Find the enthalpy change of combustion of a number of alcohol's' so that you can investigate how and why the enthalpy change is affected by the molecular structure of the alcohol.

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Aim

        The aim is to find the enthalpy change of combustion of a number of alcohol’s’ so that you can investigate how and why the enthalpy change is affected by the molecular structure of the alcohol.                                                                                   

Background Knowledge

Combustion is principally the oxidation of carbon compounds by oxygen in air to form CO2 if there is a sufficient amount of oxygen. The hydrogen in a compound forms H2O. Combustion produces heat as well as carbon dioxide and water. The enthalpy change of combustion is the enthalpy change that occurs when 1 mole of a fuel is burned completely in oxygen.  I can use an enthalpy cycle to work the combustion value out only if you have the right information.  The energy contained in the bonds of the products is less than the energy contained in the bonds of the reactants. The difference in energy is released as heat. Energy releasing reactions are called exothermic reactions. Calorimetery is a way to determine the amount of heat produced in a reaction. Calorimeters are devices to measure heat released by a reaction.  The temperature of the calorimeter increases as heat is released by the reaction.  


For any reaction to take place bonds must be broken and made Bond breaking requires energy whilst bond making releases energy. Bonds between different atoms require or release different amounts of energy when broken or made because they are different in strength.  Chemical bonding is the electrical attraction between atoms or ions. When you break a bond you have to do work in order to overcome these attractive forces. To break the bond completely you need (theoretically) to separate the atoms or ions so they are an infinite distance apart. The quantity of energy needed to break a particular bond in a molecule is called the bond enthalpy. Breaking bonds is endothermic- needs energy. Making bonds energy is given out- exothermic. When the fuels are burnt the reaction involves both making and breaking new bonds. In these combustion reactions the energy taken in during the bond breaking steps is less than the energy given out in the bond making steps so the overall reaction is exothermic. I have drawn up the theoretical values at what the standard enthalpy of combustion for a mole of each of the alcohol’s is and written equations to show what the reactants and products are in each of the experiments.

This information was obtained from a data book done under standard conditions (1 atm, 298K).
        The following equation is helps to work out the enthalpy change of combustion:

Energy transferred to water by burning fuel (Kj mol-1)= (mass of water (g) x temperature rise (0C) x 4.2 (J))

Prediction

        From my background knowledge I can form a prediction about my results.  I am predicting that the alcohol’s with a greater number of carbon atoms within the molecule to have a higher enthalpy change of combustion than the ones with less.  This means as the number of carbon’s increase so does the enthalpy change of combustion.  This means that butan-1-ol will have the highest enthalpy change of combustion and methanol will have the lowest.

For any reaction to take place bonds must be broken and made, bond breaking requires energy while bond making releases energy. Bonds between different atoms require or release different amounts of energy when broken or made because they are different in strength.  There are much more bonds to make and break in bigger alcohol’s this therefore means that bigger alcohol’s will have a high bond breaking enthalpy and therefore will have a very high bond making enthalpy.  This in turn means that they will have a high enthalpy change of combustion.  It is also due to an increase in the number of carbon atoms and hydrogen atoms.

Plan

        I am going to write a plan on how to find the enthalpy changes of combustion of alcohol’s so that I can draw conclusions about how these values are affected by their molecular structure.  I will work out the enthalpy change of combustion by using the fact that 4.2J of energy is required to raise the temperature of 1g of water by 1oc.

Apparatus: -

  • Spirit burner to burn the alcohol
  1. Methanol
  2. Ethanol
  3. Propan-1-ol
  4. Butan-1-ol

  • Aluminium calorimeter ( is highly thermal conductive)

  • Tripod stand to balance the aluminium can on it

  • Electronic balance to 2 decimal places, I used this type of balance as this gives me enough detail to be accurate but not so much that it would be difficult to handle the result detail.

  • 4-heat proof mats to act as draught/heat shields to reduce energy/heat loss.

  • Goggles used for safety to prevent damage to the eye.

  • 250 cm3 measuring cylinder to keep results more accurate.

  • Thermometer set to 10c intervals, this is being used to keep my results accurate but not confusing

  • 200 ml of water this is because it gives a reasonable temperature rise.

  • Matches to light the wick
  • Mortar (used for, see diagram)
  • Gauze (used for, see diagram)

Diagram: -

Note – the heatproof mats cover all 4 sides of the equipment. No heat mat covers the top of the experiment (for safety reasons).  


Preliminary work and results: -

        I carried out the practical to work out the enthalpy combustion of ethanol.  I repeated the experiment twice.

I used 150 ml of water.

I am going to show how to work out the enthalpy of combustion of ethanol trial 1:

Energy transferred = 150 x 4.2 x (36 – 16) = - 12600 J

Moles of fuel used ----- Mole = Mass (g)/ RMM g mol-1

RMM of Ethanol (C2H5OH) = (12 x 2) + (1 x 6) + (1 x 16)

                                                = 46

Mass = 212.50 – 210.44 = 2.06 g

Moles = 2.06/46 = 0.0448 moles (to 3 s.f.)

Amount of energy released per mole of fuel burned/ Enthalpy of combustion -------------------

(1/0.0448) x 12600 = -281250 j

- 281250/1000 = 281.25 kJ mol-1 

I repeated the same steps for working out the enthalpy for ethanol trial 2.

Ethanol trial 2 standard enthalpy of combustion = -294.39 kJ

                                                                                               mol-1 

 

The average for these 2 results is =  (-294.39 + -281.25)/2

                                                           = - 287.82 kJ mol-1 

% Error in the form of heat loss is very high.  The average enthalpy change of combustion for ethanol is very low compared to the value given in the databook, which is –1367 kJ mol-1.

% Error = (1367 – 287.82) = - 1079.18

        = (1079.18/1367) x 100 = 78.9%

From this preliminary work I know how I am going to change my plan to make there be less errors in the experiment and to make my results more accurate.  I will do this in a number of ways that will be explained in more detail later on but some examples are to use draught shields to prevent heat loss.  Keeping the amount of water the same and preventing soot build up by putting the wick closer to the calorimeter.


Method: -

I plan to measure the enthalpy change by burning the alcohol, using a spirit burner, I will then use the heat produced during the combustion of the alcohol to heat 200 cm3 of water that will be situated in a aluminium calorimeter directly above the burning alcohol. The calorimeter is made of aluminium as aluminium has a high thermal conduction value, this basically means that it is a good conductor of heat so a lot of the heat the aluminium receives will be passed on to the water which I am then able to measure.

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During the experiment I will be taking a number of measurements, I will firstly take the initial temperature of the water and initial mass of the alcohol I will then burn the alcohol until an increase in temperature of 20oc has occurred in the water I will then reweigh the alcohol.

The measurements
* Mass of alcohol burned (g)
* Temperature increase (
oc)

        

This will tell me what mass of alcohol is used during combustion to cause the temperature increase of 20oc in the water, I can then work out the energy released per mole and compare these values and see ...

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