Unit 2 - Hydrocarbons T5

Question 1

Homologous series

Answer 1

Group of chemically similar compounds which can be represented by the same general formula. Physical properties can change gradually through the series.

Question 2

Hydrocarbons

Answer 2

Compounds made of hydrogen and carbon only

Question 3

Alkanes

Answer 3

Hydrocarbons that are joined only by single bonds - have the same general formula -CnH2n+2.
Examples of alkanes are candle wax, paraffin and petrol.

Question 4

Hydrocarbon prefixes 1-8

Answer 4

Meth -
Eth -
Prop- 
But-
Pent-
Hex-
Hept-
Oct-

Question 5

Branched hydrocarbons

Answer 5

Hydrocarbons with side groups attached to the main chain.

Question 6

Naming branched alkanes

Answer 6

Part 1 - what carbon branch is on - number closest to the side that branch is on. If there are two branches on carbon 3 say 3,3.
Part 2 branch - length of branch ending in yl ie ethyl or propyl. If there are two of same branch say Di or potentially tri - if there are two or more different branches put one that’s prefix comes first in the alphabet first I.e 2 ethly, 3,3 dimethyl.
Part 3 name of longest chain I.e hexane.

Question 7

Isomers

Answer 7

Isomers are molecules which have the same molecular formula but different structural formulae.

Question 8

Making isomers

Answer 8

Can move chain from end to coming off a middle one
When identifying them - one will often have different no. of carbons, one will be the same and another will have a different no. of hydrogens

Question 9

Alkenes

Answer 9

Alkenes are hydrocarbons in which each molecule has the functional group of one double bond between a pair of carbon atoms.
They are used in plastics.
General formula = CnH2n.
They are unsaturated as they have a double bond.

Question 10

Functional group

Answer 10

The part which makes it behave (or function) as a member of a particular chemical family.

Question 11

Naming branched alkenes

Answer 11

Similar as alkanes however branches are numbered from side closest to double bone and end chain is name ends with name of chained alkene.

Question 12

Naming alkene

Answer 12

Position of double bond decides alkenes name as in but -1 - ene it is in the first position, in but - 2 - ene double bond is in middle position.

Question 13

Alkene isomer

Answer 13

Similar for alkanes however position of double bond can be moved to a different position however bear in mind that alkene can be flipped and bond be in same position. Cycloalkanes are also isomers of alkenes.

Question 14

Cycloalkanes

Answer 14

Ring of carbons connected by single bonds - so it is saturated. First member of the series is cyclopropane.
General formula is CnH2n.

Question 15

Naming cycloalkanes

Answer 15

No way of numbering carbons so it is 1 word, if there are two of the same methyl branches on cycle ut and then it will be named dimethylcyclobutane, if it is single then it is just methylcyclobutane.

Question 16

Cycloalkenes

Answer 16

Cyclo with a double bond between carbons, general formula is CnH2n-2.

Question 17

Naming cycloalkenes

Answer 17

We start with the double-bonded carbons and number in the direction that gives the branch the lowest number. Then for naming state what carbon branch is on, type of branch then name of cycloalkene.

Question 18

Reactions of hydrocarbons - combustion

Answer 18

All hydrocarbons (including alkanes, alkenes and cycloalkanes) can undergo combustion reactions with oxygen to give the same two products. Hydrocarbon fuels burn when they react with oxygen in the air. As all hydrocarbons only contain the elements carbon and hydrogen, the only products will be oxides of these elements. So as long as enough oxygen is present for complete combustion, the two products formed will be carbon dioxide (CO2) and water (H2O). The word equation for this reaction is shown below.
Hydrocarbon + oxygen ——> CO2 + H2O

Question 19

Testing for combustion

Answer 19

Set up experiment with hydrocarbon being burned going into funnel, through tube into test tube with cobalt chloride paper - will turn blue to pink if water is produced. Draw a tube from that test tube into another test tube with limewater in it. If CO2 is produced the limewater will turn milky white from colourless.

Question 20

Incomplete combustion

Answer 20

When a hydrocarbon is burned with a limited supply of oxygen it will not be burned properly- the substances produced are carbon monoxide, water and carbon (soot).

Question 21

Addition reactions

Answer 21

Alkenes are more reactive than alkanes and cycloalkanes because they have a double bond. While alkanes, alkenes and cycloalkanes undergo combustion reactions with oxygen, only alkenes can participate in addition reactions.
In an addition reaction, the double bond of the alkene partially breaks when the reactant molecule attacks and adds on across it.

Question 22

Addition reaction test

Answer 22

Putting hydrocarbon into bromine water is a test for unsaturation. Unsaturated Alkenes turn bromine water orange to colourless where as saturated alkanes do not.

1. Two test tubes of bromine water are set up and alkane added to one and alkene to the other.
2. Test tubes are shaken.
3. Alkane remains orange brown, alkene turned colourless.

Question 23

Types of addition reactions

Answer 23

Hydrogen addition - hydrogenation.
Water addition turning alkenes to alcohols - hydration .
Bromine addition
Bromination.

Question 24

Catalytic cracking

Answer 24

Crude oil is a mixture of many different hydrocarbon molecules of varying carbon chain length. Most of these molecules are straight and branched chain alkanes.
The shorter chain alkanes are in high demand for use in fuels – in particular for blending together to make petrol.
Most of the alkanes however, are long chain compounds and are not particularly useful as fuels as they burn slowly, producing a lot of pollution.
These hydrocarbons can be broken down into shorter chain, more useful molecules by a process called catalytic cracking.

Question 25

Cracking equations

Answer 25

Split alkane into smaller alkane and alkene. Ensure that carbons and hydrogen in smaller hydrocarbons are equal to large hydrocarbon.

Question 26

Setup of catalytic cracking experiment.

Answer 26

Bunsen burner with test tube over top of it perpendicularly. Inside test tube is mineral wool soaked in paraffin at bottom and aluminium oxide catalyst in middle which speeds up the reaction. Test tube is stoppered with tude going from top of test tube into beaker with water and upside down test tube. Reaction has taken place if a gas is produced from the liquid paraffin.

Question 27

Care taken in catalytic cracking experiment

Answer 27

Boiling tube is loosely clamped as it will expand on heating.
Delivery tube must be removed from the water before heating is stopped to avoid suckback.

Question 28

Gases in hydrocarbons

Answer 28

Weak intermolecular attractions makes substances gases. Smaller hydrocarbons have weaker inter molecular attractions, thus they are gases.

Question 29

Alcohols

Answer 29

Alcohols, also known as alkanols, have the hydroxyl functional group –OH. They are a homologous series and have the general formula CnH2n-1OH . Their names all end in –ol.
Most common alcohol is ethanol - found in alcoholic drinks.

Question 30

Fermentation.

Answer 30

This process uses enzymes produced by yeast as a catalyst and is the way that all alcoholic beverages are produced for human consumption.
C6H12O6 —> CH3Ch2OH +2CO2.

Question 31

Hydration of ethene

Answer 31

A hydrogen atom and a hydroxyl group from the water molecule add on to the ethene molecule across the double bond. Because the process involves the addition of water, it is known as hydration.
CH2CH2 + H2O —> CH3CH2OH

Question 32

Naming alcohols

Answer 32

The rules for naming an alcohol are the same as before. The hydroxyl functional group is the most important part of the molecule and the carbon chain is numbered to give it the lowest branch number. Unnecessary for methanol and ethanol, as they will look the same no matter what.

Question 33

Uses of Alcohols

Answer 33

Alcohols have a wide variety of uses. While ethanol is most famous for its presence in alcoholic drinks, many other alcohols exist and are commonly used as fuels and solvents. Propan-2-ol is widely used in hand gels and disinfectant wipes. Alcohols are particularly useful as fuels due to their high flammability and the fact that they burn cleanly.

Question 34

Energy from alcohols - energy changes in reactions.

Answer 34

Alcohols are flammable compounds made up of carbon, hydrogen and oxygen. When they burn, they release energy in an exothermic reaction. This is the opposite to a reaction which takes in heat energy (called an endothermic reaction).
Alcohols can be made from plants, and as a result can be classed as biofuels.

Question 35

Biofuels

Answer 35

Wood is an example of a biofuel, it is fuel that has recently being produced from a natural product that made its carbon structure from carbon dioxide in the air. When it burns it releases the same amount of carbon dioxide back into the atmosphere. As we can keep growing more wood it is described as an infinite or renewable resource.

Question 36

Calculating energy released when fuels burn

Answer 36

Eh = c x m x delta T
Eh is the energy produced.
c = specific heat capacity of water always 4.18
m = mass of water in kg , 1cm3 is 1 gram.
Delta T = the change in temperature

Question 37

Investigating energy in alcohols

Answer 37

Have liquid fuel in a spirit burner. Have a beaker with water on top of it.
With a thermometer, measure temperature of water before and after experiment.
Set up experiment as shown with 200cm3 of water and time for two mins, once 2 mins is up measure temperature of water again. Use energy equation to find energy of the alcohol.
Minimise errors - use windshield, don’t remove lid until last moment, give it a plentiful supply of oxygen and use a windsheild.

Question 38

Carboxylic acids

Answer 38

AKA alkaloic acid all contain carboxyl functional group - COOH. Their names end in oic acid.
General formula - CnH2n+1COOH.
USES- vinegar and preservatives.