rates of reactions, maxwell boltzmann distribution curves

Question 1

why does increasing the temperature, increase the rate of reaction?

Answer 1

at a higher temperature, more particles have energy at/above the activation energy. so more collisions are successful, result in a reaction.

Question 2

what are 4 things you must remember when drawing a maxwell boltzmann distribution curve?

Answer 2

1. always start at the origin
2. reach a peak
3. draw on the Ea and shade on the right
4. never quite reach the X axis

Question 3

if there were two temperatures on the graph (T2>T1), what differences would there be between the 2 curves?

Answer 3

when compared to T1, T2s features would be:
- a lower curve, with the same origin and ending point
- Ea would start at the same point along the x axis
- there would be more particles at/above the Ea because of the lower curve

Question 4

how does a catalyst work?

Answer 4

a catalyst speeds up the rate of reaction by providing an alternative route with a lower activation energy (Ea).

Question 5

what happens when you represent a catalyst on a maxwell boltzmann distribution curve?

Answer 5

the Ea line shifts more to the left when compared with the reaction with no catalyst.

Question 6

why does the Ea line shift to the left when a catalyst is present?

Answer 6

with a catalyst these particles are at/above the new lower Ea so more collisions will result in a reaction, therefore the Ea line needs to cover more of the total area.

Question 7

what are the two types of catalysts?

Answer 7

1. heterogenous catalyst
2. homogenous catalyst

Question 8

what states are heterogenous and homogenous catalysts in when compared to the reactant?

Answer 8

heterogenous: catalyst and reactant are in different states
homogenous: catalyst and reactant are in the same state

Question 9

explain how a heterogenous catalyst works.

Answer 9

1. reactant adsorbs onto catalyst surface
2. bonds in the reactants weaken and break
3. new bonds form
4. product desorbs from the surface of the catalyst

Question 10

explain how a homogenous catalyst works.

Answer 10

the catalyst reacts with the product to give an intermediate/transition state.
this then reacts further to make the product and regenerate the catalyst.
on a bond enthalpy diagram this would be shown as two curves with a platform in the middle. the Ea of these two curves will add up to less than the curve without a catalyst.

Question 11

on a maxwell boltzmann distribution curve, where is the most probable energy of the particles?

Answer 11

at the peak of the curve, since this is the highest proportion

Question 12

what effect does raising concentration and pressure have on the rate of reaction?

Answer 12

molecules are more closely packed together this means that a collision is more likely to occur between two molecules, this means the rate of reaction is quicker

Question 13

define activation enthalpy?

Answer 13

Ea is the minimum energy required by a pair of colliding particles for the collision to result in a reaction

Question 14

define what a catalyst is

Answer 14

a catalyst is a molecule that speeds up the rate of reaction by providing an alternative route with a lower activation energy.
a catalyst is used up in one step, then remade in the next.

Question 15

what are the 6 methods for measuring reaction rates?

Answer 15

1. measure volume of gas given
2. measure mass lost
3. remove measured volumes at known time intervals and perform titrations for the concentrations
4. colourimetry
5. pH measurement
6. conductivity (before and after reaction, you can compare presents of ions)

Question 16

describe how a homogenous catalyst works

Answer 16

• the catalyst reacts with the reactant to form an intermediate/transition state
• this then reacts further to give the products and then regenerate the catalyst
• the activation energy of the two steps adds up to less than the activation energy of the reaction without a catalyst

Question 17

describe how a heterogenous catalyst works

Answer 17

1. reactant adsorbs onto catalyst surface
2. bonds in reactants weaken and break
3. new bonds are formed
4. products desorb from the catalysts surface

Question 18

what would an enthalpy profile diagram look like if the reaction involved a homogenous catalyst?

Answer 18

the profile would have two humps, pausing in the middle at an intermediate stage