GCSE OCR Gateway Organic Chemistry C6.2c Combustion reactions

GCSE OCR Gateway Organic Chemistry C6.2c Combustion reactions

C6.2c To be able to predict the formulae and structures of products of reactions of the first four and other given members of the homologous series of alkanes, alkenes and alcohols

Reactions to include combustion; addition of bromine and hydrogen across a double bond; oxidation of alcohols to carboxylic acids using potassium manganate(VII)

1.    Combustion of Alkanes

Alkanes burn completely in pure oxygen to form just water and carbon dioxide.

This is true of all alkanes. 

For example:

C₃H₈ (g)  +   5O₂ (g)   ⟶   3CO₂ (g)   +   4H₂O (l)

In the exact molar proportions (e.g. 10cm³ propane and 50 cm³ oxygen), this mixture of gaseous propane and pure oxygen will explode with some violence.  This property of explosive violence is true of all the gaseous alkanes with the correct molar proportions of oxygen.

You can tell an alkane is burning completely because it will burn with a pale blue flame.

The effect is seen when you open the air hole of a Bunsen burner and the methane flame changes from yellow to blue (See the photo below:)

You can find help to balance alkane combustion equations from my blog here

Incomplete combustion more often occurs in air since there is insufficient oxygen for complete combustion. 

The alkane will burn with a yellow flame because some hydrocarbon is converted into incandescent carbon, and that gives the flame its yellow colour.

For the past century and a half the combustion of alkane hydrocarbons has been the major fuel for transport whether octane based fuel in the internal combustion engine or diesel in the diesel engine or kerosine in jet engines. 

This irresponsible burning of valuable carbon based molecules cannot continue indefinitely because the earth has only a limited resource of such fuels.

The time is fast approaching when burning these fossil fuels for transport will change to other forms of propulsion whether hydrogen powered vehicles or electric powered engines. 

2.    Combustion of Alkenes

Alkenes contain a higher ratio of carbon to hydrogen than do alkanes so we would expect alkene to burn with a yellow flame incompletely to form carbon, carbon dioxide and water.  Here is an example of incomplete combustion of an alkene

C₃H₆ (g)  +   3½O₂ (g)   ⟶   C(s)     +      2CO₂ (g)   +   3H₂O (l)

Air does not contain enough oxygen and even pure oxygen is not probably not enough. 

If an alkene were to burn completely then here is the equation for the combustion of propene:

C₃H₆ (g)  +   4½O₂ (g)   ⟶   3CO₂ (g)   +   3H₂O (l)

Note that to balance the oxygen you will need to accept half a mole of oxygen in your equation or double all molar stoichiometric values as below.

2C₃H₆ (g)  +   9O₂ (g)   ⟶   6CO₂ (g)   +   6H₂O (l)

3.    Combustion of Alcohols

Alcohols burn completely in air with a pale blue flame to produce water vapour and carbon dioxide. 

You have probably seen this effect if you have seen a Christmas pudding set alight through the combustion of a liqueur or whisky of 40% strength or greater. 

Ethanol is the alcohol involved and it easily burns completely in air.

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

As the carbon chain increases the alcohol becomes more difficult to burn completely and eventually the alcohol changes from a liquid to a waxy solid. 

Alcohols up to octanol (C₈H₁₇OH(l)) will burn in air but they burn incompletely with yellow not blue flames. 

Brazil adds the alcohol ethanol at 10% of its petrol fuel to supplement its use.

In my next post, I’m going to talking about the addition reactions of alkenes with bromine and hydrogen.