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Combustion of Alcohols

Key Concepts

  • Combustion is a chemical reaction in which oxygen gas is a reactant.

  • An alcohol is an organic (carbon) compound containing the elements carbon (C), hydrogen (H), and oxygen (O).

Complete Combustion of alcohols:

  • Excess O2(g) is present to react.

  • Reaction products are CO2(g) and H2O(g).

  • alcohol + oxygen gas → carbon dioxide gas + water vapour

  • Alcohol combusts (burns) with a clean flame.

Incomplete Combustion of alcohols:

  • Insufficient O2(g) is present, that is, there is excess alcohol present.
        O2(g) is the limiting reagent in the reaction.

  • Reaction products are either CO(g) and/or C(s) as well as H2O(g).

  • example: alcohol + oxygen gas → carbon solid + carbon monoxide gas + water vapour

  • Alcohol combusts (burns) with a smokey or sooty flame due to the production of solid carbon.

Combustion of alcohols releases energy making alcohols useful as fuels.

Example of Complete Combustion

Ethanol, C2H5OH(l) or CH3-CH2OH(l), widely used as a fuel, combusts in excess oxygen to produce carbon dioxide gas and water vapour.
Write a balanced chemical equation to represent the complete combustion of ethanol.

  1. Write the word equation for the complete combustion of ethanol.

    Complete combustion of alcohols produces carbon dioxide gas and water vapour.

    general equation: reactants products
    general word equation: alcohol + oxygen gas carbon dioxide gas + water vapour
    word equation for this reaction: ethanol + oxygen gas carbon dioxide gas + water vapour

  2. Write the formula for each reactant and product in the word equation:

    Reactants Products
    ethanol :

    oxygen gas :

    CH3-CH2OH(l) or C2H5OH(l)

    O2(g)

      carbon dioxide gas :

    water vapour :

    CO2(g)

    H2O(g)

  3. Write the unbalanced chemical equation by substituting the formula for the name of each reactant and product in the word equation:

    word equation : ethanol + oxygen gas carbon dioxide gas + water vapour
    unbalanced chemical equation : C2H5OH(l) + O2(g) CO2(g) + H2O(g)

  4. Balance the chemical equation:

    unbalanced chemical equation : C2H5OH(l) + O2(g) CO2(g) + H2O(g)  
    No. C atoms : 2 1 C atoms NOT balanced
    Need to multiply the number of carbon dioxide molecules by 2 to balance the carbon atoms.
      C2H5OH(l) + O2(g) 2CO2(g) + H2O(g)  
    No. C atoms : 2 = 2 C atoms balanced
    Are hydrogen atoms balanced?
    No. H atoms : 5+1 = 6 2 H atoms NOT balanced
    Need to multiply the number of water molecules by 3 to balance the hydrogen atoms.
      C2H5OH(l) + O2(g) 2CO2(g) + 3H2O(g)  
    No. H atoms : 5+1 = 6 = 3x2 = 6 H atoms balanced
    Are oxygen atoms balanced?
      C2H5OH(l) + O2(g) 2CO2(g) + 3H2O(g)  
    No. O atoms : 1 + 2 =3 2x2 + 3x1 = 7 O atoms NOT balanced
    Need to multiply the number of oxygen molecules (O2) by 3 to balance the oxygen atoms.
      C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(g)  
    No. O atoms : 1 + 3x2 =7 = 2x2 + 3x1 = 7 O atoms balanced

  5. The balanced chemical equation for the complete combustion of ethanol is:

    C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)

Example of Incomplete Combustion

In a particular experiment, excess ethanol was combusted in limited oxygen to produce soot (solid carbon), carbon monoxide gas and water vapour.
Write a balanced chemical equation to represent the incomplete combustion of ethanol in this experiment.

  1. Write the word equation for the incomplete combustion of methane:

    general equation: reactants products
    general word equation: alcohol + oxygen gas solid carbon + carbon monoxide + water vapour
    word equation
    for this reaction:
    ethanol + oxygen gas solid carbon + carbon monoxide + water vapour

  2. Write the formula for each reactant and product in the word equation:

    Reactants   Products
    ethanol :

    oxygen gas :

    C2H5OH(l)

    O2(g)

      solid carbon :

    carbon monoxide gas:

    water vapour :

    C(s)

    CO(g)

    H2O(g)

  3. Write the unbalanced chemical equation by substituting the formula for the name of each reactant and product in the word equation:

    word equation: ethanol + oxygen gas solid carbon + carbon monoxide + water vapour
    unbalanced chemical equation: C2H5OH(l) + O2(g) C(s) + CO(g) + H2O(g)

  4. Balance the chemical equation:

    unbalanced chemical equation : C2H5OH(l) + O2(g) C(s) + CO(g) + H2O(g)
    No. C atoms : 2 = 1 + 1 =2 C atoms balanced
    Check to see if hydrogen atoms are balanced.
      C2H5OH(l) + O2(g) C(s) + CO(g) + H2O(g)
    No. H atoms : 5+1 =6 2 H atoms NOT balanced
    Need to multiply the number of water molecules by 3 to balance the hydrogen atoms.
      C2H5OH(l) + O2(g) C(s) + CO(g) + 3H2O(g)
    No. H atoms : 5+1 =6 = 3x2 =6 H atoms balanced
    Check to see if oxygen atoms are balanced.
      C2H5OH(l) + O2(g) C(s) + CO(g) + 3H2O(g)
    No. O atoms : 1 + 2 = 3 1 + 3x1 =4 O atoms NOT balanced
    Need 1 more O atom on the left hand side of the equation.
    1 atom of O is half an oxygen molecule (½O2)
    Add ½ to the number of O2 molecules on the left hand side, that is, 1 + ½ = 1½ or 3/2
      C2H5OH(l) + O2(g) C(s) + CO(g) + 3H2O(g)
    No. O atoms : 1 + (1½x2=3) = 1 + 3x1 O atoms balanced

  5. The balanced chemical equation for the incomplete combustion of methane gas in this experiment is 1:
    C2H5OH(l) + 3/2O2(g) → C(s) + CO(g) + 3H2O(g)


What would you like to do now?


1The actual ratio of products formed can vary, so there is more than one way to balance the chemical equation for the incomplete combustion of an alcohol.

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