Alcohols and Esters: Question 7

Worked solution

Part (a): Balancing the combustion equation

Complete combustion of an alcohol in excess oxygen produces carbon dioxide and water only. Balancing carbon, hydrogen and then oxygen for propan-1-ol, $\text{C}_3\text{H}_8\text{O}$:

$\text{C}_3\text{H}_8\text{O} + 4\tfrac{1}{2}\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$

Doubling every coefficient removes the fraction, giving whole-number coefficients:

$2\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + 9\text{O}_2 \rightarrow 6\text{CO}_2 + 8\text{H}_2\text{O}$

(Check: left-hand side has $2\times3=6$ C, $2\times8=16$ H, $2\times1+9\times2=20$ O; right-hand side has $6\times1=6$ C, $8\times2=16$ H, $6\times2+8\times1=20$ O. Balanced.)

Part (b): Complete vs incomplete combustion

Complete combustion occurs when there is an excess (plentiful supply) of oxygen, producing only carbon dioxide and water. Incomplete combustion occurs when the oxygen supply is limited (insufficient), so the fuel is not fully oxidised: this produces carbon monoxide, $\text{CO}$, and/or solid carbon (soot), $\text{C}$, in addition to water.

Part (c): Why alcohols burn more cleanly

Propan-1-ol’s molecular formula, $\text{C}_3\text{H}_8\text{O}$, already contains one oxygen atom bonded within the fuel molecule itself. This means that, per carbon atom, propan-1-ol needs less external oxygen from the air to be completely oxidised to $\text{CO}_2$ and $\text{H}_2\text{O}$ than a hydrocarbon of similar chain length, such as propane ($\text{C}_3\text{H}_8$), which contains only carbon and hydrogen and relies entirely on external oxygen. Because propan-1-ol is therefore less easily starved of oxygen during burning, it tends to burn with a less smoky (cleaner) flame, producing less unburnt carbon.

Final answers

• (a) $2\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + 9\text{O}_2 \rightarrow 6\text{CO}_2 + 8\text{H}_2\text{O}$.
• (b) Complete combustion: excess oxygen. Incomplete combustion: limited oxygen, producing $\text{CO}$ and/or $\text{C}$ (soot).
• (c) The oxygen atom already present in propan-1-ol lowers the external oxygen demand per carbon atom compared with a hydrocarbon of similar chain length, so propan-1-ol burns with less soot.