Alkanes Flashcards
What is the molecular formula of isooctane?
C8H18
What is the systematic name for isooctane?
2,2,4 trimethyl octane
Draw the displayed formula of an isooctane molecule
Describe the electronegativity of carbon and hydrogen
The electronegativities are very similar so the C-H bonds are non polar. As a result of this, alkanes are non polar molecules and have no partial positive and negative charges.
Given that each carbon atom forms four covalent bonds, what is the bonding arrangement(shape) of methane?
tetrahedral with a bond angle of 109.5 degrees
Because alkanes are non polar, what electrostatic forces do they exhibit?
temporary induced dipole-dipole forces between the molecules
As the bond length of the alkane molecule increases…?
the boiling point also increases
As the bond length of the alkane molecule increases…?
the boiling point also increases
What does the boiling point of a substance show?
It’s a good indication of the strength of forces between the particles in it.
High boiling points=?
strong intermolecular forces
What are van der waal’s forces?
they are a weak form of intermolecular force caused by electrons creating instantaneous dipole-induced dipole forces .
How do van der waals forces increase?
They increase with increasing number of electrons/ increasing chain length
When are van der waal’s forces the highest?
van der waal’s forces are highway when molecules have lots of surface contact. straight chain molecules can pack closer so have more surface contact.
The boiling point of alkanes varies with what 2 factors?
chain length
branching
Explain how chain length affects the boiling point of alkanes
Temporary induced dipole-dipole forces will be very small for an alkane such as methane but will increase
as the size of the molecules increase
Therefore, the boiling points of the alkanes increases with the molecular size, due to the increased temporary induced dipole-dipole forces
Explain how branching affects the boiling point of alkanes.
Branched alkanes normally exhibit lower boiling points than unbranched alkanes containing the same number of carbon atoms. Temporary induced dipole-dipole forces can only operate over very short distances between one molecule and its neighbouring molecules. The unbranched alkanes have greater van der Waals’ forces of attraction because of their greater surface areas.
Draw the skeletal formulae of two branched isomers of hexane and state the type of isomerism shown by these branched isomers.
chain isomerism
What is the change from a solid to a gas called?
deposition
What is the change from a gas to a solid called?
sublimation
What is the difference between evaporation and boiling ?
Evaporation involves the change of liquid to gas, but unlike boiling, evaporation only occurs at the surface and takes place at the temperatures below boiling the point. Boiling occurs at a specific temperature and takes place when the vapour pressure reaches the external atmospheric pressure
The temperature of any substance is directly proportional to what?
the mean kinetic energy of the particles
Describe the graph
- Between 1 & 2, the particles are vibrating and gaining kinetic energy and the temperature rises
- Between 2 & 3, all the energy goes into breaking intermolecular attractions - there is no increase in kinetic energy or temperature
- Between 3 & 4, the particles are moving around and gaining in kinetic energy
- Between 4 & 5, the substance is boiling, so intermolecular attractions are breaking and there is no increase in kinetic energy or temperature
- From 5 & 6, the particles are moving around rapidly and increasing in kinetic energy
At absolute zero, how much kinetic energy to the particles have?
none
Why do liquids cool as they evaporate?
When a liquid evaporates (which occurs below the boiling point), some KE is used to overcome forces between particles to allow the particle to escape. As a consequence, the mean KE of the remaining particles is lower and so the T is lower.
What are the 3 types of intermolecular forces?
van der waal’s
permanent dipole- dipole forces
hydrogen bonding
How are van der waal’s formed?
formed by an instantaneous dipole that causes an induced dipole
How are permanent dipole- dipole forces formed?
Polar molecules have permanent dipoles.The molecule will always have a negatively and positively charged end. Forces between two molecules that have permanent dipoles are called permanent dipole - dipole forces.
The + end of the dipole in one molecule and the - end of the dipole in a neighbouring molecule are attracted towards each other.
Compare the boiling point of propanone and butane.
Butane and propanone have the same number of electrons
Butane is a nopolar molecule and will have induced dipole forces
Propanone is a polar molecule and will have permanent dipole forces
Therefore, more energy is required to break the intermolecular forces betwen propanons molecules than between butane molecules
So, propanone has a higher boiling point than butane
How does hydrogen bonding occur
Hydrogen bonding is the strongest form of intermolecular bonding
For hydrogen bonding to take place the following is needed:
A species which has an O, N or F (very electronegative) atom bonded to a hydrogen
When hydrogen is covalently bonded to an O, N or F, the bond becomes highly polarised
The H becomes so + charged that it can form a bond with the lone pair of an O, N or Fatom in another molecule
give the name of the alkane with the lowest boiling point with molecular formula C5H12
dimethylpropane
how is crude oil and natural gas formed?
by the slow decay of marine animals and plants over millions of years under heat and pressure in the absence of air
What does crude oil mainly consist of?
alkanes
Why must crude oil be partially separated?
It has no use in its raw form, so to provide useful products, its components must be partially separated.
How is crude oil separated?
by fractional distillation
What feature of crude oil is used to separate the compounds?
the different boiling points
Describe fractional distillation
-Firstly, crude oil is vaporised and then the vapour is passed through a tower which is hot at the bottom and cold at the top
-As the vapour rises, it cools
-Molecules will condense at different heights in the tower as they have different boiling points
-The larger the molecule(with higher boiling points)the lower down the column it condenses
How does the size of the hydrocarbon link to its boiling point? Why?
The longer the alkane chain , the more electrons and the stronger the van der waals forces that need to be broken
What is a temperature gradient: What does this mean for the hot vapour as it ascends the column?
A temperature gradient is the difference or change in temperature with the highest temperature at the bottom of the column and the lowest at the top. This means that the hot vapour cools and condenses as it ascend the column.
Where will higher boiling point molecules collect? Why? Will these be light or heavy molecules?
The higher boiling point molecules will be collected at the bottom of the column as they are the longest molecules with the greatest Van der waal’s forces. These will be the heavy molecules
What is cracking
Cracking is when long chain hydrocarbons are broken down into smaller molecules or compounds
What is always a product of cracking
an alkene
In terms of supply/demand explain why cracking occurs
To make use of excess larger hydrocarbons and supply demand for shorter ones, longer hydrocarbons are cracks. The products of are more useful than the starting materials.
Give the general formula for cracking
longer alkanes—>shorter alkanes+alkenes(+ hydrogen sometimes)
How are the mixture of products of cracking separated out by?
fractional distillation
What are the two types of cracking and what are they dependent on?
catalytic cracking
thermal cracking
there are determined on the products you would like to make
How does catalytic cracking work?
It uses zeolite crystals and the mixture is heated to about 450°C and slightly above atmospheric pressure.
Zeolites have microscopic pores and channels in them, what does this mean?
This means they have a very high surface area
What does catalytic cracking produce?
branched hydrocarbons
cyclic hydrocarbons
aromatic hydrocarbons
How are the products of catalytic cracking used?
they are useful as fuels because short chain alkanes burn more reactively and are more volatile
How does thermal cracking work?
This used very high temperatures(800-900degrees celsius) and high pressures(5000kPa-8000kPa) to crack the molecules. This is needed as the bonds in the hydrocarbon molecules are very strong.
What does thermal cracking produce?
alkenes and short chain alkanes
How are the products of thermal cracking used?
to produce polymers
Give a balanced equation to show the thermal cracking of tetradecane.(C14H30)
C14H30–>C10H22 + C4H8
Give a balanced equation to show the catalytic cracking of C15H32 forming ethene, propene and octane.
C15H32—>C8H18 + 2(C2H4) + C3H6
What is an incomplete combustion?
burning an alkane in a limited supply of oxygen in an exothermic reaction
What are the products of incomplete combustion?
solid carbon(soot;C)
gaseous carbon monoxide(CO)
water
these are pollutants
Write an equation for the incomplete of butane that has only a solid product and water only.
C4H10 + 21/4O2—>4C+5H2O
Write an equation for the incomplete of butane that has only a gaseous product and water only.
C4H10 + 9/2O2 —> 4CO + 5H2O
What is the chemical formula of carbon monoxide?
CO
How is carbon monoxide formed?
from incomplete combustion
What problems does carbon monoxide cause?
Binds irreversibly to haemoglobin.
Toxic
Dizziness
Nausea
Results in suffocation
What are the ways to reduce the problems caused by CO?
Make sure any gas appliances are regularly serviced
and get CO detector
What is the chemical formula of carbon(soot)?
C
How is carbon(soot) formed?
From incomplete combustion of hydrocarbons
What problems are caused by carbon(soot)?
They pass deep into the lungs and aren’t removed
Global dimming
What are the ways to reduce the problems caused by C?
Use advanced combustion technologies
What is the chemical formula of sulphur dioxide?
SO2
How is sulphur dioxide formed?
during combustion of fuels containing sulphur
What are the problems caused by sulphur dioxide?
forms acid rain
its toxic and can cause respiratory problems
What are the ways to reduce the problems caused by sulphur dioxide?
Sulphur is usually removed from the fuels before burning or SO2 can be removed from fumes after burning
What is the chemical formula of nitrogen oxides ?
NOx
How are nitrogen oxides formed?
Sparks and very high temperatures in an engine causes N2 from the air to react with O2. This can form NO and NO2. We call these NOx.
What are the problems caused by nitrogen oxides?
NOx can form acid rain when they react with water and oxygen
What are the ways to reduce the problems caused by nitrogen oxides?
catalytic converters in cars can reduce the amount of NOx
What is the chemical formula of unburnt hydrocarbons?
CnH2(n+2)
How are unburnt hydrocarbons formed?
Formed by fuels passing through the engine unburnt
What are the problems caused by unburnt hydrocarbons?
They are carcinogenic(can cause cancer)and are air pollutants
What are the ways to reduce the problems caused by unburnt hydrocarbons?
-careful mixing of fuel reduces unburnt hydrocarbons
-they can also be removed in catalytic converters
How do catalytic converters work?
Catalytic converters remove CO, NOx and unburnt hydrocarbons from the exhaust gases, turning them into CO2, N2 and H2O
Describe and explain the features of catalytic converters
Converters have a ceramic honeycomb coated with a thin layer of catalyst metals (Pt, Pb, Rh). This is cheaper and gives a large surface area.
Write an equation to show how CO and NO react with each other in a catalytic converter
2CO + 2NO → 2CO2 + N2
Some fuels contain sulfur. During the combustion of the fuel, what is formed?
sulphur dioxide is formed
What is sulfur dioxide?
It’s an acidic oxide and can react with water to form acid rain.
It is toxic and can cause respiratory problems.
As a result of the toxicity of sulphur dioxide, what is usually done?
sulphur is usually removed from the fuel before burning; or SO2 is removed from fumes after burning flue gas desulfurization)
How does flue gas desulfurization work?
Waste gases pass through scrubber.
This is calcium oxide or calcium carbonate.
CaO+SO2–>CaSO3 or
CaCO3+SO2–>CaSO3+CO2
This is an acid-base reaction
The reaction forms gypsum [calcium sulfate(IV), CaSO3]. Gypsum can make plasterboard
what do greenhouse gases do?
they absorb and emit infra-red radiation.
How do greenhouse gases heat up the atmosphere?
IR radiation excites the bonds C=O, C-H and O-H in carbon dioxide, methane and water vapour.
This causes the bonds to vibrate more vigorously because they vibrate at the same frequency as IR.
The molecules then re-emit the energy in all directions as heat. This heats up the atmosphere.
What are the greenhouse gases?
• Carbon dioxide
• Hydrocarbons
• Water vapour
• Methane
• Nitrous oxide (N2O)
What is the ‘greenhouse effect’ of a given gas dependent on?
Its atmospheric concentration: the greater the concentration of the gas, the more molecules there are to absorb IR radiation.
Its ability to absorb IR radiation: some gases absorb and re-emit IR radiation more strongly than others.
what is the greenhouse effect?
The greenhouse effect is a natural process that occurs when gases in the Earth’s atmosphere trap heat from the sun, warming the planet.
How does the greenhouse effect work?
Gases in the atmosphere absorb the sun’s heat, trapping it and preventing it from escaping into space
Why is the greenhouse effect important?
The greenhouse effect keeps the Earth’s temperature at a level that supports life
What causes the greenhouse effect ?
Human activities are increasing the concentration of greenhouse gases in the atmosphere, which is boosting the greenhouse effect
What are the consequences of the greenhouse effect?
The greenhouse effect is the primary driver of climate change, which is leading to a rise in average temperatures, more extreme weather events, and other consequences
What is the evidence for climate change?
rising sea levels
melting glaciers
shrinking arctic sea ice
ocean acidification
ice cores
increasing frequency of extreme events
what does carbon neutral mean?
“An activity that has no net annual carbon (CO₂) emissions to the atmosphere.”
what is a biofuel
A fuel that is derived from recently living material, such as plants or animal waste.
what is a carbon footprint
“The total amount of carbon dioxide (CO₂) and other greenhouse gases emitted over the full life cycle of a product, service, or event.”
What is a free radical substitution?
Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)
what is needed for a free radical substitution to occur?
Since alkanes are very unreactive, ultraviolet light (sunlight) is needed for this substitution reaction to occur
How many steps does free radical substitution consist of and what are they?
3
initiation
propagation
termination
What is initiation and what happens during it?
In the initiation step the CI-Cl or Br-Br is broken by energy from the UV light
Each atom takes one electron from the covalent bond
This produces two radicals in a homolytic fission reaction
What is propagation and what happens during it?
This refers to the progression (growing) of the substitution reaction in a chain reaction
• Free radicals are very reactive and will attack the unreactive alkanes
• A C-H bond breaks homolytically
• An alkyl free radical is produced
• This can attack another chlorine / bromine molecule to form a halogenoalkane and regenerate the chlorine / bromine free radical
what is homolytic fission
Homolytic fissions is where each atom gets one electron from the covalent bond
what is the termination step and what happens?
This is when the chain reaction terminates (stops) due to two free radical reacting together and
forming a single unreactive molecule
what is ozone
an allotrope of oxygen
what is the formula of ozone
O3
What is rhe ozone layer and what does it do?
The ozone layer is a very high area of the atmosphere rich in ozone.
The ozone layer absorbs ultraviolet radiation from the Sun and prevents it from reaching the surface of the Earth.
Without the ozone layer, plant and animal life as we know it would struggle to exist.
Ozone acts like a big pair of sunglasses filtering out most of the harmful UV radiation.
What is uv radiation divided into?
UV radiation is divided into a, b, and c.
C is the most harmful and is blocked out completely by ozone.
A is the weakest and only a small amount is absorbed by ozone. This is the one that gives you a tan and ages your skin prematurely.
How is ozone made?
Ozone is made by ultraviolet light reacting with oxygen molecules to form oxygen free radicals
The oxygen free radical then reacts with molecular oxygen to form ozone
What is the formula for ozone formation?
what is ozone depletion?
-When an ozone molecule absorbs ultraviolet light it dissociates to form an oxygen molecule and an oxygen atom
-The natural formation and degradation of ozone by UV radiation is a reversible reaction
what is the overall formula for ozone depletion?
What are CFCs?
Chlorofluorocarbons (CFCs) are halogenoalkane molecules.
In CFCs some of the hydrogens in the alkane have been replaced by chlorine and fluorine atoms.
What are CFCs useful as?
CFCs are useful refrigerants or inert molecules.
what is used to break C-Cl bonds in CFCs
UV light can break C-Cl bonds in CFCs to form chlorine free radicals.
Can the C-F bond in CFCs be broken?
no
The C - F bond is too strong to be broken by UV.
what happens to the chlorine free radicals?
The chlorine radicals react with ozone and decompose it
Then
The chlorine oxide radical can react with ozone and decompose it
A chlorine radical is then produced which can go on to continue this reaction many thousands of time.
