1-Rates of Chemical Change

Question 1

What are the factors that affect the rate of chemical reaction?

Answer 1

Concentration, pressure, temperature, surface area, catalysts

Question 2

How do you work out the rate of reaction?

Answer 2

Rate of reaction=change in conc/change in time

Question 3

How can you measure the rate of reaction when a gas is given off?

Answer 3

Measuring the volume of gas produced

Measuring the change in mass of a mixture

Question 4

How do you measure the rate of reaction by measuring the volume of gas produced?

Answer 4

Measure the volume of gas produced at regular intervals, collecting gas in a measuring cylinder or gas syringe by water displacement
Plot concentration by time

Question 5

How do you measure the change in mass of a mixture?

Answer 5

Measure the decrease in mass of total reacting mixture as the reaction proceeds using a balance
Many opportunities for experimental error such as splashes

Question 6

What is used to measure colour change?

Answer 6

Colorimeter

Question 7

How does a colorimeter work?

Answer 7

Appearance of a coloured product, or loss of coloured reactant, is used

Question 8

Why is a colorimeter used?

Answer 8

Allows more subtle changes to be detected

Question 9

How is the rate of chemical reaction measured in titrimetric analysis?

Answer 9

Aliquots of the reaction mixture is removed and another reagent added to quench the reaction or slowed down by putting it in an ice bath
Aliquot are then titrated

Question 10

What affects a solutions conductivity?

Answer 10

Number and type of ions in a solution

Larger ions are slower moving

Question 11

How does conductimetric analysis work?

Answer 11

When a reactant takes place in a solution, the ionic balance changes and the resulting changes in conductivity, showing the changes in concentration

Question 12

What is a general rate equation? What does m stand for?

Answer 12

Rate= k [A]m [B]n

The order of reaction with respect to A

Question 13

What does [ ] mean in the rate equation?

Answer 13

Concentration in moldm-3

Question 14

What does k stand for in the rate equation?

Answer 14

Rate constant

Question 15

How do you work out the overall order?

Answer 15

The sum of the indices

m+n

Question 16

What is the rate determining step?

Answer 16

The slowest reaction when a reaction involves several steps

Question 17

<p>What is the overall order of a reaction related to?</p>

Answer 17

<p>The molecularity of the rate determining step (number of particles involved)</p>

Question 18

<p>How is the rate measured in the iodine clock reaction?</p>

Answer 18

<p>The time taken for the two colourless solutions to turn blue-black is recorded. The rate of reaction is proportional to 1/t</p>

Question 19

<p>Why are rates of reactions studied?</p>

Answer 19

<p>Understand what is going on
Understand how to change the rate
Provide evidence of mechanisms</p>

Question 20

<p>How do you work out the order of a reaction with respect to a product or reactant?</p>

Answer 20

<p>Found out by experiment by measuring the changes in concentration
Plotting concentration (y) against time (x)</p>

Question 21

<p>Why does the rate of reaction decrease over time?</p>

Answer 21

<p>The number of particles of the reactants decrease therefore there are fewer collisions</p>

Question 22

<p>How do you find the rate of reaction at a particular time?</p>

Answer 22

<p>A tangent is drawn and the gradient calculated</p>

Question 23

<p>How do you calculate the gradient?</p>

Answer 23

<p>Gradient= y/x</p>

Question 24

<p>Why is the value of the rate of reaction negative?</p>

Answer 24

<p>The concentration is decreasing as time goes on</p>

Question 25

How can the order be determined after experiment?

Answer 25

The rate is plotted against the concentration

Question 26

What does the concentration against time and rate against concentration graphs look like for a reaction of zero order?

Answer 26

Conc vs time- straight negative line | Rate vs conc- horizontal line

Question 27

What does the concentration against time and rate against concentration graphs look like for a reaction of first order?

Answer 27

I

Question 28

What does the concentration against time and rate against concentration graphs look like for a reaction of second order order?

Answer 28

I

Question 29

What is the rate determining step? What do you use it in?

Answer 29

The slowest step in a reaction | The rate equation

Question 30

How is the rate constant worked out?

Answer 30

Substituting certain values into the rate equation

Question 31

How can you determine the order of a reaction?

Answer 31

Half-lives Comparing graphs Comparing results

Question 32

What happens when a reaction is second order?

Answer 32

Doubling the concentration of a reactant quadruples the rate of reaction

Question 33

What happens when a reaction is second order?

Answer 33

Doubling the concentration of a reactant doubles the rate of reaction

Question 34

How do you work out the values to plot initial rate against concentration?

Answer 34

Series of experiments using different initial concentrations of the reagent, with every other factor unchanged For each experiment, a conc vs time graph is plotted and a tangent drawn at t=0

Question 35

What is a half-life?

Answer 35

The time taken for half a reactant to be used up during a reaction process

Question 36

What can half-lives determine?

Answer 36

The order of the reaction

Question 37

What are the half-lives of a first order reaction?

Answer 37

Constant | Independent of the concentration

Question 38

How are half-lives worked out?

Answer 38

Using a graph | Initial concentration halved and find corresponding time

Question 39

What can half-lives be used for?

Answer 39

Work out the age of something, for example with radioactive decay or carbon dating

Question 40

What does the half-life of a second order reaction depend on?

Answer 40

Initial concentrations of reactants

Question 41

What is the pattern of half-lives in a second order reaction?

Answer 41

Having the concentration doubles the half-life | Inversely proportional to the initial concentration of reactants

Question 42

Why don't some reactions occur at room temperature?

Answer 42

Many particles don't collide with sufficient energy to overcome the activation energy barrier

Question 43

What does the profile for a catalysed reaction look like in comparison to an uncatalysed reaction?

Answer 43

l

Question 44

What is the definition of a catalyst?

Answer 44

A substance that alters the speed of reaction without being used up in the process (often used up in secondary reactions)

Question 45

What is a homogenous catalyst? Give an example

Answer 45

In the same phase as the reactants | Chlorine free-radicals in UV light

Question 46

What is a heterogenous catalyst? Give an example

Answer 46

In a different phase from the reactants | In the Haber process solid iron catalyses gases

Question 47

Why do catalysts have a large surface area?

Answer 47

Reacting mixtures are normally absorbed onto the surface so it allows effects to be maximised

Question 48

Describe how a heterogenous catalyst works

Answer 48

Reactant molecules are absorbed at active sites, wearing the bonds within the molecules Bonds between reacting atoms form, producing product molecules Product molecules leave the surface area in desorption

Question 49

What does the area under the curve on a distribution graph show?

Answer 49

Total number of particles | Shaded part shows number of particles with sufficient energy

Question 50

What is the rate-determining step?

Answer 50

The slowest step in a multistep reaction

Question 51

How are theoretical mechanisms supported?

Answer 51

The predicted rate law from the mechanism much match the rate law achieved experimentally

Question 52

What is a SN1 reaction?

Answer 52

Substitution Nucleophilic addition First order

Question 53

Explain the mechanism of the reaction when 2-iodo-2-methylbutane is refluxed with aqueous potassium hydroxide

Answer 53

The electrons in the C-I bond move toward the iodine atom (slow step), forming a carbocation The hydroxide ion forms a bond with the carbocation

Question 54

What is a SN2 reaction?

Answer 54

Substitution Nucleophilic addition Second order Rate equation involves both the reagents

Question 55

Explain the mechanism for the reaction between bromomethane and aqueous potassium hydroxide

Answer 55

Hydroxide ion joins the central carbon atom as the bromine is leaving Part of the energy required to break the C-Br bond is supplied by the formation of C-OH

Question 56

What sort of reaction occurs between iodine and propanone?

Answer 56

Substitution

Question 57

How does a colorimeter measure the reaction between iodine and propanone?

Answer 57

Calibrated using standard iodine solutions | More lift is transmitted as the reaction mixture changes from light brown to a paler colour

Question 58

How can a titration be used to determine the mechanism between iodine and propanone?

Answer 58

Reaction mixture quenched with sodium hydrogen carbonate then titrated against sodium thiosulphate Results can be used to find out that it is first order with respect to propanone and hydrogen and zero order with respect to iodine

Question 59

What can be determined from the titration between iodine and propanone to find out the mechanism?

Answer 59

Iodine is not in the rate determining step | Hydrogen ions act as a catalyst as they are regenerated during the reaction

Question 60

Describe the mechanism for the reaction between iodine and propanone

Answer 60

1- H+ ion protonates the oxygen atom in propanone 2- Electrons in C=0 partly shift to form a carbocation 3- Slowly, the strong C-H bond is broken, the another carbon atoms stabilises it by releasing a proton to from H3O+ 4- Iodine molecule acts as an electrophile and undergoes electrophilic addition reaction 5- Water molecule then removes proton in acid-base reaction