The aim of this experiment is to compare the amount of energy released when a number of alcohols are burned.

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Aim

The aim of this experiment is to compare the amount of energy released when a number of alcohols are burned.  

Introduction

An alcohol is a series of organic, homologous compounds with the general formula CnH2n+1OH.  Alcohols react with the oxygen in the air to form water and carbon dioxide.

In this coursework we are going to carry out the experiment on a maximum of eight alcohols.  On the next page I have drawn the structure of each of these alcohols, as the structure will determine the formula of the alcohol.  This is because the formula includes the number of carbon atoms and hydrogen atoms in one molecule of the alcohol, so by drawing the structure of each alcohol we can clearly see the number of carbon and hydrogen atoms in its molecule.  This will give its formula.  The formula of each alcohol will be needed when working out the equation for the burning of the alcohols.  

What affects the amount of heat released by the alcohol, in this coursework?

Volume of water – if there is more water, more energy will be required to raise the temperature of it by a fixed temperature.  And if there is a small volume of water, less energy will be required to raise the temperature by the same value.  This is because when there is more water, there are more water particles that have to be provided with energy so that they move around quicker.  But when there is less water there are less water particles, so not as much energy has to be inputted to make the particles move around.      

Distance between burner and can – when there is a larger distance between the two, the alcohol will have to release more energy to raise the temperature of water by a certain temperature because there will be more heat loss in the air between the burner and can.  When the burner is closer, less energy is released to raise the temperature of the water, because less heat loss will occur.  This is because when the distance is shorter, there is less air in between, so overall, there will be less heat loss to the air during the experiment when the can is closer.  

Hypothesis

Energy changes in reactions are caused by the breaking and making of chemical bonds.  Breaking chemical bonds takes in energy and making chemical bonds releases energy.  In any chemical reaction, bonds in the reactants must first be broken.  This requires energy.  New bonds are then formed.  This releases energy.  If the energy released is greater than the energy taken in, the reaction will be exothermic.  

The idea of energy changes in bond breaking and bond making is in the case of the reaction of hydrogen and oxygen to form water:

2H2 (g)        +        O2 (g)                                 2H2O (l)

Here, 1 oxygen molecule and 2 hydrogen molecules are broken.  This leaves 4 hydrogen atoms and 2 oxygen atoms free.  They then make new bonds.  2 of the hydrogen atoms join to an oxygen atom, and the other 2 hydrogen atoms join to the other oxygen atom.  This forms two water molecules.  The reaction is exothermic as the energy released in making new bonds is greater than the energy taken in to break the original bonds.  

Above is an energy diagram for the reaction between methanol and oxygen from the air.  I have created it using the calculated values, which are based upon the bond energies for each bond in the alcohol.  It shows that when an alcohol is burnt in air, the reaction is exothermic, therefore the energy released when new bonds are formed, is greater than the energy taken in to break the old bonds.    

 

Equations

CH3OH (l)   +           1½O2 (g)                CO2 (g)          +        2H2O (g)

Methanol                Oxygen                Carbon dioxide        Water

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

Ethanol                Oxygen                Carbon dioxide        Water

C3H7OH (l)        +          4½O2 (g)                3CO2 (g)             +           4H2O (g)  

Propan-1-ol                Oxygen                Carbon dioxide        Water

C4H9OH (l)        +           6O2 (g)                  4CO2 (g)             +           5H2O (g)  

Butan-1-ol                Oxygen                Carbon dioxide        Water

C5H11OH (l)        +          7½O2 (g)                  5CO2 (g)             +           6H2O (g)  

Pentan-1-ol                Oxygen                Carbon dioxide        Water

C6H13OH (l)        +           9O2 (g)                  6CO2 (g)             +           7H2O (g)  

Hexan-1-ol                Oxygen                Carbon dioxide        Water

C7H15OH (l)        +           10½O2 (g)                  7CO2 (g)             +           8H2O (g)  

Heptan-1-ol                Oxygen                Carbon dioxide        Water

C8H17OH (l)        +           12O2 (g)                  8CO2 (g)             +           9H2O (g)  

Octan-1-ol                Oxygen                Carbon dioxide        Water  

   

Calculating the bond energies

In order to work out how much energy will be taken in and released when bonds are broken and formed, I have to explain the bonds involved and how the calculated values for H will be achieved.  I am going to explain the process using the example of methanol as the alcohol.  

  1. First we have to be aware of the bond energy values for the various bonds involved in the burning of the alcohols.  From this data I can then calculate the energy taken in and released in the reaction, depending on the number of each bond in each alcohol.
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(in kJ)

C–C         =  347

C–O         =  358

C–H        =  413

O=O         =  498

O–H         =  464

C=O         =  805

  1. Methanol:        

      +        1½ O=O                                O = C = O  

                        

                                 +  

                                        

                                                                           H — O — H

                                                                           H — O — H

  1. To calculate the energy taken in when the bonds on the left of the equation above are broken, all the different bonds have to be counted and multiplied by the bond energy for one of ...

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