Energy from fuels — IB Chemistry HL Revision Notes & Topical Questions

Section 1 of 34Combustion

Combustion

• Combustion a rapid exothermic oxidation needing a fuel, oxygen and an ignition source.

Products

• Products releases heat and light.

Complete combustion

• Complete combustion occurs in plenty of oxygen; fuel + oxygen → carbon dioxide + water.

Incomplete combustion

• Incomplete combustion occurs in limited oxygen; carbon monoxide or carbon (soot) forms instead of CO₂.

Metals

• Metals all oxidise, but only reactive or finely divided metals combust, forming basic ionic oxides.

Non-metals

• Non-metals p-block non-metals form acidic covalent oxides.

Organic fuels

• Organic fuels they do not ignite spontaneously because of a high activation energy, which makes them safe to store.

Section 2 of 34Incomplete combustion

Flame

• Flame a Bunsen burner gives a blue non-luminous flame for complete combustion and a yellow luminous flame for incomplete combustion.

Carbon monoxide

• Carbon monoxide colourless, odourless and toxic, binding irreversibly to haemoglobin so oxygen cannot be carried.

Soot

• Soot carbon forms with very limited oxygen; fuels with a higher percentage of carbon burn sootier.

Never hydrogen

• Never hydrogen hydrogen is preferentially oxidised, so incomplete combustion never produces H₂.

Where it occurs

• Where it occurs car engines and faulty boilers.

Section 3 of 34Specific energy and energy density

Specific energy

• Specific energy the energy released per unit mass of fuel (kJ kg⁻¹).

Energy density

• Energy density the energy released per unit volume (kJ dm⁻³).

Hydrogen

• Hydrogen the highest specific energy, about 142 MJ kg⁻¹, but a low energy density by volume.

Liquid fuels

• Liquid fuels liquid hydrocarbons have a high energy density, which suits transport.

Alkane chain length

• Alkane chain length longer chains give more CO₂ per gram and less energy per gram, with methane having the highest specific energy.

Section 4 of 34Fossil fuels

Fossil fuels

• Fossil fuels coal, oil and natural gas, formed from buried remains of plants and animals;
  • they are finite and non-renewable.

Coal

• Coal cheap and long-lasting, but releases CO₂, SO₂ and particulates and mining destroys habitats.

Oil

• Oil easy to store and transport with a high energy density, but releases CO₂, SO₂ and CO and risks spills.

Natural gas

• Natural gas the cheapest, burning relatively cleanly, but it produces CO₂ and is costly and hazardous to store.

Choice

• Choice depends on availability, cost, intended use and energy requirements.

Section 5 of 34Carbon dioxide and the greenhouse effect

Rising CO₂

• Rising CO₂ caused mainly by burning fossil fuels, with deforestation and livestock (methane) also contributing.

Greenhouse gases

• CO₂, CH₄, N₂O and water vapour absorb infrared radiation.

Natural effect

• Natural effect the greenhouse effect keeps Earth warm enough for life.

Enhanced effect

• Enhanced effect human activity adds greenhouse gases, trapping more heat and causing global warming.

IR absorption

• IR absorption a molecule absorbs infrared only if its vibration changes the dipole;
  • N₂ and O₂ are inactive but CO₂ is active.

Section 6 of 34Biofuels

Renewable

• Renewable a biofuel can be replaced as quickly as it is used.

Bioethanol

• Bioethanol made by fermenting glucose, C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

Biodiesel

• Biodiesel made by transesterification of vegetable oils or fats with methanol using an alkaline catalyst, giving methyl esters.

Biogas

• Biogas made by anaerobic breakdown of organic waste, mostly methane and CO₂.

Carbon neutral

• CO₂ released on burning equals CO₂ absorbed during growth, though full life-cycle emissions are positive.

Disadvantages

• Disadvantages land competes with food crops and fuel crops need fertiliser and energy.

Section 7 of 34Hydrogen–oxygen fuel cell

Fuel cell

• Fuel cell an electrochemical cell that converts the chemical energy of a fuel and oxidant directly into electrical energy without combustion.

Operation

• Operation it works continuously while fuel and oxygen are supplied.

Acid electrolyte

• Acid electrolyte anode H₂ → 2H⁺ + 2e⁻, cathode O₂ + 4H⁺ + 4e⁻ → 2H₂O.

Alkaline electrolyte

• H₂ + 2OH⁻ → 2H₂O + 2e⁻ and O₂ + 2H₂O + 4e⁻ → 4OH⁻.

Overall

• 2H₂ + O₂ → 2H₂O, E = +1.23 V, with water the only product.

Advantages

• Advantages no combustion, no nitrogen oxides, and high efficiency.

Disadvantages

• Disadvantages hydrogen is flammable, needs heavy storage, and is mostly made from fossil fuels.

Section 8 of 34More from the specification

1Combustion stoichiometry

• Balancing complete combustion equations of organic compounds (e.g. propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O) to determine energy output.

2Carbon dioxide emissions and climate impact

• Burning fossil fuel resources releases significant greenhouse gas emissions, elevating atmospheric carbon dioxide levels.

3Specific energy vs energy density

• Specific energy measures energy released per unit mass (MJ/kg), whereas energy density measures energy per unit volume (MJ/dm³.

4Renewable biofuels

• Bioethanol and biogas derived from contemporary biomass provide carbon-neutral alternatives to finite fossil reserves.

5Compound

• Two or more elements chemically combined in a fixed ratio.

6Chemical formula

• Symbols and numbers showing the relative number of atoms of each element.

Section 9 of 34What combustion is

What combustion is

Definition

• Definition a rapid exothermic oxidation reaction, usually called burning.

Requirements

• Requirements a fuel, oxygen and an ignition source such as a spark, flame or heat build-up.

Products

• Products heat and light.

Two types

• Two types complete combustion in sufficient oxygen and incomplete combustion in limited oxygen.

Section 10 of 34Combustion of metals

Oxidation

• Oxidation all metals oxidise, but not all combust.

Unreactive metals

• Unreactive metals copper only forms a coating, black CuO on heating.

Reactive metals

• Reactive metals s-block metals burn and form basic ionic oxides with pH above 7 in water.

Surface area

• Surface area a high surface area, such as iron powder in a sparkler, helps a metal burn.

General equation

• General equation metal + oxygen → metal oxide, for example 2Mg(s) + O₂(g) → 2MgO(s) with a bright white flame.

Potassium

• 4K + O₂ → 2K₂O with a lilac flame.

In exams

• In exams give the typical oxides (Na₂O, Fe₂O₃), even though sodium forms Na₂O₂ and iron forms Fe₃O₄.

Section 11 of 34Combustion of non-metals

Oxides

• Oxides p-block non-metals form covalent oxides, usually acidic with pH below 7.

General equation

• General equation non-metal + oxygen → non-metal oxide.

Sulfur

• S(s) + O₂(g) → SO₂(g) with a blue flame.

White phosphorus

• P₄ + 5O₂ → P₄O₁₀.

Section 12 of 34Complete combustion of hydrocarbons and alcohols

Fuels

• Fuels hydrocarbons and alcohols release large amounts of energy.
  • (especially alkanes)

Safe storage

• Safe storage they do not ignite spontaneously because of a high activation energy.

Excess oxygen

• Excess oxygen all carbon and hydrogen is oxidised to carbon dioxide and water.

Equation

• Equation fuel + oxygen → carbon dioxide + water.

Flame

• Flame a Bunsen burner shows a blue non-luminous flame.

Examples

• CH₄ + 2O₂ → CO₂ + 2H₂O, C₃H₈ + 5O₂ → 3CO₂ + 4H₂O and C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

Complete combustion of hydrocarbons and alcohols

Write the products CO₂ and H₂O.

Balance carbon (one CO₂ per C) and hydrogen (one H₂O per two H).

Count the oxygen on the product side and balance with O₂, remembering any oxygen already in the fuel.

Section 13 of 34Alcohols as fuels

Flame

• Flame ethanol burns with an almost invisible flame.

Source

• Source ethanol can be made sustainably by fermentation of sugars.

Energy density

• Energy density lower than petrol, so vehicles need a larger tank or more frequent refuelling.

Blending

• Blending mixing with petrol or diesel raises energy density and makes the flame visible.

Land use

• Land use using farmland for fuel reduces land for food, raising socio-economic and ethical issues.

Balancing

• Balancing remember the oxygen already present in the alcohol.

Section 14 of 34Incomplete combustion

Product	Propane equation	Features
Carbon monoxide	C3H8 + 3½O2 → 3CO + 4H2O	Colourless, odourless, toxic; binds irreversibly to haemoglobin
Carbon (soot)	C3H8 + 2O2 → 3C + 4H2O	Very limited oxygen; sooty yellow flame

Conditions

• Conditions limited oxygen in car engines and faulty boilers; water is still formed.

Flame

• Flame a Bunsen burner gives a yellow luminous flame.

Soot

• Soot fuels with a higher percentage of carbon by mass burn sootier.

Hydrogen

• Hydrogen never formed, as hydrogen is oxidised in preference to carbon.

Symptoms

• Symptoms carbon monoxide poisoning causes dizziness, unconsciousness and possibly death.

Section 15 of 34Standard enthalpy changes and combustion energy

Standard conditions

• 100 kPa, 1.00 mol dm⁻³ solutions and standard states, usually at 298 K.

Combustion, ΔHc°

• Combustion, ΔHc° one mole of substance burned completely in excess oxygen; always negative.

Formation, ΔHf°

• Formation, ΔHf° one mole of compound from its elements in standard states; zero for an element.

Neutralisation

• H⁺(aq) + OH⁻(aq) → H₂O(l), about −57 kJ mol⁻¹ for strong acid and base.

Spirit burner

• Spirit burner energy gained by the water is q = mcΔT, assuming no heat loss.

Sign

• Sign q for the water is positive but the enthalpy change of the combustion reaction is negative.

Error

• Error heat losses make the measured value too small.

Section 16 of 34More from the specification

More from the specification

Greenhouse Gases and Global Climate Change

• The combustion of carbon-based fossil resources releases greenhouse gases—predominantly carbon dioxide (CO₂) and methane (CH₄)—which trap infrared radiation in the atmosphere and intensify the enhanced greenhouse effect.

Methane and Organic Fossil Resources

• Natural gas consists primarily of methane, an organic hydrocarbon fuel extracted from fossil reserves that burns efficiently but contributes to global warming if released directly into the atmosphere.

Direct Alcohol and Hydrogen Fuel Cells

• Electrochemical fuel cells convert chemical energy from fuels like hydrogen or organic methanol directly into electrical power through catalytic oxidation, offering high efficiency and reduced atmospheric emissions.

Sustainable Alternatives to Depleting Fossil Fuels

• Transitioning from finite fossil resources to renewable biofuels, green hydrogen,
• and carbon-neutral synthetic fuels mitigates global warming and curbs environmental degradation.

Global warming

• Rise in the Earth's average temperature caused by extra greenhouse gases.

Chemical formula

• Symbols and numbers showing the relative number of atoms of each element.

Fossil

• The preserved remains or traces of an ancient organism.

Renewable

• Can be replaced as quickly as it is used

Non-renewable

• Cannot be replaced in a human lifetime

Fossil fuel

• A fuel formed from the remains of living things, such as coal, natural gas and petroleum.

Greenhouse effect

• Absorption and re-emission of thermal energy by atmospheric gases, which reduces energy lost to space.

Section 17 of 34Fossil fuels

Fossil fuels

• Fossil fuels coal, oil and natural gas, made of hydrocarbons from the buried remains of plants and animals formed millions of years ago.

Non-renewable

• Non-renewable finite supplies that cannot be replaced quickly.

Choice of fuel

• Choice of fuel depends on availability, cost, intended use and energy requirements.

Section 18 of 34Fossil fuels compared

Fuel	Advantages	Disadvantages
Coal	Cheap, widely available, long-lasting, safe to store, convertible to liquid and gas fuels, ash useful for roads	Releases CO2 (warming), SO2 (acid rain) and particulates (global dimming); hard to transport; mining harms habitats and miners
Oil	Easy to store and transport, impurities removed easily, high energy density, separated by fractional distillation and cracked to alkanes and alkenes	Releases CO2, SO2 and CO; smog and global dimming; spills and drilling risks; uneven global distribution
Natural gas	Cheapest, high specific energy, relatively clean, no acid rain	Produces CO2; costly and slow to extract; costly and hazardous to store under pressure

Section 19 of 34Chain length, energy and carbon dioxide

Chain length, energy and carbon dioxide

Specific energy

• Specific energy the energy released per unit mass of fuel.

More carbon

• More carbon longer alkanes make more CO₂ per gram.
  • (and more CO or soot if incomplete)

Stronger London forces

• Stronger London forces longer chains are less volatile.

Less energy per gram

• Less energy per gram methane has the highest specific energy.

Combustion type

• Combustion type both complete and incomplete combustion are exothermic, but longer chains are more likely to burn incompletely.

Section 20 of 34Calculating the CO₂ produced

1

Write the balanced equation for combustion.

2

Find the moles of fuel from mass ÷ M_r.

3

Use the equation ratio to find the moles of CO₂.

4

Find the mass of CO₂ from moles × M_r.

• Ratio
  • 1 mol of C₄H₁₀ gives 4 mol of CO₂, for example.

Section 21 of 34Rising carbon dioxide levels

Causes

• Causes combustion of fossil fuels, mainly for electricity generation, transport and construction.
  • (about 85 % of fossil fuel use)

Other sources

• Other sources deforestation and livestock farming, where cattle release methane.

Other natural sources

• Other natural sources respiration, wood burning, and the action of acids on carbonates or thermal decomposition of carbonate rocks.

Monitoring

• Mauna Loa has measured CO₂ since 1958 and methane since 1984.

Pattern

• Pattern a steady rise with annual fluctuations from seasonal photosynthesis, which removes CO₂ from the air.

Section 22 of 34The greenhouse effect

Role

• Role keeps Earth warm enough for life; without it temperatures would swing wildly, as on Mars.
  • (about 20 °C to −153 °C)

Greenhouse gases

• CO₂ , CH₄ (livestock, landfill, mining), nitrogen oxides (fossil fuel impurities) and water vapour.
  • (respiration and burning)

1

The Sun emits short-wave radiation that reaches the surface.

2

The surface absorbs it and re-emits long-wave infrared.

3

Some infrared escapes to space and some is absorbed by greenhouse gases.

4

These gases re-radiate in all directions, trapping heat.

Section 23 of 34Enhanced greenhouse effect

Cause

• Cause human activity such as burning fossil fuels raises greenhouse gas levels.

Effect

• Effect more heat is trapped, so Earth's average temperature rises, causing global warming.

Other pollutants

• SO₂ comes from sulfur impurities in fuel, and nitrogen oxides from nitrogen and oxygen reacting at high temperatures in engines.

Fuel choice

• Fuel choice natural gas gives the most energy per gram and least CO₂ per unit of energy of the fossil fuels.

Section 24 of 34More from the specification

More from the specification

Transition metals redox chemistry

• Transition metals redox chemistry variable oxidation states and partially filled d-orbitals enable transition metals to catalyze diverse industrial synthetic reactions.

Complete vs incomplete hydrocarbon combustion

• Complete combustion in excess oxygen yields carbon dioxide and water;
  • incomplete combustion in limited oxygen produces toxic carbon monoxide and soot particulates.

Fuel cells and electrochemical oxidation

• Hydrogen-oxygen fuel cells react hydrogen with oxygen across a proton-exchange membrane to generate electricity with zero direct carbon emissions,
• producing pure water.

Combustion enthalpy of alkanes and alcohols

• Standard enthalpy of combustion becomes increasingly exothermic with longer carbon chains due to the formation of more stable C=O and O-H bonds in products.

Incomplete combustion producing carbon monoxide

• Incomplete combustion producing carbon monoxide burning propane or organic hydrocarbon compounds in limited oxygen yields toxic carbon monoxide gas alongside soot.

Electrochemical fuel cells

• Electrochemical fuel cells fuel cells combine hydrogen fuel and oxygen at electrodes, generating clean electrical power without combustion flames.

Fossil fuel resources and conservation

• Fossil fuel resources and conservation finite fossil fuel resources require conservation because burning organic compounds releases greenhouse gases into the atmosphere.

Metal

• An element with a lattice of positive ions in a sea of delocalised electrons, usually shiny, conductive and malleable.

Compound

• Two or more elements chemically combined in a fixed ratio.

Carbon monoxide

• A toxic gas that binds to haemoglobin.

Chemical formula

• Symbols and numbers showing the relative number of atoms of each element.

Section 25 of 34Renewable and non-renewable resources

Renewable

• Renewable can be replaced as fast as, or faster than, it is used, so it is sustainable and effectively infinite.

Non-renewable

• Non-renewable finite, cannot be replaced within a short time and is not sustainable, like fossil fuels.

Reasons for biofuels

• Reasons for biofuels to reduce pollution, help with climate change and respond to depleting fossil fuels.

Origin

• Origin biofuels are made from organic compounds produced by biological carbon fixation.

Section 26 of 34Bioethanol and biodiesel

Bioethanol

• Bioethanol plants form glucose by photosynthesis, 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, and fermentation turns it into ethanol.

Carbon neutral

• CO₂ released on burning equals CO₂ absorbed in photosynthesis.

Biodiesel

• Biodiesel made from renewable vegetable oils by transesterification, converting triglycerides to methyl esters that are less viscous.

Catalysts

• Catalysts acid or alkali such as NaOH or KOH (deprotonates the alcohol), with alkali more common.
  • (protonates the carbonyl group)

Equilibrium

• Equilibrium the reaction is reversible, so excess methanol shifts it towards products.

Yield

• Yield up to 98 % under ideal conditions.

Section 27 of 34Biogas

Formation

• Formation microorganisms break down organic waste in anaerobic conditions.

Composition

• Composition mostly CH₄ and CO₂ with traces of H₂S, depending on the waste.

Carbohydrates

• C₆H₁₂O₆ → 3CO₂ + 3CH₄, giving 50.0 % methane.

Fatty acids

• 4C₁₇H₃₃COOH + 34H₂O → 21CO₂ + 51CH₄, giving 69.4 % methane.

Section 28 of 34Biofuels compared with fossil fuels

Advantages	Disadvantages
Carbon neutral, renewable and sustainable if crops are replanted	Costly to convert engines and machinery
Reduce greenhouse gas emissions and pollution	Harvesting and transporting crops is expensive
Reduce landfill; biodiesel and biogas can use waste	Land use conflict with food and possible deforestation
Create jobs and income	Crops deplete soil and need fertilisers and pesticides
	Lower specific energy than fossil fuels

1Whole process

• Whole process growing, harvesting, transporting and processing use energy, so real savings are smaller than the carbon-neutral argument suggests.

2Ethics

• Ethics waste-based biodiesel and biogas are preferred to food crops.

Section 29 of 34Fuel cells

Fuel cell

Fuel cell an electrochemical cell in which a fuel donates electrons at one electrode while oxygen gains electrons at the other.

Voltage

• Voltage oxidation of the fuel sets up a potential difference.

Use

• Use increasingly used in vehicles in place of petrol or diesel engines.

Continuous operation

• Continuous operation it works as long as hydrogen and oxygen are supplied, because energy is not stored in the cell.

Section 30 of 34Hydrogen–oxygen fuel cell

Construction

• Construction separate inlets for H₂ and O₂, a semi-permeable membrane, an aqueous NaOH electrolyte and a water outlet.

Negative electrode

• 2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻, E = −0.83 V.

Positive electrode

• O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq), E = +0.40 V.

Overall

• 2H₂(g) + O₂(g) → 2H₂O(l), E = +1.23 V, water the only product.

Advantages

• Advantages no harmful emissions, room temperature operation, no nitrogen oxides and efficient conversion to electricity.

Disadvantages

• Disadvantages flammable hydrogen, heavy thick-walled tanks, low energy density by volume and mostly fossil-fuel-derived hydrogen.

Section 31 of 34Methanol fuel cell

Fuel

• Fuel methanol is the source of H⁺ instead of hydrogen gas, with a proton exchange membrane electrolyte.

Half equations

• CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻ and O₂ + 4H⁺ + 4e⁻ → 2H₂O.

Overall

• CH₃OH + 1.5O₂ → CO₂ + 2H₂O.

Advantages over hydrogen

• Advantages over hydrogen easier to store and transport, low pressure and temperature, higher energy density by volume and can be made from biomass.

Disadvantages

• Disadvantages toxic and flammable, lower voltage, methanol crossing the membrane, expensive catalysts and CO₂ production.

Section 32 of 34More from the specification

More from the specification

Complete combustion of hydrocarbons

• In an excess supply of oxygen, hydrocarbons such as propane (C₃H₈) undergo complete combustion with a clean blue flame to produce carbon dioxide (CO₂) and water vapour (H₂O).

Incomplete combustion and pollutants

• Under restricted oxygen supply, incomplete combustion produces toxic carbon monoxide (CO) gas, unburnt hydrocarbons,
• and black carbon soot particulates, often burning with a yellow, smoky flame.

Combustion of reactive metals

• Reactive metals burn exothermically in oxygen to form solid basic metal oxides rather than gaseous oxides.
  • (e.g. 2Mg + O₂ → 2MgO)

Greenhouse gases and atmospheric impact

• The release of combustion gases, particularly carbon dioxide and methane, enhances the greenhouse effect, trapping infrared radiation and driving global climate change.

Carbon dioxide

• A gas that turns limewater cloudy.

Complete combustion

• Burning in plenty of oxygen to give carbon dioxide and water.

Incomplete combustion

• Burning in limited oxygen to give CO, soot and water.

Carbon monoxide

• A toxic gas that binds to haemoglobin.

Global warming

• Rise in the Earth's average temperature caused by extra greenhouse gases.

Chemical formula

• Symbols and numbers showing the relative number of atoms of each element.

Combustion

• Burning in oxygen

Metal

• An element with a lattice of positive ions in a sea of delocalised electrons, usually shiny, conductive and malleable.

Hydrocarbon

• A compound containing hydrogen and carbon only.

Section 33 of 34Key terms

Term	Meaning
Combustion	A rapid exothermic oxidation with a flame.
Specific energy	Energy released per unit mass of fuel.
Energy density	Energy released per unit volume of fuel.
Biofuel	A fuel made from biological material.
Fuel cell	A cell converting fuel and oxygen directly to electrical energy.
Greenhouse gas	A gas that absorbs infrared radiation in the atmosphere.

Term	Meaning
Complete combustion	Burning in excess oxygen to give CO2 and H2O.
Incomplete combustion	Burning in limited oxygen to give CO or C.
Soot	Carbon formed in incomplete combustion.
Carbon monoxide	A toxic gas that binds to haemoglobin.
Standard enthalpy of combustion	Enthalpy change for one mole burning completely in excess oxygen.

Term	Meaning
Fossil fuel	A fuel formed from the remains of ancient organisms.
Non-renewable	Cannot be replaced quickly once used.
Greenhouse effect	Natural warming from gases absorbing infrared radiation.
Enhanced greenhouse effect	Extra warming from increased greenhouse gas levels.
Global dimming	Reduction of sunlight reaching Earth due to particulates.

Term	Meaning
Renewable resource	A resource replaced as fast as it is used.
Transesterification	Conversion of triglycerides to methyl esters with methanol.
Biogas	Methane-rich gas from anaerobic breakdown of waste.
Fuel cell	A cell that continuously converts fuel and oxygen to electricity.
Carbon neutral	Net zero CO2 over the fuel's life cycle.

Section 34 of 34Exam technique

1

Saying incomplete combustion produces hydrogen.

2

Saying biofuels are always carbon neutral — energy used in growing and processing adds emissions.

3

Forgetting that CO binds irreversibly to haemoglobin, not just that it is toxic.

4

Thinking a fuel cell stores energy — it converts the energy of a continuous fuel supply.

1

Saying incomplete combustion produces hydrogen.

2

Forgetting oxygen already in an alcohol when balancing.

3

Giving the oxide of sodium as Na₂O₂ in an exam where the typical oxide is expected.

4

Writing the combustion of a non-metal as producing a basic oxide.

1

Saying longer-chain alkanes release more energy per gram — they release less.

2

Forgetting to use the mole ratio from the balanced equation in a CO₂ mass calculation.

3

Saying the greenhouse effect itself is harmful — it is the enhancement that causes warming.

4

Attributing sulfur dioxide to the hydrocarbon itself — it comes from sulfur impurities.

1

Saying a biofuel is always carbon neutral without considering growing and processing energy.

2

Forgetting that biodiesel transesterification is reversible and needs excess methanol.

3

Saying a fuel cell stores energy like a battery.

4

Saying a hydrogen fuel cell is emission-free regardless of how the hydrogen is made.