topic 9: kinetics I

Question 1

what is the activation energy`

Answer 1

the minimum energy which particles need to collide to start a reaction

Question 2

when can reactions only occur

Answer 2

when collisions take place between particles having sufficient energy. the energy is
usually needed to break the relevant bonds in one or either of the reactant molecules

Question 3

what does the Maxwell Boltzmann energy distribution show

Answer 3

the spread of energies that molecules of a gas or liquid have at a particular temperature

Question 4

what does the area under the curve represent

Answer 4

the total number of particles present

Question 5

what does the Emp represent

Answer 5

the most probable energy (not the same as mean energy)
highest peak of the curve

Question 6

at the beginning of the curve (steep part of the curve), what does it represent

Answer 6

A few have low energies because collisions cause some particles to slow down

Question 7

why should the energy distribution go through the origin

Answer 7

because there are no molecules with no
energy

Question 8

why should the energy distribution never meet the x axis

Answer 8

as there is no maximum energy for molecules

Question 9

what do only a few particles have

Answer 9

energy greater
than the activation energy

Question 10

what is the label of the y - axis

Answer 10

fraction of molecules with energy

Question 11

how can a reaction go to completion if few particles have energy greater than EA

Answer 11

particles can gain energy through collisions

Question 12

what happens as the temperature increases

Answer 12

the distribution shifts
towards having more molecules with higher energies

Question 13

when temperature increases, why does the total area under the curve remain constant

Answer 13

because the total number of particles is constant

Question 14

when temperature increases, what happens to the Maxwell Boltzmann distribution curve

Answer 14

it goes lower and to the right

Question 15

what is the rate of reaction defined as

Answer 15

the change in concentration of a substance in unit time
- its usual unit is mol dm-3s-1

Question 16

how do you work out the reaction rate

Answer 16

gradient of tangent to curve

Question 17

in the experiment between sodium thiosulfate and hydrochloric acid, how is reaction rate normally measured

Answer 17

1/time where the time is the time taken for a cross placed underneath the reaction mixture to disappear due to the cloudiness of the sulfur
this is an approximation for rate of reaction as it does not include concentration. We can use this because
we can assume the amount of sulfur produced is fixed and constant

Question 18

what happens when you increase concentration and pressure

Answer 18

there are more particles per unit volume and so the particles collide with a greater frequency
and there will be a higher frequency of effective collisions

Question 19

what happens to the shape of the energy distribution curve when concentration increases

Answer 19

curves do not change (i.e. the peak is at the same energy) so the Emp and mean energy do not change
they curves will be higher, and the area under the curves will
be greater because there are more particles
more molecules have energy > EA
(although not a greater proportion)

Question 20

what happens to the rate curves the higher the conc/ temp/ surface area

Answer 20

steeper gradient

Question 21

what happens as the temperature increases

Answer 21

at higher temperatures the energy of the particles increases. they collide more frequently and more often with energy greater than the activation energy. more collisions result in a reaction

Question 22

what is the effect of increasing surface area

Answer 22

will cause successful collisions to occur more frequently between the
reactant particles and this increases the rate of the reaction

Question 23

what is a catalyst

Answer 23

a substance increases reaction rates without getting used up they do this by providing an alternative route or mechanism with a lower activation energy

Question 24

what happens to the reaction rate when the activation energy is lowered

Answer 24

more particles will have energy > EA, so there will be a higher frequency of effective collisions. the reaction will be fast

Question 25

what are heterogeneous catalyst

Answer 25

in a different phase from the reactants

Question 26

what state are heterogenous catalyst normally in

Answer 26

usually solids
whereas the reactants are gaseous or in solution. the reaction occurs at the surface of the catalyst.

Question 27

how does heterogeneous catalyst work

Answer 27

• adsorption of reactants at active sites on the surface
  may lead to catalytic action
• the active site is the place where the reactants adsorb on to the surface of the catalyst
• this can result in the bonds within the reactant molecules becoming weaker, or the molecules being held in a more reactive configuration
• there will also be a higher
  concentration of reactants at the solid surface so leading to
  a higher collision frequency

Question 28

what is the effect of pressure on heterogeneous catalysts

Answer 28

• increasing pressure has limited effect on the rate of heterogenous catalysed reactions because the reaction
  takes place on surface of the catalyst
• the active sites on the catalyst surface are already saturated with
  reactant molecules so increasing pressure wont have an effect

Question 29

how are catalysts used industrially

Answer 29

speeds up the rate allowing lower temperature to be used
(and hence lower energy costs) but have no effect on equilibrium

Question 30

what are the environmental benefits of catalysts

Answer 30

• catalysed reactions can occur at lower temperature so less fuel needed and fewer emissions from
  fuels
• catalysed reaction enables use of an alternative process with higher atom economy so meaning fewer raw materials needed and less waste products are produced