The aim of this experiment is to investigate the enthalpy change of combustion for a variety of alcohols and determine how the number of carbon atoms affects the combustion of the alcohol.

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The enthalpy change of combustion of alcohols

Aim

The aim of this experiment is to investigate the enthalpy change of combustion for a variety of alcohols and determine how the number of carbon atoms affects the combustion of the alcohol.

        ‘The standard enthalpy change of combustion is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions. For example a pressure of 100 kilopascals, a temperature of 298 Kelvin, solutions must have a concentration of 1.0 mol/dm3 and also the reactants and products must be in the physical states e.g. solid, liquid or gas, that are normal for these conditions.’1.   An example of a combustion reaction of methane is show below.

 CH4(g) + 2O2                  CO2 (g) + 2H2O(l)

Prediction & Development of problem

I predict that the enthalpy change of combustion will increase as the number of carbon atoms within the alcohol increases. Alcohols form a homologous series.  A homologous series is formed by molecules with different carbon chain lengths but with the same functional group and all have similar chemical properties.  These aliphatic alcohols have the general formula CnH2n+1OH, (where n is the number of carbon atoms present in the molecule.)  The alcohols are given names of the alkane with the corresponding number of carbon atoms with the final 'e' being replaced with 'ol'.  Alcohols all have similar properties, for example ethanol and methanol is freely miscible in water.  'Miscibility is the measure of how easily a liquid mixes.'2  When water and ethanol mix, some of the hydrogen bonds between the molecules in the separate liquids are broken, hydrogen bonds then form between the water and the ethanol.  The miscibility of alcohols in water decreases with increasing hydrocarbon chains.  Alcohols can also be used as fuels as they have high enthalpies of combustion.  However there are other chemicals, which are better fuels and are cheaper and are therefore more widely used.  Unleaded petrol contains about 5% methanol the advantages of methanol are that it burns cleanly however it is more toxic than ethanol and produces less energy per litre than petrol.  Ethanol is used as a fuel in the form of methylated spirit.  This spirit burns with a light blue flame. Due to its volatiltity, which is explained below, the flame can be hard to detect in sunlight, so when filling stoves accidents can occur.  However alcohols are relatively light therefore campers can prefer carrying these to other chemicals.  In some areas methanol is used for racing cars and ethanol is blended with petrol to make cheaper motor fuel.  More useful fuels can generally be made from chemicals with large hydrocarbon chains; therefore the fractional distillation of crude oil is more widely used to produce fuels.  Alcohols are more widely used as solvents as they contain a polar hydroxyl group.  They also mix with many other compounds, and will dissolve in some ionic compounds.  More importantly in this investigation is the volatility of alcohols.  'Volatility is the ease with which a liquid turns into a vapour'3.  The attractive forces between the C-H atoms etc are known as covalent bonds. These intramolecular forces are relatively strong in alcohols        

                                Covalent bond

                

                                                      C-H

There are also intermolecular attractions between the alcohol molecules known as hydrogen bonds.  Hydrogen bonds are caused by permanent dipole to dipole attractions within the molecules. Dipoles are caused by the difference in electronegativity between two atoms.  Elecctronegativity is the ability of an atom to attract the bonding electrons from within a covalent bond towards itself.  Oxygen is  more electronegative than hydrogen and carbon and therefore attracts the electrons towards itself creating a partially negative charge on the carbon atom this in turn means that the hydrogen atom and carbon atom become partially positively charged.

 

The movement of electrons due to the electronegativity of one or more atoms creates a partial charge      or       .This is known as a dipole.  The molecule as a whole is said to be polarised.  As the dipoles are permanent it can be said that the molecule is permanently polarised.

Due to the polarisation of the molecules, alcohols have generally higher boiling and melting points than you might predict  points and hence a low volatility and are liquid at room temperature.  

When reacted with oxygen, which has a much greater electronegativity than hydrogen, the remaining hydrogen  atoms (which are not bonded to the oxygen atom,) readily form new stronger bonds with the oxygen atoms forming H2O.  The carbon atoms also form bonds with the oxygen atoms forming carbon dioxide.

The combustion of alcohols is an exothermic reaction.  The law of conservation of energy states that energy can neither be created or destroyed it is only transferred from one form to an other.  For a reaction to take place bonds must be broken and new bonds formed.  When bonds are broken energy is absorbed, this is an endothermic process.  When bonds are formed energy is released usually in the form of heat and light, this is known as an exothermic reaction.  As in the combustion of alcohols, if the energy released when bonds are formed is greater than the energy absorbed in breaking the bonds the overall reaction is exothermic.  The bonds, which are broken in this reaction between an alcohol and oxygen, are C-C, C-H, 0=0 and C-O.  The bonds made are C=O and H-O.  However the bond energies between the bonds which are formed are greater than the bond energies of the bonds which are broken.  Particularly the C=O bond which is almost twice as large as the bond energy within the C-C bond.  The significance of this is that as the carbon chains increase in length more C-C bonds are broken, however more C=O bonds are formed. As the carbon chain length increases the difference between the bond energies of bonds broken and bonds formed increases in favour of the exothermic reaction (bonds formed). I calculated the predicted value of the enthalpy change of combustion using bond energy calculations to support my prediction.  I have also decided to investigate whether the enthalpy of combustion is effected by whether the alcohol is a straight chain or a branched alcohol.  I have therefore also calculated the overall bond energy for the isomer of butanol, butan-2-ol.  

Theoretical bond enthalpy calulations

BOND        (KJ/mol)

H-H                +436

C-C                +350

C=C                +610

C-H                +410

O=O                +496

O-H                +460

C-O                +360

C=O                +740

   Methanol   CH3OH

                    H

H-C-O-H

    H

CH3OH(l) + 1.5O2(g)               CO2 (g) + 2H2O(g)                        

=3(C-H) + C-O + O-H + 1.5(O=O)               =2(C=O) + 4(H-O)                                

=3(410) + 360 + 460 + 1.5(496)                 =2(740) + 4(460)

=1230 + 360 + 460 + 744                       =1480 + 1840

=+2794KJ/mol    (bonds broken)               =-3320KJ/mol   (bonds formed)

                                 H =+2794 -3320

                            =-526KJ/mol

A Hess cycle could also be used to calculate the enthalpy change in combustion.  Hess's Law states that 'The total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place, providing the initial and final conditions are the same.'4  An example of this is shown below;

        CH3OH(l) + 1.5O2(g)                   CO2(g) + 2H2O(g)

H f (CH3OH)                              H f (CO2)                2*  H f (H2O)        

                        C(g)     +    2H2(g)    +    2O2(g)           

                

 H C(methanol) = -    H f (CH3OH) +    H f (CO2) + 2    H f (H2O)

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   Ethanol        CH3CH3OH

                    H H

                H-C-C-O-H

    H H

   

  CH3CH2OH(l) + 302(g)                      2CO2(g) + 3H2O(g)

=5(C-H) + C-C + C-O + O-H + 3(O=O)      =4(C=O) + 6(H-O)

=5(410) + 350 + 360 + 460 + 3(496)            =4(740) + 6(460)

=2050 + 350 + 360 + 460 + 1488                   =2960 + 2760

=+4708KJ/mol   (bonds broken)                   =-5702KJ/mol   (bonds formed)

                        H =+4708-5702

                            =-994KJ/mol

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