Alkanes

Question 1

Is an alkane saturated or unsaturated?

Answer 1

• saturated

- they only contain single covalent bonds

Question 2

What are the 3 types of alkanes?

Answer 2

• Straight
• Branched
• Cycloalkanes

Question 3

Smaller alkanes are what at room temperature?Do they have a high or low boiling point?

Answer 3

Gas, and have a low boiling point

Question 4

Larger alkanes are what at room temperature?

Do they have a high or low boiling point?

Answer 4

Liquid, and have a high boiling point

Question 5

What forces are between molecules of alkanes?

Answer 5

Van Der Waals

Question 6

Why is it the longer the chain, the stronger the VDW

Answer 6

Because there’s more molecular surface area and more electrons to interact.
Also as it gets longer, it takes more energy to overcome the VDW and seperate the molecules.

Question 7

Why does a branch chain isomer have a lower boiling point than straight chain?

Answer 7

Because branch chain can’t pack closely together, there’s s smaller molecular surface area so the VDW are reduced

Question 8

Why are alkanes unreactive?

Answer 8

• strong single bonds

* no polar bond

Question 9

In plentiful supply of alkanes burn to produce?

Answer 9

Carbon dioxide and water
-> it’s an exothermic process bc heat is given out. Oxidation of alkanes is the basis for their use as fuels
• Natural gas = heating purposes
• Diesel = power
- they burn by a free radical reaction in a gas phase, they have to be vapourised before burned

Question 10

Hetrolytic Fission?

Answer 10

Two different substances are formed

Question 11

Homolytic Fission

Answer 11

Two electrically uncharged radicals are formed. A radical is a species with an unpaired electron. Because its unpaired it’s highly reactive

Question 12

Which stage are free radicals produced?

Answer 12

Initiation stage

Question 13

General formula for an alkane?

Answer 13

CnH2n+2

Question 14

What is Carbon Capture and Storage?

Answer 14

Capture : they capture CO2 from fossil fuels, industrial plants or power stations and store it underground rather than releasing it into the air
Storage: underground rocks act as huge sponge that soak up unwanted CO2

Question 15

How to minimise climate change?

Answer 15

• providing the government with scientific evidence global warming is taking place
• investigating solutions to environmental problems e.g carbon capture &&storage
• monotoring progress against initiatives

Question 16

In limited supply of oxygen, alkanes burn to produce?

Answer 16

• carbon monoxide and water

Question 17

What is the problem with carbon monoxide?

Answer 17

• it’s poisonous

• the oxygen in the blood is carried by haemoglobin, CO is better at binding to haemoglobin than oxygen is
• if you breathe in air with a high concentration of CO it will bind to haemoglobin in bloodstream before oxygen can
• means less O2 will reach cells and you suffer symptoms e.g headaches, fatigue and it can be fatal

Question 18

Halogens react with alkanes in what type of reaction?

Answer 18

• photochemical, which is started by light ( UV )

Question 19

What is a free radical substitution reaction?

Answer 19

• where a hydrogen gets replaced by a chlorine or bromine

Question 20

What is the initation stage?

Answer 20

• sunlight provides enough energy to break the Cl-Cl bond = photodissociation
• the bond splits equally and each atom keeps one electron
• atom becomes highly reactive free radical because it’s unpaired

Question 21

What happens in the propagation stage?

Answer 21

• the radical attacks a molecule
• the new radical can attack a Cl2/Br2 and produce the product and a radical
( start with a radical, end with a radical )

Question 22

What happens in termination stage?

Answer 22

• all the free radicals are used up

Question 23

What is a problem with substitution reactions?

Answer 23

• you don’t just get the chloromethane for e.g but another by product
• if there is too much chlorine the remaining H2 atoms on the chloromethane will be swapped do chlorine
• to avoid this, the best way is to use excess methane bc there’s a greater chance of chlorine radical colliding with methane not chloromethane

Question 24

Separating hydrocarbons inside crude oil?

Answer 24

• crude oil must be separated first by F.D to obtain useful bits : technique relies on differences in boiling points of liquids

Question 25

How does F.D work?

Answer 25

* crude oil is vaporised at 350 degrees/ 65OK, enters the column at the bottom and rises up through the trays * the largest hydrocarbons don't vapourise bc the B.P are too high * as the crude oil goes up, gets cooler bc of different chain lengths - each fraction condenses @ different temperatures * hydrocarbons w/lowest B.P don't condense ( they're drawn off as gases )

Question 26

Uses of the fractions?

Answer 26

* fuels | * processed to make petrochemicals

Question 27

What is cracking?

Answer 27

•breaking long chain alkanes into smaller hydrocarbons, involves breaking the C-C bond • this is bc the shorter alkanes are more in demand • heavier fractions are passed over a catalyst at high temps and moderate pressure • this breaks them into smaller molecules • using a catalyst cuts costs because the reaction is done at lower temps+ pressure and speed up reactions • alkanes and alkenes are formed - the products of cracking are more valuable than the starting materia

Question 28

What is the bond angle around the saturated hydrocarbons?

Answer 28

109.5 - the shape is a tetrahedral as a result of the 4 bond pairs of electrons equally repelling

Question 29

The problem with straight chain alkanes?

Answer 29

• they auto ignite; when fuel/ air mixture is compressed they explode without being ignited by spark. This extra explosion causes 'knocking' in the engine - to get rid of the knocking and make combustion efficient, shorter branched, Cycloalkanes + arenes are added to petrol which increases octane rating

Question 30

How do manufacturers covert straight chain alkanes into branched chain alkanes and cyclic hydrocarbons?

Answer 30

* isomerism | * reforming

Question 31

What is isomerism?

Answer 31

* heat straight chain alkanes with a catalyst stick on inert Al3O2 * alkanes break up and join back together as branched isomers. * molecular sieve used to seperate isomers ( zeolite )

Question 32

What is reforming?

Answer 32

* converts alkanes into cyclic hydrocarbons * uses a catalyst made of platinum * catalyst = rhodium/ platinum

Question 33

Fossil fuels?

Answer 33

* combustion of F.F is exothermic - gives lots of energy which is used to generate most of worlds power stations * F.F they are important raw materials in chemical industry for plastics, lubes

Question 34

Relying on fossil fuels?

Answer 34

* we rely on F.F to produce energy for transport, heating etc * finite amount of F.F and they're running out so its not sustainable to keep using F.F * burning F.F contribute to global warming/climate change bc it releases CO2 = greenhouse gas

Question 35

Alternative sources?

Answer 35

* Bioethanol : produced from plants made by fermentation of sugar from crops e.g maize - has no overall emission into atmosphere bc all CO2 that's released when fuel is burned is removed by crops when they grow * Biodiesel : comes from plants, can be used in Diesel engines. Made by refining renewable fats/oils

Question 36

Problem with using crops?

Answer 36

* possible developed countries they will create a huge demands, poorer countries may see this as a way of making money and will convert farming land to producing crops for fuel so they won't grow food to eat * forests are being cleared for room for the biofuel crops but they absorb less CO2 than forests so it defeats the main object for growing the crops

Question 37

Greenhouse effect keeping us alive?

Answer 37

* gases H20, ozone, CO2, CH4, nitrous oxide absorb/ remit IR. Effect occurs when solar radiation is absorbed by Earths surface, converted to heat/emitted into atmosphere as IR. * some IR escapes into outer space, some absored/re-emitted back towards earths surface by G.H.G. Returing waves of IR radiation warm troposphere/earths surface * G.H.E creates equilibrium. Earths surface/ atmospheric gases absorb energy same rate as it radiates energy maintaining steady temp

Question 38

Greenhouse effect?

Answer 38

* IR strikes a molecule, causes bonds to bend/vibrate. IR is absorbed by bending and stretching of bonds * molecule gains K.E by this absorption of IR radiation and this K.E may be transmitted to other molecules e.g O2 which causes general heating of atmosphere • IR is absorbed by C=O and C-H bonds in H20, CO2/CH4 -> these contribute to global warming. The effect of these is dependent on how long they stay in atmosphere and ability to absorb IR

Question 39

Minimising Climate change?

Answer 39

CarbonCapture + Storage • Capture : CO2 from the F.F's, power stations/ industrial plants then store underground rather than releasing into atmosphere • Storage : rocks underground can act as a huge sponge to soak up unwanted CO2 or reacting with metal oxides to form stable carbonate minerals

Question 40

Minimising climate change

Answer 40

* providing scientific evidence to government that global warming is taking place * investigating solutions to environmental problems e.g CCS * monitoring the progress against initiatives e.g Kyto protocol

Question 41

Why is it important to control global warming?

Answer 41

* it can lead to sea levels rising by the melting of ice caps which leads to flooding * global warming means there's more heat energy in system so it could lead to stormier and less predictable weather * some places could have less rainfall which could bring drought and crop failure