Combustion basics

• Combustion = reaction of a fuel with oxygen releasing energy.

• Fuels in this topic: reactive metals, non-metals, hydrocarbons, alcohols.

• Most combustion reactions are exothermic because strong bonds form in products such as $\ce{CO2}$ and $\ce{H2O}$.

• You must be able to identify combustion as oxidation and write a balanced equation.

• Typical examples:

  • Magnesium: $\ce{2Mg + O2 -> 2MgO}$

  • Carbon: $\ce{C + O2 -> CO2}$

  • Methane: $\ce{CH4 + 2O2 -> CO2 + 2H2O}$

  • Ethanol: $\ce{C2H5OH + 3O2 -> 2CO2 + 3H2O}$

This diagram shows the balanced combustion of methane in oxygen, with atoms rearranging to form carbon dioxide and water. It is useful for checking atom conservation and product identification in complete combustion. Source

Writing combustion equations fast

• For complete combustion of a hydrocarbon: products are always $\ce{CO2}$ and $\ce{H2O}$.

  • General form: $\ce{C_xH_y + (x + \frac{y}{4})O2 -> xCO2 + \frac{y}{2}H2O}$

• For complete combustion of an alcohol, products are still $\ce{CO2}$ and $\ce{H2O}$, but the fuel already contains oxygen, so less $\ce{O2}$ is needed than for a similar hydrocarbon.

• Fast balancing strategy:

  • Balance C first.

  • Then balance H.

  • Balance O last.

  • Multiply through if you need whole-number coefficients.

• Watch for exam traps:

  • Do not put $\ce{CO}$ or $\ce{C}$ in complete combustion.

  • Include state symbols only if the question asks or data are given.

Incomplete combustion

• Incomplete combustion happens when there is a limited supply of oxygen.

• Organic fuels may produce:

  • Carbon monoxide, $\ce{CO}$

  • Carbon (soot), $\ce{C}$

  • Water, $\ce{H2O}$

• Examples:

  • $\ce{2CH4 + 3O2 -> 2CO + 4H2O}$

  • $\ce{CH4 + O2 -> C + 2H2O}$

• Key consequences:

  • Less complete energy release

  • Formation of toxic carbon monoxide

  • Formation of particulates / soot

• Carbon monoxide is dangerous because it is poisonous.

• Larger hydrocarbons are generally more likely to burn incompletely because they are harder to mix fully with oxygen.

Fossil fuels: strengths and weaknesses

• Fossil fuels = coal, crude oil, natural gas.

• They are non-renewable because they form over millions of years.

• Advantages:

  • High energy output

  • Established infrastructure

  • Often cheap and easy to transport/store

• Disadvantages:

  • Release $\ce{CO2}$, increasing the greenhouse effect

  • Can undergo incomplete combustion, producing $\ce{CO}$ and soot

  • Cause environmental, social, economic and ethical concerns

• Relative ideas you should know:

  • Natural gas / methane generally gives less $\ce{CO2}$ per unit energy than coal or larger hydrocarbons because it has a higher H:C ratio.

  • Fuels with more carbon per unit mass tend to add more $\ce{CO2}$ to the atmosphere when burned.

  • Energy released per unit mass is an important comparison point when evaluating fuels.

Evaluating $\ce{CO2}$ emissions from fuels

• In exams, compare fuels by the amount of carbon dioxide formed when they burn.

• Core idea: Every carbon atom in the fuel becomes one carbon atom in $\ce{CO2}$ in complete combustion.

• Therefore:

  • 1 mol C in a fuel produces 1 mol $\ce{CO2}$

  • More moles of carbon burned $\rightarrow$ more moles of $\ce{CO2}$

• Useful exam method:

  • Write the balanced combustion equation.

  • Use the equation to find moles of $\ce{CO2}$.

  • Convert to mass of $\ce{CO2}$ if needed.

  • For evaluation questions, compare $\ce{CO2}$ released per unit mass or per unit energy.

• Key judgement points:

  • A fuel may be energetically useful but still be environmentally worse if it releases more $\ce{CO2}$.

  • Methane is often favored over coal because it is cleaner-burning and produces less $\ce{CO2}$ per unit energy.

Greenhouse effect link

• $\ce{CO2}$ is a greenhouse gas.

• Higher atmospheric $\ce{CO2}$ leads to an enhanced greenhouse effect.

• This means more infrared radiation is retained by the atmosphere, increasing global temperatures.

• IB focus: know the link, not just the definition:

  • Burning fossil fuels $\rightarrow$ more $\ce{CO2}$

  • more $\ce{CO2}$ $\rightarrow$ stronger greenhouse effect

• Do not confuse:

  • Greenhouse effect = heat trapping by gases

  • Ozone depletion = a different environmental issue

Biofuels

• Biofuels are fuels made from recently living material.

• They are considered renewable because the carbon in them was fixed from the atmosphere over a short timescale.

• This carbon is fixed by photosynthesis:

  • $\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}$

• Why biofuels may reduce net atmospheric $\ce{CO2}$:

  • Plants remove $\ce{CO2}$ first by photosynthesis.

  • Burning the biofuel later returns this recently fixed carbon.

• Advantages:

  • Renewable source

  • Potentially lower net increase in atmospheric $\ce{CO2}$

  • Can reduce dependence on fossil fuels

• Disadvantages:

  • Need land, water and energy to produce

  • May compete with food production

  • Still releases $\ce{CO2}$ when burned

  • Overall sustainability depends on how the crop is grown and processed

This diagram shows photosynthesis using sunlight, carbon dioxide and water to produce carbohydrates and oxygen. It is the key visual link for explaining why biofuels are described as using recently fixed carbon. Source

This diagram shows carbon moving between the atmosphere, organisms, and combustion processes. It helps compare fossil fuels with biofuels by showing whether carbon is being released from long-term stores or from a short-term cycle. Source

Renewable vs non-renewable

• Renewable energy source = replaced on a human timescale.

• Non-renewable energy source = not replaced quickly enough for continued use.

• In this topic:

  • Biofuels = renewable

  • Coal, crude oil, natural gas = non-renewable

• Evaluation questions often want a balanced judgement, not a one-sided answer.

• Good exam style:

  • give one advantage

  • give one disadvantage

  • then make a reasoned conclusion

Fuel cells

• A fuel cell converts chemical energy directly into electrical energy.

• This is different from combustion in a power station, where chemical energy is first converted to heat.

• IB examples: hydrogen fuel cells and methanol fuel cells.

• Main advantage of fuel cells:

  • More direct energy conversion

  • Less reliance on combustion

• Main limitations:

  • Fuel production and storage can be difficult

  • Some fuels are still obtained using fossil energy

  • Cost and infrastructure can limit use

This schematic shows a hydrogen fuel cell with anode, cathode, electron flow through the external circuit, and ion movement through the membrane. It is ideal for learning how fuel cells produce electrical energy directly from a redox process. Source

Hydrogen fuel cell equations

• You must be able to write half-equations and the overall equation.

• Anode (oxidation): $\ce{H2 -> 2H+ + 2e-}$

• Cathode (reduction): $\ce{\frac{1}{2}O2 + 2H+ + 2e- -> H2O}$

• Overall: $\ce{H2 + \frac{1}{2}O2 -> H2O}$

• Key point: the only chemical product is water.

• Important evaluation point:

  • The fuel cell itself does not emit $\ce{CO2}$ when using hydrogen.

  • But the overall environmental benefit depends on how the hydrogen is produced.

Methanol fuel cell equations

• You may also be asked for methanol as the fuel.

• Anode (oxidation): $\ce{CH3OH + H2O -> CO2 + 6H+ + 6e-}$

• Cathode (reduction): $\ce{\frac{3}{2}O2 + 6H+ + 6e- -> 3H2O}$

• Overall: $\ce{CH3OH + \frac{3}{2}O2 -> CO2 + 2H2O}$

• Compared with hydrogen fuel cells:

  • Methanol is easier to store as a liquid.

  • It still produces $\ce{CO2}$.