Chemistry--Hydrogen bond－bxzbthf的部落格

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Chemistry--Hydrogen bond

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Experiment1Add 10cm^3 hexane to 10cm^3 ethanolQ1: Is the mixing process exothermic or endothermic?Q2: Why there is a temperature change on mixing the two liquids?What is the effect has been produced by the hexane on the intermolecular forces between the ethanol molcules?Experiment2Add 20cm^3 hexane to 10cm^3 ... 顯示更多 Experiment1 Add 10cm^3 hexane to 10cm^3 ethanolQ1: Is the mixing process exothermic or endothermic?Q2: Why there is a temperature change on mixing the two liquids?What is the effect has been produced by the hexane on the intermolecular forces between the ethanol molcules?Experiment2 Add 20cm^3 hexane to 10cm^3 ethanol Q1: Why the hexane has to be used in excess? Q2:Find the value of the hydrogen bond strength( in KJ per mole) Molar mass of ethanol:46.1 Molar mass of hexane: 86.2 Density of ethanol :0.81gcm^-3 Density of hexane :0.66gcm^-3 Specific heat capacity of ethanol:2.44Jg^-1K^-1 Specific heat capacity of hexane:2.26Jg^-1K^-1 initial temperature:26oC final temperature:23oC Q3: Comment on the reliability of the value. Experiment3 Add10cm^3 trichloromethane to 10cm^3 propanone Q1: Account for the energy change

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I. endothermic. the temperature decreases. ethanol is a polar, protic solvent, which is able to form intermolecular hydrogen bonds. when hexane, which is miscible with ethanol, is added, the hexane molecules are inserted in between ethanol molecules. H-bonding between ethanol molecules is disrupted. when the "bond" is broken (attraction is overcome to separate attracting molecules), energy is absorbed. thus average kinetic energy (and speed too) of all molecules decreases. temperature thus drops. II. to ensure all (at least most) of the hydrogen bonds between ethanol molecules are broken / interrupted. mass of ethanol = 8.1 g no. of mole of ethanol = 0.1757 mole maximum no. of mole of H-bond = 0.1757 mole & mass of hexane = 13.2 g m1, c1: mass and heat capacity of compound 1 Ti, Tf: initial and final temp. m1c1Ti + m2c2Ti = m1c1Tf + m2c2Tf + (heat change) 8.1x2.44x23 + 13.2x2.26x23 = 8.1x2.44x26 + 13.2x2.26x26 + heat change heat change = -67.428 J bond enthalpy = 67.428 / 0.1757 = 383.8 J/mol = 0.3838kJ/mole assumptions are made, like: - maximum no. of H-bond =/= actual no. of H-bond; - actual no. of H-bond interrupted; - non-H-bond interaction between ethanol and hexane also releases energy (less but still present)... III. the Hydrogen on trchloromethane is highly electropositive, owing to strong electron-withdrawing effect of three chlorine atoms; oxygen on propanone has lone pairs; H-bond can be formed. so the temperature will... you should know this, and i'm not going into details.

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