3.2 Physical chemistry

Question 1

Describe an exothermic enthalpy profile

Answer 1

reactant line above products line

Question 2

Describe an endothermic enthalpy profile

Answer 2

reactant line below products line

Question 3

What is activation energy?

Answer 3

the minimum amount of energy required for a reaction to take place

Question 4

What are the standard conditions?

Answer 4

298 K
100 kPa

Question 5

What is the enthalpy change of formation?

Answer 5

the energy required to form 1 mole of a compound from its elements

Question 6

What is the enthalpy change of combustion?

Answer 6

energy required for the complete combustion of 1 mole of a substance

Question 7

What is the enthalpy change of reaction?

Answer 7

the energy change associated with a given reaction

Question 8

What is the enthalpy change of neutralisation?

Answer 8

energy required to form 1 mole of water from neutralisation

Question 9

Define ‘average bond enthalpy’ ?

Answer 9

the mean energy needed for 1 mole of a given type of gaseous bonds to undergo homolytic fission - breaking of a covalent bond

Question 10

What is an endothermic reaction (in terms of bond breaking/making)

Answer 10

more energy needed to break bonds than make new bonds
reaction takes in energy

Question 11

What is an exothermic reaction (in terms of bond breaking/making)

Answer 11

more energy released when new bonds formed than needed to break bonds
reaction releases energy

Question 12

What factors affect a rate of reaction?

Answer 12

temperature
pressure
catalyst
surface area
concentration

Question 13

Describe the effect of concentration on the rate of reaction, in terms of collisions.

Answer 13

increased concentration - more molecules in same volume
molecules will be closer together so greater chance of them colliding with enough energy to overcome activation energy
so more frequent collisions

rate increased

Question 14

Describe the effect of pressure on the rate of reaction, in terms of collisions.

Answer 14

increased pressure - molecules are pushed closer together
same number of molecules, in smaller volume
more collisions occur with sufficient energy to overcome activation energy

rate increases (only for gaseous)

Question 15

Define a ‘catalyst’

Answer 15

a substance that increases the rate of reaction without being used up. it provides an alternative route with a lower activation energy

Question 16

What is a homogenous catalyst?

Answer 16

a catalyst in the same state/phase as the reactants
e.g. liquid catalyst with liquid reactants

Question 17

What is a heterogenous catalyst?

Answer 17

catalyst in a different state/phase as the reactants
e.g. liquid reactants with solid catalyst

Question 18

What are the economic and sustainability benefits of using a catalyst?

Answer 18

catalysts lower energy demands of processes - reduce costs, less fossil fuel needs to be burnt(combusted) to generate required energy, lower CO2 emissions

Question 19

Name some techniques that can be used to investigate reaction rates.

Answer 19

• titrations (monitoring concentration changes)
• using gas syringe to measure gas volume of products
• monitoring a reaction on a balance - mass of substance formed/decreasing mass of reactants

all over a period of time

Question 20

Analyze the Boltzmann distribution

Answer 20

The area under the curve = total number of molecules in a sample
There are no molecules with 0 energy
x axis is not touched
only molecules greater then Ea have enough energy to react

x axis = energy
y axis = number of molecules with a given energy

Question 21

Describe and explain what happens to the Boltzmann distribution if temperature increases

Answer 21

the peak of the curve is lower and moves to the right - a higher energy
Ea section increases vertically, but Ea line does not move

more collisions take place in a certain length of time - molecules have more kinetic energy (moving faster)
higher propertion of molecules have an energy greater than the Ea - more successful collisions occur
rate of reaction increases

Question 22

Describe and explain what happens to the Boltzmann distribution if a catalyst is added

Answer 22

The curve shape does not change but the catalyst lowers the Ea

more molecules will overcome the new lower Ea of the reaction.
there will be more successful collisions in a certain length of time
rate of reaction will increase

Question 23

What is dynamic equilibrium?

Answer 23

exists in a closed system when the rate of forward reaction is equal to the rate of the reverse reaction
concentrations of products and reactions stay the same

Question 24

What is the effect of increasing concentration on equilibrium?

Answer 24

reactant - equilibrium shifts the the right, forming more products
products - equilibrium shifts to left, forming more reactants

Question 25

What is the effect of pressure on equilibrium?

Answer 25

increasing - equilibrium shifts to the side with fewer moles
decreasing - equilibrium shifts to side with more moles

Question 26

What is the effect of changing the temperature of an exothermic reaction?

Answer 26

increasing - equilibrium shifts in endothermic direction (left)

decreasing - equilibrium shifts in exothermic direction (right)

Question 27

What is the effect of changing the temperature of an endothermic reaction?

Answer 27

increasing - equilibrium shifts in endothermic direction (right)

decreasing - equilibrium shifts in exothermic direction (left)

Question 28

What is the effect of a catalyst on equilibrium?

Answer 28

does not affect equilibrium position
speeds up rate of forward and reverse reaction equally

Question 29

What is the Haber process equation?

Answer 29

N2(g) + 3H2(g) <=> 2NH3(g)

Question 30

Explain how there must be a compromise between chemical equilibrium and reaction rate?

Question 31

What does Kc value indicate?

Answer 31

> 1 = equilibrium towards reactants
1 = dynamic equilibrium
<1 = equilibrium towards products