To investigate the different factors affecting the rates of reaction. The rate of reaction is the rate of loss of a reactant or the rate of formation of a product during a chemical reaction.

Chemical reaction:

Sodium thiosulphate + hydrochloric acid  ---->  sodium chloride  +  water  +   sulphur dioxide   +   sulphur

Apparatus:

• 1 X    conical flask
• 1 X    beaker
• 1 X    gauze
• 1 X    heatproof mat
• 1 X    tripod
• 1 X    stopwatch
• 2 X    measuring cylinders

• 1 X    Bunsen burner

• 1 X    white tile
• 1 X    pair of tongs
• 1 X   pipette
• Pair of goggles

Method:

• Mark X on a white tile with a black marker so that it is clearly visible even after a transparent flask is placed on top of it

• Place conical flask with 10cc of sodium thiosulphate

• Put 1M Hydrochloric acid into conical flask until solution is opaque

• Repeat this procedure for 0.8M, 0.6M, 0.4M, 0.2M hydrochloric acid

• Record the times for each solution of different concentrations of hydrochloric acid until solution is opaque and the X on the tile cannot be visible.

Diagram:

Fair test:

• In order for my findings to be clear and precise the experiment must be conducted in a fair manner.

This means;

• Ensuring the same standard each time for judging when the X has disappeared.

• Making sure that the measuring cylinders for the HCl and thiosulphate will not be mixed up and the two reactants do not get into any contact before the experiment

• The amount of Sodium thiosulphate will be 10cc each time, and the amount of Hydrochloric acid will be altered.

• Making sure the X is visible for the person looking through the flask and that the same person is used every time to look through it as allowing different people to take a judgement every time may cause inaccuracies in the readings as different people have different eyesight powers

• During the heating stage of the experiment, a blue flame will be used throughout.

• Also the same Bunsen burner and gas tap will be used to maintain a fair continuity throughout the experiment

• Another factor taken into consideration when gathering data for this experiment is the need to collect sufficient amounts of it. This requires the experiment to be carried out more that once with 2 or more repeats. I will also carry out a preliminary experiment to evaluate on changes which I will alter to ensure accurate and precise readings in the final experiment

 All of these precautions will make my final results more reliable and keep anomalies at a minimum so thus make the entire investigation more successful

Safety:

A pair of goggles must be worn during the heating part of the experiment in order to protect the eye as I am dealing with corrosive and dangerous chemicals.

 An apron or lab-coat must also be worn at all times during the experiment to protect the skin and clothing.

When handling hot beakers and measuring cylinders a pair of tongs will be used. A gauze and heatproof mat will be used while heating to avoid any damage to the equipments

Normal lab safety rules also apply to this experiment such as conducting it in a clean and spacious environment.

Scientific theory of collisions:

Using the scientific theory of collisions and the chemical equation I can explain exactly what happens:

Once I remove these spectator ions (the ions which do not collide or react I get):

If I link this equation to the collision theory I find that I was left with the ions, which do collide and actually result in a reaction. The sulphur which, is produced is the substance that makes the solution cloudy, sulphur dioxide is the foul smell which is evident in the gases given off

The sulphur and the sulphur dioxide were the result of the collision between 2H+ and SO

Predictions: 

I predict that as the concentration of the hydrochloric acid is altered or increased the rate of reaction will increase. This can be justified by relating to the collision theory. When the concentration of one of the reactants is increased, the particles will have more energy and thus move faster. Therefore they will collide more often and with more energy. Particles with more energy are more likely to overcome the activation energy barrier to reaction and thus react successfully.

Also, as the concentration of HCl is increased the time taken for the solution to be opaque will decrease. This is due to the collision theory. I

Ref: In the Chemistry for AQA (separate award) by Ann and Patrick Fullick ISBN: 9780435583910, it states that:

 “Increasing the concentration increases the frequency of collisions between reacting particles. If a solution is more concentrated it will have more particles of the reactants moving around it. The more “crowded” the reactant particles are, the more likely it is that they will bump into each other and a reaction will take place as the solutions of reacting particles are made more concentrated there are more particles per unit volume. This increases collisions between reacting particles are therefore reaction is more likely to occur.”

 All this can be understood better with full understanding of the collision theory itself:

Only a small percent result in a reaction to take place. This is due to the energy barrier to overcome which is great. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy required by a particle 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.

Activation Energy:

Varying the variables:

Here are some of the other variables, which are written in the book, 

Ref: In the Chemistry for AQA (separate award) by Ann and Patrick Fullick ISBN: 9780435583910

These variables may be altered, and the outcomes of their alterations are also given

Here are some of them that are mentioned in the book:

Concentration:

• The more concentrated the solution, the more particles are present and the greater the chance of successful collisions. Therefore if concentration is increased, the reaction rate is faster.

Pressure:

• If the pressure is increased the particvles in the gas are pushed closer. This increases the concentration and thus increasing the rate of reaction

Surface area:

• If a solid If the solid has a large surface area then there are more opportunities for collisions to occur between the reactants (solid and liquid only) because there are more opportunities for collisions to occur.

Temperature of reactions:

• When the temperature is low, the particles do not have much energy so the collision is unlikely to occur. However on heating, particles take in energy causing them to move faster and collide more often. Because the collisions have more energy they are more likely to be successful in breaking and reforming bonds therefore the rate of reaction will increase

Collision Theory:

            Low concentration/ low-pressure          High concentration/high pressure

Preliminary experiment (results):

I have done a preliminary experiment, which I will evaluate and analyse with my predictions.

Preliminary experiment (conclusion):

From the preliminary experiment results, it is very clear to conclude that as the concentration of HCl increases, the time taken for the solution to become opaque decreases. This is due to the frequent collisions of the particles in the solution with a high concentration. The frequent collisions from the high concentration of HCl produces more sulphur faster and thus making the solution opaque and the X on the white tile to disappear.

My predictions were quite true and remain conclusive to the fact that the concentration of one of the reactants affects the rate of reaction overall

However I did encounter some problems whilst conducting the experiment which, I will evaluate upon to ensure a more fair and accurate experiment in the final test.

Some of these errors include the making of the different concentrations of HCl acid. I did this using water and the HCl acid. This was done by mixing the required amount of HCl acid with some water to dilute it. Some of these include:

To make:

1M HCl acid   ------------->  add required amount of HCl acid with no water

0.8M HCl acid ------------>  add 8cc of Hcl and 2cc of water

0.6M HCl acid ------------>  add 6cc of HCl and 4cc of water

0.4M HCl acid ------------> add 4cc of HCl and 6cc of water

0.2M HCl acid ------------> add 2cc of HCl and 8cc of water

As all the solutions of Hydrochloric acid were made by hand and not ready made,  and human errors are inevitable therefore leading to changes in the time for solution to become opaque thus leading to an unreliable source of data. To fix this problem I can do two things:

1. Prepare the solution of the different concentrations of Hydrochloric acid to be ready made by a lab technician or someone senior with the appropriate tools

2. Prepare the concentrations using a magnetic stirrer when mixing the dilute solutions. By doing this I will be guaranteed a reliable solution with the right dilution strengths

I chose these concentrations of HCl acid as they are in lower frequent intervals, therefore leading to narrower margin of error and also since HCl acid is quite a strong and corrosive acid the lower the concentrations i.e. the dilute they are the safer the experiment is. It would also be easier to identify a trend from these intervals of HCl acid.

Another fault found whilst conducting the experiment is the fact that an ordinary black marker pen was used. This would mean that as the conical flask was placed each time a new concentration was tested some of the black faded causing the person who was looking through the flask to judge the time unreliably due to errors in the qualitative source. In the real experiment, drawing on the white tile with a permanent black marker would eliminate this error and make the data source more reliable and accurate

I can draw a graph to represent the preliminary experiment data.

Another factor taken into consideration will be the fact that more data needs to be collected to make the graph more clear, precise and accurate. I will also take an average of this data source to identify any anomalous results (if any).

Another form of representing the preliminary experiment data is by bar graphs. Above is a bar graph showing time for taken for solution to be opaque with different concentrations of Hydrochloric acid. At 1M of HCl acid it took the least amount of time for solution to become opaque due to kinetic theory of particles with varying concentrations. The trend suggests that as the concentration of HCl decreased the time taken for solution to become opaque increased. This is because there were less frequent collisions between particles and therefore the rate of reaction was slow thus leading to increase in the time taken for person to see the black cross marked on the white tile.