Fuels - Topic 8

Question 1

Hydrocarbons:

Answer 1

compounds that contain carbon and hydrogen only

Question 2

What is crude oil?

Answer 2

• a complex mixture of hydrocarbons
• containing molecules in which carbon atoms are in chains or rings
• an important source of useful substances (fuels and feedstock for the petrochemical industry)
• a finite resource

Question 3

Explain the change in boiling points of alkanes:

Answer 3

• boiling points increase as length of chain increases
• as the chain is longer so there are more places for IMFs so more forces to be broken

Question 4

What does crude oil contain and how can these substances be separated?

Answer 4

• crude oil is a mixture which contains a large number of different liquids which have dif. boiling points
• these types of mixtures can be separated by fractional distillation

Question 5

The easier something is to boil…

Answer 5

the more flammable it is

Question 6

As the boiling points of the fractions increases what happens to their properties?

Answer 6

• appearance darkens
• ease of lighting becomes more difficult
• smokiness of the flame increases
• viscosity (stickiness) increases
• thicker liquids and not runny

Question 7

How is crude oil separated by fractional distillation in the lab?

Answer 7

• if crude oil is heated the liquids with the lowest bp’s will evaporate first and pass up the flask and fractionating column
• this vapour then passes down the side-arm and through the condenser
• this cools down the vapour and so it eventually condenses as a liquid which forms in the conical flask
• if the mixture is heated very gently between a number of temp ranges - different liquids can be collected at dif. temps - these are called fractions

Question 8

How is crude oil separated by fractional distillation industrially?

Answer 8

• crude oil is heated/boiled and then it enters the fractionating column
• as they rise up the column the temp becomes lower
• when the temps becomes just below the bp of the a substance that substance will melt and a liquid will be collected in a tray at that level in the column
• at different levels a mixture of liquids called fractions are collected - fractions will consist of a mixture of liquids containing similar sized molecules with similar boiling points
• substances with low boiling points will remain gases even at the top of the column as the temp is higher than their boiling points
• these gases are called petroleum gases and will be composes of the shorter chains of alkanes e.g. ethane, methane, propane etc.
• some v. long alkanes are still liquid even at high temps as the both of the column - this fraction is called residue

Question 9

Uses of (petroleum) gases:

Answer 9

• domestic heating
• cooking

Question 10

Uses of petrol:

Answer 10

fuel for cars

Question 11

Uses of kerosene:

Answer 11

fuel for aircraft

Question 12

Uses of diesel oil:

Answer 12

fuel for some cars and trains

Question 13

Uses of fuel oil:

Answer 13

• fuel for large ships
• used in some power stations

Question 14

Uses of bitumen:

Answer 14

• surface roads - tar
• roofs

Question 15

Examples of some fractions of crude oil:

Answer 15

• (petroleum) gases
• petrol
• kerosene
• diesel oil
• fuel oil
• bitumen

Question 16

Organic compound:

Answer 16

compounds which contain carbon

Question 17

Homologous series:

Answer 17

a group of chemical compounds which share certain characteristics

Question 18

What do compounds in the same homologous series share?

Answer 18

• each compound had the same functional group and therefore the same general formula
• have similar chemical properties
• differ by CH2 units in molecular formulae from neighbouring compounds
• show gradual trend in physical properties (as exemplified by their boiling points)

Question 19

The complete combustion of hydrocarbon fuels is a reaction in which…

Answer 19

• CO2 and H2O are produced
• energy is given out - exothermic

Question 20

Which alkanes are used as fuels and why?

Answer 20

the shorter alkanes are used as fuels are they react easily with oxygen

Question 21

When does complete combustion take place?

Answer 21

when an alkane burns in sufficient oxygen

Question 22

Balanced equation for complete combustion of ethane:

Answer 22

Question 23

Balanced equation for complete combustion of hexane:

Answer 23

Question 24

How can impurities in some hydrocarbon fuels result in the production of sulphur dioxide?

Answer 24

• most fuels inc. coal, carbon and/or hydrogen may also contain some sulphur
• when the fuels are burnt in oxygen this sulphur can react with the oxygen to form sulphur dioxide

Question 25

How is acid rain caused?

Answer 25

when sulphur dioxide dissolves in rain water

Question 26

What are the problems with acid (sulphuric acid) rain?

Answer 26

- damages buildings and statues (made of limestone) - reduces the growth of or kills trees and crops - cause pH of water tables to be lower which kills fish

Question 27

What happens when fuels are burned in engines?

Answer 27

- When burned in engines at high temperatures nitrogen and oxygen from the air combine to produce nitrogen monoxide - When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide - nitrogen monoxide and nitrogen dioxide are pollutants

Question 28

Advantages of using hydrogen as a fuel in cars rather than petrol:

Answer 28

- Use of hydrogen – petrol is from crude oil, a finite resource - Only produces water – no CO2 produced which contributes to global warming

Question 29

Disadvantages of using hydrogen as a fuel in cars rather than petrol:

Answer 29

- Expensive - Difficult to transport and store hydrogen - as H2 takes up a lot of space since it is a gas - Dangerous – hydrogen can be explosive

Question 30

What are petrol, kerosene, diesel oil and methane?

Answer 30

non-renewable fossil fuels

Question 31

Where are petrol, kerosene and diesel oil obtained from?

Answer 31

obtained from crude oil

Question 32

Where is methane obtained from?

Answer 32

found in natural gas

Question 33

What does the fractional distillation of crude oil provide in general?

Answer 33

fractional distillation of crude oil provides a surplus of the long chain hydrocarbons e.g. decane but a shortage of short chains hydrocarbons e.g. methane

Question 34

What does cracking involve?

Answer 34

cracking involves the breaking down of larger, saturated hydrocarbon molecules (alkanes) into smaller, more useful ones, some of which are unsaturated (alkenes)

Question 35

What type of reaction is the cracking of hydrocarbons?

Answer 35

thermal decomposition

Question 36

General cracking equation:

Answer 36

longer alkane → shorter alkane + short-chain alkene

Question 37

What is the process of cracking?

Answer 37

- Hydrocarbons can be cracked to produce smaller, more useful molecules - heat the hydrocarbons to vaporise them - the vapours are then: - either passed over hot catalyst - mixed with steam and heated to a v. high temp so that thermal decomposition reactions can occur

Question 38

What are the products of cracking?

Answer 38

alkanes + alkenes (unsaturated hydrocarbons)

Question 39

Why is cracking necessary?

Answer 39

- vital process as it converts long chain molecules which are less useful into short chain molecules which are more useful - demand for smaller chained alkanes is much greater for longer chained alkanes as they are more useful - demand > supply - shorter chained hydrocarbons ignite more easily and so are more useful as fuels

Question 40

When does incomplete combustion take place?

Answer 40

Incomplete combustion happens when the supply of air or oxygen is poor

Question 41

How much energy is released in incomplete combustion?

Answer 41

Less energy is released than during complete combustion

Question 42

Products of incomplete combustion:

Answer 42

- carbon monoxide - carbon (soot) - water

Question 43

Incomplete combustion of propanol example:

Answer 43

C3H8 + 3O2 → 2CO + C + 4H2O

Question 44

What is the carbon released as in incomplete combustion?

Answer 44

carbon is released as fine black particles (soot)

Question 45

Harmful impacts of soot:

Answer 45

- Soot can cause breathing problems and it blackens buildings - It may block boilers and other appliances, or cause a fire

Question 46

Harmful impacts of carbon monoxide:

Answer 46

- Carbon monoxide is a toxic gas - It is absorbed in the lungs and binds with the haemoglobin in the red blood cells - this reduces the capacity of the blood to carry oxygen - Carbon monoxide causes drowsiness, and affected people may fall unconscious or even die

Question 47

Why can the incomplete combustion of hydrocarbons produce carbon and carbon monoxide?

Answer 47

less oxygen so carbon atoms cannot be completely oxidised to produce CO2 instead CO and O are produced

Question 48

Why can the incomplete combustion of hydrocarbons produce carbon and carbon monoxide?

Answer 48

- If there’s not enough oxygen, some of the fuel doesn’t burn – this is partial combustion - solid particles of soot (carbons) and unburnt fuel are released - Carbon monoxide (CO) is also released when there isn’t enough oxygen to produce CO2 instead

Question 49

How does carbon monoxide behave as a toxic gas?

Answer 49

carbon monoxide is a colourless and odourless gas which, if breathed in, prevents red blood cells carrying oxygen around your body, which leads to death

Question 50

What problems can be caused by the incomplete combustion producing carbon monoxide and soot in appliances that use carbon compounds as fuels?

Answer 50

- Carbon monoxide causes health problems - Soot causes global dimming