Kinetics I - Topic 9

Question 1

what is the activation energy

Answer 1

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

Question 2

what does the Maxwell Boltzmann distribution show

Answer 2

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

Question 3

draw and label the Maxwell Boltzmann distribution curve

Answer 3

pg 1 chemrevise

Question 4

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

Answer 4

Particles can gain energy through collisions

Question 5

what is the effect on increasing temperature on Maxwell Boltzmann curve

Answer 5

• the distribution shifts towards having more molecules with higher energies
• the Emp and mean energy shift to higher energy values although the number of molecules with those energies decrease
• The total area under the curve should remain constant
  because the total number of particles is constant
• a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases

Question 6

draw the Maxwell Boltzmann curve for increasing and decreasing temperature

Answer 6

pg 1 chemrevise

Question 7

what is the rate of reaction

Answer 7

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

Question 8

what is the initial rate

Answer 8

the rate at the start of the reaction where it is fastest

Question 9

what does increasing the concentration and pressure do to the rate

Answer 9

At higher concentrations (and pressures) there are more particles per unit volume and so the particles collide at a greater frequency and there will be a higher frequency of successful collisions increasing rate of reaction

Question 10

explain how does increasing the concentration affect the Maxwell Boltzmann curve

Answer 10

• If concentration increases, the shape of the energy distribution curves do not change (i.e. the peak is at the same energy) so the Emp and mean energy do not change
• The peak of the curves will be higher, and the area under the curves will be greater because there are more particles

Question 11

what is the affect of increasing temperature on the rate of reaction

Answer 11

At higher temperatures the energy of the particles increases. They collide more frequently and more often with energy greater than the activation energy. this causes the rate of reaction to increase

Question 12

what is the effect of increasing surface area on the rate of reaction

Answer 12

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

Question 13

how do catalyst increase the rate of reaction without being used up

Answer 13

by providing an alternative route with a lower activation energy, so if the activation is lower more particles will have energy greater than the activation so there will be a higher frequency of successful collisions

Question 14

what are heterogeneous catalyst

Answer 14

catalyst that are in a different phase (state) from reactants

Question 15

properties of heterogeneous catalyst

Answer 15

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

Question 16

what is a catalyst

Answer 16

A catalyst is a substance that accelerates the rate of a chemical reaction without being permanently altered by the reaction itself

Question 17

define adsorption

Answer 17

Reactant molecules attach to the catalyst surface.

Question 18

how do heterogenous catalysts work

Answer 18

1) adsorption of reactant at active sites on the surface leads to a catalytic action
2) This can result in the bonds within the reactant molecules becoming weaker, or the molecules being held in a more reactive configuration
3) new bonds will form between reactive species
4) Desorption occurs and the Product molecules detach from the catalyst surface.

Question 19

what are Homogeneous catalyst

Answer 19

catalyst that is in the same phase (state) as the reactants.

Question 20

properties of homogeneous catalysts

Answer 20

• This typically involves an aqueous catalyst in a solution of aqueous reactants.
• The reaction occurs throughout the mixture.

Question 21

how do homogenous catalyst work

Answer 21

1) Reactants combining with the catalyst to form an intermediate species.
2) The intermediate reacting to form products and regenerate the catalyst.

Question 22

effect on pressure on heterogenous catalysts

Answer 22

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 23

environmental benefits of catalyst

Answer 23

• 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

Question 24

economic benefits of catalyst

Answer 24

• Lower production costs - Catalysts often allow reactions to occur at lower temperatures, thereby reducing energy consumption.
• Increased efficiency - Faster reaction rates mean that more product can be produced in a shorter amount of time.

Question 25

key features of catalyst

Answer 25

• They are highly efficient - A small quantity of catalyst can facilitate the reaction of a large amount of reactants.
• They are usually very specific - Most catalysts only work for particular reactions.
• They participate in the reaction but are regenerated at the end.