14 - rates and equilibrium

Question 1

Define rate of reaction

Answer 1

Change in concentration of a reactant or product in a given time

Question 2

What is the equation for rate of reaction?

Answer 2

Rate = delta c / delta t

Where c is concentration and t is time

Question 3

What is the unit of rate of reaction?

Answer 3

mol dm^-3 s^-1

Question 4

What 4 things increase the rate of reaction?

Answer 4

If you increase the following, you increase rate of reaction:

Concentration (pressure for gases)

Surface area

Temperature

You can also increase rate of reaction by adding a catalyst

Question 5

Define collision theory

Answer 5

Reactions occur when reactant particles collide provided that they collide with a minimum kinetic energy (the activation energy)

Question 6

What statement do you always put at the end of an explanation to why rate of reaction is increased?

Answer 6

“There are more frequent successful collisions”

Question 7

How does orientation come into collision theory?

Answer 7

Molecules must collide in the correct orientation in order for a reaction to happen

Question 8

How does increasing concentration / pressure increase rate of reaction?

Answer 8

There are more molecules per unit volume so you get “more frequent successful collisions”

Question 9

How does increasing temperature increase rate of reaction?

Answer 9

Increasing temperature increases the kinetic energy of molecules so you get “more frequent successful collisions”

Question 10

How does increasing surface area increase rate of reaction?

Answer 10

More surface area that can collide so there are “more frequent successful collisions”

Question 11

Define catalyst

Answer 11

A substance that increases the rate of reaction without being used in the process by providing an alternative route for the reaction to lower the activation energy

Question 12

What happens to the catalyst throughout the reaction?

Answer 12

It initially reacts and forms different intermediate products but it then regenerates at the end so none has been used up

Question 13

What is the difference between a homogeneous and a heterogeneous catalyst?

Answer 13

Heterogeneous catalyst is in different state of matter to the reactants

Homogeneous catalyst is the same state

Question 14

What do “adsorbed” and “desorbed” mean?

Answer 14

Adsorbed means stuck to the surface

Desorbed means unstuck from the surface

Question 15

Give 3 examples of heterogeneous catalysts

Answer 15

V~2 O~5 (vanadium pentoxide)

Iron

Nickel

Question 16

How do heterogeneous catalysts work?

Answer 16

Reactants are ADSORBED onto surface of catalyst

Reaction takes place, old bonds weaken and new bonds form

Products are DESORBED from catalyst and diffuse away

Question 17

Give an example of a homogeneous catalyst

Answer 17

The Cl• radical in ozone depletion

Question 18

What are the 2 main environmental effects of using catalysts?

Answer 18

Less energy used so less finite fossil fuels need to be used

Also, less fossil fuels so less polluting emissions like CO~2

Question 19

Give a well known reaction which uses a catalyst

Answer 19

Haber process

Question 20

What is the Boltzmann distribution?

Answer 20

It shows energy on the x axis and shows how many molecules will have that energy on the y axis

It gives a smooth curve

Question 21

Which molecules on the Boltzmann distribution can react?

Answer 21

The ones with higher energy than the activation energy

Question 22

What is the area under the curve equal to in the Boltzmann distribution?

Answer 22

The total number of molecules

Question 23

describe the Boltzmann graph

Answer 23

Starts at origin
-Shows that no molecules can have 0 energy

important to remember the line never touches the x axis, even at the end as it approaches 0, since no molecules can have 0 energy

Question 24

What is a reversible reaction?

Answer 24

Products and reactants are in equilibrium and the reaction doesn’t go to completion

Question 25

How do you show a reversible reaction in an equation?

Answer 25

Draw an equals sign

Put a forward facing half arrow on the top stick

Put a backwards facing half arrow on the bottom stick

Question 26

Define dynamic equilibrium

Answer 26

Where the rate of the forwards reaction is equal to the rate of the backwards reaction

Question 27

What is equilibrium position?

Answer 27

Extent of a reaction at equilibrium

symbol: A triple equals sign, then “m”

Question 28

What is Chatelier’s principle?

Answer 28

When a system in dynamic equilibrium is subjected to change, the equilibrium position will shift to minimise the change

Question 29

What happens to equilibrium position when concentration of reactants is increased?

Answer 29

Increasing conc. of reactants favours forwards reaction

Decrease this favours the backwards reaction

Question 30

When is equilibrium position affected by pressure?

Answer 30

Only when gases are present

Question 31

What happens to equilibrium position when pressure is increased?

Answer 31

The reaction where the product has less moles is favoured when pressure is increased

Question 32

What happens to equilibrium position when temperature is increased?

Answer 32

What happens to equilibrium position when temperature is increased?

Question 33

How can you see which reaction is endothermic in an exam question?

Answer 33

The enthalpy of the forwards reaction will be given

Question 34

What are the pressure and temperature for the Haber process?

Answer 34

High pressure and low temperature to favour the forward reaction

Question 35

What are the reasons for not making the temperature too low and not making the pressure too high in the Haber process?

Answer 35

Very high pressure is dangerous

Very low temperature would decrease rate of reaction so a compromise needs to happen

Question 36

What catalyst is used in the Haber process?

Answer 36

Iron catalyst

Question 37

What are the actual values for the temperature and pressure of the Haber process?

Answer 37

350 - 500 C temperature

100 - 200 atm (atmospheres) pressure

Question 38

What percentage of reactants in the Haber process are turned into ammonia?

Answer 38

Only 15% of reactants

Question 39

What do square brackets mean?

Answer 39

The concentration of the substance inside the square brackets

Question 40

What is K~c and what does it tell us?

Answer 40

K~c is the equilibrium constant and it tells us if you favour reactants or products

Question 41

How do you calculate K~c for a given reaction?

Answer 41

You write the reaction equation

You calculate K~c by multiplying the concentrations of the products on the numerator

Then you multiply all concentrations of reactants and put on the denominator

Any “big” numbers in the reaction equation are turned into powers

Question 42

What is the value of K~c which tells you a reaction favours reactants or products?

Answer 42

If K~c < 1, reactants are favoured

If K~c > 1, products are favoured

If K~c = 1, products and reactants are favoured equally

Question 43

Give 2 features of a reversible reaction when dynamic equilibrium is set up

Answer 43

Rate of forwards reactions equals rate of backwards reaction

Concentrations of all substances REMAIN CONSTANT

Question 44

For a reaction in equilibrium between an acid and alkali

What effect does adding a DIFFERENT acid have and why?

Answer 44

Favours the reaction producing the alkali

Because concentration of H+ ions is greater