Chapter 5 - Rate of Reaction

Question 1

What are some examples of slow reactions?

Answer 1

• Rusting of iron in air
• Fermentation
• Decomposition

Question 2

What are some examples of moderately fast reactions?

Answer 2

• Baking
• Reaction of Group I metals with water
• Contact process to manufacture sulfuric acid
• Haber process to manufacture ammonia

Question 3

What are some examples of very fast reactions?

Answer 3

• Explosion of petrol-air mixture

- Precipitation of silver chloride from aqueous sodium chloride and silver nitrate

Question 4

How can the rate of a chemical reaction be determined?

Answer 4

By measuring any property that changes over the course of the reaction

• amount of product formed
• amount of reactant(s) used up

Question 5

What are some common methods to determine the rate of a chemical reaction?

Answer 5

• Measuring mass or volume changes when a gas is evolved
• Absorption using a spectrometer when there is a colour change
• Measuring pH changes when there is a change in acidity
• Measuring electrical conductivity when there is a change in the concentration of ions

Question 6

Graph of a property against time: The instantaneous rate at any stated point in time is given by the ________ at that time

Answer 6

gradient of the graph

Question 7

The rate of a chemical reaction usually ________ with time as reactants are ________

Answer 7

decreases

progressively used up

Question 8

Formula to calculate rate of reaction

Answer 8

Rate of reaction = Change in (property) / Time taken

Question 9

What are the units for rate of reaction?

Answer 9

g/s
mol dm^-3/s
cm^3/s

Question 10

For a chemical reaction to occur, reacting particles must…

Answer 10

1. collide with each other

2. collide with energy that is equal to or greater than the activation energy

Question 11

What is activation energy?

Answer 11

The minimum amount of energy needed for the particles to react

Question 12

What are effective collisions?

Answer 12

Collisions between reacting particles with energy greater than the activation energy, that result in the formation of products

Question 13

What are the factors affecting the rate of a chemical reaction?

Answer 13

• Temperature of the reactants
• Concentration of the reactants
• Pressure of gaseous reactants
• Particle size or surface area of reactants
• Presence of a catalyst

Question 14

What is the effect of temperature on the rate of reaction?

Answer 14

Increased temperature → increased rate of reaction

rise in 10°C doubles the rate of reaction

Question 15

Explain the effect of temperature on the rate of reaction

Answer 15

Particles at higher temperature → Particles gain kinetic energy and move faster → Collisions between particles more frequent → Effective collisions more frequent → Higher speed of reaction

• As temperature increases, the particles gain kinetic energy and move faster, resulting in more collisions per second between the reactant particles
• A greater proportion of reactant particles also possesses the activation energy required to react with one another
• The reactant particles collide more frequently and more energetically, hence increasing the frequency of effective collisions

Question 16

What is the effect of concentration on the rate of reaction?

Answer 16

Increased concentration → increased rate of reaction

Question 17

Explain the effect of concentration on the rate of reaction

Answer 17

More concentrated solution → More particles occupying a given volume → Collisions between particles more frequent → Effective collisions more frequent → Higher speed of reaction

• The higher the concentration of reactants, the more reactant particles per unit volume
• The reactant particles collide more frequently, resulting in an increase in frequency of effective collisions

Question 18

What is the effect of pressure on the rate of reaction?

Answer 18

Increased pressure → increased rate of reaction

Question 19

Explain the effect of pressure on the rate of reaction

Answer 19

Gas at high pressure → Reacting gas particles are closer together → Collisions between particles more frequent → Effective collisions more frequent → Higher speed of reaction

• When the pressure of a gas is increased, the number of gas particles per unit volume is increased
• The gas particles collide more frequently, resulting in an increase in frequency of effective collisions

Question 20

What is the effect of particle size (surface area) on the rate of reaction?

Answer 20

Smaller particle size (larger surface area) → increased rate of reaction

Question 21

Explain the effect of particle size (surface area) on the rate of reaction

Answer 21

Smaller particles size → Greater surface area for reacting particles to collide into → Collisions between particles more frequent → Effective collisions more frequent → Higher speed of reaction

• For solid reactants, only the particles on the surface can come into contact with the surrounding reactant and take part in collisions
• When a solid is in powder form, the total surface area increases
• The gaseous or liquid reactant particles collide more frequently with the powdered solid, resulting in an increase in frequency of effective collisions

Question 22

What is the effect of presence of catalyst on the rate of reaction?

Answer 22

Presence of catalyst → increased rate of reaction

Question 23

Explain the effect of presence of catalyst on the rate of reaction

Answer 23

• Catalysts provide an alternative reaction path of lower activation energy
• A greater fraction of the particles will possess sufficient energy to overcome the activation energy, so the frequency of effective collision increases

Question 24

What is a catalyst?

Answer 24

A substance that

• increases the speed of a chemical reaction
• remains chemically unchanged at the end of the reaction

Question 25

What are the characteristics of a catalyst?

Answer 25

- Changes the speed but not the yield of a chemical reaction - Only a small amount is needed to change the speed of reaction - Not used up during the reaction: The same amount of catalyst is present at the beginning and at the end of the reaction - Physical appearance may change at the end of the reaction, but the chemical properties remain unchanged - Lowers the activation energy of a reaction - Selective in its action: Different catalysts speed up different reactions - Impurities can prevent it from working: The catalyst is "poisoned" or "inactivated"

Question 26

Use of iron as a catalyst in the industry

Answer 26

Manufacture of ammonia from nitrogen and hydrogen (Haber process)

Question 27

Use of platinum or rhodium as catalysts in the industry

Answer 27

Catalytic converters of car exhausts | - Catalyses the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen

Question 28

Use of aluminium oxide or silicon dioxide as catalysts in the industry

Answer 28

Production of hydrogen in the cracking process