Chapter 16 - Rates Of Reactions

Question 1

Rate of reaction

Answer 1

The change in concentration per unit time of any one reactant or product

Question 2

Average rate of reaction

Answer 2

The average rate of the reaction over the entire experiment

Question 3

Instantaneous rate of reaction

Answer 3

The rate of the reaction at a particular moment in time

Question 4

What are the factors affecting the rates of reactions?

Answer 4

Nature of reactants
Particle size
Concentration
Temperature 
Catalysts

Question 5

Why does the nature of the reactants affect the rate of reaction?

Answer 5

Reactions involving the breaking of covalent bonds have a higher activation energy as energy is required to break the bonds before new bonds can be formed. Ionic reactions just involve the coming together of charged particles and will not involve the breaking of bonds and so will have higher reaction rate and lower activation energy

Question 6

Why does particle size affect the rate of reaction?

Answer 6

The smaller the particle size, the greater the surface area exposed and so greater number of collisions, and also, the number of these collisions that are effective increases. Therefore there is increased reaction rate

Question 7

Why does concentration increase reaction rate?

Answer 7

If concentration increases so to does the number of collisions because particles are closer together. As a result, so will the number of effective collisions. Therefore there is an increase in reaction rate. Also, as the reaction proceeds, the concentration of reactants will decrease and so the rate of reaction will decrease over time

Question 8

Why does temperature affect the rate of reaction?

Answer 8

At an increased temperature, there is an increase in the number of collisions due to the increased energy of the particles. The increased energy of the collisions means more collisions reach the activation energy.

Question 9

Catalyst

Answer 9

Since that alters the rate of a chemical reaction but is not consumed in the reaction

Question 10

Negative catalysts/inhibitors

Answer 10

Slow down reaction

Question 11

Homogeneous catalysts

Answer 11

Both the reactants and the catalysts are in the same phase, i.e. there is no boundary between the reactants and the catalyst

Question 12

Phase

Answer 12

Specific state of matter that has uniform chemical and physical properties

Question 13

Heterogeneous catalysts

Answer 13

The reactants and catalysts are in different phases, i.e. there is a boundary between the reactants and the catalysts

Question 14

Autocatalysts

Answer 14

One of the products of the reaction acts as a catalyst for the reaction

Question 15

Give an example of a homogeneous catalyst

Answer 15

The decomposition of hydrogen peroxide solution by iodide ions; they are all in the same, liquid phase

Question 16

Give an example of a heterogeneous catalyst

Answer 16

The decomposition of hydrogen peroxide (liquid) by manganese dioxide (solid)

Question 17

Give an example of an autocatalyst

Answer 17

The reaction between potassium permanganate (KMnO4) and Fe2+ ions leads to the formation of Mn2+ ions which act as an autocatalyst for the reaction

Question 18

What are the two general mechanisms by which catalysis takes place?

Answer 18

1. The Intermediate Formation Theory

2. The Surface Adsorption Theory

Question 19

The Intermediate Formation Theory

Answer 19

States that a catalyst works through the formation of an intermediate compound. In the reaction, one (or sometimes more compounds) combine with the catalyst to form the intermediate compound. This then reacts with the other reactant to form the final product and regenerate the catalyst

Question 20

Give an example of a reaction the demonstrates the Intermediate Formation Theory

Answer 20

The oxidation of potassium sodium tartrate using hydrogen peroxide. Catalysed by the presence of Co2+ ions (pink)

Question 21

The Surface Adsorption Theory

Answer 21

The accumulation of substances only at the surface of another substance (heterogeneous)

Question 22

What are the stages of the Surface Adsorption Theory?

Answer 22

1. Adsorption stage
2. Reaction on the surface
3. Desorption stage

Question 23

Catalytic converter

Answer 23

Device fitted in the exhaust system of a motor vehicle which contains catalysts to convert pollutants in the exhaust gases to less harmful substances

Question 24

What does a catalytic converted look like?

Answer 24

It consists of a ceramic honeycomb inside a stainless steel case. The honeycomb is lined with platinum, palladium and rhodium catalysts

Question 25

What is the benefit of the honeycomb structure of the catalytic converter?

Answer 25

It provides a large surface area on which the reactions can occur

Question 26

Catalyst poison

Answer 26

Substance that makes a catalyst inactive

Question 27

What can poison the catalysts in a catalytic converter?

Answer 27

Lead (Pb)

Question 28

What are the assumptions of the Collision Theory?

Answer 28

○For the reaction to occur, reacting particles must collide with each other
○Collision only results in the formation of products if a certain minimum energy is exceeded in the collision

Question 29

Effective collision

Answer 29

One that results in the formation of new products

Question 30

Activation energy

Answer 30

The minimum energy that colliding particles must have for a reaction to occur, i.e, the minimum energy required for effective collisions between particles to occur

Question 31

Reaction profile diagram

Answer 31

A graph which shows the change in energy of a chemical reaction with time as the reaction progesses

Question 32

Activated complex

Answer 32

Intermediate substance formed by the collision of molecules with enough energy to react

Question 33

Exothermic or endothermic reaction? : products have less energy that the reactants

Answer 33

Exothermic

Question 34

Exothermic or endothermic reaction? : products have more energy that the reactants

Answer 34

Endothermic

Question 35

Why do catalysts affect the rate of a reaction?

Answer 35

They provide an alternative reaction pathway with a lower activation energy. Therefore, more reactant molecules now possess the energy required for effective collisions to occur and the rate of reaction incfreases

Question 36

Why do transition metals make good catalysts?

Answer 36

They have valence electrons in two shells instead of one and they can easily lend and take away electrons from other molecules