COMPARING THE ENTHALPY CHANGE OF COMBUSTION OF DIFFERENT FUELS.

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Comparing the Enthalpy Changes of Combustion of Different Alcohols.

COMPARING THE ENTHALPY CHANGE OF COMBUSTION OF DIFFERENT ALCHOHOLS.

Aim

      The aim of my experiment will be to find out which alcohols have a higher enthalpy change of combustion. The comparison of the enthalpy changes of these fuels will then determine the alcohol efficiency and effectiveness. I will experiment on the first 5 consecutive primary alcohols. These are; methanol, ethanol, propan-1-ol, butan-1-ol, pentan-1-ol and hexan-1-ol. The reason why I chose these fuels is because they are the most reliable and accurate fuels to compare within the group; which also have the smallest variable, ‘add one carbon’ each time, to the aliphatic chain. All combustion reactions are exothermic which is why I am expecting all the values for the enthalpy change of combustion to always be negative.

CH3OH + 1.5O2           CO2 + 2H2O                                                                METHANOL

CH3CH 2OH+ 3O2             2CO2 + 3H2O                                                          ETHANOL

CH3CH 2 CH 2OH+ 4.5O2             3CO2 + 4H2O                                          PROPAN-1-OL

CH3CH 2 CH 2 CH 2OH+ 6O2         4CO2 + 5H2O                       BUTAN-1-OL

CH3CH 2 CH 2 CH 2 CH 2OH+ 7.5O2        5CO2 + 6H2O            PENTAN-1-OL  

CH3CH 2 CH 2 CH 2 CH 2 CH 2OH+ 9O2              6CO2 + 7H2O          HEXAN-1-OL

Prediction

        I believe that hexan-1-ol will have the highest enthalpy of combustion because there are more bonds to break and form and the energy released when bonds form is greater than energy needed to break bonds, this means that the reaction is exothermic and there is a higher temperature rise.

The molecular formulas are shown below. The general formula for alcohols series are CnH2n+1OH.This means you move across the series the amount of carbon and hydrogen atoms increase. This also means that the relative atomic mass increases. Although more energy is needed to break up hexan-1-ol than that of methanol, the energy released when new bond form is greater due to its higher mass and higher length of aliphatic chain, therefore it’s more exothermic. The stored energy of the reactants is higher than the stored energy of the reaction. The difference in energy is released to the surroundings when the fuel and oxygen react.

       CH3OH + 1.5O2               CO2 + 2H2O                                                                METHANOL

       CH3CH 2 CH 2 CH 2 CH 2 CH 2OH+ 9O2          6CO2 + 7H2O    HEXAN-1-OL

As seen above in methanol only 1CO2 and 2H2O have been formed, which is a lot smaller than the new bonds formed in hexan-1-ol, 6CO2 and 7H2O are formed and energy released from these are greater and make the reaction more exothermic.  

                                                                                               

I have completed a preliminary test in my module developing fuels as an activity 1.2. I have experimented using the two fuels methanol and hexane. My results show that hexane’s “1227.8 kjmol-1” enthalpy change of combustion is a lot higher than that of methanol’s “387.1kjmol-1”; this is because hexane has many more bonds to be broken down and new ones to be made. This is because, the size of the molecule (hexane) is much larger compared to molecule (methane).

      When hexane reacts, all O-H bonds must be made, which gives a more exothermic reaction. Fuels that contain oxygen like methanols are called oxygenate and have lower enthalpy change of combustion.

Diagram

        This is the way I will set out my apparatus, which I have listed below.

Equipment

  • Copper calorimeter; with a base diameter of at least 10 cm³. This is where I will put the 200cm³ of water. The copper is a good conductor therefore it will allow more of the energy to get to the water compared to some metals.
  • Clamp stand; this will be used to hold the copper calorimeter right above the flame which will ensure the heat and energy from the flame isn’t wasted at all. The clamp stand is the best choice because a metal tripod will heat up itself and energy will be lost.
  • Metal stirrer; to stir the heated water in order to get the heat equally dispersed in water.
  • Electronic balance; this would weigh the spirit burner to two decimal places which allows higher accuracy when calculating an accuracy.
  • Draught excluders; which are tiles put around the experiment to prevent heat loss and are safer to use as it will help prevent things set on fire.
  • Goggles; these would be worn to protect eyes as some fuels are irritant*.
  • Measuring cylinder; this will be used to measure 200cm³ of water, with increments of 1cm3 to a good degree of accuracy.
  • 0-100°C thermometer; this will be used to measure the waters temperature rise; I have chosen a 0-100°C because it’s more accurate; it has increments of 1°C which will result in a smaller percentage error.
  • Spirit burners containing;
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  1. Methanol
  2. Ethanol
  3. Propan-1-ol
  4. Butan-1-ol
  5. Pentan-1-ol

These are the fuels which will be burned to heat up the water in the calorimeter.

Method

Firstly I will put on my protective clothing and my goggles because of the dangers explained further on.

  • I will then add the fuels which I will compare; into the spirit burner, which I had already emptied, washed and dried before hand.
  •  I will then weigh my spirit burner and lid filled halfway with the liquid fuel.
  • I will place the spirit burner on a heat proof mat for ...

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