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I. DESIGN
Purpose
To determine the relation between enthalpy of Combustion and the number of carbon atoms
in an alcohol by finding out enthalpy of combustion for three alcohols, namely, Ethanol,
Propanol and Methyl Spirit.
Hypothesis
As the number of carbon atoms in hydrocarbon chain increases the heat released on burning
of alcohol will accordingly increase. i.e, as size of hydrocarbon increase the Standard
enthalpy of combustion will become more negative. (Since combustion is an exothermic
process of enthalpy of combustion will necessarily be negative).
Theory
All the hydrocarbons are essentially composed of Carbon and Hydrogen and can have one or
more functional groups. As the number of carbon atoms increase so does the molar mass of
hydrocarbon leading to higher Van der Waal’s force of attraction this causes stronger
intermolecular bonding. At the same time more the Carbon atoms in the hydrocarbon, more is
the number of C-H as well C-C bonds that will need to be broken when combustion takes
place.
Heat of combustion is the energy released when hydrocarbons are burnt. This process on
molecular level involves breaking up of C-C and C-H bonds. So higher is the number of C-H
and C-C bonds higher is the heat of combustion for the hydrocarbon.
General formula of Alcohol can be represented as CnH2n+1OH. It is clear from the general
formula that as n increases so will overall molar mass of alcohol and for every increase in
Carbon atom it signifies the increase of –CH2- group in alcohol.
When alcohols are burnt the reaction is exothermic and heat is released. Theoretically this
fact can be used to capture the heat and use it through calorimetry the enthalpy of
Combustion. This is given by formula,
Enthalpy change during combustion = ∆H = mC∆T
Where, m = mass of substance,
C = Specific heat capacity of substance
Temperature change, ∆T = Final temperature – Initial Temperature
And, Standard Enthalpy of Combustion, ∆H0comb = ∆H/n
Where, n = number of moles of combusting substance.
Variables
Independent Variables : Number of Carbon atoms in alcohol
Controlled Variables: Temperature Change, Mass of distilled water, distance of burner top
from the base if calorimeter (Water container in this case 250 ml beaker).
Dependent Variable: Mass of alcohol burnt
Materials and Apparatus
•
Lighter
•
Tongs
•
Weighing Machine (accuracy 1 grams)
•
Spirit Burner
•
250 ml beaker (accuracy:1 ml )
•
Tripod
•
Heat Cover
•
Thermometer (accuracy: 0.5 oC)
•
Alcohols
1. Ethanol: C2H5OH
2. 2-Propanol: CH3CHOH-CH3
3. Methyl Spirit: CH3OH
Procedure
1. Place spirit burner on the table and than on top of it place tripod to support the 250
ml beaker.
2. Put water into 250 ml beaker and note down the weight in grams before and after
putting the water into the beaker.
3. Take initial temperature of water in degree Celcius
4. Put alcohol in the spirit burner and take the weight in grams
5. Place heat cover, covering the spirit burner
6. Light the spirit burner
7. Let burner, burn until the temperature of water becomes around 63oC.
8. Take the temperature of the water using thermometer in degree Celcius
9. Take the weight of the spirit burner in grams
10. Repeat steps 1-8 three times for each alcohol.
11. Calculate the temperature rise after each cycle, and loss of alcohol in spirit burner
after each cycle in degree Celcius
12. Calculate the heat of combustion from above raw data obtained from all the trials
of an alcohol in kJ
13. Calculate moles of alcohol used for burning
14. Calculate the Standard Enthalpy of combustion and take mean of a...
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