sodium chloride + sulphur + sulphur

dioxide + water

Na2S2O3(aq) + 2HCL(aq) = 2NaCL(aq) + S(s) + SO2(g) + H2O(l)

This reaction will only occur where the particles of the reactants meet and combine or as I have I have said before the “collision theory”.

∙Prediction

By increasing the concentration of hydrochloric acid when reacting it with sodium thiosulphate the reaction rate will be faster. I know this as you can see from my background information ...

This is a preview of the whole essay

Sodium thiosulphate + hydrochloric acid = sodium chloride + sulphur + sulphur

dioxide + water

Na2S2O3(aq) + 2HCL(aq) = 2NaCL(aq) + S(s) + SO2(g) + H2O(l)

This reaction will only occur where the particles of the reactants meet and combine or as I have I have said before the “collision theory”.

∙Prediction

By increasing the concentration of hydrochloric acid when reacting it with sodium thiosulphate the reaction rate will be faster. I know this as you can see from my background information I have written. I found out the reason for the reaction rate to increase is because by increasing the concentration of hydrochloric acid it will cause the substance to contain more particles, Then when the substance is mixed together with the sodium thiosulphate there will be more particles. The more particles will increase the chances for them to collide and also the amount they collide. Due to this the reaction rate will then increase. This variable is continuous and independent and I have decided to test the variable of concentration.

I predict that by doubling the concentration of hydrochloric acid, the rate of the reaction will double. Therefore when there is twice as many particles in the same volume, the reaction will be twice as fast because there will be twice as many particles colliding.

∙Key Factors

The main factors that I fell would have a influence on the experiment:

∙Temperature – When increasing the temperature of chemicals provides them with more energy, and usually speeds up the reaction. Because the particles of the chemicals are moving around faster, they bump into each other more often and harder and will therefore react together more quickly

∙Concentration – When increasing the concentration of chemicals in a solution, then there are more particles of that chemical in the same volume of solution that can react. This means that they are closer together and more likely to collide with the other chemicals. The more the concentration the chemical, the faster the reaction rate will be.

∙Equipment used –

∙Time – The longer the particles of the two chemicals are mixed together the more time they have to move around and collide into each other causing the reaction to take place for longer. So the longer the chemicals are together, a larger reaction will take place

∙Volume – The larger the volume of chemical there are the more particles in that chemical, and therefor when the chemical is mixed with another chemical its more likely for the particles to collide with the other chemical.

∙The one factor I have decided to deliberately change:

From the list I have made above I have decided to keep all the factors the same apart from the factor of concentration. I am investigating how the concentration of hydrochloric acid will have an affect on the reaction rate. I will start with the hydrochloric acid at a concentration of 2 molar. Then decreasing it with distilled water seven times.

∙The Factors I Will Keep the Same to Insure a Fair Test

∙ I am only going to allow the reaction of the hydrochloric acid and sodium thiosulphate to take place until I am unable to see the cross underneath the flask. This is when I will stop the stop clock and record the time, I will not allow the time to carry on.

∙Another factor that I will keep the same to ensure a fair test will be the volume of the chemicals. The volume of the sodium thiosulphate will be kept the same through out the whole experiment, I have decided to keep the volume at 20cm3. I have to change the concentration of the hydrochloric acid by adding distilled water to it each time, but I will always make sure that the total volume of them together will also be 20cm3.

∙The equipment- I will be using all the equipment listed underneath to test the variable of concentration.

1 * Conical flask 0 – 250 cm3

2 * Measuring cylinder 0 – 25 cm3

2 * Measuring cylinder 0 – 10 cm3

3 * beaker 0 – 250 cm3

White paper

Black marker pen

Distilled water

Stop clock

Sodium thiosulphate 0.2 molar

Hydrochloric acid 2 molar

Pipette

∙Method

I am going to test eight different concentrations for hydrochloric acid. I shall use distilled water to dilute the hydrochloric acid for different concentrations.

The hydrochloric acid will be added to the sodium thiosulphate, and together they will react forming a cloudy solution. As these react it will form sulphur, and the more sulphur formed the cloudier the solution will get.

The Sodium thiosulphate is going to be a constant variable that I will keep the same through out all of the experiments. I will use 20cm3 of sodium thiosulphate in each experiment I do.

Hydrochloric acid is an input variable that I will change its concentration in each experiment I do. I will start off with the hydrochloric acid not being concentrated at all with any distilled water. I will only put 20cm3 of hydrochloric acid into the conical flask. The next experiment I will lower the concentration by only putting 15cm3 of hydrochloric acid and 5cm3 of distilled water. Here is a table to show the eight different concentrations I am going to use.

These are the steps I will follow to carry out my experiment.

Put safety goggles and lab coat on, make sure hair is tied back. Tidy everything on the bench and set out all the equipment that I will need during the experiment.
Place a piece of white paper underneath the conical flask with a large black cross on it that will sit exactly under the flask.
Measure the volumes of hydrochloric acid and distil water I will need to create the concentration I want in separated measuring cylinders. This is to prevent contamination. Use pipettes to get the measurements as exact as possible. Make sure you use two pipettes, one for the distilled water and one for the hydrochloric acid.
Poor them both into the conical flask.
Measure out the 20cm3 of sodium thiosulphate in another clean measuring cylinder. Using a different pipette for getting the measurement exact.
Pour the 20cm3 of sodium thiosulphate into the conical flask, as I do this begin the stop clock.
Watch the black cross from above the conical flask. As soon as I can not see the cross through the solution stop the stop clock.
Record the time, clean the conical flask out thoroughly and then I will be able to begin the experiment again with then next concentration.

∙Diagram

Please see separate sheet.

∙Safety Measures.

To ensure my safety and the safety of others around me I will carry out my experiment on and clear beach. I will place all the equipment I need on the beach I am working on so I know where everything thing is when I need it.

Through out the whole experiment I will make sure I wear safety goggles to protect my eyes from any of the chemical that could get in them. I will also be wearing a lab coat to prevent and chemicals coming in contact with them in the case of a spillage or breakage.

While the reactions are taking place between the hydrochloric acid and sodium thiosulphate a little sulphur dioxide is formed as a by-product. This is a toxic gas but in very small quantities it shouldn’t do any harm. Due to this fact I will try not to inhale this gas extensively. Luckily I am not an asthmatic so the sulphur dioxide shouldn’t affect me at all.

∙Table of results

After I had worked out how I was exactly going to carry out the experiment I was ready to start. I made sure that all the safety facts that I had mentioned above were carried out to insure top safety for others and me. I have decided to carry out the whole experiment twice, so I get two sets of results which I can add together and dived by two to find the average of them to receive more accurate results.

Here are my results from both attempts of the experiment. After I received these I went down the two rows to see if there were any anomalous results. I could notice an odd result by using my prediction ‘by increasing the concentration of hydrochloric acid when reacting it with sodium thiosulphate the reaction rate will be faster’. Therefor when I decrease the concentration of hydrochloric acid the results of the number of seconds it took for the cross to disappear should increase. This is because a higher concentration of hydrochloric acid will contain more particles causing more collisions causing the reaction rate to be faster.

There should not be any anomalous results if I had carried out the experiment correctly and accurately, and have been very accurate with all my measurements. I did this by using the correct equipment. For example, when measuring 20cm3 of sodium thiosulphate I used a 25cm3 measuring cylinder instead of a 100cm3 measuring cylinder that would not give me as much of a accurate result as the 25cm3 one. I made sure when reading all measurement in cylinders they were at eye level, I also made sure they were sat on a flat surface and read the reading from the bottom of the meniscus. As I hoped, I did not collect any anomalous results that stick out from the rest.

I was now able to find the average of the two sets of results I did to find a more precise reading for the time taken for the cross to disappear for each concentration I tested.

The results that I have collected do not have any obvious anomalous results but from the line graph I drew I can see the results do not go up in a nice smooth steady line, this is because my results are not completely accurate. I would be almost impossible to get total exact results but to improve mine I could repeat the experiment another two times and then find the average of the four sets of results. I could also increase the amount of concentration changes of hydrochloric acid I do. This will give me a wider range of results.