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Aqueous Reactions Study Guide

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Updated on Sep 24, 2011

Introduction

Water is the most abundant (and important, besides oxygen) substance on Earth. It is found in large amounts in cells and blood. Water is an excellent solvent and has a high boiling point, high surface tension, high heat vaporization, and low vapor pressure. Three key types of reactions occur in water: precipitation, acid-base, and oxidation reduction.

Water: The Common Solvent

Water is the common solvent for many reasons. First, the O-H bonds in H2O are highly polar, and water forms networks of hydrogen bonds between hydrogen and oxygen atoms of different molecules. The polarity of water is designated by the partial positive (δ+) hydrogen and partial negative (δ–) oxygen in Figure 6.1. This polarity allows ions to be soluble and stable in a solution, a requirement for biological activity and when carrying ions through the body. When a substance is dissolved in water, an aqueous solution is formed. An aqueous solution is a homogeneous mixture of a substance with water as the solvent.

Figure 6.1 Water

Figure 6.1 Water

One key property of a solution is its electrical conductivity or ability to conduct electricity. When a substance, a solute, is dissolved is water, a solvent, ions may or may not be formed. A strong electrolyte is formed when the solute completely ionizes (the substance completely separates into ions), such as sodium chloride (a soluble salt), hydrochloric acid (strong acid), or sodium hydroxide (strong base). A weak electrolyte is formed when the solute partially ionizes, such as acetic acid (weak acid) or ammonia (weak base). A nonelectrolyte is a substance that dissolves in water but does not ionize, such as sugar or alcohol. Most soluble, nonacid organic molecules are nonelectrolytes.

How does a chemist know whether a solute will be a strong or weak electrolyte? Strong acids and bases are strong electrolytes, and weak acids and bases are weak electrolytes. Soluble salts are also strong acids. Table 6.1 can determine when a substance is soluble.

Table 6.1 Solubility Rules for Aqueous Solutions

Table 6.1 Solubility Rules for Aqueous Solutions

 

What happens if solid lead (II) nitrate, Pb(NO3)2, is placed in water? According to the solubility table, nitrate compounds are soluble with no exceptions, so lead (II) nitrate would separate into its ions Pb2+ and NO3.

What happens if solid silver bromide is placed in water? The solubility table states that bromide compounds are soluble with the exception of silver, lead (II), and mercury (I) ions. Therefore, the silver bromide would be insoluble and remain a solid.

Precipitation Reactions

Table 6.1 can also be used to determine the outcome of precipitation reactions. A precipitation reaction occurs when two soluble compounds are mixed and it produces one or more insoluble compounds.

Table 6.1 Solubility Rules for Aqueous Solutions

Table 6.1 Solubility Rules for Aqueous Solutions

Example:

Write the net ionic equation for the reaction of lead (II) nitrate and sodium chloride.

Solution:

Write the molecular equation. The molecular equation shows the reactants and products as molecules. The solubility rules are used to determine if a product is insoluble. Precipitation reactions are examples of a double-displacement reaction. Therefore, the cation of the first molecule (Pb2+) combines with the anion of the second molecule (Cl) to produce the first product. The cation of the second molecule (Na+) combines with the anion of the first molecule (NO3) to produce the second product.

Pb(NO3)2(aq) + NaCl(aq) → PbCl2(s) + NaNO3(aq)

The molecular equation can be balanced after the products are written:

Pb(NO3)2(aq) + 2NaCl(aq) → PbCl2(s) + 2NaNO3(aq)

Next, the ionic equation is written. The ionic equations show the strong electrolytes (soluble compounds as predicted by the solubility rules) as ions. Of course, solid compounds are not separated.

Finally, spectator ions are removed (canceled out) to reveal the net ionic equations. Spectator ions are ions not involved in the reaction. The net ionic equation shows only the species that are directly involved in the reaction (i.e., the spectator ions are not included).

Pb2+(aq) + 2Cl(aq) → PbCl2(s)

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