Experiment P08
Chemicals:
Hydrogen bonding
Ethanol, C2H5OH (20 cm3)
Cyclohexane, C6H12, (35 cm3)
Ethyl ethanoate, CH3COOCH2CH3 (30 cm3)
Trichloromethane, CHCl3 (35 cm3)
Apparatus:
10 cm3 and 25 cm3 measuring cylinders,
50 cm3 insulated beaker,
-10 - 110 oC thermometer.
Hazard Warning: Ethanol, cyclohexane and ethyl ethanoate are flammable,
Trichloromethane is harmful,
Principle:
Breaking or formation of intermolecular hydrogen bonds between molecules in
liquids would cause an enthalpy change when the liquids are mixed.
This experiment is to investigate such enthalpy changes and to measure
approximate strengths of hydrogen bonds formed between molecules of ethanol
and those between molecules of trichloromethane and ethyl ethanoate using
simple calorimetric methods.
You may find this formula useful.
ΔH = ms(t2-t1)
Where s is the specific heat capacity.
The following physical data may be useful:
Specific heat capacity of glass = 0.78 kJ kg-1 K-1
Liquid
Formula
Molecular
Density
Specific heat capacity
mass
kg dm-3
kJ kg-1K-1
Ethanol
CH3CH2OH
46
0.81
2.44
Cyclohexane
C6H12
84
0.78
1.83
Trichloro-
CHCl3
119.5
1.48
0.98
88
0.90
1.92
methane
Ethyl
CH3CO2CH2CH3
ethanoate
P.1
Experiment P08
Hydrogen bonding
Name:
Seat No.:
Date:
Grade:
Procedure:
Part A
a.
To discover the existence of hydrogen bonds between ethanol molecules
Using a measuring cylinder, add 10 cm3 of ethanol into an insulated 50 cm3 beaker, Measure
the temperature of the liquid.
b.
Then add 10 cm3 of cyclohexane to the ethanol in the beaker, mix well and record the
lowest temperature attained.
1.
Why should the beaker be insulated?
2.
Is the mixing process endothermic or exothermic?
3.
Account for the temperature change.
Part B
To measure the strength of hydrogen bond formed between ethanol molecules
Repeat steps a and b in part A above using the same volume of ethanol but 25 cm3 of
cyclohexane. From the temperature drop estimate the hydrogen bond strength
(in kJ per mole) in ethanol.
1.
2.
Comment on the reliability of the hydrogen bond strength obtained.
P.2
Experiment P08
Hydrogen bonding
Name:
Seat No.:
Date:
Part C
To discover the formation of hydrogen bond between molecules of ethyl ethanoate
and trichloromethane
a.
Measure 10 cm3 of ethyl ethanoate into an 50 cm3 insulated beaker. Record its temperature.
b.
Add to this 10 cm3 of trichloromethane and mix well. Record the highest temperature
attained.
1.
Is the mixing process exothermic or endothermic?
2.
Account for the temperature change.
Part D
To estimate a value for the strength of hydrogen bond between ethyl ethanoate
and trichloromethane
Repeat steps a and b in part C above using either one liquid in excess (Using 25 cm3
trichloromethane). From the temperature change estimates the strength of the hydrogen
bond formed between molecules of ethyl ethanoate and trichloromethane.
P.3