1. AND SOLUTION STOICHIOMETRY
AQUEOUS REACTIONS
AND
SOLUTION STOICHIOMETRY

2. GENERAL PROPERTIES OF AQUEOUS SOLUTIONS
A solution in which water is the dissolving medium is called an aqueous solution.
A solution is a homogeneous mixture of two or more substances.
The substance in the greatest quantity is called the solvent.
The other substances in the solution are called solutes.

3. Electrolytic Properties
Sugar and salt are both soluble in water, but solutions of each are very different.
One way in which they may be different is in electrical conductivity.
The salt solution is a good conductor of electricity and the sugar solution is not.
The presence of ions in aqueous solutions can make them good conductors.
Ions carry electrical current from one electrode to another, completing an electrical circuit.

4. Electrolytic Properties (cont’d.)
The conductivity of a solution can indicate the presence of ions in the solution.
The lack of conductivity of a solution can indicate the lack of ions in solution.
When NaCl dissolves in water, the solution contains Na+ and Cl- ions, surrounded by water molecules.
A substance (such as NaCl) whose aqueous solutions contain ions is called an electrolyte.
NaCl is an ionic compound and sugar is molecular. Even though sugar dissolves in water it does not produce ions.

5. Ionic Compounds in Water
Ionic compounds dissociate in water into their component ions as it dissolves.
As an ionic compound dissolves, the ions become surrounded by the polar water molecules. The ions are said to be solvated.
The solvation process stabilizes the ions in solution and prevents the cations and anions from recombining.

6. Molecular Compounds in Water
Molecules dissolved into water usually remain intact (whole), but dispersed throughout the solution.
Most molecular compounds are non-electrolytes.
A few molecular compounds have aqueous solutions that contain ions.
The most important of these solutions contain acids. For example, when HCl dissolves in water to form hydrochloric acid, it ionizes or dissociates into H+ and Cl- ions.

7. Strong and Weak Electrolytes
The two categories of electrolytes differ in their ability to conduct electricity.
Strong electrolytes are solutes that exist in solution completely as ions.
Essentially all soluble ionic compounds (such as NaCl) and a few molecular compounds (such as HCl) are strong electrolytes.
Weak electrolytes (such as acetic acid) are solutes that exist in solution mostly as molecules with only a small fraction in the form of ions.
When a weak electrolyte such as acetic acid ionizes in solution, the reaction is written as follows:
CH3COOH(aq) CH3COO-(aq) + H+(aq)

8. Strong and Weak Electrolytes (cont’d)
Because HCl is a strong electrolyte, the equation for the ionization of HCl is written as follows:
HCl(aq) H+(aq) + Cl-(aq)
The absence of a reverse arrow indicates that the H+ and Cl- ions have no tendency to recombine in water to form HCl molecules.
For now, remember that soluble ionic compounds (composed of metals and nonmetals) or compounds containing the ammonium ion (NH4+) are strong electrolytes.
(see sample exercise 4.1)

9. Precipitation Reactions
Reactions that result in the formation of an insoluble product are called precipitation reactions.
A precipitate is an insoluble solid formed by a reaction in solution.
Precipitation reactions occur when certain pairs of oppositely charged ions attract each other so strongly that they form an insoluble ionic solid.
Guidelines concerning the solubilities of common ionic compounds must be followed to predict whether certain combinations of ions will form insoluble compounds. (See Table 4.1 or the handout you received in class)

10. Precipitations Rxn. (cont’d.)
To predict whether a precipitate forms when you mix aqueous solutions of two strong electrolytes, you must:
1. note the ions present in the reactants
2. consider the possible combinations of the cations and anions
3. use the solubility guidelines to determine if any of the combinations are insoluble

11. Exchange Reactions Also known as metathesis reactions
Reactions in which positive and negative ions appear to exchange partners (such as single and double replacement reactions) are called exchange reactions or metathesis reactions – Greek for “to transpose”.
Precipitate reactions conform to this pattern, as do many acid-base reactions. (see sample exercise 4.3)

12. Ionic Equations
An equation showing the complete chemical formulas of the reactants and products without showing their ionic character, is called a molecular equation.
An equation written with all the soluble strong electrolytes shown as ions, is called a complete ionic equation.
Ions that appear in identical forms among both the reactants and products of a complete ionic equation are called spectator ions. These ions are present, but play no role in the reaction.
Note: if every ion in a complete ionic equation is a spectator ion, then no reaction occurs.

13. Ionic Equations (cont’d.)
When spectator ions are omitted from the equation (they cancel out like algebraic quantities), what is left is a net ionic equation
A net ionic equation includes only the ions and molecules directly involved in the reaction.
The following steps summarize the procedure for writing net ionic equations
1. Write a balanced molecular equation for the reaction.
2. Rewrite the equation to show the ions that form in solution when each soluble strong electrolyte dissociates into its component ions. Only strong electrolytes dissolved in aqueous solution are written in ionic form.
3. Identify and cancel or omit spectator ions. (see sample exercise 4.4)