Combustion of Alkanes

Complete Combustion occurs when there is an excess supply of oxygen, ensuring that every carbon atom in the alkane is fully oxidized to carbon dioxide ($CO_2$).

The general equation for complete combustion of an alkane $C_nH_{2n+2}$ is: $C_nH_{2n+2} + \left(\frac{3n+1}{2}\right)O_2 \rightarrow nCO_2 + (n+1)H_2O$

Incomplete Combustion happens when the oxygen supply is limited, leading to products where carbon is not fully oxidized, such as carbon monoxide ($CO$) or solid carbon (soot).

Incomplete combustion is less efficient than complete combustion because it releases less energy per mole of fuel and produces toxic or polluting side products.

Carbon Monoxide (CO) is a colorless, odorless, and highly toxic gas that binds irreversibly to haemoglobin in red blood cells, preventing the transport of oxygen around the body.

Nitrogen Oxides ($NO_x$) are formed in high-pressure, high-temperature environments like car engines where atmospheric nitrogen ($N_2$) reacts with oxygen ($O_2$); these contribute to acid rain and photochemical smog.

Sulfur Dioxide ($SO_2$) results from sulfur impurities within the fuel reacting with oxygen; it dissolves in atmospheric water to form sulfuric acid, a primary component of acid rain.

Particulates (Soot) are solid carbon particles that can cause respiratory problems in humans and contribute to global dimming by reflecting sunlight away from the Earth.

Catalytic Converters are fitted to car exhausts to reduce pollutant emissions using precious metals like platinum, palladium, or rhodium on a ceramic honeycomb structure to maximize surface area.

They facilitate the reaction between carbon monoxide and nitrogen monoxide to produce less harmful gases: $2CO(g) + 2NO(g) \rightarrow 2CO_2(g) + N_2(g)$

Flue Gas Desulfurization (Scrubbing) is used in power stations to remove $SO_2$ by reacting it with an alkaline slurry, such as calcium oxide ($CaO$) or calcium carbonate ($CaCO_3$).

This process converts the gaseous pollutant into a solid waste product, calcium sulfite or sulfate, which can be safely disposed of or used in construction: $CaO(s) + SO_2(g) \rightarrow CaSO_3(s)$

Balancing Equations: Always balance carbon atoms first, then hydrogen atoms, and finally oxygen atoms. If you end up with a fraction for oxygen (e.g., $6.5 O_2$), you can leave it as a fraction or double the entire equation.

State Symbols: In combustion reactions, water is often produced as a gas ($H_2O(g)$) due to the high temperatures involved, though standard enthalpy definitions assume liquid water ($H_2O(l)$).

Pollutant Sources: A common mistake is stating that nitrogen oxides come from the fuel. Remember that $N_2$ comes from the air and only reacts because of the extreme heat of the engine.

Sanity Check: If a question mentions 'limited oxygen', ensure your products include $CO$ or $C$, never $CO_2$ alone.