Topic 16- Further kinetics

Question 1

Define the term rate of reaction

Answer 1

change in concentration (of any reactant or product) per unit of time. state what is being monitored, usually production of a product

Question 2

How could you calculate the rate of a reaction at a given instant?

Answer 2

rate of reaction = change in concentration/ change in time

Question 3

What are the methods of determining the rate of reaction experimentally?

Answer 3

• measuring the volume of gas evolved
• measuring the change in mass of a reaction mixture
• monitoring the change in intensity of colour of a reaction mixture (colorimetry)
• measuring the change in concentration of a reactant or product using titration
• measuring the change in pH of a solution
• measuring the change in electrical conductivity of a reaction mixture

Question 4

How can you determine the rate constant and expression for a reaction?

Answer 4

only experimentally

Question 5

What affects the value of a rate constant for a specific reaction?

Answer 5

temperature

Question 6

Do species need to be in the chemical equation to be in the rate equation?

Answer 6

no- species in the chemical equation may be excluded and species not in the chemical equation e.g. catalysts,

Question 7

Define the term order of reaction (with respect to a given product)

Answer 7

the power to which a species’ concentration is raised in the equation

Question 8

Define the term overall order of reaction

Answer 8

the sum of the orders of reaction of all species in the rate expression e.g. total order= x + y (x and y are the indices)

Question 9

How would you calculate the units of the rate

constant?

Answer 9

units of rate= moldm-3s-1
units of concentration= moldm-3
rearrange equation to get k= and sub in the units and cancel out

Question 10

What would a rate concentration graph look like for a zero order of reaction?

Answer 10

parallel straight line across- no curve, all the points plotted in a line

Question 11

What would a rate concentration graph look like for a first order of reaction?

Answer 11

straight line going from the origin diagonal (equal between the y and x axis)

Question 12

What would a rate concentration graph look like for a second order of reaction?

Answer 12

curved line upwards, not equal

Question 13

What would a concentration time graph look like for a zero order of reaction?

Answer 13

negative correlation, straight line going down

Question 14

What would a concentration time graph look like for a first order of reaction?

Answer 14

curved line with negative correlation (steeper than zero order)

Question 15

What would a concentration time graph look like for a second order of reaction?

Answer 15

curved line with negative correlation (steeper than first order)

Question 16

How can you tell the difference from first order and second order concentration time graphs?

Answer 16

• the concentration time graph for a first order reaction has a constant half life
• the concentration time graph for a second order reaction has increasing half lives with a decreasing concentration

Question 17

How could you find the rate expression using the initial rate method?

Answer 17

• do a series of experiments whilst varying the concentration, so the concentration of just one reactant changes each time
• plot a graph of concentration against time for each reactant and use a tangent at t=0 to find the initial rate of the reaction
• compare rates and concentrations between each experiment to find order of reactions and overall rate equation

Question 18

What must you add to react with iodine as it is produced for an iodine clock reaction?

Answer 18

known moles of sodium thiosulfate and a little starch (reacts with a 1:2 ratio)

Question 19

When does the starch turn a blue-black colour in an iodine clock reaction and why?

Answer 19

When all the sodium thiosulfate (Na2S2O3) has been used up and so iodine is produced- which reacts with starch leading to a blue-black colour

Question 20

How can you calculate the rate of reaction from the data from an iodine clock reaction?

Answer 20

record the time taken for the colour change to occur and use rate=1/t (used as the initial rate)

Question 21

What is roughly the effect of a 10K temperature change on the rate of reaction?

Answer 21

doubles the rate

Question 22

What is true of the half life of a first order reactant (concentration against time graph)?

Answer 22

half life is constant

Question 23

What is the rate-determining step?

Answer 23

the slowest step in a reaction mechanism, which determines the overall rate of the equation

Question 24

How does the rate determining step link to the species involved in the rate equation?

Answer 24

any species involved in the rate determining step appear in the rate expression, species only involved after the rate determining step do not appear in the rate expression

Question 25

Define a homogeneous catalyst

Answer 25

In the same phase as the reactants

Question 26

Define a heterogenous catalyst

Answer 26

In a different phase to the reactants

Question 27

What is activation energy, Ea

Answer 27

The minimum energy that colliding particles must possess for a reaction to occur

Question 28

What does activation energy represent?

Answer 28

The amount of energy that the colliding particles must obtain to reach the energy level of the transition state

Question 29

What is a catalyst?

Answer 29

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

Question 30

How does a catalyst work?

Answer 30

Provides an alternative route for the reaction that has a lower activation energy than the original route

Question 31

What is the half-life of a reaction?

Answer 31

The time taken for the concentration of the reactant to fall to one-half of its initial value

Question 32

Define the instantaneous reaction rate

Answer 32

The gradient of a tangent drawn to the line of the graph of concentration against time. The instantaneous rate varies as the reaction proceeds, except for zero order

Question 33

What is an elementary reaction?

Answer 33

When a reaction takes place by a single collision between the two reactant particles to form the products

Question 34

What can you determine from the equation of an elementary reaction?

Answer 34

The rate equation

Question 35

Can you determine the rate equation from any stoichiometric equation for a reaction?

Answer 35

No, only if it’s elementary

Question 36

What is the rate determining step?

Answer 36

The slowest step in a reaction

Question 37

What is the Arrhenius equation?

Answer 37

k=Ae(-Ea/RT)

Question 38

State the meaning of each component in the Arrhenius equation

Answer 38

A=pre-exponential factor, a measure of the rate at which collisions occur
Ea=activation energy
R=gas constant
T=absolute temperature, in kelvin

Question 39

What is the natural log of the Arrhenius equation?

Answer 39

lnk=-Ea/RT 1/T + lnA

Question 40

What would the graph look of lnk against 1/T?

Answer 40

a straight line with a gradient of -Ea/R

Question 41

What are the different methods for measuring the rate of reaction experimentally?

Answer 41

• colorimeter at suitable intervals to measure colour change
• gas syringe to measure gas evolved
• change in mass of the reaction mixture
• titration to measure the time it takes fir end-point to occur

Question 42

How would you draw a rate concentration graph?

Answer 42

plot [A] against time, draw tangents at different values —> draw a secondary graph of rate against [A]