The Ozone Story Flashcards
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?
Why?
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?
No
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)?
Why?
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?
Why?
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?
Why?
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?
Why?
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
- Nucelophile attacks electron deficient carbon atom in C-Hal bond
- Nucelophile donates its lone pair to C to form a dative covalent bond
- 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
Product(s)
Reaction conditions
Equation:
R-Hal + H2O → R-OH + H+ + Hal-
Product:
Alcohol
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
Product(s)
Reaction conditions
General Equation:
R-Hal + OH- → R-OH + Hal-
Product:
Alcohol
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
Product(s)
Reaction conditions
General Equation:
R-Hal + NH3 → R-NH2 + Hal- + H+
Product:
Amine
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+)
