Table 2 shows the data of the substances:
Energy absorbed to break hydrogen bond
= energy absorbed from test-tube (E1) + energy absorbed from ethanol (E2) + energy absorbed from cyclohexane (E3)
Energy = mc△T
E1 = [14.44 ÷1000]．[0.78]．[4.2] kJ
= 0.04731 kJ
E2 = [(0.81．4) ÷1000]．[2.44]．[1.7] kJ
= 0.01344 kJ
E3 = [(0.78．9) ÷1000]．[1.83]．[2.7] kJ
= 0.03469 kJ
E = E1 + E2 + E3
= 0.09544 kJ
Strength of hydrogen bond of ethanol = E ÷ no. of moles of ethanol
= E ÷ [(0.81．4) ÷ 46]
= 1.36 kJmol-1
Precaution
1.> The test-tube should be used instead of beaker because the smaller in size of test-tube can reduce heat loss or heat gain from surroundings. For
same reason, there is cotton wool between test-tube and beaker.
2.> Since both ethanol and cyclohexane are easily vapourize, the volume of substances added may be affected, so the transfer process and the
experiment should be done as fast as possible.
3.> The ethanol - cyclohexane mixture should be stirred gently & carefully with the thermometer, to ensure heat is evenly distributed.
4.> Ethanol & cyclohexane are flammable, it should be ensure that there is no fire around.
Discussion
Hydrogen bond is defined as the the intermolecular force (or intramolecular force) formed between H atom attached to F, O, or N and the lone pair electrons on F, O or N. The more electropositive (δ+) on H, the stronger is the hydrogen bond.
In this experiment, there are 2 assumptions made - it is assumed that no heat loss to (or gain from) the surroundings and all hydrogen bonds between ethanol molecules are broken.
It should be noticed that among ethanol molecules, hydrogen bonds can be present due to the presence of lone pair electrons on O attached to H:
However, among cyclohexane molecules, there is only relatively much weaker van der Waal's force but no H-bond:
After mixing these two substances, hydrogen bonds formed between ethanol molecules are broken because cyclohexane molecules separate ethanol molecules and hence interrupt its hydrogen bonds from forming:
To ensure that all the hydrogen bonds are broken, excess cyclohexane is added.
However, in this experiment, hydrogen bond strength obtained is not reliable because we can't ensure that all hydrogen bonds are broken. If we want to have a more reliable result, we can improve the experiment by several methods:
< 1.> Repeat the experiment by adding more cyclohexane until the temperature becomes constant which indicates hydrogen bonds are no longer
present.
< 2.> Since ethanol is a polar molecule whereas cyclohexane is non-polar, they are immiscible. To ensure all hydrogen bonds are broken, it's better to
use 2 substances which are miscible, with only ethanol can form hydrogen bond.
< 3.> Ethanol used should be as pure as possible, say, 99% to eliminate the effect of hydrogen bonds formed between water molecules.
When hydrogen bonds are broken, energy is absorbed, temperature drops. Since hydrogen bond is much stronger than van der Waal's force, the slightly increase in temperature caused by formation of van der Waal's force between ethanol & cyclohexane molecules can be compensated by the highly decrease in temperature caused by breaking of hydrogen bond among ethanol molecules. Hence, the approximate value of hydrogen bond strength of ethanol can be obtained by this calorimetric method.
Conclusion
From this experiment, we can conclude that hydrogen bond is present between ethanol molecules due to the presence of hydroxyl group, ──OH. The approximate strength of hydrogen bond of ethanol can be measured by using simple calorimetric method.
