Chemistry - Hydrocarbons

What is covalent bonding?

Bonding between two or more non-metals, which share electrons to complete their outer shells

How can a covalent bond be represented?

With a dot and cross diagram or a stick diagram

How do you draw a dot and cross diagram for a covalent bond?

Determine how many electrons are in those atoms outer shells
Work out how many electrons need to be shared between the atoms
Draw the atom’s outer shells, with them overlapping
Assign dots to one atom/element and crosses to the other
Draw the shared electrons as pairs of dots and crosses in the overlapping area
Fill the rest of the outer shell

How do you draw a stick diagram?

Draw the dot and cross diagram
Work out which atoms are sharing electrons, these will be connected with lines
Work out how many pairs of electrons in the shared area, this is the number of lines connecting the atoms (eg. 2 pairs looks like this =)

What is a hydrocarbon

A molecule made of only carbon and hydrogen

What is the order of alkanes

Methane, Ethane, Propane, Butane (monkeys eat peanut butter), Pentane, Hexane, Septane, Octane etc

What is a homologous series

a group of organic compounds that have the same functional group, share a general formula, and exhibit similar chemical properties

How do you form any alkane?

CnH2n+2

What stops the molecules in water from leaving the substance

Intermolecular Forces

What are intermolecular forces

The forces of attraction that act between molecules (this does not mean the molecules are chemically bonded together).

What do intermolecular forces depend on

Size of the molecule, the number of electrons, how close the molecules can get to each other (so a branched one will be weak because its branches get in the way

What does boiling point mean

The temperature at which a substance changes from a liquid to a gas

What does flammability mean

How easily a substance ignites

The more flammable a substance is, _______

The easier it ignites

What is viscosity

How sticky a substance is, and how easily it flows

The more viscous a substance is _______

The stickier it is, and the less easily it flows

Why are some alkanes more viscous than others

Alkanes with longer chains are often more viscous, as the molecules can get closer together. The chains can also become entangled and wrapped around each other, making the substance thicker

Why are some alkanes more flammable than others

Smaller alkanes have weaker intermolecular forces, so they are more easily ignited, as it takes less energy to overcome them

Why do some alkanes have a higher boiling point than others

Smaller alkanes have weaker intermolecular forces, so they are more easily ignited, as it takes less energy to overcome them

Why is carbon monoxide produced when burning fuel

Not enough oxygen for all carbon to burn into carbon dioxide, so some form carbon monoxide

Why is carbon monoxide a problem

It is a highly toxic gas, which binds to the haemoglobin in the blood, starving organs of oxygen.
It is also colourless, tasteless, and odourless, which makes it very hard to detect

Why is carbon dioxide produced when burning fuel

Carbon bonds with O2 in the air in complete combustion

Why is carbon dioxide a problem

It contributes to the enhanced greenhouse effect, meaning that enough of the sun's heat can leave the atmosphere, and becomes trapped

Why is sulphur dioxide produced when burning fuel

It is a naturally occurring impurity in most fossil fuels. When they are burnt, sulphur is released, which reacts with oxygen in the air

Why is sulfur dioxide a problem

It causes respiratory problems when inhaled, and reacts with water in the air, forming acid rain. This can erode rocks, buildings, and damaged vegetation and aquatic life

Why are nitrous oxides produced when burning fossil fuels

In extremely high temperature (eg. when burning fossil fuels in an engine) , N2 and O2 can dissociate into N and O. This means that they are more reactive, and will bond to form nitrous oxides

Why are nitrous oxides a problem

They are a greenhouse gas, 265x more powerful than carbon dioxide at trapping heat, which means that they accelerate global warming and the enhanced greenhouse effect

Why are carbon particulates produced when burning fossil fuels

In incomplete combustion, there isn’t enough oxygen to form CO2, so carbon particulates (eg. soot) are formed

Why are carbon particulates a problem

If ingested or inhaled, they irritate the lung tissue and can enter the bloodstream, causing heart problems, and cancer. When they build up, they damage the quality of the air and contribute to global dimming (blocking sunlight)

What is complete combustion

When all the carbon in the fuel reacts with oxygen to form carbon dioxide

What is incomplete combustion

Not all the carbon in the fuel reacts with oxygen to form carbon dioxide. Instead, it may form carbon monoxide, or carbon particulates

What colour flame is produced in complete combustion

Blue

What colour flame is produced in complete combustion

Orange

What is produced when an alkane is burnt in oxygen?

Carbon dioxide and water

Write the chemical symbol equation for methane (complete combustion)

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)

Write the chemical symbol equation for ethane (complete combustion)

C2H6(g) + 3.5O2(g) -> 2CO2(g) + 3H2O(g)

Write the chemical symbol equation for propane (complete combustion)

C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(g)

Write the chemical symbol equation for butane (complete combustion)

C4H10(g) + 6.5O2(g) -> 4CO2(g) + 5H2O(g)

What is the pattern of complete combustion of alkanes

Number of O2 = 1.5 x C + 0.5
Number of CO2 = Number of C
Number of H2O = Number of C + 1

Write an example of a chemical symbol equation for the incomplete combustion of butane

C4H10 + 5O2 -> 5H2O +2CO2 + CO +C

What is fractional distillation?

The process that separates mixtures of liquids with different boiling points into useful fractions

How does fractional distillation work

A mixture with liquids of different boiling points is heated and evaporated
It is pumped into a large fractional tower/column, which has different condensers at different heights. The middle of it is often in a spiral shape.
The vapour, as it has different boiling points, will condense at different points as it gets further away from the heat source.
This means it comes out of different condensers

What is the order of crude oil fractions

Gas, petrol, naphtha, kerosine, gas oil, bitumen (Guinea pigs nearly kindly go bowling)

What is a liebig condenser

A condenser tube with an outer tube which cold water circulates around to cool and condenser the vapour faster and more effectively

For which fractions of crude oil does the demand exceed the supply

Gas, petrol

For which fractions does the supply exceed the demand

Bitumen, kerosine, naptha

What are cracking reactions and how do they help solve these problems

A reaction which breaks the bonds between carbon atoms in a long chain alkane, to make a smaller, lighter one, using heat. This helps solve the problem as the smaller alkanes are higher in demand, and cracking reactions increase the supply

How do cracking reactions work

Heat the hydrocarbon until it vaporizer
a) Catalytic cracking - pass the vapour over a catalyst (e.g. aluminium oxide or a porous pot), cause the hydrocarbon to crack
b) mix the vapour with steam, and heat to a very high temperature, causing it to crack

What is produced it a cracking reaction

A shorter alkane and an alkene

True or false: Alkenes can have multiple double bonds

False - they can only have one double bond

Are alkenes a homologous series

Yes

How do you form any alkene?

CnH2n

What is a saturated hydrocarbon

A hydrocarbon that contains only single bonds

What is an unsaturated hydrocarbon

A hydrocarbon that contains a double bond or triple bond

Are alkanes or alkenes more reactive

Alkenes - the double bonds are stressed and one of them can break very easily, allowing them to form saturated compounds

What are the 6 reactions that can happen with alkenes and what are their products (using ethene as an example) - BCIHSC

Bromination ->C2H4Br2
Chlorination -> C2H4Cl2
Iodination -> C2H4I2
Hydrogenation -> C2H6 (ethane)
Reaction with steam -> C2H5OH (ethanol)
Combustion -> CO2 + H2O

What is addition polymerisation

The double bond between the carbons breaks, allowing it to make a new bond with another monomer. They join and form a long chain molecule called a polymer

What does the monomer ethen look like

n

What does the polymer ethene look like

____ _____

            n (with brackets around it)

What does n stand for

The number of repeating units

What happens when you mix a polymer in bromine water

No reaction - the bromine stays clear because the polymer is saturated

Why do polymers have higher boiling and melting points than monomers

The polymers are much larger molecules than monomers, this means that they have more electrons per molecule, so the intermolecular forces between them are stronger. This means that they take more energy to overcome

Why do polymers soften over a range of temperatures rather than having a sharp melting point

The polymer chains are different lengths, this means that the number of electrons in different parts of the polymer are different. Therefore, the intermolecular forces vary from place to place, so some take less energy to overcome than others

What are LDPE’s

Low Density Polyethene’s (can be stretched, are soft)

What are HDPEs

High density polyethenes (are very rigid)

Why are HDPEs and LDPEs different

LDPEs have branches -> the molecules cannot get very close together ->Intermolecular forces are weaker ->less energy is needed to pull them apart

HDPE don’t have branches -> they chains can be packed tightly together -> intermolecular forces are stronger -> it takes a lot of energy to pull them apart

What are thermosoftening polymers

Polymers which soften with heat and can be remolded into another shape

What are thermosetting polymers

Polymers which burn or char when heated

What is the difference between thermosoftening and thermosetting polymers

Thermosoftening are long polymer chains which are not joined to each other, but may be tangled -> the intermolecular forces are weak and can be easily overcome -> the polymer melts easily

Thermosetting polymers are long chains which are connected to each other by covalent bonds (cross links) -> the bonds are INCREDIBLY hard to break -> when exposed to heat, the material burns