36: An Introduction to Rate of Reaction

Question 1

What is chemical kinetics?

Answer 1

Chemical kinetics is the branch of chemistry that measures the rates of chemical reactions, studies the factors that influence the rates and interprets the results at the molecular level.

Question 2

List two ways to measure the rate of a chemical reaction.

Answer 2

The rate of reaction can be measured by the rate of reactant consumption or the rate of product formation.

Question 3

What is the definition of rate of reaction?

Answer 3

Rate of reaction is the change in concentration or amount of a reactant or a product per unit time.
Rate = (change in concentration or amount of a reactant or product) / time.

Question 4

List two ways to determine the rate of reaction by measuring the rate of product formation.

Answer 4

1. Measure the amount of product formation over a fixed time. The larger the amount of product produced, the faster the rate of reaction.
2. Measure the length of time it takes to produce a fixed amount of product. The smaller the length of time, the faster the rate of reaction.

Question 5

How can the instantaneous rate of reaction be determined graphically?

Answer 5

Draw a tangent to the curve at the required time. The slope of the tangent is the instantaneous rate of reaction.

Question 6

How can the average rate of reaction be determined graphically?

Answer 6

Draw a line connecting the end-points of the curve in the required time period. The slope of the line is the average rate of reaction during this time period.

Question 7

How can the initial rate of reaction be determined graphically?

Answer 7

Draw a tangent to the curve at time = 0 s. The slope of the tangent is the initial rate of reaction.

Question 8

List 4 possible units for measuring the rate of reaction.

Answer 8

mol dm^-3 s^-1, mol dm^-3 min^-1, mol s^-1, cm³ s^-1,

Question 9

The chemical equation of a reaction is X₂(g) -> 2X (g). Explain the ratio between the rate of consumption of reactant and the rate of production of the product.

Answer 9

The ratio is 1:2.
The mole ratio of X on two sides o the equation is 1:2. As 1 mole of X₂ gas is consumed, 2 moles of 2X gas are produced. Hence the rate of consumption of the reactant is half the rate of production of product.

Question 10

List 4 methods for following the progress of a reaction.

Answer 10

1. Measuring the change in the volume of a gaseous product or reactant.
2. Measuring the change in mass of the reaction mixture.
3. Measuring the change in pressure of the reaction mixture.
4. Measuring the change in colour intensity of the reaction mixture.

Question 11

State one limitation of measuring the change of mass of the reaction mixture to follow the progress of the reaction.

Answer 11

The reaction must involve a gaseous product or gaseous reactant, and the set-up must allow gaseous products to escape / allow gaseous reactants to enter.

Question 12

State one limitation of measuring the change of colour intensity of the reaction mixture to follow the progress of the reaction.

Answer 12

There must be only 1 coloured reagent.

Question 13

List 3 requirements for the experimental set-up measuring the change in volume of gaseous products.

Answer 13

1. The reactants are kept separate initially.
2. The start time can be accurately recorded by mixing the reactants without loosening the stopper.
3. Any gas product formed is tightly stoppered in the chamber and will not escape.

Question 14

Explain the levelling off of the total volume of carbon dioxide produced in the reaction where calcium carbonate reacts with excess hydrochloric acid.

Answer 14

The reaction finishes and no more gas is produced. This is because all calcium carbonate is used up as it is the limiting reagent.

Question 15

Describe the experimental set-up used to follow the progress of a reaction by measuring the change in mass of the reaction mixture.

Answer 15

The reaction mixture is put in a conical flask stoppered with cotton wool, and the beaker is put on an electronic balance.

Question 16

State the use of the cotton wool in measuring the change in mass of a reaction mixture.

Answer 16

It prevents the reaction mixture from spilling out of the flask, but also allows gaseous products to escape into the air.

Question 17

Explain whether measuring the change in mass of the reaction mixture is a suitable method to follow the progress of the reaction between magnesium and hydrochloric acid.

Answer 17

No.
In the reaction between magnesium and hydrochloric acid, hydrogen gas is the only gaseous product produced. However, it has a very low molar mass, and its escape does not cause a significant decrease in mass of the reaction mixture.

Question 18

State a limitation for measuring the progress of a gas-liberating reaction by measuring the change in mass of reaction mixture.

Answer 18

The gaseous product must have a relatively high molar mass for the decrease in mass of reaction mixture to be significant.

Question 19

State three precautions of measuring the change of pressure of the reaction mixture to follow the progress of a gas-liberating reaction.

Answer 19

The container must have a fixed volume and be able to withstand high pressure. The whole set-up should be stopper tightly to ensure there are no leaks.

Question 20

State two advantages of using a pressure sensor and data-logger to record the change in pressure over recording the pressure manually with a bourdon gauge.

Answer 20

1. The pressure readings can be taken continuously by the data-logger.
2. The curve of the variation of pressure with time can be automatically plotted.

Question 21

Name the apparatus used to measure the colour intensity of a reaction mixture.

Answer 21

Colorimeter

Question 22

Explain the choice of filter colour of a colorimeter.

Answer 22

The complementary colour should be selected for the filter to generate monochromatic light of that colour. This ensures that the provided colour of light is absorbed most by the coloured chemical species in the sample.

Question 23

What is the chemical equation of the reaction between propanone and iodine water?

Answer 23

CH₃COCH₃ (aq) + I₂ (aq) -> CH₃COCH₂I (aq) + H⁺ (aq) + I^- (aq)

Question 24

In acidic solution, propanone and iodine water react to form a colourless solution.
State two ways to follow the progress of the reaction.

Answer 24

1. By measuring the change in colour intensity using a colorimeter.
2. By quenching the reaction with alkali, and measuring the time taken for sodium thiosulphate solution to turn turbid when the reaction mixture is added. (this gives a measure of the amount of iodine left in the reaction mixture.)

Question 25

What is absorbance?

Answer 25

Absorbance is a **measure of the colour intensity** of a sample, and determined by comparing the intensity of the incident light to the intensity of light after it has passed through the sample. **The darker the colour of the sample, the higher than absorbance**.

Question 26

What is the chemical equation when ethanedioate ions (C₂O₄^2-) react with permanganate ions?

Answer 26

2MnO₄^- (aq) + C₂O₄^2- (aq) + 16H⁺ -> 2Mn²⁺ (aq) + 10CO₂ (g) + 8H₂O (l)

Question 27

Ethanedioate ions react with permanganate ions to produce Manganese(II) ions, water, and carbon dioxide. State four ways to follow the progress of the reaction.

Answer 27

1. By measuring the change in **colour intensity** of the reaction mixture. 2. By measuring the change in **gas pressure** of the reaction mixture in a **rigid sealed container**. 3. By measuring the change in **volume** of the reaction mixture in a **expandable sealed container**. 4. By measuring the change in **mass** of the reaction mixture in an **open container**.

Question 28

Ethanedioate ions react with permanganate ions to produce Manganese(II) ions, water, and carbon dioxide. Explain the change in absorbance of the reaction mixture over time.

Answer 28

The absorbance of the reaction mixture **falls** with time as the purple permanganate ions are consumed, and the colour intensity of the reaction mixture decreases.

Question 29

What is the chemical equation when sodium thiosulphate solution reacts with dilute hydrochloric acid?

Answer 29

Na₂S₂O₃ (aq) + 2HCl (aq) -> 2NaCl (aq) + SO₂ (aq) + S (s) + H₂O (l)

Question 30

Describe the procedure which can be used to study the rate of reaction between sodium thiosulphate solution and dilute hydrochloric acid.

Answer 30

1. Place the sodium thiosulphate solution in a conical flask and put the flask on a **cross** drawn on a piece of white paper. 2. Quickly add hydrochloric acid to the flask and start the stopwatch. 3. Look at the cross from above. The cross grows fainter as sulphur is formed and precipitated. 4. Stop the stopwatch when the cross becomes invisible.

Question 31

What is the average rate of reaction from the start to the turbid stage when sodium thiosulphate solution is reacted with dilute hydrochloric acid?

Answer 31

(Amount of sulphur needed to obscure the cross) / time to reach the turbid stage.

Question 32

What is the definition of quenching?

Answer 32

Quenching refers to **stopping the reaction** in a sample before analysis.

Question 33

List 3 ways in which a reaction can be quenched.

Answer 33

1. Cooling in ice rapidly 2. Diluting with a large volume of **ice-cold water** to lower the temperature and concentrations of the reactants. 3. **Removing the catalyst** of the reaction.

Question 34

State one precaution when following the progress of a reaction by titrimetric analysis.

Answer 34

The titration has to be carried out **rapidly** even after quenching the sample of reaction mixture.

Question 35

Methyl ethanoate can be hydrolysed using sodium hydroxide solution. Describe the steps to follow the progress of the reaction by titrimetric analysis.

Answer 35

1. Take a small sample of known volume from the reaction mixture quickly after the reaction has started. 2. Quench the sample by **dilution** with a **known** volume of ice-cold water and immediately record the time. 3. Repeat steps 1 and 2 at regular intervals. 4. Determine the concentration of sodium hydroxide remaining in each sample by titrating it with standard hydrochloric acid.

Question 36

State a limitation of titrimetric analysis.

Answer 36

It is only suitable to follow the progress of **slower** reactions taking over 1 hour to complete.