CGP EXAM QUESTIONS: OZ

Question 1

Many covalent molecules have a permanent dipole, due to differences in electronegativities

Define the term electronegativity

Answer 1

The ability of an atom to attract the bonding electrons in a covalent bond

Question 2

Many covalent molecules have a permanent dipole, due to differences in electronegativities

What are the trends in electronegativity as you go across a period and down a group in the periodic table?

Answer 2

Electronegativity increases across a period and decreases down a group

Question 3

Which of the following molecules is polar?

A) H₂O

B) Br₂

C) CCl₄

D) SF₆

Answer 3

A) H₂O

Question 4

Draw a diagram to show the shape of Boron trichloride (BCl₃) a covalently bonded molecule. Mark any partial charges on your diagram

Answer 4

Question 5

Draw a diagram to show the shape of Dichloromethane (CH₂Cl₂) a covalently bonded molecule. Mark any partial charges on your diagram

Answer 5

Question 6

Explain whether or not Boron trichloride (BCl₂) or Dichloromethane (CH₂Cl₂), both covalently bonded molecules, have an overall dipole.

Answer 6

The polar bonds in BCl₃ are arranged so that they cancel each other out, so the molecule has no overall dipole
CH₂Cl₂ does have an overall dipole because the polar bonds are not orientated so they are pointing in opposite directions so they don’t cancel each other out

Question 7

The molecules in the table all have the molecular formula C₅H₁₂

Explain the differences in the boiling points of these molecules

Answer 7

The boiling point of a substance depends on the energy needed to overcome the intermolecular forces between the molecules. Pentane is the most linear molecule so it has the greatest surface contact, and so has the strongest instantaneous dipole-induced dipole bonds. This gives it the highest boiling point
The surface contact of 2-methylbutane is less than that of pentane and that of 2,2-dimethylpropane is smaller still, meaning that these substances have weaker instantaneous dipole-induced dipole bonds and consequently lower boiling points

Question 8

What intermolecular bonds are present in methane?

A) Instantaneous dipole-induced dipole bonds only

B) Permanent dipole-permanent dipole bonds only

C) Both instantaneous dipole-induced dipole bonds and permanent dipole-permanent dipole bonds

Answer 8

A) Instantaneous dipole-induced dipole bonds only

Question 9

State and explain which of Ammonia (NH₃) and methane (CH₄) will have the higher boiling point.

Answer 9

Ammonia will have the higher boiling point
Because it can form hydrogen bonds
There are stronger/take more energy to overcome than the intermolecular bonds between the molecules of CH₄

Question 10

State and explain which of water (H₂O) and hydrogen sulfide (H₂S) will have the higher boiling point.

Answer 10

Water will have the higher boiling point
Because it can form hydrogen bonds
There are stronger/take more energy to overcome than the intermolecular bonds between the molecules of H₂S​

Question 11

State and explain which of butane and propan-1-ol will have the higher boiling point.

Answer 11

Propan-1-ol will have the higher boiling point
Because it can form hydrogen bonds
There are stronger/take more energy to overcome than the intermolecular bonds between the molecules of butane

Question 12

An organic compound used as antifreeze is ethane-1,2-diol
Its structure is shown below

The boiling point of ethane-1,2-diol is 197^oC, whereas the boiling point of ethanol is 78^oC. Suggest a reason for this difference

Answer 12

Ethane-1,2-diol has stronger intermolecular bonds because there are two alcohol groups, twice as many as in ethanol. Therefore ethane-1,2-diol can form twice as many hydrogen bonds as ethanol

Question 13

Nitrogen monoxide (NO) and ozone (O₃) sometimes react to produce nitrogen dioxide (NO₂) and oxygen (O₂)

A collision between the two molecules does not always lead to a reaction. Explain why.

Answer 13

The molecules don’t always have enough energy
Collisions don’t always happen in the right orientation

Question 14

Explain why an increase in temperature results in an increase in the rate of a reaction. Draw a suitable graph to support your answer

Answer 14

At higher temperatures, more particles will have the activation energy and will be able to react
So collisions that result in a reaction/have the activation energy will happen more frequently

Question 15

The graph below shows the rate that oxygen is evolved when hydrogen peroxide decomposes at 25^oC

Which of the curves, X, Y or Z shows the rate that oxygen is evolved when the same amount of hydrogen peroxide decomposes at 15^oC? Explain your answer

Answer 15

Y
The same amount of hydrogen peroxide is decomposed, so the same amount of oxygen will be evolved (so it can’t be curve Z)
Curve Y shows that oxygen is evolved more slowly and reaction rates are slower at lower temperatures

Question 16

Explain how homogenous catalysts speed up chemical reactions

Answer 16

The homogenous catalyst forms intermediate compounds
The activation enthalpy needed to form the intermediates (and to form the products from the intermediates) is lower than that needed to make the products directly from the reactants

Question 17

Enzymes are proteins that catalyse specific biological reactions

Draw a fully labelled enthalpy profile for an enzyme-catalysed reaction and an uncatalysed reaction

Answer 17

Question 18

A student follows the rate at which magnesium reacts with hydrogen chloride by measuring the volume of gas evolved over 5 minutes. The reaction is:
Mg_(s) + 2HCl_(aq) –> MgCl_2(aq) + H_2(g)

Her results are shown in the table below
Plot a graph to follow the course of the reaction

Answer 18

Question 19

A student follows the rate at which magnesium reacts with hydrogen chloride by measuring the volume of gas evolved over 5 minutes. The reaction is:
Mg_(s) + 2HCl_(aq) –> MgCl_2(aq) + H_2(g)

Her results are shown in the table below
What is the rate of formation of hydrogen after 50 seconds?

Answer 19

0.15 cm³ s^-1 (± 0.005 cm³ s^-1)

Question 20

A student follows the rate at which magnesium reacts with hydrogen chloride by measuring the volume of gas evolved over 5 minutes. The reaction is:
Mg_(s) + 2HCl_(aq) –> MgCl_2(aq) + H_2(g)

Her results are shown in the table below
What other suitable method could the student have used to follow this reaction?

Answer 20

measuring the mass loss
measuring the change in pH
taking samples and titrating
measuring a temperature change

Question 21

A student follows the rate at which magnesium reacts with hydrogen chloride by measuring the volume of gas evolved over 5 minutes. The reaction is:
Mg_(s) + 2HCl_(aq) –> MgCl_2(aq) + H_2(g)

Her results are shown in the table below
Explain how the rate of the reaction changes over time

Answer 21

The rate of reaction decreases over time because as the concentration of reactants decreases, they are likely to collide and react less frequently

Question 22

2-bromopropane reacts with water in the following reaction:
CH₃CHBrCH₃ + H₂O –> CH₃CH(OH)CH₃ + HBr

Name this type of reaction

Answer 22

nucleophilic substitution

Question 23

2-bromopropane reacts with water in the following reaction:
CH₃CHBrCH₃ + H₂O –> CH₃CH(OH)CH₃ + HBr

Give the systematic name of the organic product

Answer 23

propan-2-ol

Question 24

2-bromopropane reacts with water in the following reaction:
CH₃CHBrCH₃ + H₂O –> CH₃CH(OH)CH₃ + HBr

Under the same conditions, 2-iodopropane was used in place of 2-bromopropane in the reaction above. What difference would you expect in the rate of the reaction? Explain you answer

Answer 24

The reaction would be faster with 2-iodopropane
This is because the C-I bond is weaker than the C-Br bond

Question 25

2-bromopropane reacts with water in the following reaction:
CH3CHBrCH3 + H2O –> CH3CH(OH)CH3 + HBr

Draw the mechanism for the reaction between 2-bromopropane and aqueous potassium hydroxide.

Answer 25

diagram

Question 26

When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.

Write an overall equation to show the formation of bromomethane from methane and bromine.

Answer 26

CH₄ + Br₂ –(uv)–> CH₃Br + HBr

Question 27

When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.

Write down the two equations in the propagation step for the formation of CH₃Br

Answer 27

Br• + CH₄ –> HBr + •CH₃
•CH₃ + Br₂ –> CH₃Br + Br•

Question 28

When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.

Explain why a tiny amount of ethane is found in the product mixture

Answer 28

Two methyl radicals bond together to form an ethane molecule

Question 29

When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.

Write the equation for the formation of ethane in this reaction

Answer 29

•CH₃ + •CH₃ –> CH₃CH₃

Question 30

The ‘ozone layer’ lies mostly between 15 and 30 km above the Earth’s surface

Explain how ozone forms in this part of the atmosphere

Answer 30

Ozone is formed by the effect of UV radiation from the Sun on oxygen molecules
The oxygen molecules split to form oxygen free radicals
which react with more oxygen molecules to form ozone

Question 31

The ‘ozone layer’ lies mostly between 15 and 30 km above the Earth’s surface

How does the ozone layer absorb harmful radiation without getting permanently destroyed?

Answer 31

The ozone molecules absorb harmful UV radiation, breaking down to form an oxygen molecule and a free oxygen radical (O₃ + hv –> O₂ + O)
The radical produced forms more ozone with an O₂ molecule (O₂ + O –> O₃) so ozone is continually destroyed and remade

Question 32

The ‘ozone layer’ lies mostly between 15 and 30 km above the Earth’s surface

Write equations to show how the CFC CCl₄ could catalyse the destruction of ozone in the stratosphere

Answer 32

CCl₄ + hv –> •CCl₃ + Cl•

Cl• + O₃ –> ClO• + O₂
ClO• + O₃ –> Cl• + O₂

Overall: 2O₃ –> 3O₂

Question 33

Explain why large amounts of ground-level ozone can be a problem. Why are these problems more frequently encountered in heavily industrialised areas?

Answer 33

E.g. it causes photochemical smog which can cause respiratory problems
cause harm to animals/plants/materials

Ground-level ozone forms as a result of sunlight acting on mixtures of nitrogen dioxide and hydrocarbons, which are emitted by power stations/vehicles

Question 34

Ozone is a molecule that absorbs ultraviolet light with a wavelength of 255nm

What frequency of ultraviolet radiation does ozone absorb?

Answer 34

Wavelength in m = 255 x 10^-9 = 2.55 x 10^-7m
v = c/λ
v = (3.00 x 10⁸)/(2.55 x 10^-7) = 1.18 x 10¹⁵ Hz

Question 35

Ozone is a molecule that absorbs ultraviolet light with a wavelength of 255nm

What is the energy of the ultraviolet light that ozone absorbs?

Answer 35

E = (hc)/λ
= ((6.63 x 10^-34) x (3.00 x 10⁸))/(2.55 x 10^-7)
= 7.80 x 10^-19 J

Question 36

What is the concentration of ozone in ppm if its percentage concentration by volume is 0.000021%?

Answer 36

0.21 ppm

Question 37

Calculate the energy absorbed when one molecule of H₂O changes from its ground electronic level to the next level, given that the frequency of radiation absorbed is 1.80 x 10¹⁵ Hz

Answer 37

E = hv
= (6.63 x 10^-34) x (1.80 x 10¹⁵)
= 1.19 x 10^-18 J