Rates Of Reaction

Question 1

What is the rate of reaction?

Answer 1

The change in concentration of a reactant/product per unit time

Question 2

As a reaction proceeds, it starts off at a fast pace and then it slows down, why?

Answer 2

After some time, the reactants become less concentrated as they are being used up and therefore the amount of reactants decreases

Question 3

How does the reaction proceed according to the graph?
Curve

——————-
/
/
/

Answer 3

Steep initially indicated a fast rate and as the rate decreases it levels off
The reaction is complete when the graph levels off or has no slope

Question 4

Two graphs of concentration, have to draw half conentration of reactant. Distinguish

Answer 4

Start of reaction, the half concentration is not as steep and will level off sooner and at a lower level

Question 5

What factors affect the rate of reaction?

Answer 5

Nature of reactants
Concentration
Temperature
Particle size
Catalyst

Question 6

How does the nature of reactants affect rate of reaction?

Answer 6

Ionic - fast due to the fact you don’t have to break bonds before new ones can be formed unlike covalent

Question 7

How does particle size affect the rate of reactions?

Answer 7

The larger the surface area, the faster the rate of reaction will be. Smaller particles [powders] have a larger surface area and therefore are faster than large lumps

Question 8

What is needed for a dust explosion to occur?

Answer 8

Enclosed space
Oxygen
Source of ignition

Question 9

How does concentration affect the rate of reaction?

Answer 9

The higher the concentration, the faster the reaction and more product is formed in a shorter period of time.
Directly proportional

Question 10

How does temperature affect the rate of reaction?

Answer 10

The higher the temperature, the faster the reaction.

Question 11

What is a catalyst?

Answer 11

It’s a substance that alters the rate of a chemical reaction without being used up in the reaction

Question 12

Calcium carbonate [marble chips] reacts with hydrochloric acid according to the equation
CaCO3 + 2HCL + CO2 + H2O
Using simple experiments involving marble chips and HCL describe how you could demonstrate the effects of particle size

Answer 12

Equal masses of small particles [powder] and larger particles [lumps] were added to conical flask
HCL of equal volume and concentration from graduated cylinder is added
Time taken is noted - where there is no more gas bubbles, the vigour of the reaction is observed and small particles reacts quicker

Question 13

Calcium carbonate [marble chips] reacts with hydrochloric acid according to the equation
CaCO3 + 2HCL + CO2 + H2O
Using simple experiments involving marble chips and HCL describe how you could demonstrate the effects of concentration

Answer 13

Equal volumes of HCL of different concentrations [80cm3 + fill rest with water]
Equal masses of equal sized particle [powder]
Time taken is noted - no gas bubbles, the vigour is observed and the higher concentration is faster

Question 14

Catalytic converters are used in cars

Identify one reaction that is catalysed in the catalytic converter in a care and state one environmental benefit

Answer 14

2CO + 2NO = 2CO2 + N2
Benefit :
Decline in smog and acid rain
Decrease carbon monoxide emissions
Decline in killing of fish by NO
Decline in toxicity

Question 15

Name one element used as a catalyst in a catalytic converter

Answer 15

Palladium, Platinum, Rhodium

Question 16

What type of catalysis is involved in a catalytic converter?

Answer 16

Heterogenous, adsorption

Question 17

What is activation energy?

Answer 17

The minimum energy which colliding molecules need before they can react

Question 18

Give an example of a reaction involving a named catalyst

Answer 18

2H2O2 - 2H2O + O2

Catalyst -MnO2

Question 19

Why is leaded petrol not suitable for a car fitted with a catalytic converter

Answer 19

It is a catalytic poison
Catalytic converter is made up of a large surface area and coated with three metal catalysts. Lead occupies their active sites and poisons them

Question 20

In a reaction between zinc and hydrochloric acid it was observed that the rate of production of hydrogen decreased with time. Give two reasons why

Answer 20

As reaction goes on

• concentration of reactants decreases as they are being used up
• less reactants means less effective collisions to form products

Question 21

What is homogenous catalysis and give an example

Answer 21

When the catalyst and reactants are in the same phase

Example - KMno4 reacts with Fe^2+ ions with catalyst Mn^2+. They are all in solution

Question 22

What is heterogenous catalysis and give an example

Answer 22

When the catalyst and the reactants are in different phases

Example - methanol vapour reacts with oxygen gas with heated platinum as catalyst

Question 23

What is autocatalysis and give an example

Answer 23

It’s when a product of the forward reaction catalyses the reaction
KMnO4 reacts with Fe^2+ ions and product Mn^2+ catalyses the reaction

Question 24

What are features of catalysts?

Answer 24

They are not used up however may be physically changed
Specific
Can be reused
Can be poisoned
Act on forward and reverse directions of the same reactions

Question 25

Suggest how catalysts can differ

Answer 25

Greater/smaller surface area

Purer

Question 26

How could you reduce the rate of reaction that takes place in solution

Answer 26

Low concentration

Low temperature

Question 27

In the oxidation of methanol using platinum wire as catalyst -
State one observation made during this experiment

Answer 27

Platinum wire glows

Flame pops

Question 28

In the oxidation of methanol using platinum wire as catalyst -
Identify two major products

Answer 28

Methanal

H2O

Question 29

What term is usually used to describe the attachment of liquids or gaseous molecules to a solid surface?

Answer 29

Adsorption surface

Question 30

Explain how adsorption surface affects rate of oxidation

Answer 30

Reacting substances adsorb onto surface of catalyst and this causes their concentration to build up and increases the rate of reaction.

Question 31

Give one way that catalysts increase the rate of reaction

Answer 31

Lower activation energy

Question 32

Name a substance that could poison the catalysts of the catalytic converter

Answer 32

Lead

Question 33

Explain clearly why there is an almost instantaneous reaction /quick between sodium chloride and silver nitrate

Answer 33

BONDS
One metal and one non metal
IONIC BONDING - don’t need to break bonds to form new ones

Question 34

When do you use curvey graph

Answer 34

-manganese dioxide and marble chips

Question 35

Explain effective collisions

Answer 35

When particles reach activation energy and react to form product

Question 36

In a reaction mixture, what effect if any does an increase in temperature have on

• no. of collisions
• effectiveness of collisions
• activation energy

Answer 36

• inc
• inc
• lowers

Question 37

Give another example of heterogenous catalysis

Answer 37

Decomposition of Hydrogen peroxide

Question 38

Why is the graph steepest at the beginning

Answer 38

At the beginning of the reaction is when the concentration is highest therefore there is more collisions

Question 39

Why does the rate of chemical reactions decrease over time?

Answer 39

Concentrations decrease as reactants used up

Question 40

The oxidation of potassium sodium tartrate by hydrogen peroxide catalyses by cobalt (II) ions provides evidence for the intermediate formation theory. State observations and explain how they prove theory

Answer 40

Pink - green - new substance made
Back pink - intermediate is formed
Bubbling/fizzing - means intermediate is reacting