GENERAL FORMULA
FOR ALCOHOLS IN C n H2n+1
HOMOLOGOUS SERIES
Example:
ETHANOL C2 H 2 X 2 + 1 OH
C2 H5 OH
As you can see from the molecular formulas in the table, each alcohol differs from the previous member by a – CH2 unit –
e.g. C2H5OH + CH2 unit C3H7OH
ethanol propanol
In my investigation I will also be looking at heat of combustion. To explore the heats of combustion of this series of alcohols I firstly need to define what the term actually means.
HEAT OF COMBUSTION
The heat evolved when 1 mole of the compound is burned completely in oxygen under standard conditions.
The most frequently used alcohol in chemistry is ethanol, mainly because it is the only non-poisonous alcohol. When any alcohol is burnt bonds are broken and formed, likewise in most chemical reactions it involves energy changes where bonds are re-arranged. In order to break the bonds energy must be supplied, and this type of reaction is called endothermic, where heat energy is taken into the reaction from its surroundings to break the bonds. However when new bonds are formed heat energy is then released from the reaction, and this type is called exothermic, where heat energy is lost.
Energy needed to
break bonds
Energy released when
new bonds are formed
Reactants
Heat Change
(KJ/mole)
Products
This energy level diagram shows an example of an exothermic reaction, where the energy released is higher than the energy needed to break the initial bonds. The heat of combustion is therefore shown by the heat change the reactants required to form the products (in Kilojoules per mole). In this instance, the products would become more stable because energy has been lost.
Burning Alcohols
Here is an equation to show what happens when an alcohol is burnt in the presence of oxygen:
e.g. Ethanol
C2H5OH (l) + O2 (g) 2CO2 (g) + 3H2O (l)
This equation shows us that carbon dioxide (CO2) and water (H2O) are formed when the ethanol is burnt, and this is true for all alcohols. The products will always be CO2 and H2O but obviously varying in the amounts produced. This process is called complete combustion because all of the carbon (C) is converted into CO2 and all of the hydrogen (H) is converted into H2O, and it occurs when there is sufficient oxygen in the surroundings.
When looking at either endothermic or exothermic reactions, energy level diagrams can be drawn. They show if energy is being lost or gained during the heat change from the reactants to the products.
Energy
C2H5OH (l) + 3O2
2CO2 (g) + H2O (l)
Course of reaction
This example of an energy level diagram is showing that energy has been lost when ethanol is burnt. The exact heat of combustion can also be worked out by calculating the sum of the average values for each bond using the molecular structure of ethanol. In this particular reaction the reactants have lost energy, making the products more stable. This is an exothermic reaction.
Prediction
From using my knowledge on the subject of alcohols I am able to predict that the heat of combustion will increase as the molecular structures become larger, as the series of alcohols increases:
e.g. Methanol Hexanol
H H H H H H H
H – C – O – H H – C – C – C – C – C – C – H
H H H H H H H
(lower heat of combustion) (higher heat of combustion)
To justify this prediction I will make some specific calculations using average values of each bond in the structural formulae to calculate their heats of combustion. I have chosen to calculate only three heats of combustion from my series of six alcohols to see if I can see a pattern emerging. I will be working out the heats of combustion of Ethanol, Butanol and Hexanol, and if my prediction is accurate their heats of combustion should increase as their structures become larger.
Table Showing Average Values for Bonds
Equation for the heat of combustion of ethanol:
C2H5OH + 2O2 2CO2 + 3H2O
ETHANOL OXYGEN CARBON WATER
DIOXIDE
This equation is balanced, so I will now work out the total bond energy values for each compound, using their structural formulas. Here are the reactant’s totals:
ETHANOL
H H
H – C – C – O – H
H H
-
C – H = 5 x 413 = 2065 KJ
1 C – C = 1 x 347 = 347 KJ
1 C – O = 1 x 336 = 336 KJ
1 O – H = 1 x 464 = 464 KJ
3212 KJ/mole
OXYGEN
O = O
O = O
-
O = O = 2 x 498 = 996 KJ
996 KJ/mole
TOTAL OF REACTANTS = 3212
+ 996
4208 KJ/mole
The following calculations are for the products from the equation:
CARBON DIOXIDE
O = C = O
O = C = O
-
C = O bonds = 4 x 805 = 3220KJ
3220KJ/mole
WATER
O
H H
- O – H bonds = 2 x 464 = 928KJ
928KJ/mole
TOTAL OF PRODUCTS = 3220
+ 928
4148KJ/mole
