Combustion as oxidation

A combustion reaction is an exothermic reaction where a substance reacts with oxygen, usually after heating or ignition. A fuel is a substance that releases useful energy when it reacts, most often through combustion.

Combustion counts as a redox process. An oxidizing agent causes another species to be oxidized by accepting electrons; in combustion, oxygen usually takes that role. A reducing agent causes another species to be reduced by donating electrons; here, the fuel is usually the reducing agent.

That helps explain why a fuel with a reasonably high activation energy is useful. Activation energy is the minimum energy barrier that reacting particles must overcome before a reaction can proceed. If the barrier is high enough, the fuel can be stored and transported without reacting with air at room temperature. But we still need it to burn readily when we deliberately ignite it — petrol that ignites in an engine is useful; petrol that ignites on the shelf is not.

Combustion of metals and non-metals

Reactive metals burn in oxygen and form metal oxides. For example:

$2Mg(s) + O_2(g) \to 2MgO(s)$

Magnesium is oxidized because magnesium atoms lose electrons to form magnesium ions. Oxygen is reduced because oxygen atoms gain electrons to form oxide ions. Written as electron transfer:

$Mg \to Mg^{2+} + 2e^-$

$O_2 + 4e^- \to 2O^{2-}$

Non-metals can burn in oxygen too, forming non-metal oxides. Sulfur, which can be present as an impurity in fossil fuels, burns to form sulfur dioxide:

$S(s) + O_2(g) \to SO_2(g)$

Sulfur dioxide can be oxidized further in air and then react with water to form acidic products. So combustion is not just about energy; it also affects air quality.

Complete combustion of hydrocarbons

A hydrocarbon is an organic compound made only of carbon and hydrogen atoms. An alkane is a saturated hydrocarbon with only carbon–carbon single bonds and the general formula $C_nH_{2n+2}$, where n is the number of carbon atoms in one molecule of the fuel (dimensionless whole number).

A complete combustion reaction is a combustion reaction where a carbon-containing fuel burns in excess oxygen to form carbon dioxide and water only as carbon-containing and hydrogen-containing products. For an alkane:

$C_nH_{2n+2} + \frac{3n + 1}{2}O_2 \to nCO_2 + (n + 1)H_2O$

Methane, for instance, burns completely like this:

$CH_4(g) + 2O_2(g) \to CO_2(g) + 2H_2O(l)$

The state of water depends on the conditions. Hot exhaust gases may contain steam, $H_2O(g)$, while cooler conditions may give liquid water, $H_2O(l)$. In equations, balance the atoms first; then add state symbols if the conditions are known.

Complete combustion of alcohols

An alcohol is an organic compound containing a hydroxyl group, –OH, bonded to a saturated carbon atom. Straight-chain alcohols often met in this topic can be written as $C_nH_{2n+1}OH$, where n is the number of carbon atoms in one molecule of the alcohol.

Alcohols also burn completely to form carbon dioxide and water:

$C_nH_{2n+1}OH + \frac{3n}{2}O_2 \to nCO_2 + (n + 1)H_2O$

For ethanol:

$C_2H_5OH(l) + 3O_2(g) \to 2CO_2(g) + 3H_2O(l)$

When deducing combustion equations, I always tell students to use this order: balance carbon, then hydrogen, then oxygen last. Fractions for oxygen are acceptable, but you may multiply the whole equation by 2 if whole-number coefficients are preferred.

What changes when oxygen is limited?

Incomplete combustion is a combustion reaction in which there is insufficient oxygen for all carbon atoms in the fuel to be oxidized fully to carbon dioxide. Some carbon then ends up as carbon monoxide, $CO$, or as elemental carbon, $C$.

A limiting reactant is the reactant that is completely used up first and therefore limits the amount of product formed. In complete combustion, oxygen is usually available in excess, so the fuel is often the limiting reactant. In a poorly ventilated flame, blocked boiler, charcoal burner indoors, or badly adjusted engine, oxygen can become limiting.

When methane burns with too little oxygen, the flame may look yellow or smoky instead of clean blue. You might also see black soot on a cool surface, with less heat released. Carbon monoxide may form too, and that’s the dangerous part: it is colourless and odourless, so your senses won’t detect it reliably.

Equations for incomplete combustion of hydrocarbons

For an alkane forming carbon monoxide:

$C_nH_{2n+2} + \frac{2n + 1}{2}O_2 \to nCO + (n + 1)H_2O$

For an alkane forming carbon soot:

$C_nH_{2n+2} + \frac{n + 1}{2}O_2 \to nC + (n + 1)H_2O$

For methane, the two idealized incomplete combustion equations are:

$CH_4(g) + 1.5O_2(g) \to CO(g) + 2H_2O(l)$

$CH_4(g) + O_2(g) \to C(s) + 2H_2O(l)$

Real flames don’t follow one tidy equation. Complete combustion, carbon monoxide formation and soot formation can all happen at the same time in different parts of the flame.

Equations for incomplete combustion of alcohols

For an alcohol forming carbon monoxide:

$C_nH_{2n+1}OH + nO_2 \to nCO + (n + 1)H_2O$

For an alcohol forming carbon soot:

$C_nH_{2n+1}OH + \frac{n}{2}O_2 \to nC + (n + 1)H_2O$

For example, incomplete combustion of ethanol to carbon monoxide is:

$C_2H_5OH(l) + 2O_2(g) \to 2CO(g) + 3H_2O(l)$

Health risk

Carbon monoxide is a toxic gas composed of one carbon atom and one oxygen atom that binds strongly to haemoglobin in red blood cells. Haemoglobin is a protein in red blood cells that transports oxygen around the body. Once carbon monoxide binds to haemoglobin, the blood carries less oxygen, so tissues may be starved of oxygen even while the person is breathing.

Carbon soot causes problems as well. Fine carbon particles can be inhaled and are associated with respiratory harm. Limiting oxygen therefore changes both the chemistry and the risk: less $CO_2$ is formed, more $CO$ and particulate carbon are produced, and the useful energy released is lower than for complete combustion.

What counts as a fossil fuel?

A fossil fuel is a non-renewable carbon-based fuel formed from ancient biological material over geological time. The three main examples are coal, crude oil and natural gas.

A non-renewable resource is a resource used faster than natural processes can replace it on a human timescale. Coal, crude oil and natural gas are finite. After they are extracted and burned, carbon that was stored underground moves into the active carbon cycle, mainly as carbon dioxide.

Comparing carbon dioxide released by different fuels

To work out how much carbon dioxide a fuel adds to the atmosphere, start with the balanced combustion equation and molar masses. The key piece is the mole ratio between the fuel and $CO_2$.

For propane:

$$C_3H_8(g) + 5O_2(g) \to 3CO_2(g) + 4H_2O(l)$$

One mole of propane produces three moles of carbon dioxide. One mole of propane has a mass of about 44 g, while three moles of carbon dioxide have a mass of about 132 g. So burning 1 g of propane produces about 3 g of $CO_2$. Don’t guess from the size of the molecule; use the equation.

Fuels with more carbon per unit energy generally add more $CO_2$ for the same useful energy output. Hydrogen, if produced without fossil fuels, forms water rather than carbon dioxide when used in a fuel cell or burned. Methane produces less $CO_2$ per unit energy than coal, but it is still a fossil fuel when obtained from natural gas reserves.

Specific energy and incomplete combustion tendency

Specific energy is the energy released per unit mass of fuel, usually measured in $MJ\ kg^{-1}$. A fuel with high specific energy releases more energy for the same carried mass, which is especially important in transport.

The standard enthalpy of combustion, $\Delta H^\theta_c$, is the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions, with all substances in their standard states; its usual unit is $kJ\ mol^{-1}$. A more negative value shows that more energy is released per mole during complete combustion.

Specific energy and $CO_2$ production do not always identify the same “best” fuel. Coal, for example, is convenient where it is locally available, but it has relatively high greenhouse gas emissions and often more pollutants. Natural gas has high specific energy and burns more cleanly than coal, yet it still releases $CO_2$, and methane leakage is a serious concern.

Larger hydrocarbons are more likely to undergo incomplete combustion. Their stronger London dispersion forces give them higher boiling points and lower volatility, so they vaporize and mix with oxygen less readily. With poor mixing, some carbon atoms do not meet enough oxygen during the short time available in a flame or engine cylinder.

Carbon dioxide and the greenhouse effect

A greenhouse gas is an atmospheric gas that absorbs infrared radiation emitted by Earth and re-emits energy in different directions, including back toward Earth’s surface. The greenhouse effect is the warming influence caused when greenhouse gases reduce the rate at which infrared energy escapes to space.

Carbon dioxide matters because $CO_2$ absorbs infrared radiation associated with molecular vibrations. Nitrogen and oxygen make up most of the atmosphere, but they are not significant infrared absorbers in the same way. Even a relatively small concentration of $CO_2$ can have a large climate effect when its concentration increases.

The link is not “$CO_2$ makes sunlight hotter”. Sunlight reaches Earth mainly as visible and shorter-wavelength radiation. Earth then emits infrared radiation. Greenhouse gases interact strongly with some of this outgoing infrared radiation, shifting the energy balance and raising average global temperature.

Implications of burning fossil fuels

The implications are chemical, but they are also environmental, economic, ethical and social.

• Environmental: increased atmospheric $CO_2$, global warming, ocean acidification, habitat loss, particulates, sulfur oxides and nitrogen oxides depending on fuel and conditions.
• Economic: fossil fuels have existing infrastructure and provide reliable energy, but climate damage, healthcare costs and pollution control are expensive.
• Ethical: the benefits and harms are unevenly distributed. Some communities contribute little to emissions but are highly vulnerable to climate impacts.
• Social: energy access supports development, transport, heating and industry, so replacing fossil fuels must consider reliability, affordability and fairness.

That is the central challenge of the topic: fossil fuels are chemically useful because they store a lot of energy and are easy to use, but the products and scale of their combustion create long-term global problems.

Renewable and non-renewable energy sources

A renewable energy source is an energy source replenished by natural processes at a rate comparable to, or faster than, its use by humans. Solar, wind, hydroelectric, geothermal and sustainably managed biomass are examples.

A non-renewable energy source is an energy source with a finite supply that is not replaced on a human timescale. Fossil fuels fit this category because they take millions of years to form, yet humans burn them over years to centuries.

A biofuel is a fuel made from recently living biological material. “Recently” matters: biofuels are not automatically pollution-free, but their carbon has usually come from the atmosphere over years or decades, rather than being locked underground for geological time.

Biological carbon fixation and photosynthesis

Biological carbon fixation is the conversion of inorganic carbon, such as carbon dioxide, into organic carbon compounds by living organisms. In green plants, photosynthesis carries this out.

Photosynthesis is a biological process in which plants use light energy to convert carbon dioxide and water into glucose and oxygen. Learn this equation:

$6CO_2(g) + 6H_2O(l) \to C_6H_{12}O_6(aq) + 6O_2(g)$

Glucose stores chemical energy in covalent bonds. Organisms can use it directly, build it into biomass, or ferment it to make alcohol fuels such as ethanol.

Fermentation is an anaerobic biological process in which microorganisms convert glucose into ethanol and carbon dioxide. The equation is:

$C_6H_{12}O_6(aq) \to 2C_2H_5OH(aq) + 2CO_2(g)$

Ethanol then burns like other alcohols:

$C_2H_5OH(l) + 3O_2(g) \to 2CO_2(g) + 3H_2O(l)$

This does release $CO_2$. Ethanol can have a lower net carbon footprint than a petroleum fuel because some of the $CO_2$ released during combustion was absorbed recently as the plant grew. That claim still depends on the whole life cycle: farming, fertilizers, processing, transport and land-use change all matter.

Advantages and disadvantages of biofuels

Biofuels are useful because they can fit into parts of the existing fuel infrastructure. They can also be made from crops, plant waste or other biomass, and they may improve energy security for countries that import crude oil.

The trade-offs are real. Biofuels compete for land, water and agricultural resources. If forests are cleared to grow biofuel crops, the carbon balance may become worse, not better. Monoculture farming can reduce biodiversity, and food prices may rise if food crops or farmland are diverted to fuel production.

A strong evaluation avoids the lazy phrase “biofuels are carbon neutral” unless the evidence supports it. Better wording is: biofuels can reduce net $CO_2$ emissions when the rate of biological carbon fixation and the production pathway offset the emissions from growing, processing and burning the fuel.

What a fuel cell does

A fuel cell is an electrochemical cell that converts chemical energy from a continuously supplied fuel and oxidant directly into electrical energy. It does not have to burn the fuel first to produce heat, which can make it efficient and clean at the point of use.

A primary cell is an electrochemical cell containing a fixed amount of reactants that cannot be recharged under normal use. A fuel cell works differently: its reactants come from outside the cell. As long as fuel and oxidant keep arriving, and the products are removed, it can keep producing current.

The electrode names follow the same rules as other electrochemical cells. Oxidation occurs at the anode, and reduction occurs at the cathode. That sentence is worth memorising.

Hydrogen fuel cell

In an acidic hydrogen fuel cell, hydrogen is oxidized at the anode:

$H_2(g) \to 2H^+(aq) + 2e^-$

Oxygen is reduced at the cathode:

$O_2(g) + 4H^+(aq) + 4e^- \to 2H_2O(l)$

To combine the half-equations, multiply the hydrogen half-equation by 2 so the electrons cancel:

$2H_2(g) \to 4H^+(aq) + 4e^-$

Then add:

$2H_2(g) + O_2(g) \to 2H_2O(l)$

The useful product is electrical energy. The chemical product is water, which is why hydrogen fuel cells are often called clean at the point of use.

The wider environmental judgement depends on how the hydrogen is made. Electrolysis is a process in which electrical energy drives a non-spontaneous chemical reaction; for water, it produces hydrogen and oxygen:

$2H_2O(l) \to 2H_2(g) + O_2(g)$

If the electricity comes from renewable sources, this can be a low-carbon route. Much industrial hydrogen, though, is made by reforming hydrocarbons, which can produce carbon monoxide and then carbon dioxide:

$CO(g) + H_2O(g) \rightleftharpoons CO_2(g) + H_2(g)$

So always separate “no $CO_2$ from the fuel cell exhaust” from “no $CO_2$ in the whole production chain”.

Methanol fuel cell

A direct methanol fuel cell is a fuel cell in which methanol is oxidized directly at the anode. Since methanol is a liquid at room temperature, it is easier to store and transport than hydrogen gas.

In an acidic methanol fuel cell, the anode half-equation is:

$CH_3OH(aq) + H_2O(l) \to CO_2(g) + 6H^+(aq) + 6e^-$

The cathode half-equation is:

$1.5O_2(g) + 6H^+(aq) + 6e^- \to 3H_2O(l)$

Adding the half-equations gives:

$CH_3OH(aq) + 1.5O_2(g) \to CO_2(g) + 2H_2O(l)$

Methanol fuel cells have a practical storage advantage because liquid methanol releases much more energy per unit volume than compressed hydrogen. The drawbacks are still significant: methanol is toxic, catalysts can be expensive, and the cell still produces carbon dioxide.

Approximate fuel energy densities and specific energies; compressed hydrogen is highest by mass but lowest by volume in this set.

Comparing fuel cells with primary cells

The main difference is where the reactants come from. In a primary cell, the reactants are sealed inside and eventually run out. In a fuel cell, fuel and oxidant are fed in continuously, so the cell behaves more like a small chemical power station than a disposable battery.

Energy density is the energy released per unit volume of fuel, usually measured in $MJ\,dm^{-3}$. Specific energy is the energy released per unit mass of fuel, usually measured in $MJ\,kg^{-1}$. Hydrogen has very high specific energy but low energy density unless compressed or liquefied, so storage volume and tank mass matter. Methanol has lower specific energy, but it is much easier to store as a liquid. Fuel choice is never made from one number alone.