Solubility and Ionic Equations Aqueous Solutions

Adding compounds to water Many molecular and ionic compounds dissolve in water. Ionic compounds dissolve and dissociate into their component ions. Ionize: another word for the dissociation of ionic compounds

Types of Electrolytes A strong electrolyte dissociates completely. A strong electrolyte is present in solution almost exclusively as ions. Strong electrolyte solutions are good conductors. A nonelectrolyte does not dissociate. A nonelectrolyte is present in solution almost exclusively as molecules. Nonelectrolyte solutions do not conduct electricity. A weak electrolyte dissociates partially. Weak electrolyte solutions are poor conductors. Different weak electrolytes dissociate to different extents.

Electrolytic Properties of Aqueous Solutions Electrolytes dissociate to produce ions. The more the electrolyte dissociates, the more ions it produces.

Identifying “Strong” and “Weak” Electrolytes 1. Most salts are strong electrolytes. (ionic compounds) 2. Most acids are weak electrolytes (Exceptions: the strong acids)

Identifying “Strong” and “Weak” Electrolytes (Cont.) 3. The common strong bases are the hydroxides of the alkali metals and the heavy alkaline earths. Ammonia (NH3) is a weak base. 4. Most other substances are nonelectrolytes. (Molecular compounds)

Four –Step Path to writing Ionic Equations (Precipitates) Decide what ions are present in the solutions 2. Identify the two possible precipitates formed by combining a cation from one solution with the anion from the other.

Four –Step Path to writing Ionic Equations (Precipitates, Cont.) 3. Use the solubility rules to decide whether either or both of the possible solids will precipitate. 4. Write a balanced net ionic equation for any precipitation reaction that occurs. - no reaction occurs if all of the possible products are soluble.

Opposite Charges Attract!

Driving Forces “Driving forces” pull ions out of solution because insoluble products of reactions are formed. Driving forces: Formation of a solid Formation of a gas (H2S, CO2, H2 etc.) Formation of water Formation of a weak acid or base

Formation of a Solid Double replacement reactions resulting in the formation of a precipitate that is insoluble. General Form: AX + BY AY + BX   Ex. CaCl2(aq) + Na2SO4(aq) CaSO4(s) + 2 NaCl(aq)

Acids and Bases Arrhenius Acid – a substance that produces H+ ions Arrhenius Base – a substance that produces OH- ions when dissolved in water BrØnsted-Lowrey Acid – donates protons BrØnsted-Lowrey Base – accepts protons * Acids – sour taste * Base – bitter taste, slippery feel

Chemistry Students “Play Ball”

Acid-Base Reactions Strong Acids and Bases ionize completely HNO3(aq)  H+(aq) + NO3-(aq) HCl(aq)  H+(aq) + Cl-(aq) NaOH(aq)  Na+(aq) + OH-(aq) Ionization of strong acids and bases

Conductivity Testing Strong acids and bases are strong electrolytes. When a conductivity tester is inserted in an aqueous solution of a strong acid or base, the light bulb readily lights up.

Acid-Base Neutralization Reactions HCl (aq) + NaOH(aq)  H2O(l) + NaCl (aq) HNO3 (aq) + NaOH(aq)  H2O(l) + NaNO3 (aq) Neutralization occurs when a solution of an acid and a base are mixed. Notice we form a salt (NaCl, NaNO3 ) and water. Salt = ionic compound whose cation comes from a base and anion from an acid. Neutralization between acid and metal hydroxide produces water and a salt.

Equations for weak acids and bases have “double arrows” indicating that there are very few ions in solution

Acid–Base Reactions: Net Ionic Equations HCl + NaOH  H2O + NaCl In the reaction above, the HCl, NaOH, and NaCl all are strong electrolytes and dissociate completely. The actual reaction occurs between ions. Na+ and Cl– are spectator ions. H+ + Cl– + Na+ + OH–  H2O + Na+ + Cl– H+ + OH–  H2O A net ionic equation shows the species actually involved in the reaction.

Indicators Indicators are commonly used to tell when a neutralization is complete, or if a solution is acidic or basic. Phenol red is … … and red in basic solution. … orange in neutral solution … … yellow in acidic solution …

Neutralization of a strong acid with strong base gives a salt and water: This net ionic equation applies only to strong acids and bases

Weak acids and bases Weak acids and bases only partially ionize when in aqueous solution. Conductivity tester lights up “weakly” The brightness of light is experimental verification of the classification as a strong or weak electrolyte.

Weak acids and bases are weak electrolytes because less than 100% of the molecules ionize.

Weak acids and bases are in dynamic equilibrium in solution Consider the case of acetic acid: Two opposing reactions occur in solution: the ionization of the acid, called the forward reaction, and the recombination of ions into molecules, called the reverse reaction. Chemical or dynamic equilibrium results when the rate of the forward and reverse reaction are equal.

Weak acid + Strong base The neutralization of a weak acid with a strong base involves a strong and weak electrolyte Consider the neutralization of acetic acid with NaOH:

Note that in ionic equations the formulas of weak electrolytes are written in “molecular” form

Acid-base reactions may produce a gas Sulfide and carbonate ions can react with H+ in a similar way to OH-. (OH- makes water) 2HCl (aq) + Na2S(aq)  H2S(g) + 2NaCl(aq) 2H+(aq) + S2-(aq)  H2S(g) HCl(aq) + NaHCO3(aq)  NaCl(aq) + H2O(l) + CO2(g) The reactions are driven to completion because a gas escapes and is unavailable for the reverse reaction.

(CO2 and SO2 are produced by the decomposition of H2CO3 and H2SO3, respectively)

Polyprotic Acids