hydrogen bonds Essay
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Nov 19th, 2019

hydrogen bonds Essay

70136540Determining the Approximate Hydrogen Bond Strengths in Water and EthanolIntroduction:Intermolecular forces operate between neighbouring molecules. The forces result from the action of the kinetic energy of atoms and the positive and negative charges on different parts of a molecule that affect its neighbours.The three main categories of intermolecular forces are dipole-dipole interaction, van der Waals forces and the hydrogen bond.Dipole-dipole interactions is a week intermolecular force between the molecules which contain a permanent dipole. Ramsden E.N. described, molecules lead the molecules to pack in such a way that partial positive charges will be adjacent to partial negative charges.

Dipole -dipole intermolecular forces are stronger than van der Walls forces. )Van der Waals forces are distance dependent, repulsive force of attraction between atoms and molecule that are neither ionic nor covalent bond. Van der Walls forces weakest of the intermolecular forces. E.N. Ramsden said, dipole-dipole interaction between polar molecules are one type of van der Waals force.

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(2000, P.109)( bonding occurs when a hydrogen atom is covalently bonded to an electronegative element such as oxygen, nitrogen and fluorine. It is a specific type of dipole-dipole attraction and the strongest type of intermolecular force.The aim of the experiment is to determine of the hydrogen bond strengths in water and ethanol, the understanding of intermolecular forces and its categories.Materials:WaterMeasuring cylinderEthanolBeakerCyclohexaneThermometerConical flaskBunsen burner Methods:Part 1:A conical flask was filled with 240 cm¶џ water using a measuring cylinder and fixed the lower clump then the thermometer fixed in the upper clamp so that the bulb was about 2 cm clear of the bottom of the flask. A bunsen burner is placed under the flask and initial temperature recorded and timing started. Temperature is recorded every half minute until the water was boiling steadily and every two minutes afterwards. Water is allowed to boil for exactly ten minutes and Bunsen burner removed to allow the water to cool. Volume of the remaining water measured accurately and recorded.Part 2:10 cm¶џ of ethanol added into insulated 50 cm¶џ beaker and the temperature measured using a thermometer and recorded. 10 cm¶џ of cyclohexane measured and its initial temperature recorded then added to ethanol in the beaker mixed well and the lowest temperature recorded.Results:Part 1: Measuring the intermolecular Forces Between Ethanol MoleculesInitial mass of water: 240 cm¶џFinal Mass of Water: 158 cm¶џTable 1: Temperature of Boiling Water Over TimeTime / Second Temperature/ °C 0 2530 3060 4890 55120 64150 73180 82210 89240 97270 103390 105510 105630 105750 105870 106Graph 1: Heating Curve for Water-12096751371600Temperature (°C)00Temperature (°C)18192753267075Time ( Minutes)00Time ( Minutes)Energy required to turn 1 mole of liquid water into to gaseous water = 40.67 kjmol”№Graph 2:25050751887855H‚‚Se00000H‚‚Se00031242001421130H‚‚Te00H‚‚Te18859501668780H‚‚S00H‚‚S1247775478154H‚‚O00H‚‚O1 mole of hydrogen bonds strength in water: 11.85 kJmol”№Part 2: Measuring the intermolecular Forces Between Ethanol MoleculesInitial Temperature of 10 cm¶џ of Ethanol: 24 °CFinal Temperature of Ethanol mixed with 10 cm¶џ of Cyclohexane: 21 °CFinal Temperature of Ethanol mixed with 25 cm¶џ of Cyclohexane: 21 °CDiscussion:Every atom prefers to stay at its lowest energy state called theground states. The energy level of the atom depends on the number of protons, electrons and number of the particle charged. The lowest energy can be obtained when the valence shell of the atom has the highest possible number of electrons. There are two ways, atom can achieve full valance shell through an ionic bond or covalent bonds. Otter C. said, the oppositely charged ions attract each other strongly called an ionic bond and bonds formed by sharing electrons are called covalent bonds. (2008, P.37)Electronegativity is a measure of the ability of an atom in a molecule to attract electrons in a c bond to itself said, Otter C. (2008, P.40). Differences in electronegativity can use to predict how polar covalent or ionic bond will be. Large electronegativity difference leads to an ionic bond while small electronegativity difference leads to a polar covalent bond. If there is no electronegativity difference between two atoms, that leads to non-polar covalent bond.Bond polarity describes the sharing of the electrons between atoms in a covalent bond. Otter C. stated, a nonpolar covalent bond occurs when the electrons are shared equally between two atoms and a polar covalent bond occurs when an atom has a greater attraction for the electrons than the other atoms. (2008, P.40)Covalent bond and ionic bonds are result of intramolecular forces, however, intermolecular forces play important role on the physical properties such as melting point, boiling point, vapour pressure, evaporation and surface tension.Water molecule contains polar bonds forming polar molecule. Markgraf B. (2018) explained, oxygen atom in water molecule has six electrons in its outer shell however there is a room for eight electrons. Two hydrogen atoms in water form covalent bond with the oxygen atom by sharing their two electrons hence eight available bonding electrons in the molecule, two are shared with each of the two hydrogen atoms leaving four free. The shared electrons stay between the hydrogen atoms and the oxygen atom. Moreover, that the hydrogen side of the water molecule has a positive charge, while the other side where the free electrons are has a negative charge. Hence, the water molecule is polar and is a dipole. Hydrogen bonds are much stronger than other kinds intermolecular force that are induced dipole and permanent dipole. Therefore, due to large electronegativity covalent bond formed and leads to permanent dipole.Hydrogen bonds play important role determining the shapes, properties and functions of molecules. Hydrogen bonding is stronger in the solid state, while minimal in the gaseous state. Melting and boiling point are also, consequences of intermolecular hydrogen bonding. Also, the strength of the hydrogen bond depends on the electronegativity of the atoms. Greater the electronegativity difference of the atom, grater the strength of hydrogen bond.Intermolecular forces affect melting point and boiling point of a covalent substance because energy is needed to overcome these forces and to separate molecules when the substance melts and then boils. If the between molecules are strong, then the melting point and boiling point of the substance tend to be high. If the intermolecular forces between molecules are weak, then the melting point and boiling point of the substance tend to be low.In water the intermolecular forces are due to van der Waals, permanent dipole and hydrogen bonding. Hydrogen bonding in water is strong because, there are two lone pairs of electrons of oxygen and two slightly positively charged hydrogen atoms per oxygen therefore intermolecular force maximised.Otter C. said, water has unusually high values for its boiling point and enthalpy change of vaporisation. (2008, P.102) The enthalpy change of vaporisation is a measure of the energy put into a liquid to overcome the intermolecular forces and then turn one mole of molecules to from liquid to vapour. This can be explained as; strong attraction produced by the hydrogen bonds which is special case of polar dipole force supplies very strong effect to keep water molecule in a liquid state until more kinetic energy applied in an increased temperature to break the hydrogen bonds to free the water molecules as gas. Regards to data from graph 2, hydrogen bonding contribution to the In the experiment when ethanol is mixed with cyclohexane the temperature dropped observed. In an ethanol molecule, hydrogen and oxygen atom are bonded by polar bond due to the electronegativity difference between oxygen and hydrogen atoms then one ethanol molecule and a hydrogen atom the alcohol group of another ethanol molecule formed. Hence intermolecular force very strong. Cyclohexane has six carbon so than formed by carbon and hydrogen bonds hence electronegativity difference therefore cyclohexane formed as a non polar bond. This is because the non-polar cyclohexane pushes the ethanol molecules further apart, which in turns decreases the strength of the hydrogen bonds. The action pushing the ethanol molecules apart requires energy, which is the reason why the temperature decreases.Appendix: Gradient = (97-25) /4 = 18Joules Per Minute = 240 x 4.18 x 18 = 18057.60 J = 18057.60/ 1000 = 18.0576kJKilo Joules for 10 minutes = 18.0576x 10 = 180.576 kJMr of H‚‚O = 18Moles of Water Boiled Away : (240-160)/18 = 4.44Energy required to turn 1 mole of liquid water into to gaseous water = 180.576/4.44 = 40.67 kjmol”№Hydrogen bonding contribution to the vapourization energy = 40.7-17.0=23.71 mole of hydrogen bonds in water: 23.7/2= 11.85”T = 24-21 = 3 °CJ = (8.1 x 3 x 2.44 ) + ( 19.5 x 3 x 1.83)J = 166.347Moles of ethanol = 8.1/ 46 = 0.1761Hydrogen Bond Strengths in Ethanol = 166.347/ 0.1761= 944.617 Jmol”№ = 944.617/1000 = 0.945 k Jmol”№References:Ramsden, E.N. (2000) A-Level Chemistry, Fourth Edition. Cheltenham, Nelson Thornes Ltd 2000Solomons, G (2008) Organic Chemistry, Ninth Edition. USA, John Wiley & Sons, IncOtter, C. & Stephenson, K (2008) Chemical Ideas. Essex, HeinemannContinued heating of the water after the ice has completely melted will now increase the kinetic energy of the liquid molecules and the temperature will rise. Assuming that the atmospheric pressure is standard, the temperature will rise steadily until it reaches 100°C. At this point, the added energy from the heat will cause the liquid to begin to vaporize. As with the previous state change, the temperature will remain at 100°C while the water molecules are going from the liquid to the gas or vapor state. Once all the liquid has completely boiled away, continued heating of the steam (remember the container is closed) will increase its temperature above 100°C.The stronger the intermolecular forces, the more energy it takes to overcome these interactions and to cause the substance to boil. Therefore, molecules with strong bonds have high boiling points, and molecules with weak bonds have lower boiling points.The four types of intermolecular forces, listed from strongest to weakest, are ionic, hydrogen bonding, dipole-dipole interactions and Van Der Waals dispersion forces, or London forces.

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