chemistry - fuels & earth science: fuels (8.1 - 8.17)

8.1 what are hydrocarbons?

compounds that contain carbon & hydrogen ONLY

8.2 crude oil

complex mixture of hydrocarbons

contains molecules in which carbon atoms are in chains/rings

important source of useful substances (fuels & feedstock for petrochemical industry)

finite resource

8.3 crude oil separated by fractional distillation - why?

crude oil not runny enough/ignited easily enough to be used as fuel

separated into simpler, more useful mixtures

8.3 crude oil separated by fractional distillation - how?

diff. hydrocarbons have diff. boiling points

1. crude oil heated strongly to evaporate it

2. hot vapours piped into bottom of column

in fractionating column: (hottest at bottom, coldest at top)

3. vapours rise through column & cool

4. vapours condense when reach part of column below their boiling point

5. liquid falls into tray & piped away

6. vapours with lowest boiling points don’t condense - leave top as mixture of gases

7. bitumen has highest boiling point - leaves at bottom as hot liquid

8.4 order of fractions

gases

petrol

kerosene

diesel oil

fuel oil

bitumen

8.4 uses of fractions - gases

domestic heating

domestic cooking

8.4 uses of fractions - petrol

fuel for cars

8.4 uses of fractions - kerosene

fuel for aircraft

8.4 uses of fractions - diesel oil

fuel for some cars

fuel for trains

8.4 uses of fractions - fuel oil

fuel for large ships

in some power stations

8.4 uses of fractions - bitumen

surfacing roads & roofs

8.5 how hydrocarbons in diff. fractions differ in number of carbon & hydrogen atoms molecules contain, boiling points, ease of ignition, viscosity

8.5 what homologous series are hydrocarbons in diff. fractions from?

(mostly) from alkane homologous series

8.6 homologous series

series of compounds which:

• have same general formula (e.g. alkanes = C_nH_2n+2)

• differ by CH₂ in molecular formulae from neighbouring compounds

• show gradual variation in physical properties (shown by boiling points)

• have similar chemical properties

8.7 complete combustion of hydrocarbon fuels

carbon dioxide + water produced

energy given out

8.8 why can incomplete combustion of hydrocarbons produce carbon & carbon monoxide?

some carbon atoms:

• fully oxidised to carbon dioxide

• only partially oxidised to carbon monoxide

• released as smoke & soot (carbon)

incomplete combustion of hydrocarbon fuels

limited supply of air/oxygen

carbon + carbon monoxide + water produced

energy given out (less than complete combustion)

8.9 how does carbon monoxide behave as toxic gas?

combines with haemoglobin in RBCs - prevents oxygen combining

reduces amount of oxygen in blood stream - makes people sleepy/ unconscious/can cause death

8.10 problems caused by incomplete combustion in appliances that use carbon compounds as fuel - producing carbon monoxide

reduces amount of oxygen in bloodstream - makes people sleepy/ unconscious/can cause death

(causes problems if appliances unventilated)

8.10 problems caused by incomplete combustion in appliances that use carbon compounds as fuel - producing soot

blocks pipes carrying waste gases away from appliance

blackens buildings

causes breathing problems if collects in lungs

(causes problems if appliances poorly maintained/unventilated)

8.11 impurities in some hydrocarbon fuels - production of sulfur dioxide

hydrocarbon fuels may contain sulfur compounds (impurities)

hydrocarbon fuel burnt - sulfur reacts with oxygen, forms sulfur dioxide

8.12 acid rain - formed

sulfur dioxide dissolves in rain water - forms sulfuric acid

8.12 acid rain - problems

makes soil acidic - crops don’t grow well

causes excess acidity in rivers & lakes - prevents fish eggs hatching, kills fish & insects

reacts with calcium carbonate - increases rate of weathering of limestone/ marble buildings & breaks down their structures

increases rate of corrosion of metals

8.13 fuels burned in engines - produce pollutants

car engines - fuel mixed with air & ignited inside engine

oxygen & nitrogen in air react together at high temps.

produces oxides of nitrogen = pollutants

8.14 hydrogen as fuel in cars instead of petrol - advantages

renewable resource: by-product of cracking, produced by reacting methane (from natural gas) with steam

• petrol from crude oil = finite resource

environmental benefits: produces water vapour but no carbon dioxide

• petrol produces carbon dioxide = greenhouse gas → global warming, climate change

easily ignited

combustion releases large amounts of energy

8.14 hydrogen as fuel in cars instead of petrol - disadvantages

difficult to store in large amounts: gas at room temp.

• petrol = liquid at room temp.

8.15 petrol, kerosene, diesel oil - renewable/non-renewable

non-renewable fossil fuels

8.15 petrol, kerosene, diesel oil - obtained from

crude oil

8.15 methane - renewable/non-renewable

non-renewable fossil fuel

8.15 methane - found in

natural gas

8.16 cracking

breaks down larger, saturated hydrocarbon molecules (alkanes)

forms smaller, more useful hydrocarbon molecules - some unsaturated (alkenes)

saturated - type of hydrocarbon, bond

alkanes

carbon atoms joined by single covalent bonds (C-C)

unsaturated - type of hydrocarbon, bond

alkenes

contain carbon-carbon double bond (C=C)

8.17 why is cracking necessary?

crude oil separated by fractional distillation - volume of each fraction usually doesn’t match demand

demand for smaller hydrocarbon molecules > demand for larger hydrocarbon molecules - shorter hydrocarbon molecules more useful

cracking produces shorter chained alkanes from longer chained alkanes