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Unformatted text preview: CH116 - General Chemistry 2 - Solutions to HW Problems - Chapter 10 HW: Chapter 10: Problems 36, 38, 40, 42, 44 , 46, 50, 58, 66, 68, 74, 76, 82, 84, 86, 88, 90 The IMFs (intermolecular forces) to consider can be divided into the following categories (see below). (i) Ionic: Metal salts are often ionic in character. Metals form cations in compounds and combine with ions to make ionic compounds. When a compound is ionic, ionic forces (opposite charges attract) are the only types of IMFs present. Covalently bonded compounds are formed by sharing one or more electron pairs between bonding atoms. Sharing leads to multiple types of IMFs, based on the nature of the bond and the type of sharing (equal or unequal, the latter forms polar bonds). (ii) When electrons are shared equally (true covalent bond), only fleeting dipoles are created which rapidly switch between either atom in the bond, creating momentary dipoles that help hold molecules together. These are called London dispersion forces. Not all molecules with polar bonds are polar. When bond dipoles cancel, the end result is a nonpolar molecule and these are bonded together, once again by London dispersion forces. (iii) When the presence of polar bonds results in the molecule being polar overall, the most predominant form of intermolecular attraction are the dipole-dipole forces. The molecule may also contain some true covalent bonds, and at these parts of the molecule the IMFs continue to be the London dispersion forces. (iv) When ionic compounds are dissolved in water, they dissociate (does not matter whether dissociation is complete or incomplete). The ions formed in solution are attracted to the opposite poles of the water molecule (which is a polar compound) and these forces are referred to as ion-dipole attractions. (Note: Solids cannot have ion dipole attractions unless you have a homogenous mixture (like a melt) of an ionic and a polar compound. (v) Hydrogen bonding can be exhibit by ceratin molecules. When a highly electronegative atom such as F, O or N is bonded to a hydrogen, a strong dipole is formed. This coupled with the lone pairs present on the N, O and F atoms has a tremendous potential for attracting the positive hydrogen in the dipole of the next molecule. Hydrogen bonds tend to be stronger forces than dipole dipole attractions between polar molecules, and explain the reason why water is a liquid at room temperature and has a rather high boiling point in comparison to several other, larger molecules. Note that whenever molecules are formed by electron sharing between atoms, London forces will always be present, even though the primary attarction comes from dipole-dipole interactions in molecules that are polar. In many instances, Lewis structures have to be drawn that indicate the presence of lone pair(s) and affect the shape of the molecule. A summary of molecule types, shapes and polarities is given below:...
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