BIG AND SMALL – COVALENT MOLECULESWhen 2 or moreatoms are covalently bonded to form molecules, they move as one and themol...
While at the other end which has very few electrons, a slightly positive charge will be theresult.This is very temporary a...
boiling point increase  means stronger intermolecular attraction down groupstrong intermolecular forces  because down th...
From the graph above, explain what the state of matter which the compound being heated isin between time D and E.It is in ...
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Intermolecular forces of attraction

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Introduces the concept of covalent bonding with macro-molecules and simple covalent molecules.

Next, it covers inter-molecular attraction but explaining how temporary dipoles form

Finally, heating and cooling curves together with an explanation for how energy is absorbed or given out during boiling or freezing

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Intermolecular forces of attraction

  1. 1. BIG AND SMALL – COVALENT MOLECULESWhen 2 or moreatoms are covalently bonded to form molecules, they move as one and themolecule is considered one unit all by itself.The energy of a typical single covalent bond is ~80 kilocalories per mole (kcal/mol).However, this bond energy can vary from ~50 kcal/mol to ~110 kcal/mol depending on theelements involved. Once formed, covalent bonds rarely break spontaneously.This is due to simple energetic considerations; the thermal energy of a molecule at roomtemperature (298 K) is only ~0.6 kcal/mol, much lower than the energy required to break acovalent bond.Molecules come in different sizes depending on the extensiveness of the number of covalentbonds formed per atom. A molecule formed completely of carbon would have a very largesize, while that formed of hydrogen and oxygen has a formula of just H2O. The smallest level is the diatomic molecule which is the elemental form of group 7 atoms as they just need to form just one covalent bond to attain octet electronic configuration. Very large molecules are called macro-molecules and theycan contain over 1000 atoms and more.Look at how many water molecules can fit in the space of ONE diamond molecule.There is very little intermolecular forces for diamond or for the sand on the seashore, becauseone molecule is so big… but it is very significant for simple covalent compounds! Temporary dipoles An molecule has an electron cloud combining the electrons from the atoms that make up the molecule As electrons are very mobile, the majority of them may move to oneend of the cloud at an instant, and cause a temporary negative charge to form at that end.
  2. 2. While at the other end which has very few electrons, a slightly positive charge will be theresult.This is very temporary and electrons move very very fast and… the polarity may be reversedvery quickly The uneven distribution of electrons can occur in atoms of noble gases even, such as helium!Because the end are slightly charged, consider what happens hereThe positive end repels electrons from a nearby molecule to the opposite end and induces atemporary dipole to form on it!And even if the electrons move to the other end, the attraction is still maintained!And this doesn’t just occur between 2 molecules, remember the image of water molecules allbunched up earlier?A large number of molecules can be held together by intermolecular forces Molecular size and strength of van der waal forcesLet us look at the boiling point of the noble gaseshelium -269°Cneon -246°Cargon -186°Ckrypton -152°Cxenon -108°Cradon -62°C
  3. 3. boiling point increase  means stronger intermolecular attraction down groupstrong intermolecular forces  because down the group, the number of electrons increase,and the size of the atom becomes bigger. More electrons  means that the temporary dipoles are stronger Larger size of atom  more surface area to interact with other moleculesMelting and boiling pointsMelting and boiling points represent the temperature at which a change of state occurs.Heat, once absorbed as energy, contributes to the overall internal energy of the object. Oneform of this internal energy is kinetic energy; the particles begin to move faster, resulting in agreater kinetic energy.This more vigorous motion of particles is reflected by a temperature increase. The reverselogic applies as well. Energy, once released as heat, results in a decrease in the overallinternal energy of the object.1) At temperatures lower than its melting point if a solid is heated, its KE increases2) Just at the melting point, heat supplied is used to overcome the attractive forces betweenparticles, therefore temperature remains the same.3) After melting is completed, the additional heat increases the KE of the particles.
  4. 4. From the graph above, explain what the state of matter which the compound being heated isin between time D and E.It is in both liquid and gaseous phaseIt is only after point E that all of the compound has been converted to gaseous stateFlat line indicate that temperature is constantAs the compound is melting between point D and E. energy supplied is used to break bondsonlyAdditional stuffThe big energy change when water freezes is in the potential energy ofinteractions between the water molecules. In the ice, the molecules arrange totouch in a way that lowers this energy. In the liquid, the arrangement is lessregular and the energy is not lowered as much.Freezing is a change in the ordering, or structure of the molecules. An ice crystalhas less spatial symmetry (specific crystal axes are defined in space) than water(every direction is as good as every other direction). There is an energyassociated with this transition -- 80 calories per gram of ice are needed to meltice at 0C at ordinary pressure, and 80 calories per gram of water are given offduring the freezing process.http://van.physics.illinois.edu/qa/listing.php?id=1730

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