Chapter 4 - Chapter 4 Page #1 Before we can understand...

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Chapter 4 Page #1 Ÿ Before we can understand solution reactions, we need to discuss the nature of solutions in which water is the dissolving medium, or solvent. These solutions are called aqueous solutions. 4.1 Water, the Common Solvent Ÿ Liquid water consists of a collection of H2O molecules. An individual H2O molecule is “bent” or V-shaped, with an H-O-H angle of about 105 degrees. Ÿ The O-H bonds in the water molecule are covalent bonds formed by electron sharing between the oxygen and hydrogen atoms. However, the electrons of the bond are not shared equally between these atoms. Oxygen has a greater attraction for electrons does hydrogen. Ÿ Because the oxygen atom has a greater attraction for electrons, the shared electrons tend to spend more time close to the oxygen than to either of the hydrogens. Thus the oxygen atom gains a slight excess of negative charge, and the hydrogen atoms become slightly positive. Ÿ Figure 4.1 (page 92), where delta indicates a partial charge (less than one unit of charge) Ÿ Because of this unequal charge distribution, water is said to be a polar molecule. It is this polarity that gives water its great ability to dissolve compounds. Ÿ Polar molecule - a molecule that has a permanent dipole moment. Ÿ Polar covalent bond - a covalent bond in which the electrons are not shared equally because one atom attracts them more strongly than the other one Ÿ Figure 4.2 (page 92), the “positive ends” of the water molecules are attracted to the negatively charged anions and that the “negative ends” are attracted to the positively charged anions and that the “negative ends” are attracted to the positively charged cations. This process is called hydration . The hydration of its ions tends to cause a salt to “fall apart” in the water, or to dissolve. Ÿ When ionic substances (salts) dissolve in water, they break up into the individual cations and anions. For instance, when ammonium nitrate (NH4NO3) dissolves in water, the resulting solution contains NH4 + and NO3 - ions floating around independently. The symbol (aq) indicates that the ions are hydrated by unspecified numbers of water molecules. Ÿ The solubility of ionic substances in water varies greatly. Ÿ The differences in the solubilities of ionic compounds in water typically depend on the relative affinities of the ions for each other (these forces hold the solid together) and the affinities of the ions for water molecules [which cause the solid to disperse (dissolve) in water]. Ÿ The most important thing to remember at this point is that when an ionic solid does dissolve in water, the ions are dispersed and are assumed to move around independently. Ÿ Water also dissolves many nonionic substances. Ÿ
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This note was uploaded on 08/26/2011 for the course CHEM 1A 05611 taught by Professor Anderson during the Spring '09 term at UCSB.

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Chapter 4 - Chapter 4 Page #1 Before we can understand...

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