R1.3: Energy From Fuels

Question 1

What do thermochemical reactions need?

Answer 1

Fuel - reactive metals, nonmetals, organic compounds

Oxygen - oxidation reaction

Source of ignition - not necessarily spark or flame -> could be buildup of heat in volatile liquid or exposure to high levels of oxygen

Question 2

Explain the combustion of metals

Answer 2

metal + oxygen → metal oxide

All metals can oxidized, not all combust
-> need high surface area

Less reactive metals -> will not combust
More reactive metals (s block) -> combust in air
-> form ionic oxide, basic

Eg: magnesium
magnesium + oxygen -> magnesium oxide
2Mg (s) + O2(g) -> 2MgO (s)

Question 3

Explain the combustion of non-metals

Answer 3

Non-metal + oxygen -> non-metal oxide

Nonmetals -> variety of oxidation states

P-block -> covalent oxides, acidic

Eg: sulfur
Sulfur + oxygen -> sulfur dioxide
S (s) + O2 (g) -> SO2 (g)

Question 4

Explain the combustion of organic compounds

Answer 4

Many OC -> fuel -> large amount of energy
No spontaneous combustion -> save to transport/store

Combustion of hydrocarbons:
Eg: methane
Methane + oxygen -> carbon dioxide + water
CH4 (g) + 2O2 (g) -> CO2 (g) + 2H2O (l)

Combustion of alcohols:
Eg: ethanol
Ethanol + oxygen -> carbon dioxide + water
C2H5OH (l) + 3O2 (g) -> 2CO2 (g) + 3H2O (l)
- not as energy dense -> mixed with gasoline/diesel

Longer hydrocarbon chains:
Increased carbon content -> more CO2/CO/C
Stronger London dispersion forces -> less volatile -> less energy dense
-> incomplete combustion more likely to

Question 5

How can you compare the energy of different fuels?

Answer 5

Specific energy of a fuel (kJ/kg) = energy released rom fuel / mass of fuel consumed

Question 6

What is complete combustion?

Answer 6

Fuels burnt in excess of oxygen
All carbon and hydrogen oxidized
Products: CO2 and water

Fuel + oxygen -> carbon dioxide + water

Question 7

What is incomplete combustion?

Answer 7

Limited supply of oxygen
Still produce water, but produced carbon monoxide/carbon (not fully oxidized)

Often in car engines

Fuel + oxygen -> carbon monoxide + water

Question 8

Why is incomplete combustion not wanted?

Answer 8

Carbon Monoxide -> colorless and odorless

Toxic and poisonous gas - binds irreversibly to haemoglobin
-> limit haemoglobin ability to transport oxygen

No oxygen transport -> dizziness, lose consciousness
If CO not removed -> die

Carbon -> produced when very reduced supply of O2 -> soot
Mainly hydrocarbons from benzene (high carbon content)

Question 9

What is fossil fuel?

Answer 9

Fuel formed from the remains of dead plants and animals undergone reduction under pressure
-> contains hydrocarbons + N/O/S (from amino acids)
-> coal, oil, natural gas

Non-renewable
Oil + gas -> aerobic decomposition of marine organisms
Coal -> terrestrial vegetation
Crude oil -> mix of straight-chained, branched, cyclic and aromatic hydrocarbons

Question 10

What are the advantages and disadvantages of different types of fossil fuels?

Answer 10

COAL:
Advantages:
Cheap, abundant, long-lasting, relatively safe
Converted into liquid fuels, gases
Feedstock for organic chemicals
Energy dense
Safer than nuclear

Disadvantages:
High pollution
Associated with global warming, acid rain, pollution
Difficult to transport
Issues with mining (pollution, habitat destruction, safety, etc)

OIL:
Advantages:
Easy to store and transport
Easily purified/processed
Feedstock for organic chemicals
Energy dense

Disadvantages:
Pollution
Global warming, acid rain, smog, oil spills
Safety issues with drilling
Limited lifespan

NATURAL GAS:
Advantages:
Cheap, east to store + transport
Energy dense
Relatively clean

Disadvantages:
Pollution
Global warming
Expensive + time intensive to store
Safely issues with storage
Limited lifespan

Question 11

Why is the CO2 in our atmosphere increasing?

Answer 11

Increase in:
Electricity generation, transport, construction, deforestation/agriculture, livestock feeding

CO2 + methane -> contribute to atmospheric carbon level -> increase global temps
Increasing CO2 because of increased combustion of fossil fuel

Question 12

What are greenhouse gases?

Answer 12

Gases that absorb radiation emitted from the earths surface, trapping it in the atmosphere so that it is not lost to space

Average temp on earth -> regulated by steady-state equilibrium of energy reaching earth and energy radiated back by earth
- incoming radiation reflected/absorbed

Greenhouse gas absorb radiated infrared heat and ré-radiate it back to earth -> important in temp regulation

Eg: CO2, methane, nitrous oxides, water vapor

Question 13

What is the greenhouse effect?

Answer 13

Shortwave radiation -> strike earth -> absorbed + re-emitted from surface as infrared radiation -> passes through atmosphere/absorbed by greenhouse gases and re-emitted to earth
Reduces thermal energy lost in space + keeps earth warm
=> greenhouse effect

Greenhouse gas concentration increase due to human activity -> more thermal energy trapped -> earth average temperature rise -> global warming (>0.5°C in past 40 years)
=> enhanced greenhouse effect

Question 14

Why do greenhouse gasses absorb infrared radiation?

Answer 14

Molécules -> vibrate as bonds stretch and bend

Energy associated with bond vibration = infrared

Stretching + bending -> change in dipole moment -> vibration are infrared active

Question 15

Explain the formation of biofuel by photosynthesis

Answer 15

Biofuels -> biological fixation of carbon over a short period of time through photosynthesis

6CO2 (g) + 6H2O (l) -> C6H12O6 (s) + 6O2 (g)
Endothermic (+2816 kJ/mol)

Question 16

How do we get energy from biomass?

Answer 16

Respiration, wood burning, waste material combustion
C6H12O6 (s) + 6O2 (g) -> 6CO2 (g) + 6H2O (l)

Plant material also decomposed by bacteria into biogas
(Methane + CO2 from anaerobic decomposition)

Liquid fuel from fermentation
C6H12O6 (s) -> 2C2H5OH (l) + 2CO2 (g)
Then ethanol burned (or + gasoline = gasohol)
2C2H5OH (l) + 3O2 (g) -> 2CO2 (g) + 3H2O (l) = -1371kJ

Question 17

What are some key difference between renewable and non renewable resources?

Answer 17

Renewable:
Won’t run out, replaced easily
Infinite
Sustainable -> produced at the same rate/faster than used

Nonrenewable:
Will run out, limited
Finite
Not sustainable

Question 18

Advantages and disadvantages of biofuel

Answer 18

Advantages:
Reduce pollution/waste
Carbon neutral
Renewable
Sustainable
Opportunity for less developed countries

Disadvantages:
High costs
Uses up land -> developed centuries don’t have the space
Increase fertilizer use

Question 19

What is bioenthanol?

Answer 19

Plants absorb atmospheric CO2 -> glucose (via photosynthesis) -> ethanol (via fermentation)

Question 20

What is biodiesel?

Answer 20

Made from renewable vegetable oils rather than non-sustainable petrochemicals

Natural triglyceride oils -> esters of methanol
Reversible reaction -> excess methanol used to drive equilibrium to right

Eg: biodiesel made from grapeseed oil made by transesterificaiton
Triglyceride + methanol -> fatty acid methyl ester + glycerol
- Acid protonating carbonyl group
- Alkali deprotonating alcohol/methanol (more common)

Question 21

What is biogas?

Answer 21

Renewable fuel released when organic matter broken down by microorganisms in the absence of oxygen

Methane + CO2 + small amount of hydrogen sulphide/others
- amounts depends on type of waste used

C6H12O6(s) -> 3CO2(g) + 3CH4(g)

Question 22

What is a fuel cell?

Answer 22

An electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode

Structure:
Two electrode: negative anode and positive cathode
Separated by solid/liquid electrolyte
Electrically charged particles move between two electrodes
Catalysts -> often used to speed up reaction

=> electricity generated when oxygen and hydrogen combine

Alkaline fuel cell (NaOH, KOH):
Anode: H2 + 2OH- -> 2H2O + 2e-
Cathode: O2 + 2H2O + 4e- -> 4OH-

Acidic fuel cell (H3PO4):
Anode: H2 -> 2H+ + 2e-
Cathode: O2 + 4H+ + 4e- -> 2H2O

Question 23

What is a hydrogen oxygen cell?

Answer 23

Structure:
Reaction chambers -> seperate inlets for H and O2
Outlet for water (product)
Electrolyte - aqueous sodium hydroxide
Semipermeable membrane seperating H and O2

Half equations:
2H2(g) + 4OH–(aq) → 4H2O (l) + 4e–
O2(g) + 2H2O + 4e–→ 4OH–(aq)
Overall equation:
2H2(g) + O2(g) → 2H2O (l)

Energy produced given as electricity
Unlike rechargeable batteries -> don’t need external source of electricity for charging -> non-polluting

Question 24

What are the advantages and disadvantages of a hydrogen-oxygen cell?

Answer 24

Advantages:
Water only product (used in spacecraft: drinking water)
All bond energy converted into electrical energy
No harmful oxides of nitrogen produced

Disadvantages:
Hydrogen highly flammable -> safety hazard
Think walled cylinders + pipes needed to store H -> pricey
Production of hydrogen -> byproduct of crude oil -> relies on non-renewable resource
High energy density but is a gas -> larger volume taken up

Question 25

What is a methanol fuel cell?

Answer 25

Same as hydrogen oxygen except methanol instead of hydrogen Structure: Reaction chamber -> separate inlets of methanol and oxygen Outlet for CO2 and water (products) Electrolyte (usually proton exchange membrane) Semi permeable membrane -> separate H and O2 Half equations: CH3OH (aq) + H2O (l)  →  CO2 (g) + 6H+ (aq) +  6e– O2 (g) + 4H+ (aq) + 4e– →  2H2O (l) Overall equation: CH3OH (aq) + 1.5O2 (g) → CO2 (g) +2H2O (l)

Question 26

What are the advantages and disadvantages of a methanol fuel cell?

Answer 26

Advantages: Methanol easier to store + transport Doesn’t require high temp or pressure Membrane longer lifespan -> aqueous environment Greater energy density Methanol can be produced via fermentation -> cleaner Disadvantages: Methanol is toxic and highly flammable Most commonly made from non-renewable fossil fuels Lower voltage/power Low efficiency Anode require more efficiently catalyst -> expensive Produces greenhouse gas

Question 27

Why should we use fuel cells?

Answer 27

Society dependent on fossil fuels - non-renewable Fuel prices rising and resources decreasing Food, transport and electricity costs affected by fuel prices Atmosphere -> more pollution CO2 contributes to climate change change + greenhouse effect

Question 28

What are the limitations of fuel cells?

Answer 28

Storage of hydrogen -> safety Transportation of hydrogen -> expensive Feasibility of liquified hydrogen -> safety Limited life cycle of cell -> economic High production costs -> economic Toxic chemicals used in production -> safety/environmental

Question 29

How do hydrogen-oxygen/proton exchange membrane fuel cells work?

Answer 29

1. Hydrogen enters cell 2. Hydrogen is oxidized to H+ at the anode - 2H2 (g) -> 4H+ (aq) + 4e- 3. Electrons travel via external circuit 4. H+ ions travel through the electrolyte and membrane 5. Oxygen gas enters the cell 6. Oxygen is reduced at the cathode - O2 (g) + 4H+ (aq) + 4e- -> 2H2O (l) 7. Catalyst speeds up reaction between O2 and H+ 8. Water produced - 2H2 (g) + O2 (g) -> 2H2O (l)