How Does The Concentration Of Sodium Thiosulfate Affect The Rate Of This Reaction?

by ricardodossantos (student)

Candidate Number: 8085 Rate of reaction coursework

How Does The Concentration Of Sodium Thiosulfate Affect The Rate Of This Reaction?

Student name: Banudi Srikanth

Teacher name: Mrs Ogunkoya

Class/Set: 10a2

Candidate number: 8085

CONTENTS

Background Science........................................................................................................................... Page 3,4

What is a preliminary test? ............................................................................................................... Page 5

Preliminary apparatus .................................................................................................................... Page 5,6

The preliminary results table .......................................................................................................... Page 5

Preliminary safety ................................................................................................................................. Page 5

Methodology ............................................................................................................................................ Page 7

How to make it a fair test ............................................................................................................... Page 7,8

DO CONTENTS PAGE

BACKGROUND SCIENCE

The 5 Factors that affect the rate of a chemical reaction:

Concentration
Temperature
Gas pressure
Catalyst
Surface area of particles (1)

Rates of reaction:

How quickly a reaction happens is called the rate of reaction. Chemical reactions take place when two or more chemicals react with one another by colliding with each other. The increase in temperature increases the amount of collisions between particles, this fastens the reaction. If a reaction has a low rate that means the molecules combine at a slower speed than a reaction with a high rate. Particles need a minimum amount of kinetic movement if they are going to react when they collide. This is known as Activation energy.

THE COLLISION THEORY

The collision theory is the idea that different reactions happen at different rates. Reactions that occur slowly have a lower rate of reaction. Fast reactions are due to an increase of collisions; slow reactions are due to decrease in collisions. Reactions that happen quickly have a high rate of reaction (e.g. explosions).

Concentration:

The concentration of a solution is how strong the solution is. A stronger acid contains more acid particles and less water particles than a weaker acid. Increasing the concentration of a solution leads to more collisions (greater frequency of collisions) so the rate of the reaction goes up. 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 higher (faster.) Changing the concentration of the acid does not change how quickly the particles are moving (i.e. it doesn't increase the amount of energy they have.)

Temperature:

When we increase the temperature at which a reaction is taking place, the particles move more quickly. At a lower temperature, the number of collisions is lower because the particles are moving more slowly. Also when a collision occurs, there is less chance of a reaction taking place because the movement energy in the particles is less. At a higher temperature, the number of collisions is greater because the particles are moving more quickly. When a collision occurs, there is more chance of a reaction taking place because the movement energy in the particles is greater.

Gas pressure:

Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move.

That greater concentration of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often. The lower pressure decreases the rate of reaction. (2)

Catalyst: Different catalysts speed up different reactions. The table shows some common catalysts:

A catalyst provides a surface on which the reaction can take place. This increases the number of collisions between the particles of the substances that are reacting.

A catalyst lowers the activation energy (the minimum amount of energy needed for a reaction to take place). This means that the particles can react with less energy than they needed before the catalyst was added. (3)

Surface area of particles:

By increasing the surface area of the reactants, there are a higher number of reaction sites. Reaction sites are specific sites on molecules at which reactions occur. Increasing the number of reaction sites increases the number of total collisions. The greater the frequency of total collisions, the greater the frequency of effective collisions. If the frequency of effective collisions increases, so does the reaction rate.

Introduction/Aim

In this investigation, I am finding out how concentration affects the rate of reaction between Hydrochloric acid and Sodium Thiosulphate. I am going to measure the rate of reaction when hydrochloric acid is added to sodium thiosulfate. When sodium thiosulfate and Hydrochloric Acid are mixed, a yellow precipitate of sulphur is produced. The solution becomes increasingly difficult to see through as more and more sulphur is formed. This is how I plan to measure the rate of reaction. I will place a paper with a black cross underneath the solution and will stop the clock when the cross can no longer be seen. I will find out the answer by doing a preliminary test to experiment this, I will then compare and discuss the results in my evaluation then I will also state how this experiment I done could be improved, in my conclusion. But before doing the real experiment I will carry out a preliminary test.

What is a Preliminary test?

This is a test that is done before the real test. It gives an idea of the real thing; through the test you can make changes to make sure the real one is fair and reliable e.g. changing equipment or concentration of sodium thiosulfate solution. A preliminary test is more of a trial than an experiment – to find out if the equipment/method or even experiment works. A preliminary test is generally a baseline for further testing and development. It helps you refine ideas and change key concepts of the experiment.

Prediction:

Knowing the information above, I predict that as the concentration of the Sodium Thiosulphate increases, the rate of reaction will also increase. This means that the rate/concentration graph I compile from my experiments results will have a positive correlation. I believe this will happen because, according to the collision theory, as the concentration of a solution is increased the number of particles inside the solution increases. If the number of particles inside the solution is increased it makes collisions with reacting particles more likely. Also, I believe that the time/concentration graph will have a negative correlation because if my ...

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Prediction:

Knowing the information above, I predict that as the concentration of the Sodium Thiosulphate increases, the rate of reaction will also increase. This means that the rate/concentration graph I compile from my experiments results will have a positive correlation. I believe this will happen because, according to the collision theory, as the concentration of a solution is increased the number of particles inside the solution increases. If the number of particles inside the solution is increased it makes collisions with reacting particles more likely. Also, I believe that the time/concentration graph will have a negative correlation because if my prediction is true, as the concentration increases the time taken for the reaction to take place will decrease.

Preliminary Experiment:

I did a preliminary experiment to get used to the sequence of events. I got to know all the apparatus and the method.

Word Equation for the reaction:

Sodium Thiosulfate + Hydrochloric acid → Sodium Chloride + Sulphur + sulphur dioxide + water

Ionic equation for reaction:

2Na+ + S2O32- + 2H+ + 2Cl- → 2Na+ + 2Cl- + SO2 (g) + S + H2O (l)

Balanced Symbol Equation for the reaction:

Products

Reactants

Na2S203 (aq) + 2HCl (aq) → 2NaCl (aq) + S (s) +SO2 (g) +H20 (l)

I only did the experiment one, not three times so I did not get an average time for each concentration. I made sure the volume of Sodium Thiosulfate solution each time equalled to 10cm3 . The one thing I kept consistent was 10cm3 of Hydrochloric acid. I varied the concentration of the Sodium Thiosulfate and water. Below is a results table for my preliminary experiment.

Preliminary Table

Preliminary method:

If you have not already done so, put on your goggles and gloves.
Place the paper with an X onto a flat surface and put a conical flask on top.
Measure 9 cm3 of sodium thiosulphate using a pipette and put it into a flask. Then measure 1 cm3 of water using a different pipet.
Then measure 10 cm3 of hydrochloric acid using a measuring cylinder.
Add first the acid then water and place a ball of cotton wool on top of the flask to block the formed gas from getting into the air. When acid and water is added, immediately start the timer.
Look down at the cross from above the flask. When the cross disappears, stop the timer and note the time, recording it in the table.
Repeat this process using different concentrations of sodium thiosulpahte solution with water as shown in the table above.
Carefully pour the solution into the sink straight after each test.

Preliminary Safety:

Safety is a key aspect to any experiment. There are a lot of safety issues we must abide when performing this experiment. A key safety aspect was that we covered the top of the conical flask with cotton wool, making sure no gas escapes the air we breathe into as sulphur dioxide; one of the products formed from the experiment is a toxic gas. I also decided to wear goggles to protect my eyes from the acid splashing, squirting or any way entering my eye. Also be sure to tie hair back because if the sodium thiosulphate makes contact with my hair it could dye it blonde (bleach it) or if it comes into contact with my skin it would turn white and peel off. Another precaution to take measure of is wearing gloves to protect your skin as hydrochloric acid is corrosive. Afterwards, when you have done your experiment thoroughly wash away all apparatus used and pour the solution down the sink.

Apparatus:

Sodium Thiosulphate solution – (This is the variable factor being studied in the reaction)

Hydrochloric Acid
Water
Conical Flask – (to put both the hydrochloric acid and sodium thiosulphate into.)
3 Pipettes – (one for the hydrochloric acid and one for the sodium thiosulphate and one for water; this allows you to easily transfer the liquids.)
Cotton – (this blocks the top of the conical flask to make sure sulphur dioxide - the toxic gas produced doesn’t escape.)
A paper marked with an x – (to be certain when to stop the timer.)
Stop watch/Timer – (to stop the time when ‘X’ is not visible.)
Goggles – (to protect your eyes from acid entering the eyes – for safety measurements.)
Gloves – (protects your skin from hydrochloric acid; corrosive.)
2 Measuring cylinders’ (to measure the volume of the acids.)

Prediction:

I predict that the greater the concentration of Sodium Thiosulfate Solution, the faster the chemical reaction will take place. Therefore, the cross will disappear more quickly due to the cloudiness of the solution. If solutions of reacting particles are made to be more concentrated, there are a higher number of particles reacting. Meaning collisions between Hydrochloric Acid and Sodium Thiosulfate Solution are more likely to occur.

All this can be justified by the full understanding of the collision theory itself: For a reaction to occur particles have to collide with each other. Only a small per cent 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.

I think that the concentration of a solution affects 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 the successful collisions is greater and the reaction happens faster.’

I have predicted this through my acquired knowledge of the collision theory. But I must take into consideration that not all collisions results in a chemical reaction because there is not sufficient energy to overcome activation energy.

How does the concentration of Sodium Thiosulfate affect the rate of this reaction?

The Main Experiment

Safety precautions:

Safety is a key aspect to any experiment. There are a lot of safety issues we must abide when performing this experiment. The first safety precaution I used was to make sure I was standing up during my experiment, that way I will be more cautious and aware of my surroundings not causing accidents with equipment I’m using - e.g. tipping over my hydrochloric acid and sodium thiosulphate solution. A key safety aspect was that we covered the top of the conical flask with cotton wool, making sure no gas escapes the air we breathe into as sulphur dioxide; one of the products formed from the experiment is a toxic gas. I also decided to wear goggles to protect my eyes from the acid splashing, squirting or any way entering my eye. Another precaution to take measure of is wearing gloves to protect your skin as hydrochloric acid is corrosive. Afterwards, when you have done your experiment thoroughly wash away all apparatus used and pour the solution down the sink.

One change I made to my safety procedures is that I said standing during the experiment is better, as you’ll be more cautious and aware of your surroundings. I decided to add this on because as I was doing my preliminary experiment sitting was difficult, standing made it easier to move about while handling the apparatuses; making it less likely for an accident to happen i.e. spillage of chemicals.

Methodology:

If you have not already done so, put on your goggles and gloves.
Place the paper with an X onto a flat surface and put a conical flask on top.
Measure 9 cm3 of sodium thiosulphate using a pipette and put it into a flask. Then measure 1 cm3 of water using a different pipet.
Then measure 10 cm3 of hydrochloric acid using a measuring cylinder.
Add both the Sodium Thiosulfate solution and acid into the conical flask at the same time, quickly placing a ball of cotton wool on top of the flask to block the formed gas from getting into the air. When acid and water is added, immediately start the timer.
Look down at the cross from above the flask. The solution turns cloudy because we get precipitate of sulphur as the solution react. When the cross disappears, stop the timer and note the time, recording it in the table.
Repeat this process using different concentrations of sodium thiosulpahte solution with water as shown in the table above.
Carefully pour the solution into the sink straight after each test, thoroughly rinsing the measuring cylinders and conical flask with plenty of water each time.

I decided to add both the Sodium Thiosulfate Solution and Hydrochloric Acid at the same time, as it makes the outcome of the result more reliable.

Main Experiment Results Table

I have made some changes to my table. Some of these changes are that I have applied an clear subheading under each row to organise the presentation of the table, making it clear and easier to understand. One of five subheadings I have inserted is ‘Sodium Thiosulfate Solution’, under this are 3 rows of broken down areas of the subject; Volume of water (cm3), Volume of Sodium Thiosulfate (cm3) and Concentration (M).

Another adjustment I have done is added on three readings instead of one as it is one of many factors that show from my table - for my experiment to be reliable.

How to make it a fair test

To make sure my results are accurate and reliable the experiment will be kept as fair as possible by making sure the following things are kept constant:

The total volume of solution used in the reaction (10cm3 ).
The temperature of all chemicals (room temperature). To ensure this and make it a fair test, the experiment is done in the same room, same time, and same place.
The volume of Hydrochloric Acid used in the reaction (10cm3 ).
The same paper with the black cross will be used for all tests.
Each measurement will be measured accurately, by the same person (as each person has different eye-sights).
Different measuring equipment used for HCl and Na2S2O3 to avoid contamination.

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

Firstly I will keep a chemical at a constant concentration. So in this experiment we have chosen to keep Hydrochloric Acid at a constant concentration (10cm3). I will repeat the experiment of sodium thiosulphate solution and hydrochloric acid, three times. This will make sure no faults or errors are being made – through either the apparatus used, making sure the volume of each concentration is accurate and even human errors etc. By doing my experiment three times it increases the reliability of the experiment. I will also check my results with other students to make sure our results are the same or closely similar, definitely ensuring my tests to be reliable.

I will also make sure that the same person is measuring all the concentrations in each test, because each person has different methods of measuring and may be doing it – differently. Each person also has different eye-sights which may lead to inaccurate recordings. So we chose a person who has good visual perception. I will also make sure that the same person with quick reaction-time is stopping the timer when ‘X’ is not visible because again people have different reaction-times, which may result in odd results if a variation of people are taking turns to stop the clock watch.

I will make sure that the measuring cylinders for Hydrochloric Acid and Thiosulfate will not be mixed up, also in-between each use of the apparatus i.e. the measuring cylinder, is thoroughly washed as I do not want any substances to remain from my previous test, which if not done – will make my solution inaccurate.

The concentration of Sodium Thiosulphate in the beakers will be the only factor to change in the experiment.

Another thing that must be taken into consideration is that the Hydrochloric Acid and the Sodium Thiosulphate (+Water) must be poured into the conical flask simultaneously. This ensures a fair test; one more thing is that the stop-clock must be started as soon as the two solutes mix together.

Another factor which can affect my results is the conical flask. I must wash it out with water and dry it each time. This is because if there is any remaining solution, then it could alter my results and make my investigation unfair. Therefore I must clean out the flask.

One of the most important factors to consider is the temperature of my investigation. From background knowledge I know that the temperature of the reaction will affect the speed of it. So I must keep the temperature of the entire investigation constant, because otherwise the molecules will have more energy (unfairly) and will result in more collisions which are unfairly accelerated. This will tamper with my results and could make them anomalous. I must also ensure that I do not mechanically stir the solution as this will alter my results and make the investigation unfair.

Variables:

I will change the concentration of the sodium thiosulphate by changing the volume of water used to the volume of sodium thiosulphate solution. These changes may affect rate of reaction, and that will affect the time taken for the precipitate to form . I will be measuring the time taken for the cross to disappear.

The variables in this test are:

• The concentration of hydrochloric acid

• The concentration of sodium thiosulphate

• The amount of hydrochloric acid

• The amount of sodium thiosulphate

• The temperature

• The surface area of the reactants

• The size and colour of the cross

• The size of the conical flask

Temperature

I will keep the temperature the same because increasing the temperature increases the rate of reaction. I will do this by collecting all measurements on the same day and doing all tests at room temperature. If the measurements were taken on a different day, the temperature might vary.

Measuring Apparatus

I will keep the same type of measuring cylinder, because if I decide to use a different one with a different scale, it could change my results or confuse me when taking measurements.

Dependant

My independent variable is going to be the concentration of sodium thiosulphate. My dependant variable will be the rate of the reaction, or time taken for the reaction to take place. Watching the cross disappear as precipitation occurs and sulphur is formed as a solid will be the method used to realise the rate. My controlled variables will be the temperature, the concentration of hydrochloric acid, the amount of sodium thiosulphate and the amount of hydrochloric acid. I will try and control these as far as possible, and be as accurate as I can when measuring out the chemicals. Also if I did vary the volume of hydrochloric acid, I would have different rates of reactions , and there would be no reason for it because I am varying the ratio of sodium thiosulphate to water.

Independent

My dependant variable I am keeping the same is the person watching the solution and stopping the timer when the ‘X’ is not visible. I am doing this to ensure that there is no delay in time-keeping when stopping the timer. To be certain of it, I will choose a person of a quick reaction-time to do this as they will be best suited to this role. My dependent variable is the time taken.

Analysis of Graphs:

The results show that the rate of reaction decreases when the volume of sodium thiosulfate is less concentrated. From the graph we can see that as the concentration decreases, the time taken increases. The gradient decreases gradually, which shows that the time taken increases between each set interval of the concentration. These results prove my prediction correct, and also prove the particle collision theory.

Concentration of Na2S203 vs. Time

The main graph of the concentration of sodium thiosulfate (m) vs. the time of reaction in seconds has a curved line showing a negative correlation. This graph shows as you increase the concentration of sodium thiosulfate, the rate of reaction is higher. The curved line of best fit on the graph showing the average times shows that as the concentration of Sodium Thiosulphate decreases, the time increases by a bigger and bigger amount each time. This would continue to happen until the concentration was zero, and the time would be infinite. In addition, as the concentration increases, the time decreases, until it would be almost instant.

Concentration of Na2S203 vs. Time; Rate of Reaction 1/t (x amount of sulphur/time)

This graph shows that as the concentration decreases, so does the rate of reaction. This supports the first half of my prediction shown by the collision theory, which says that as the concentration increases, there will be more collisions, and so more collisions that have the energy of activation, so the rate of reaction will increase. However, it does not support my prediction that as the concentration is halved, the rate of reaction will be halved; the graphs do not support this pattern. However, the results are accurate enough to show that as the concentration increases, so does the rate of reaction, supporting my prediction.

Rate of reaction; Error Bar

On the error bar graph I have plotted the highest number and lowest number from my 3 readings and have plotted a cross for the middle reading. The distance between the two points on my error bar shows how accurate my readings were.

Anomalous results and experimental improvements

The only anomalous result that I have recorded (value of t when v=16cm3). The result may have turned out anomalous because of basic human error, or maybe because we measured the substances wrong. It may have even been because we did not clean the apparatus properly.

Apart from this, the accuracy of my experiment has been more or less accurate. Although there are a number of ways in which we could have made the results more reliable. For instance, we could have used better measuring equipment, because the apparatus we used was mainly basic equipment.

Another thing we could have done to bring more evidence is to have tried to use the hydrochloric acid as the variable substance, and used the sodium thiosulphate as the constant substance. This would have brought more evidence to support the idea that the higher concentration of a substance, the faster it will react.

I think that the evidence, which I have received, is enough to reach a suitable conclusion, but there are a few flaws to the experiment (which I have mentioned). Apart from them, the experiment is fine.

Conclusion:

The rate of reaction is manipulated by the temperature, concentration, surface area, gas pressure and catalysts. In this experiment I changed the concentration whilst keeping the temperature, surface area, gas pressure and the use of catalysts the same. I determined that if you increase the concentration of the sodium thiosulfate, the frequency of collisions increased so the rate of reaction also increased. This is because the higher the concentration of sodium thiosulphate the more frequent the collisions will be between the sodium thiosulfate and hydrochloric acid particles. Therefore bonds will break quicker and new bonds will be formed which will make new products quicker. Therefore the rate of reaction will increase.

I have found out that there is a pattern, as the concentration increases the rate of reaction also increases. This is because a higher concentration of reactants leads to more effective collisions per unit time, which leads to an increasing reaction rate.

Because a more dilute sample of hydrochloric acid there will be less molecules of actual hydrochloric acid, and more water particles. This means that there is less chance of a hydrochloric acid molecule colliding with a molecule of sodium thiosulphate, and so there are less successful collisions in any given amount of time, so the rate of reaction is slower.

The results show that as the concentration of sodium thiosulphate increases the time taken decreases, therefore the rate of reaction increases. This is because if there is less water, the solution is more concentrated. The more concentrated the solution, the quicker the precipitate will form, (faster rate of reaction), and vice versa.

If you double the concentration then you will double the frequency of collisions between hydrochloric acid and sodium thiosulfate particles thus doubling the rate of reaction.

This may create a positive correlation on my graph between the rate of reaction and the concentration. This will probably create a smooth curve as I plot the readings from the outcome of my experiment.

WORK OUT GRADIENT FOR GRAPH

Evaluation:

TALK ABOUT COLOURIMETER

How good are your results then? error sources? can we improve the existing method? are there other experimental methods?

Do your results seem consistent and accurate - always refer directly to the graph or graphs in your analysis ... do any of the sets of results not fit in with the others?, do most/all sets of results fit a pattern?, are there any particular points that don't fit the pattern? (anomalies), can some results be ignored in drawing your conclusion(s)? if so, which results and why? QUOTE DIRECTLY - WITH REFERENCE TO YOUR GRAPH(s) Discuss possible sources of error which might lead to where these or any other factor OK? in other words how suitable was the method overall? Do think the results are reliable bearing in mind any anomalies? For the hydrochloric acid - sodium thiosulphate reaction think about the precipitate, observing it etc. What further experiments, using the same method or another method,

My results are not as accurate as it could be because I have not repeated it each concentration 3 times but only done it once.

Looking at my results table, I have found an anomalous result because as I increase the concentration of the water and the sodium thiosulphate the reaction time also increases, as you go down the table. But on my third test it suddenly drops to 07.25 seconds. This may have happened due to slow reaction-time when timing. As I draw my graph I expect to clearly see an outlier because of this odd result.

I feel the method that was used to collect the results was very successful because there were no errors in my results. My results were tested again in a repeat experiment, the results of which were almost identical to the first results, only fluctuating within one second. The most difficult part of the method was measuring accurately the amount of sodium thiosulphate, HCL and deionised water in the measuring cylinders. I found this difficult because the cylinders were not as accurate as I would have liked and relied on my eye to judge when the liquid lever was equal with the intervals on the side of the cylinder.

The method used produced results accurate enough to show to show that as the concentration of sodium thiosulphate increases, so does the rate of reaction, but the method was not perfect as it was subject to human error.

The results were not as accurate as they possibly could be because of various constrictions in the lab in which the experiment was carried out. The most accurate measuring cylinders used to measure the amounts of acid and sodium thiosulphate and rely on the human eye to judge when the amount is accurate. This could have caused slight errors in the amounts of solution or acid being used. A way to correct this could be to use a more accurate piece of measuring equipment such as a pipette.

Another possible problem could come from judging when the black X has disappeared from view. The human eye is not accurate enough to judge the instant the black X disappears consistently. This could be solved by using something more accurate to measure how much precipitate has formed, and when the amount formed has blocked the X on the paper underneath the flask.

Bibliography