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Experiment investigating hydrogen bonding in different chemicals.

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Tsuen Wan Public Ho Chuen Yiu Memorial College Form 6 Chemistry Practical Experiment 8: Hydrogen Bonding Date of experiment: 27-1-2011 Objective: A. To discover the existence of hydrogen bonds between ethanol molecules B. To measure the strength of hydrogen bond formed between ethanol molecules C. To investigate the formation of hydrogen bonds between molecules of ethyl ethanoate and trichloromethane D. To measure the strength of hydrogen bonds formed between molecules of ethyl ethanoate and trichloromethane Introduction: A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond. These bonds can occur between molecules, or within different parts of a single molecule. The hydrogen bond (5 to 30 kJ/mol) is stronger than a van der Waals interaction, but weaker than covalent or ionic bonds. This type of bond occurs in both inorganic molecules such as water and organic molecules such as DNA. Intermolecular hydrogen bonding is responsible for the high boiling point of water (100 �C) compared to the other group 16 hydrides that have no hydrogen bonds. Intramolecular hydrogen bonding is partly responsible for the secondary, tertiary, and quaternary structures of proteins and nucleic acids. It also plays an important role in the structure of polymers, both synthetic and natural. ...read more.


+ energy released from trichloromethane (E2) Energy = mc?T E1 = [10 x1.48�1000].[0.98].[8] kJ = 0.116032 kJ E2 = [10x0.90�1000].[1.92].[8] kJ = 0.13824 kJ E = E1 + E2 = 0.254272 kJ Bond enthalpy/Strength of hydrogen bond of product = E � no. of moles of product = E x [(119.5+88)/ (10x1.48+10x0.9)]=2.22 kJmol-1 D.(20 cm3 trichloromethane): E1 = [10 x1.48�1000].[0.98].[9] kJ = 0.130536 kJ E2 = [20x0.90�1000].[1.92].[9] kJ = 0.31104 kJ E = E1 + E2 = 0.441576 kJ Bond enthalpy/Strength of hydrogen bond of product = E � no. of moles of product = E x [(119.5+88)/ (10x1.48+10x0.9)]=3.85 kJmol-1 D.(30 cm3 trichloromethane): E1 = [10 x1.48�1000].[0.98].[8] kJ = 0.116032 kJ E2 = [30x0.90�1000].[1.92].[8] kJ = 0.41472 kJ E = E1 + E2 = 0.530752 kJ Bond enthalpy/Strength of hydrogen bond of product = E � no. of moles of product = E x [(119.5+88)/ (10x1.48+10x0.9)]=4.63 kJmol-1 D.( 20 cm3 ethyl ethanoate): E1 = [20 x1.48�1000].[0.98].[7] kJ = 0.203056 kJ E2 = [10x0.90�1000].[1.92].[7] kJ = 0.12096 kJ E = E1 + E2 = 0.324016 kJ Bond enthalpy/Strength of hydrogen bond of product = E � no. of moles of product = E x [(119.5+88)/ (10x1.48+10x0.9)]=2.82 kJmol-1 D.(30 cm3 ethyl ethanoate): E1 = [30 x1.48�1000].[0.98].[6.5] kJ = 0.282828 kJ E2 = [10x0.90�1000].[1.92].[6.5] kJ = 0.11232 kJ E = E1 + E2 = 0.486336 kJ Bond enthalpy/Strength of hydrogen bond of product = E � no. ...read more.


< 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. Hydrogen atom and very electronegative atom (such as oxygen) are necessary for the formation of hydrogen bonds. The approximate strength of hydrogen bond of ethanol can be measured by using simple calorimetric method but improvements can be made to obtain more reliable results.. Hydrogen bond between ethanol molecules is 1.24 kJ mol-1. Hydrogen bond between ethyl ethanoate and trichloromethane is 4.24 kJ mol-1. Reference: Hands-on Science (H-Sci) Project: Chemical Safety Database of The Physical and Theoretical Chemistry Laboratory of Oxford University: http://cartwright.chem.ox.ac.uk/hsci/chemicals/hsci_chemicals_list.html Physical Chemistry I by Leung and Lee- Section 17.2 ~End of Lab Report 8~ ...read more.

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