The Ozone Story Flashcards Preview

A Level Chemistry (OCR B Salters) > The Ozone Story > Flashcards

Flashcards in The Ozone Story Deck (123):

What does PPM stand for?

Parts per million

Amount of gas particles in a sample containing 1 million particles


How can you calculate percentage composition from PPM?

ppm/1,000,000 x 100 = percentage composition

AKA ÷ by 10,000


100% = 1,000,000ppm so 1% = 10,000ppm
To convert from ppm to %, ÷ by 10,000


How can you calculate ppm from percentage composition?

% compostition/100 x 1,000,000 = ppm

AKA x by 10,000


100% = 1,000,000ppm so 1% = 10,000ppm
To convert from ppm to %, ÷ by 10,000


When might the concentration of gas in a mixture be given as percentage by volume?

When the gas is present in a high concentration

e.g. various gases that make up the air in the atmosphere


When might the concentration of gas in a mixture be given in ppm?

If the gas is present in a low concentration - less than 1% by volume


Why is high energy UV radiation bad for the skin?

What damage can it do?

Its wavelength/frequency corresponds to the energy required to break chemical bonds, such as DNA

Therefore it can damage genes and lead to skin cancer
or damage proteins and age the skin


What is the electromagnetic spectrum?

The range of wavelengths/frequencies over which electromagnetic radiation extends...


What is photodissociation?

Bond breaking caused by visible light/UV radiation


What do the surfaces of the Sun and Earth both emit?

EM radiation


What 2 equations link the 2 properties of light (i.e. wave-particle duality?)

c = λv

E = hv


Where λ = wavelength and v = frequency


How do you calculate the frequency needed to break a bond?

AKA energy needed to cause photodissociation

Convert kJ/mol into J/mol by x1000

Work out the min. energy needed to break a single bond by ÷NA

Divide energy by Planck's constant (v = E ÷ h)


How do you calculate bond enthalpy from frequency?

Calculate energy needed to break 1 bond by using E = hv

x NA to work out energy needed to break 1mol of bonds

÷1000 to give answer in kJ/mol


What are the 4 possible outcomes when molecules absorb EM raditation?

Put them in order of decreasing energy (highest energy to lowest)

Electronic transmission between energy levels

Vibration of bonds

Rotation of whole molecule

Translation of whole molecule


(Electronic transmission = requires most energy)


Absorbtion of which types of EM radiation cause which outcomes (i.e. changes in molecule)?

Match them up

Electronic transmission = Visible/UV

Vibration = IR

Rotation/Translation = Microwave


Why is the energy required to cause electronic transmission/vibration/rotation/translation given as a range despite the fact the outcomes are quantised?

The amount of energy needed to do each of these things changes depending on the chemical/substance


What 3 changes to a compound/substance can electronic transmission cause?


Put them in order of decreasing energy requirement (i.e. from highest→lowest energy requirement)

Can cause:

Ionisation - Cl2 → Cl2+ + e-

Dissociation - Cl2 → Cl• + Cl•

Release of energy + return to original state


How does the effect visible light/UV radiation have on matter differ?

Both cause electronic transmission...

Vissible light causes electrons to be excited to higher energy levels and some bonds break

UV causes electrons to be excited to higher energy levels and bonds break


What are the 2 ways covalent bonds can break?

What are the products of each type of breaking?

Homolytic fission - produces ions

Heterolytic fission - produces radicals


What is heterolytic fission?

Type of covalent bond breaking

Both electrons from a shared pair go to same species

Forms ions


What is homolytic fission?

Type of covalent bond breaking

One electron from a shared pair goes to each species

Forms radicals


What is a radical?

A species with one (or more) unpaired electron


What are curly arrows used to show?

What are the 2 types and what do they denote?

Used to show the movement of electrons

Full (double-headed) shows movement of a pair of electrons

Half (single-headed) shows movement of a single electron


What is a radical chain reaction?

A reaction in which new radicals are formed at the end of one step

These radicals then continue/propagate the reaction


What are biradicals?

Give an example

Species with 2 unpaired electrons

E.g. oxygen atoms


Dots are not usually used


What are the 3 stages of a radical chain reaction?

Briefly describe each step

Initiation - radicals formed from a stable molecule


Propagation - radical reacts + process forms new radical
Often occurs in pairs - radical formed in 1st propagation step reacts again in 2nd.


Termination - 2 radicals collide to form stable molecule (non-radical)


What is the name of the mechanism/process by which a halogen atom can substitute a hydrogen atom in an alkane chain?

Radical substitution


N.B. alkane not alkene - halogen would undergo nucleophilic substituion instead


Describe the stages of the radical chain reaction that occurs between alkanes and halogens


(e.g. methane + chlorine)

Initiation: Homolytic fission of a halogen molecule occurs in the presence of UV light
Cl2 + hv → 2Cl•

Propagation: A methyl radical is formed, which then reacts to reform the Cl•
Hence the Cl• can be thought of as a catalyst during these steps
CH4 + Cl• → CH3• + HCl
CH3• + Cl2 → CH3Cl + Cl•

Termination: Cl• or CH3• radicals collide
Cl• + Cl• → Cl2
CH3• + Cl3• → CH3Cl
CH3• + CH3• → CH3CH3

The overall equation is: Cl2 + CH4 → CH3Cl + HCl


For the radical substitution reaction between chlorine and methane, why isn't ethane included as a product in the overall equation?


Cl2 + CH4 → CH3Cl + HCl

Because it occurs as a side reaction that isn't as likely to happen

Due to the nature of radical reactions, an ethyl radical could be created out of it, which could collide with a methyl radical to produce a propyl radical... etc.

Hence it's just easier to leave it out!


What is the troposphere?

The layer of the atmosphere directly above Earth's surface


What is the stratosphere?

The layer of the Earth's atmosphere above the troposphere


i.e. Above the layer directly above the Earth's surface... 2nd layer up


Which layer of the atmosphere is ozone naturally present in?

The stratosphere


What does ozone form when it is found in the troposphere?

Is a component of photochemical smog

Hence is a secondary pollutant


Photochemical smog causes corrosion of plastics, rubber, textiles, and breathing problems


What role does ozone play in the stratosphere?

Absorbs high-energy UV radiation from the Sun

This prevents the radiation reaching the Earth's surface

High-energy UV can cause health problems such as skin cancer and cataracts


Describe the steps of the depletion of ozone by chlorine atoms in the stratosphere

Initiation: Chloroalkanes reach the stratosphere and photodissociate, forming Cl•
CH3Cl + hv → CH3• + Cl•

Propagation: Cl• reacts with ozone in a catalytic cycle involving O atoms which are also present in the stratosphere
Cl• + O3 → ClO• + O2
ClO• + O → Cl• + O2

Termination: Cl• removed from the cycle by the following reaction:
Cl• + Cl• → Cl2


Using the cancellation method, work out the overall equation for the depletion of ozone by Cl atoms in the following propagation reactions:

Cl• + O3 → ClO• + O2
ClO• + O → Cl• + O2

Combine 2 equations with all reactants on one side and all products on the other:

Cl• + O3 + ClO• + O → ClO• + O2 + Cl• + O2

Cancel out any substances that appear twice (regardless of which side) to give overall equation:

O3 + O → 2O2


Using the cancellation method, work out the overall equation for the reaction of methane with chlorine in the following propagation reactions:

CH4 + Cl• → CH3• + HCl
CH3• + Cl2 → Cl• + CH3Cl

Combine 2 equations with all reactants on one side and all products on the other:

CH4 + Cl• + CH3• + Cl2 → CH3• + HCl + Cl• + CH3Cl

Cancel out any substances that appear twice (regardless of which side) to give overall equation:

CH4 + Cl2 → HCl + CH3Cl


Which haloalkanes photodissociate most easily?


What implications does this have for the atmosphere?

Iodo- + Bromoalkanes photodissociate more easily than chloroalkanes

This is because the C-I and C-Br bonds are weaker than C-Cl bonds 

Hence they can be broken down by lower frequency radiation found in the troposphere and do not reach the stratosphere

Fluroalkanes don't photodissociate in the stratosphere because the C-F bond is too strong to be broken by the UV radiation present


Why can the Cl• radical present in the following porpagation steps of the depletion of ozone be described as a catalyst?

Cl• + O3 → ClO• + O2
ClO• + O → Cl• + O2

Because it enables the second propagation stage to take place by forming an intermediate but is reformed by the end of the overall reaction

It goes through the reaction cycle many times and is not used up/depleted


Describe how ozone is formed naturally in the stratosphere

Oxygen molecules photodissociate into oxygen atoms

O2 + hv → 2O

Ozone is formed when an oxygen atom combines with an oxygen molecule

O2 + O → O3


Describe how ozone is destroyed naturally in the stratosphere

Occurs when it absorbs high-energy UV raditation

O3 + hv → O2 + O


What is reaction kinetics?

The study of rates of reaction


What factors can affect rate of reaction?

  • Concentration
  • Pressure
  • Use of a catalyst
  • Temperature
  • Surface area
  • Particle size
  • Intensity of radiation


What is the general formula to measure rate of reaction?

Rate of reaction = change in property ÷ time taken


What properties of a reaction could be measured to determine rate?

  • Volume of gas evolved
  • Mass change
  • pH
  • Colour change (colorimetry)
  • Chemical analysis (i.e. taking samples, quenching, titration/colormetry)


What is collision theory?

Theory that explains how the frequency of collisions between particles affects the rate of reaction

i.e. reactions only occur when particles collide with the minimum amount of kinetic energy (EA)

More collisions = higher/faster rate


How does increasing concentration/pressure affect the rate of reaction?

More particles available for collisions + in closer proximity to each other so there is a higher chance of them colliding

Therefore there is a higher chance of a successful collision occuring

(Provided the particles both have the minimum energy to react)


How does increasing temperature increase the rate of reaction?

At higher temps a greater proportion of colliding particles have the sufficient energy to react so more collisions have a greater energy than the EA

Therefore there is a higher liklihood of successful collisions occuring/more successful collisions occur


How does increasing the surface area of a solid affect the rate of reaction?

When a solid is more finely divided, there is a larger surface area for reactions to take place on

This means there is a greater frequency of collisions and thus also a greater frequency of successful collisions

Rate increases


How does the use of a heterogeneous catalyst affect the rate of reaction?

Provides a surface where reacting particles may break and make bonds

Increases proximity of reactants + provides an alternate pathway with lower EA, making successful collisions/reactions more likely to take place


How do catalysts affect the rate of reaction?

Provide an alternate reaction pathway with a lower EA therefore increasing the chance of particles having the minimum amount of energy needed to successfully collide

Rate increases


How does increasing raditation intensity affect the rate of reaction?

(Photo)dissociation of bonds faster when intensity increases as bonds absorb more energy per second therefore more are broken

Increases rate


Wht general thing(s) do you need to measure in order to measure the rate of reaction

How quickly a reactant is used up or how quickly a product is formed


What is activation enthalpy?

The minimum kinetic energy required by a pari of colliding particles in order for a successful reacton to occur


What is a transition state?

The highest point on an enthalpy profile/point with highest energy on a reaction pathway

Between reactants + products forming

Where old bonds stretch and new ones start to form


What is an enthalpy profile?

Graph plotting enthalpy against the progress of a reaction


What are the main methods by which the rate of reaction can be measured experimentally?

  • Measuring vol. gas produced - done using gas syrine or displacement of water. More gas produced per unit of time = faster rate
  • Measuring mass changes - reactions which give off gas involve mass changes. Bigger change in mass per unit of time = faster rate
  • Colorimetry - measures change of intensity as coloured chemical used up/produced. Measured using a colorimeter
  • pH changes - if acid/alkali used up/produced, can be measured. pH meter can be used to monitor change. Faster change = faster rate


What effect does increasing the temperature by 10ºC (roughly) have on the rate of reaction?

It roughly doubles


What is the Maxwell-Boltzmann distribution?

What does it show?

Shows the fraction of particles with certain kinetic energy

Comparing 2 different temperatures on the distribution shows that as temp increases so does the rate of reaction


What is needed in order for a reaction to take place?

Enough molecules with a combined kinetic energy that is higher than the Ea when they collide


Why do reactions take place faster at higher temperatures?

Because a larger proportion of the colliding particles have the minimum Ea needed to react


Draw a Maxwell-Boltzmann distribution showing a catalysed and un-catalysed reaction


In Maxwell-Boltzmann distribution plots showing a catalysed and uncatalysed reaction OR reactions at 2 different temps, what is the same and what is different?

The area under the graphs is the same but the distribution is different


How do catalysts affect the enthalpy change of a reaction?

They don't

It stays the same regardless of whether a catalyst is used or not


(Ea is affected)


What aspects of equilibrium do/don't catalysts affect?

Don't affect the position/composition of equilibirum

BUT do affect the rate at which it is reached


What is a dipole?

Two opposite charges seperated by a (short) distance


What are intermolecular bonds?

The bonds found in  solid/liquids between molecules


What did CFCs used to be used for/in?

  • Refrigerants + in air can units
  • Aerosol propellants
  • Blowing agents for expanded plastics
  • Dry cleaning solvents


What is a polar bond?

A bond in which there is an uneven distribution of charge between the 2 atoms

Results in 1 atom have a partially positive (𝛿+) charge and 1 atom have a partially negative (𝛿-) charge


What is electronegativity?

A measure of the ability of an atom in a molecule to attract the electrons in a covalent bond


Electronegativity is related to the position of an atom in the period table

What are the trends in electronegativity in the periodic table?

Electronegativity increases along a period (towards the halogens)

and going up a group


What is the distribution of electrons like in a non-polar covalent molecule?

Electrons equally shared

Distance between each nucleus + electrons identical

Arrangement may also be due to equal electron affinity/electronegativity


What is the distribution of electrons like between a larger and smaller atom?

Shared electrons more strongly attracted to nucleus of smaller atom as its nucleus is closer due to electron shielding in the larger atom


What is the distribution of electrons like in atoms with different electron affinities/electronegativies?

The shared electrons are more stongly attracted to the nucleus with the larger charge + greater electron affinity/electronegativity


What is another name for a permanent dipole?

An overall dipole


If a molecule has symmetrical polar bonds, will it have an overall dipole?


If charges are arranged symmetrically around the central molecule then no overall dipole

However, electrons wil still be unevenly distributed in individual bonds

e.g. CCl4

Use bond shapes/VSEPR theory models to work it out


What are the 4 types of intermolecular bond?

Hydrogen bonds

Instantaneous dipole-induced dipole bonds

Permanent dipole-induced dipole bonds

Permanent dipole-permanent dipole bonds


Out of all of the types of intermolecular bonds, which is the strongest?

Hydrogen bonds


Which of the dipole-dipole bonds is the strongest?

Permanent dipole-induced dipole


What types of dipole-dipole attractions will be present in a molecule with an overall dipole?

Permanent dipole-permanent dipole

AND Instantaneous dipole-induced dipole


(And possibly H-bonds)


Describe a permanent dipole-permanent dipole bond

Occurs if 2 neighbouring molecules both have a permanent dipole
If so, there will be an electrostatic attraction between the charges of the dipoles


What is an instantaneous dipole?

What causes one?

Arrises due to the fact electrons in constant motion

At a particular time they may not be evenly distributed

Hence a dipole may arise due to temporarily uneven charges


What is an induced dipole?

What causes one to arise?

Occurs if non-polar/unpolarised molecule next to a dipole

Dipole attracts/repels electrons in unpolarised molecule, creating a dipole in it as well

Can't occur if no permanent or instantaneous dipoles


Describe an instantaneous dipole-induced dipole bond

If 2 neighbouring molecules don't have a permanent dipole there will still be attraction between them

This is an id-id bond

Occurs between molecules even if permanent dipoles also present

Caused by constant motion of electrons. This means they may not be evenly distributed beween atoms at a given time, causing temporarily uneven charges and, therefore, a dipole


Explain how instantaneous dipole-induced dipole bonds arise

Electrons in a molecule are in continuous, random motion

At a particular moment in time they may be unevenly distributed

This creates an instantaneous dipole

The dipole induces a dipole on a neighbouring molecule, creating an... induced dipole

There is an electrostatic attrction between the two dipoles


Why do instantaneous dipole-induced dipole bonds continously break and re-form?

Because the electron distribution in molecules is constantly chaning due to the fact they are in continuous, random motion


What factors affect the strength of instantaneous dipole-induced dipole bonds?

The no. electrons in the molecule - more means greater chance of instantaneous dipole arrising

Distance between molecules - closer packing means greater electrostatic attraction


Why do longer chain hydrocarbons have higher boiling points than shorter chains?

Because longer chains increase the no. id-id bonds


Why do branched molecules (e.g. hydrocarbons) have lower boiling points than unbranched chains?

Increased branching decreases the surface area of 1 molecule in contact with another hence the strength of id-id bonds is decreased


How does the number of electrons in a molecule affect the strength of id-id bonds?

How can this be used to explain the trend in boiling points of the halogens?

Molecules with more electrons have more chance of forming id-id bonds

The strength of id-id bonds also increases

Hence boiling points increase as you go down the halogens (Iodine = highest)


What is a chlorine reservoir?

A molecule that 'stores' chlorine and prevents it from reacting

e.g. HCl + ClONO2


When drawing a diagram to show hydrogen bonding, what should you include?

The lone pair(s)
The partial charges on the atoms

The correct bond angle around the H𝛿+ atom

Dashed/dotted lines between the atoms that are hydrogen bonded


What are the requirements for hydrogen bonding to occur?

A large dipole between a H atom and a highly electronegative atom (resulting in a  H𝛿​+)

A small electronegative atom in the other molecule - N, O, or F

Lone pair on NOF that H can line up/bond with


What is the bond angle of atoms that are hydrogen bonded?

180º - linear (around the H𝛿​+)

Because the lone pair (from the NOF) points directly at the H𝛿+


How many hydrogen bond can water form per molecule (on average)?


2 per molecule

Because O has 2 lone pairs and is bonded to 2 H, creating 2 H𝛿+

Allows 2 bonds to form to neighbouring molecules


Describe the shape and structure of ice with reference to H bonding

H bonds formed when water freezes gives ice a regular structure

The H bonds + covalent bonds around each O are arranged tetrahedrally

This arrangement of bonds around each O gives ice a very open structure

Hence, ice has a lower density than water and floats on it


What properties does H bonding give molecules?

High viscosity - for liquid to flow molecules must be able to move past each other, so frequent/easy breaking + forming of bonds required

Solubilty in water due to fact that H bonds can form between water + molecules of substance


What is the trend for boiling points in the halogens?


Boiling points increase with heavier halogen atoms + in molecules containing more halogen atoms

The larger the halogen/the more halogen atoms, the greater the overall number of electrons

This increases the number of instantaneous dipole-induced dipole bonds

This means the intermolecular bonds are stonger so more energy is needed to seperate the molecules from each other, hence they have higher boiling points


What is the trend in bond strength in the halogens?


C-Hal bonds become weaker as the size of the halogen atom increases

This makes the bond easier to break and the compounds become more reactive

Although the C-F bond is the most polar, fluroalkanes are very unreactive

This shows that it is bond strength rather than bond polarity that has the greatest effect on the reactivity of halogens (it is the determining factor in reactivity)


Describe the general reactivity of different haloalkanes


Fluoroalkanes - compounds are very unreactive

Chloroalkanes - compounds are reasonably stable in troposphere and can react to produce Cl radicals that deplete ozone

Bromo- and iodo-alkanes are reactive so are useful as intermediates in chemical synthesis


What is the general equation for the homolytic fission of haloalkanes?

R-Hal (+ hv) → R• + Hal•


Which haloalkane most easily undergoes homolytic fission?


Compounds containing the C-I bond as it has the lowest bond enthlapy


What is the general formula for the heterolytic fission of haloalkanes?

R-Hal (+ hv) → R+ + Hal-


What is a carbocation?

An ion with a positively charged carbon atom


What is a nucleophile?

A molecule or negatively charged ion with a lone pair of electrons tht it can donate to a positively charged atom to form a dative covalent bond


Means 'nucleus loving' and is attracted to areas of positive charge


What are the conditions for homolytic fission (of haloalkanes?)

Gas phase with high temps OR the presence of UV radiation (e.g. in the stratosphere)


What is a substitution reaction?

A reaction in which one atom/group in a compound is replaced by another


What is the general equation for nucleophilic substitution reactions involving haloalkanes?

R-Hal + Nu- → R-Nu + Hal-


Describe the stages of nucleophilic substitution

  1. Nucelophile attacks electron deficient carbon atom in C-Hal bond
  2. Nucelophile donates its lone pair to C to form a dative covalent bond
  3. The carbon-halogen bond breaks heterolytically, with the halogen recieveing 2 e- + forming a halide ion. This is the leaving group


How many steps is there to nucleophilic substitution if the nucleophile is an ion OR a neutral molecule?

For an ion - 1 step

For a neutral molecule - 2 steps


Draw the general reaction mechanism for the nucleophilic substitution of haloalkanes


How can haloalkanes be synthesised?

By reversing nucleophilic substitution


Draw the reaction mechanism for the nucelophilic substitution of bromobutane by hydroxide ions


Draw the reaction mechanism for the nucleophilic substitution of a haloalkane by water


What is a nucleophile substitution reaction involving what also called?

A hydrolysis reaction


Why is water able to act as a nucleophile?

Because it has 2 lone pairs on the oxygen atom


Why is ammonia able to act as a nucleophile?

Because it has a lone pair on the nitrogen atom


What is the general equation for nucleophilic substitution reaction of ammonia and a haloalkane?

R-Hal + NH3 → R-NH2 + Hal- + H+


Sometimes written as:

R-Hal + NH3 → R-NH3+ + Hal- ⇌ R-NH2 + Hal- + H+


Draw the mechanism for the reverse nucleophilic substitution of haloalkanes

(Used to synthesize them)

The halide ion (X-) acts as the nucleophile

The reaction is done in the presence of a strong acid which provides a H+ for the O in the alcohol to bond with

This causes the oxygen to have a positive charge, so it more strongly attracts electrons from the C-O bond (bond polarised)

This causes the C to have a higher partial positive charge, causing it to be attacked by the halide ion

This causes a water molecule to become the leaving group


When water is used as a nucleophile to react with a haloalkane, what is the/are the:

General equation


Reaction conditions

R-Hal + H2O → R-OH + H+ + Hal-


Reaction Condition:
Heat under reflux - this is sometimes called hydrolysis


When OH- is used as a nucleophile to react with a haloalkane, what is the/are the:

General equation


Reaction conditions

General Equation:
R-Hal + OH- → R-OH + Hal-


Reaction Conditions:
Heated under reflux with NaOH(aq) with ethanol as a solvent


When ammonia is used as a nucleophile to react with a haloalkane, what is the/are the:

General equation


Reaction conditions

General Equation:
R-Hal + NH3 → R-NH2 + Hal- + H+


Reaction Conditions:
Haloalkane heated with conc. ammonia solution in a sealed tube


What are the general conditions for heterolytic fission?

Dissolved in a polar solvent such as an ehtanol/water mixture


What must nucleophilic substitution reactions to make haloalkanes (from alcohols) be done in the presence of?

A strong acid

(To provide H+)