Comparing the enthalpy changes of combustion ofdifferent alcohols.

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Comparing the enthalpy changes of combustion of

different alcohols.

What are alcohols?

Alcohol is the common family name for the hydrocarbon group alkanols. At least one of the hydrogen groups is replaced by an –OH group. They are all organic compounds.

The general formulas for the alcohols are:

CnH(2n+1)OH

Where n represents a number.

The first and simplest member of the alkanols family is methanol.

Its molecular formula is CH3OH.

You can now see that each member of the alcohol family has a different number of carbons in its structure. They increase by one carbon atom and two hydrogen atoms each time.

The question that this piece of coursework is going to answer is

“Is there a relationship between the number of carbon atoms in a chain and the enthalpy change of combustion?”

Aim

The aim of this experiment is to prove that the longer the hydrocarbon chain the higher the amount of energy transferred to the water, therefore fewer moles of fuel will be used to achieve a temperature rise of 15°C.

The enthalpy change of combustion is the amount of energy transferred when one mole of a substance burns completely in oxygen (always –ve).

To work out the enthalpy change of combustion of the alcohols the energy output must be measured. The easiest way to do this accurately is to use the thermal energy of combustion to raise the temperature of a substance with a known specific heat capacity, (the amount of energy required to increase the temperature of 1g of a liquid by 1°C).We will be using water.

Prediction

I predict that the longer the hydrocarbon chain the more energy will be transferred to the water, therefore fewer moles of fuel will be used to achieve the same temperature rise.

So as the number of carbon atoms increases the enthalpy of combustion will become more negative.

Alcohols to be used to compare

  • Methanol        =        CH3OH
  • Pentan-1-ol        =        C5H11OH
  • Propan-1-ol         =        C3H7OH
  • Propan-2-ol        =        C3H7OH
  • Butan-1-ol        =        C4H9OH
  • Butan-2-ol        =        C4H9OH

I have chosen these alcohols because I will have a chain containing 1 carbon atom, 3 carbon atoms, 4 carbon atoms and 5 carbon atoms. I have also decided to use structural isomers of two of these, Propanol and Butanol. (Propan-1-ol and propan-2-ol and Butan-1-ol and butan-2-ol). This will allow me to compare the enthalpy of combustion of increasing chain length and the structural isomers of some of these alcohols.

Alcohol structure

Methanol

H             H

 |            ⁄

H  —        C   —        O

 |        

        H

Propan-1-ol

H        H        H                

         |         |         |          

H   —        1C  —        2C  —3C  —        H

         |         |         |                  

        OH        H        H        

Propan-2-ol

H        H        H                

         |         |         |          

H   —        1C  —        2C  —        3C  —        H

         |         |         |                  

        H        OH        H

Butan-1-ol

H        H        H        H        

         |         |         |         |        

H   —        1C  —        2C  —        3C  —        4C  —        H

         |         |         |         |        

        OH        H        H        H        

Butan-2-ol

H        H        H        H        

         |         |         |         |        

H   —        1C  —        2C  —        3C  —        4C  —        H

         |         |         |         |        

        H        OH        H        H        

Pentan-1-ol

H        H        H        H        H

         |         |         |         |         |

H   —        1C  —        2C  —        3C  —        4C  —        5C  —        H

         |         |         |         |         |

        OH        H        H        H        H

Equipment required.

  • Small copper can to act as a calorimeter
  • Thermometer (to 1°C of accuracy)
  • 100cm3 measuring cylinder
  • Spirit burners containing –  Methanol

        Pentan-1-ol

        Propan-1-ol

        Propan-2-ol

        Butan-1-ol

        Butan-2-ol

  • Digital balance (to two decimal points)
  • Draught shielding
  • Stirring rod
  • Stand and clamp
  • Ruler (to measure the height of the calorimeter above the spirit burner)
  • Pen and paper to record the results
  • Safety goggles and lab coat
  • Box of matches
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Variables.

  • Draught
  • Amount of water
  • Material of can
  • Starting temperature of water
  • Temperature rise
  • Height of the calorimeter
  • Length of wick
  • Time burnt
  • Amount of oxygen present

How will my method give me accurate and reliable results?

The plan that I have used will provide accurate results as I am using equipment that has a fairly high degree of accuracy. Another measure of accuracy is how well I can read the equipment that I am using. For example it will be easy to accurately read the reading on the digital balance but it will be harder to ...

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