OZ - kinetics *02

Question 1

what is activation enthalpy?

Answer 1

the energy that pairs of molecules must possess to react when they collide

Question 2

how must particles collide to react

Answer 2

collision theory

collide in the right direction (facing each other the right way)
collide with the minimum amount of energy needed (activation energy)

Question 3

enthalpy profile diagram - endo

Answer 3

reactants below products

Question 4

enthalpy profile diagram - exo

Answer 4

reactants above products

Question 5

Boltzmann distribution graphs - what does it show?

Answer 5

shows how molecules in a gas dont all have the same amount of energy

Question 6

Boltzmann distribution graphs - what does it look like?

Answer 6

curve starts at (0,0) cause no molecules have zero energy

a few molecules are moving very slowly

most molecules are moving at a moderate speed

some molecules have more energy than the activation enthalpy
—> these are the only ones that can react

Question 7

increasing temp. effect on rate - Boltzmann Distribution graph

Answer 7

particles, on average, have more kinetic energy and will move faster

so a greater proportion of molecules will have the activation energy and be able to react

• changes the shape of the curve —> it pushes to the right

Question 8

increasing temp. effect on rate - collision theory

Answer 8

particles on average have more kinetic energy and will move faster, so a greater proportion of molecules will have the activation energy and be able to react

so the frequency of collision that result in a reaction will increase
—> more successful collision per unit time (increases rate)

also because more kinetic energy there are more collisions that take place

(collide more are more likely to react when they collide)

Question 9

increasing conc. effect on reaction rate

Answer 9

increasing conc. of reactants in solution = the particles are closer together on average

so they collide more often

more collision = more chances to react

Question 10

increasing pressure effect on reaction rate

Answer 10

increase pressure of a gas = particles closer together on average

they collide more often = more chances to react

Question 11

catalysts effect on reaction rate

Answer 11

catalysts lower the activation enthalpy by providing a different way for the bonds to be broken and remade

activation enthalpy lower = more particles have enough energy to react

Question 12

how do you find the rate of reaction - experiments

Answer 12

monitor loss of reactant or formation of product

• vol. of gas produced
• loss of mass
• change in pH
• temperature change
• taking samples at regular intervals and analysing them by titration

Question 13

how do you find the rate of reaction - graph

Answer 13

use data from the experiment to plot a graph
- time on x-axis, change on y-axis

to find the reaction at a particular time

• draw a tangent to the curve at that time: gradient of tangent = rate of reaction
• rate units = change / unit of time

Question 14

rate of reaction - how does it change over a experiment?

Answer 14

rate always fastest at the beginning

as a reaction continues the conc. of reactants decreases

so there will be less frequent collisions between reactant particles and the rate will decrease

Question 15

role of catalysts

Answer 15

to increase rate of reaction by providing an alternative reaction pathway with a lower activation enthaply

catalyst chemically unchanged at the end of the reaction

Question 16

catalysts - two types

Answer 16

homogenous

heterogenous

Question 17

homogeneous catalyst - what?

Answer 17

a catalyst that is in the same state as the reactants

Question 18

homogeneous catalyst - how does it work?

Answer 18

forms intermediates

1. speeds up rate by forming one or more intermediate compounds with the reactants.
   - –> the products are then formed from the intermediate compounds
2. activation energy needed to form intermediates and then products LOWER than activation energy needed to make products directly from reactants
3. the catalyst is reformed at the end and is chemically unchanged

Question 19

homogeneous catalyst - enthalpy profile

Answer 19

profile would have two humps
- lower than the uncatalysed reaction hump!

intermediates are formed at the trough between the humps

Question 20

homogeneous catalyst - Boltzmann Distribution graph

Answer 20

line for activation energy moves back
- so there are molecules that have enough energy to react in catalysed reaction but not enough to react when uncatalysed

the curve stays the same

Question 21

homogeneous catalyst - example

Answer 21

chlorine radical

breakdown of ozone