Combustion of Alcohols (combustion of alkanols) Chemistry Tutorial

Key Concepts

• An alcohol is an organic (carbon) compound containing the elements carbon (C), hydrogen (H), and oxygen (O).
• Alcohols are a class of organic compounds containing the OH (hydroxyl) functional group.
• Alkanols are alcohols in which the parent hydrocarbon is an alkane.
• Combustion is a chemical reaction in which oxygen gas is a reactant.
• Combustion reactions can be classified as complete combustion or as incomplete combustion
• Complete Combustion of alcohols:

  ⚛ Excess O_2(g) is present to react.

  ⚛ Reaction products are CO_2(g) and H₂O_(g).

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

  ⚛ Alcohol combusts (burns) with a clean flame.

• Incomplete Combustion of alcohols:

  ⚛ Insufficient O_2(g) is present, that is, there is excess alcohol present.

  ⚛ O_2(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.

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Worked Example of Complete Combustion of an Alcohol

In this example we will be writing a balanced chemical equation to represent the complete combustion of ethanol.

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)

Worked Example of Incomplete Combustion of an Alcohol

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)	+	1½O2(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)