Determination Of A Rate Equation

Aim

My aim is to plan an experimental procedure, which will lead to a graphical method to determine how the concentration or volume of each component affects the rate of reaction of the equation:

2HCl(aq) + Na S O (aq)   2NaCl(aq) + SO (g) + S(s) + H O(l)

In a demonstration shown by the teacher who mixed together 50cm³ of 0.4 mol dmˉ³ Na S O (aq) with 5.0 cm³ of 0.2 mol dmˉ³ HCl(aq) diluted with 20cm³ of H O. The appearance of a precipitate turned the mixture solution milky yellow. The time taken for the mixture solution to turn completely milky white can be used to calculate the rate of reaction.

Equipment And Apparatus

1. Measuring Cylinders 25cm³, 100cm³
2. Measuring Syringe 10cm³, 5 cm³
3. Stop Clock.
4. Large Beaker.
5. Thermometer.
6. White Tile Marked With A Large Cross.
7. Distilled Water
8. 0.4 mol dmˉ³ Sodium Thiosulphate, Na S O (aq).
9. 0.2 mol dmˉ³ Hydrochloric Acid, HCl(aq).

Independent Variable And Dependent Variable.

The Independent Variable of this experiment is the concentration of one substance under investigation.

The Dependent Variable of this experiment is the rate of reaction.

Controlled Variables

Use Of Tile Marked With The Cross Use the same tile that is marked with the cross for each experiment. This is because if I use a different tile marked with a cross for each experiment may affect my results, as the tiles might have crosses different shades of black. Some crosses can be slightly darker, which could take more time to disappear, where as some crosses can be of a lighter shade, which will take less time to disappear.

Concentration Of Components Not Under Investigation If the concentration of any one chemical not under investigation is increased the rate of reaction will also increase as this is because there are more molecules so they are more likely to collide and react successfully. If the concentration of any one chemical is decreased the rate of reaction will also decrease. I will keep the chemicals not under investigation separate from the reaction and keep lids on top of the chemical containers.

Judgement Of When To Stop The Stopwatch Use the same person to make judgements of when the cross has disappeared, as different people can have different judgements based on when the cross has disappeared, which can affect my results.

Safety And Precautions

Safety for the reaction:

2HCl(aq) + Na S O (aq) 2NaCl(aq) + SO (g) + S(s) + H O(l)

To prevent any of the chemicals to get into the eye, wear eye protection such as Safety Spectacles. Also to protect the skin, wear Gloves on your hands and wear on a Laboratory Coat.

Hydrochloric Acid (HCL) This is an Irritant substance and produces a irritating vapour. To protect yourself against the irritating vapour use substance in the Fume Cupboards and in a well ventilated room. If HCL gets in contact with the skin and eyes wash thoroughly with plenty of water. Take off contaminated clothing. If split in the laboratory wash area with plenty of water.

Sodium Thiosulphate This is a Harmful chemical if ingested. If ingested drink plenty of water and seek medical advice. If Sodium Thiosulphate gets in contact with the skin and eyes wash thoroughly with plenty of water. Take off contaminated clothing. If split in the laboratory, scoop of as much solid as you can and wash area with plenty of water.

Sulphur Dioxide (SO (g)) Toxic by inhalation. Irritating to the eyes and respiratory system. If inhaled remove victim to fresh air to rest and keep warm. Also seek medical advice. Carry out the reaction in a fume cupboard to prevent the Sulphur Dioxide gas produced from escaping.

Procedure

Varying The 0.2 mol dmˉ³ Hydrochloric Acid, HCl(aq).

1. Put a large beaker on top of a white tile marked with a large cross.

2. Use a 10cm³ syringe to measure out the correct volume of 0.2 mol dmˉ³ HCl(aq) solution and transfer it to a 25cm³ measuring cylinder, make up the rest of the 25cm³ by adding distilled water. The correct volumes that are required of the 0.2 mol dmˉ³ HCl(aq) solution are written in Table 1.

3. Use a 100cm³ measuring cylinder to measure out 80cm³ of 0.4 mol dmˉ³ Sodium Thiosulphate, Na S O (aq). This is the concentration that we are going to keep constant.

4. You now have a 25cm³ measuring cylinder containing diluted 0.2 mol dmˉ³ HCl(aq) and a 100cm³ measuring cylinder containing 80cm³ of 0.4 mol dmˉ³ Sodium Thiosulphate, Na S O (aq).

5. Pour the contents of both the measuring cylinders into the large beaker on top of a white tile marked with a large cross at the same time and start the stop clock.

6. Record the time it takes for the large cross to disappear, due the appearance of the milky yellow precipitate.

7. Repeat these steps for all the volumes of 0.2 mol dmˉ³ HCl(aq) that I have varied in my set. These are found in Table 1.

8. Record the results in Table 1.

Varying The 0.4 mol dmˉ³ Sodium Thiosulphate, Na S O (aq).

1. Put a large beaker on top of a white tile marked with a large cross.

2. Use a 5cm³ syringe to measure out 5cm³ of 0.2 mol dmˉ³ HCl(aq). Pour this into a 25cm³ measuring cylinder. Make up the rest of the 25cm³ by adding distilled water. This is the concentration we are going to keep constant.

3. Use a 100cm³ measuring cylinder and measure the correct volume of 0.4 mol dmˉ³ Sodium Thiosulphate, Na S O (aq) required, this can be found in Table 2.

4. Make up to 80cm³ by adding distilled water.

5. Pour the contents of both the measuring cylinders into the large beaker on top of a white tile marked with a large cross at the same time and start the stop clock.

6. Record the time it takes for the large cross to disappear, due the appearance of the milky yellow precipitate.

7. Repeat these steps for all the volumes of 0.4 mol dmˉ³ Sodium Thiosulphate, Na S O (aq) that I have varied in my set. These are found in Table 2.

8. Record the results in Table 2.

Determination Of Rate Equation

To work out the rate equation you have to use the formula:

Rate =  1

Time

I will write the rate of each volume I vary in my range, in the appropriate tables

Order of reaction of each chemical can be worked out by comparing two experiments from each set by using the formula:

There are three types of orders of reaction, 0 Order, 1^st Order and 2^nd Order, which can be graphically displayed. The graphs below are the graphs that represent each Order.

 

The rate equation is written as:

Zero Order - The rate is constant and is unaffected by changes in concentration.

First Order - The rate doubles as the concentration doubles.

Second Order - The rate increases by four times as the concentration doubles.

You can work out the rate constant by rearranging the formula.