chap4 - Chapter 4 - Aqueous Reactions and Solution...

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Chapter 4 - Aqueous Reactions and Solution Stoichiometry Most reactions occur in liquid solutions and in nature that solvent is almost always water. Water itself is fascinating. It melts and boils at much higher temperatures than one would expect. It becomes less dense when it freezes (a very rare property) which is important because if ice were more dense than water, northern lakes would completely freeze in winter and only very basic life forms, such as microorganisms, would survive. These properties (among others) largely allow for life on Earth and will be covered in greater depth in Chapter 11. 4.1 General Properties of Aqueous Solutions Recall from Chapter 1 that a solution is a homogeneous mixture at both the macroscopic and microscopic levels. A solvent is the liquid present in largest quantity in a liquid solution. All other materials are dissolved in the solvent and are called solutes . In aqueous solution the solvent is water. Recall that aqueous means “dissolved in water.” Most people are surprised to find that pure water actually conducts electricity poorly. Add sugar and it still doesn’t conduct. Add a little salt and the conductivity increases dramatically. Why? We now examine the two basic things that can happen when a substance dissolves in water. In the case of sugar, the molecules remain intact. However, the ions in sodium chloride separate in water and help the electrons move through the solution. When an ionic compound breaks up into ions in solution the process is called dissociation . When a molecular compound breaks up into ions the process is called ionization . These terms mean different things; you should know the difference. All compounds that generate ions in solution are called electrolytes . Substances that do not generate ions are called nonelectrolytes . Molecular compounds such as acetic acid (CH 3 COOH) and hydrogen chloride (HCl) break up into ions when dissolved in water. Hydrogen chloride behaves much like ionic compounds in
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2 that it breaks up completely into ions. i.e. It completely ionizes. In contrast, only some acetic acid molecules break up into ions. Thus, acetic acid only partially ionizes. CH 3 COOH CH 3 COO - + H + The double arrow means “in equilibrium with.” Here, the reaction moves in both directions simultaneously and the amount of each species doesn’t change over time. The total concentration of each species remains constant. You’ll spend a lot of time in CHM 212 learning about this concept. Electrolytes can be divided into 2 categories. Strong electrolytes dissociate or ionize completely (100%) when dissolved in water. All ionic compounds (e.g. NaCl, Ca(NO 3 ) 2 ) and some molecular compounds (e.g. HCl, H 2 SO 4 ) are strong electrolytes. Weak electrolytes ionize less than 100% when dissolved in water. At first, this may seem like a poor definition. After all, wouldn’t it be hard to differentiate between 99% and 100%? It turns out that most weak
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This note was uploaded on 02/12/2012 for the course CHM 211 taught by Professor Staff during the Fall '08 term at Marshall.

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chap4 - Chapter 4 - Aqueous Reactions and Solution...

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