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Experiment Objective. To determine the approximate strength of hydrogen bond formed between ethanol molecules by measuring the enthalpy change on mixing it with cyclohexane.

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Introduction

Determining the approximate strength of H-bond Objective To determine the approximate strength of hydrogen bond formed between ethanol molecules by measuring the enthalpy change on mixing it with cyclohexane. Principle Ethanol contains -OH group which can form hydrogen bond among its molecules but cyclohexane hasn't. On mixing, cyclohexane will break the hydrogen bond among ethanol molecules. Breakage of hydrogen bond consumes energy thus the temperature lowers and hydrogen bond strength of ethanol can hence be calculated. Chemicals ethanol, cyclohexane Apparatus 10ml pipette, pipette filler, test-tube, cotton wool, thermometer, beaker Experimental set-up Procedure 1.> Weigh a test-tube and the weight is recorded. 2.> The initial temperature of ethanol, cyclohexane and test-tube are recorded in Table 1. 3.> A test-tube is placed into a beaker and is surrounded with cotton wool. 4.> A thermometer is then put into the test-tube. 5.> Pipette 4 cm3 of ethanol into the test-tube. ...read more.

Middle

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. ...read more.

Conclusion

< 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. ...read more.

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