Reaction rates and chemical equilibrium

Question 1

in order for a reaction to occur between particles

Answer 1

• particles must collide and must be in a correct orientation
• collisions must be sufficiently energetic for the activation energy barrier to be overcome and for bonds to be broken

Question 2

what is ea

Answer 2

the minimum energy required for a reaction to occur

Question 3

if particles have E

Answer 3

no reaction occurs

Question 4

if particles have E>EA

Answer 4

reaction should occur

Question 5

boltzmann distribution curve

Answer 5

• distribution curve at 0,0
• very few particles have high energy
• no maximum energy so line does not meet x axis
• only particles with E>EA can react on collision, shown by the shaded area on the right of the Ea

Question 6

rate of reaction

Answer 6

the change in concentration of reactants or products with time

Question 7

if concentration of reactant increases

Answer 7

• rate increases
• more particles in a given volume
• collision frequency increases

Question 8

as pressure increases

Answer 8

particles are closer together - greater number in a given volume and collide more frequently

Question 9

surface area

Answer 9

• reducing particle size for the same mass increases the surface area of a solid
• more particles exposed on the surface, greater frequency of collisions between the solid particles and mobile reactants, increasing rate

Question 10

at higher temperatures

Answer 10

• paarticles have more energy so greater number of particles have E>EA
• greater proportion of collisions lead to a successful reaction
• particles move faster and collide more frequently

Question 11

catalyst

Answer 11

• speeds up rate but remains chemically unchanged
• provides an alternative reaction route with a lower activation energy
• a greater proportion of particles will have energy greater than the new lower EA
• a greater proportion of particles will have enough energy to react when they collide and a greater proportion of collisions will be succesful

Question 12

homogenous catalyst

Answer 12

have the same physical state as the reactants

Question 13

homogenous catalysts in esterification reactions

Answer 13

sulphuric acid

Question 14

heterogenous catalysts

Answer 14

have different physical state to the reactants (usually solid with gaseous reactants)

Question 15

example of a heterogenous catalyst

Answer 15

• iron in haber process
• v2o5 in contact process
• zieglar-natta catalyst (ticl4 and al2(ch3)6 in addition polymerisation

Question 16

how do heterogenous catalysts work

Answer 16

• reactant gas adsorbs on the catalyst surface
• bonds in the reactant molecules are weakened, lowering the EA
• reaction occurs, reactant bonds break and product bonds form
• product molecules desorb from the catalyst surface

Question 17

benefits of catalysts

Answer 17

• allow lower temp use while still achieving same rate of reaction
• reduce cost of reaction
• reduce the demand for non renewable fossil fuels
• reduce the amount of co2 through combustion of fossil fuels
• are not used up and so require only small amount
• allow a wider choice of reaction routes, possibly permitting use of a higher atom economy choice

Question 18

equilibrium state

Answer 18

• can be approached from either direction
• dynamic so both reactions occur at same time
• consistency of macroscopic properties eg temp pressure
• only exists in a closed system

Question 19

dynamic equilibrium

Answer 19

rate of forwards reaction is equal to rate of backwards reaction and concentration of all reactants remain constant

Question 20

le chateliers principle

Answer 20

the position of equilibrium will shift so as to minimise the effect of any change in conditions

Question 21

if more products are formed

Answer 21

the equilibrium has shifted to the right

Question 22

if more reactants are formed

Answer 22

the equilibrium has shifted to the left

Question 23

the contact process

Answer 23

2so2 + o2 ⇆ 2so3

Question 24

increasing temp contact process

Answer 24

• position of equilibrium will changein order to minimise the effect by absorbing energy and therefore decreasing the temperature
• endothermic reaction favoured so backwards reaction
  and equilibrium will shift to left

Question 25

increasing pressure contact process

Answer 25

- will change in order to minimise the effect by reducing the number of gas molecules present - therefore will favour the forwards reaction and will shift to the right due to fewer moles of product

Question 26

increasing concentration contact process

Answer 26

- will change in order to decrease the concentration of that species so the reaction where the species is a reactant will be favoured - eg if so2 is increased the forwards reaction will be favoured and equilibrium position will shift to the right

Question 27

catalyst equlibrium

Answer 27

will not affect the position of equilibrium but does increase the rate of reactions equally

Question 28

if forward reaction if exothermic and temp increases

Answer 28

- shifts to LHS and more reactants made

Question 29

if forward reaction is exothermic and temp decreases

Answer 29

shifts to rhs and more products made

Question 30

if forwards reaction is endothermic and temp increases

Answer 30

shifts to rhs and more products made

Question 31

if forwards reaction is endothermic and temp decreases

Answer 31

shifts to lhs and more reactants made

Question 32

haber process

Answer 32

materials: hydrogen (CH4 + h2O -> CO2 + 4H2) N2 (fractional distillation of liquid air) n2 + 3H2 ⇆ 2NH3

Question 33

haber process conditions

Answer 33

450˚c 150 atm finely divided iron

Question 34

temperature haber process

Answer 34

- forwards reaction is exothermic therefore favoured by low temperatures - low temps result in low rate of reaction - compromise temperature chosen that gives a reasonable yield at a reasonable rate

Question 35

pressure haber process

Answer 35

- forwards reaction produces fewer moles of gas therefore favoured by higher pressures - this gives a faster rate of reaction - high pressure is expensive to produce and are dangerous - compromise pressure used that gives a good yield and rate at a sensible cost

Question 36

catalyst haber process

Answer 36

increases rate of both reactions equally. permits use of lower pressure and temperature whilst still achieving overall rate increases yield

Question 37

what is the equilibrium constant Kc

Answer 37

a mathematical expression for the relationship between concentrations of the substances present once a reaction has reached equilibrium

Question 38

if Kc is large

Answer 38

equilibrium is to the right so lots of products and high yield

Question 39

if Kc is small

Answer 39

equilibrium is to the left so lots of reactants and low yield