Chapter 9 - Reaction kinetics (L)

Question 1

What is the equation for rate of reaction?

Answer 1

Rate of reaction = change in concentration (of product or reactant) / time for change to happen

Question 2

What do we need to find out to work out rate of reaction?

Answer 2

Either:

• how fast one of the reactants is being used up or
• how fast one of the products is being formed

Question 3

On a concentration-time graph, how do we calculate rate of reaction?

Answer 3

By using a tangent (y/x) to find the gradient of the line. This is the rate of reaction

Question 4

If there was a table containing results on the time taken to collect 20cm3 of gas, how would you calculate the rate of reaction?

Answer 4

Volume of gas collected / time taken = rate of reaction

Question 5

What are the two requirement for a reaction to occur?

Answer 5

• The molecules must collide in the correct orientation

- The molecules must collide with the necessary activation energy

Question 6

What is it called when the shapes of molecules influence reactions?

Answer 6

Steric factor

Question 7

What is steric hinderance?

Answer 7

When the atoms/groups of atoms in a molecule hinder the course of a reaction

Question 8

When does steric hinderance mostly occur?

Answer 8

When the atoms or groups of atoms are particularly large, so they can get in the way of/block an attacking species from reacting

Question 9

What is the purpose of cotton wool blocking the top of flasks in changing mass tests?

Answer 9

So only gas can escape

Question 10

How do you calculate gradient?

Answer 10

Change in y/change in x

Question 11

What is a heterogeneous reaction?

Answer 11

A reaction in which the reactants are in different physical states (e.g. solid + solution)

Question 12

What are the 4 factors that increase rate of reaction?

Answer 12

• Higher temperature
• Higher pressure/concentration of reactants
• Higher surface area
• Presence of a catalyst

Question 13

What are four important points about the Maxwell-Boltzmann curves?

Answer 13

• Neither curve is symmetrical
• Both curves start at the origin and finish by approaching the x-axis asymptotically
• The area under both curves is the same, as the number of molecules has not changed
• The peak of T2 is to the right and lower than the peak of T1

Question 14

What are Maxwell-Bolzmann curves?

Answer 14

Curves that show the distribution of energy of particles in a reaction

Question 15

What is T2 compared to T1?

Answer 15

T2 is at a higher temperature, which means a greater distribution of particles have a higher energy, compared to T1; this will result in more particles meeting the activation energy

Question 16

What is the the effect of a catalyst on a Maxwell-Bolzmann curve?

Answer 16

The activation energy will be lowered/moved further to the left

Question 17

In gas phase, at standard conditions, how many collisions will a single particle have?

Answer 17

Between 10^9 and 10^10

Question 18

What is the effect of increasing temperature on an endothermic reaction?

Answer 18

Rate of reaction increases

Question 19

What is the effect of increasing temperature on an exothermic reaction?

Answer 19

Rate of reaction increases, unless it is a reversible reaction, in which it will benefit the backwards endothermic reaction, reducing the amount of product present at the end

Question 20

How do catalysts work?

Answer 20

They provide an alternative route for a reaction, which has a lower activation energy

Question 21

What is a catalyst?

Answer 21

A substance that increases the rate of a chemical reaction, but is chemically unchanged at the end of the reaction

Question 22

What is a heterogeneous catalyst?

Answer 22

A catalyst that is a different state to that of the reactants

Question 23

What are the 3 stages of how heterogenous catalysts work?

Answer 23

Absorption
Reaction
Desorption

Question 24

What is the absorption stage?

Answer 24

Where molecules of one or both reactants form bonds with the catalyst; this weakens the bonds within the molecules

Question 25

What is the reaction stage?

Answer 25

Where molecules react on the surface of the catalyst- the angle of collision is more likely to be favourable as one of the molecules is in a fixed position

Question 26

What is the desorption stage?

Answer 26

The now formed product molecules leave the catalyst surface site, giving space for new reactant molecules to come in