Kinetics I

Question 1

Homogeneous catalyst

Answer 1

Catalyst in the same state as the reactant

Question 2

Heterogeneous catalyst

Answer 2

catalyst in a different state to the reactant

Question 3

Advantages of heterogeneous catalysts?

Answer 3

-More frequently used in industry because they’re easier to separate (e.g. if they are solid) so they can be reused + they don’t get mixed up with the product

Question 4

Economic Benefits of catalysts?

Answer 4

Because catalysts lower activation energy it means less input energy is required so it costs less.

If the reaction can occur at lower temperatures then less money goes into protecting the reaction chamber from heat loss to the surroundings

Because most industrial processes are exothermic (eg. Haber), at lower temperatures, the equilibrium shifts right and increases the yield

Question 5

How to: ??????

1) Find initial rate
2) Find rate of reaction between two points if it is a proper curve
3) Find AVERAGE rate of reaction between two given points

Answer 5

1) Draw a tangent against the very FIRST plotted point
2) Draw separate tangents at intervals between the two time points
3) Draw tangent to curve at those two given points, add their gradients and divide by 2.

Question 6

What does the collision theory state?

Answer 6

A reaction will not take place between two particles unless they collide in the correct orientation and at least a certain minimum amount of energy (activation energy)

Question 7

What does the Maxwell-Boltzmann graph show?

What does the area under the graph show?

Answer 7

How many molecules have a given amount of energy at a particular time.
Total number of molecules in the reaction mixture

Question 8

What to note about energy of molecules at a given temperature?

Answer 8

energy is always dispersed UNEQUALLY. Eg. at 50 degrees not ever single molecule would have 40J

Question 9

On a Maxwell Boltzmann graph what can you say about molecules either side of the activation energy?

How is this depicted?

Answer 9

All molecules under the curve to the left of the activation energy will not successfully react (because they haven’t reached the activation energy required) whereas
all the molecules to the right will react (because they have energies equal to or greater than the activation energy required)

This will be depicted because the one with the lower activation energy will have a greater area under the curve to the right of the activation energy (because more molecules have the energy equal to or greater than the activation energy required)

Question 10

How does a catalyst work?

Answer 10

Provides a surface over which particles can successfully collide to react.

This provides an alternate reaction pathway which lowers activation energy.

The catalyst has an active site and provides this area for particles to bind to and when the product is formed it is released from the catalyst.

Question 11

Effects of increasing

concentration
SA
catalyst
temperature
pressure

Answer 11

X

know this - fill in later

Question 12

How does a Maxwell - Boltzmann curve showing temperature at T1 (where T1 is a lower temp) change when temperature increases?

imagine the tallish curve on the left which usually represents T1

Answer 12

The curve decreases in height and moves to the right (this is the T2 peak)

Question 13

If a Maxwell-Boltzmann is moving left or right (it looks like this as a result of the width of the curve changing), why does its height also change?

Answer 13

Because when a curve moves, the reaction itself is the same (the energies of molecules change - not the reaction itself) so there still needs to be the same number of particles in the reaction mixture and therefore the same area under the graph.

[area under graph = no. molecules in reaction mixture]

Question 14

Maxwell Boltzmann graph can only be applied to

Answer 14

GASES

Question 15

How to change the position of the activation energy on a Maxwell-Boltzmann curve?

Answer 15

add a catalyst (Ea moves left) meaning more pa than the Ea.

Question 16

What to remember about the particles on the LHS of Ea?

Answer 16

don’t think they never react because they don’t have enough energy - this isn’t true becaus eone instant later that particle could suddenly have enough energy to react

Question 17

How to get my head around the larger area under the curve and after Ea when it shifts to the right

Answer 17

you have two curves on top of each other, exactly the same shape (imagine standard MB curve)

imagine a vertical line crossing down through both curves near to when they are going down to become parallel with the x axis

if one curve moves right now more of its thickness will be on the RHS past the vertical line

Question 18

Explain why the two Maxwell-Boltzmann temperature curves (high vs low temp) are different shapes?

Answer 18

the higher temp curve has moved right and flattened because the number of particles with energy equal to or exceeding activation energy has increased but there are still the same number of particles in the reaction mixture and so the surface area under the graph must be the same

Question 19

activation energy

Answer 19

minimum collision energy needed for particles to react in a collision. If the particles have an Ea greater than or equal then they will also have to be in the correct orientation to react

Question 20

What is the order of vertical lines on a Maxwell-Boltzmann diagram?

Answer 20

first line (nearer to LHS but still on far right) - line for if there is a catalyst (Ecat)
second line (Far right) - activation energy (Ea)

Question 21

What to remember about the MZ graph on effect of temperature?

Answer 21

two graphs, one activation energy

Question 22

What to remember about the MZ graph on effect of a catalyst?

Answer 22

one graph, two activation energies

Question 23

Imagine the vertical Ea line on an MB graph - how is this represented in a reaction profile diagram?

Answer 23

horizontal line reactant, horizontal line product to the right and below

in between a small curve jumping from both lines (its height starting from the reactant line is the Ea.

Question 24

What does the reaction profile for a catalyst look like?

How many curves?!

Answer 24

TWO CURVES!

horizontal line reactant (1), horizontal line product to the right and below(2)

IN BETWEEN - a small horizontal line(3) in between and just above the first line (this is the intermediate)

small curve going from (1) to (2) [its height from the reactant line is the Ea]
another small curve going from (2) all the way down to (3)

see sergy boy’s notes

Question 25

chemguide version of what a catalyst reaction profile looks like

Answer 25

horizontal line reactant (1), horizontal line product to the right and below(2)

first tall curve from (1) to (2) - [line from top of tall curve to the reactant line is the Ea without a catalyst]

second shorter curve from (1) to (2) - [line from top of short curve to the reactant line - Ea with a catalyst]

Question 26

what do catalysts provide? and what happens to their bonds?

Answer 26

a surface on which the reactants molecules can bind, as they do, their bonds get weaker

Question 27

example of a heterogenous catalyst

Answer 27

solid iron catalyst in Haber process is in a different phase to the nitrogen and hydrogen

Question 28

two examples of reactions (other than Haber) that use heterogenous catalysts

Answer 28

oxidation of ammonia by air catalysed by platinum

manufacture of hydrogen from steam and methane catalysed by nickel

Question 29

typical reaction route with a catalyst

Answer 29

gaseous reactants onto metal to form adsorbed reactants, go on to make adsorbed products (slow step), go on to become gaseous products and the catalyst surface is free again

Question 30

explain why producing ammonia in the Haber process would be an issue if there was no iron catalyst

Answer 30

the strength of the nitrogen-nitrogen triple bond means the reaction has a very high activation energy. Without the iron catalyst this would mean it would have to occur at super high temperatures and therefore because the reaction is exothermic, favor the backwards reaction, producing less ammonia. This is unfavorable so we use an iron catalyst so the reaction can happen at a reasonable temperature and we can still get a high yield.