Aqueous Solutions

NaCl (s)  Na+ (aq) + Cl- (aq) Dissociation When a compound that is made of ions dissolves in water, the ions separate from one another. NaCl (s)  Na+ (aq) + Cl- (aq) CaCl2 (s)  Ca2+ (aq) + 2Cl- (aq)

Dissociation (cont.) Assuming 100% dissociation, a solution that contains 1 mol of NaCl contains 1 mol of Na+ ions and 1 mol of Cl- ions. A solution that contains 1 mol of CaCl2 contains 1 mol of Ca2+ ions and 2 mol of Cl- ions.

1 mol ammonium chloride 1 mol sodium sulfide 0.5 mol barium nitrate Practice Problem Write the equation for the dissolution of each of the following in water, and then determine the number of moles of each ion produced as well as the total number of ions produced. 1 mol ammonium chloride 1 mol sodium sulfide 0.5 mol barium nitrate

Precipitation Reactions What is a precipitate? A solid that is produced as a result of a chemical reaction in solution How can you tell if a precipitate will form when 2 different solutions are combined?

Will a precipitate form in these reactions? Potassium nitrate (aq) + magnesium sulfate (aq)  Barium chloride (aq) + sodium sulfate (aq) 

Net Ionic Equations Include only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution.

How to Write a Net Ionic Equation Step 1: Write a balanced chemical equation. Zinc(II)Nitrate (aq) plus Ammonium Sulfide produces Zinc(II)Sulfide and Ammonium Nitrate Balanced chemical equation: Zn(NO3)2 (aq) + (NH4)2S (aq)  ZnS (s) + 2NH4NO3 (aq)

Step 2: Convert the chemical equation into an overall ionic equation. How to Write … Step 2: Convert the chemical equation into an overall ionic equation. All soluble ionic compounds are shown as dissociated ions in solution. Precipitates are shown as solids. Overall ionic equation: Zn2+(aq) + 2NO3-(aq) + 2NH4+(aq) + S2-(aq) ZnS (s) + 2NH4+ (aq) + 2NO3- (aq)

How to Write … Step 3: To convert an ionic equation into a net ionic equation, the spectator ions are cancelled on both sides of the equation. Spectator ions: ions that do not take part in a chemical reaction and are found in solution both before and after the reaction.

Zn2+ (aq) + S2- (aq)  ZnS (s) How to Write … Overall ionic equation: Zn2+ (aq) + 2NO3- (aq) + 2NH4+ (aq) + S2- (aq)  ZnS (s) + 2NH4+ (aq) + 2NO3- (aq) Net ionic equation: Zn2+ (aq) + S2- (aq)  ZnS (s)

Practice Problems Identify the spectator ions in the reaction between KCl and AgNO3 in an aqueous solution.

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Write the balanced chemical equation, overall ionic equation, and net ionic equation for the following reactions: Mercury(II) chloride (aq) + potassium sulfide (aq)  Sodium carbonate (aq) + calcium chloride (aq) 

Write the net ionic equation for the reaction of lead(II) nitrate with hydrochloric acid. Write the net ionic equation for the precipitation reaction of aqueous potassium sulfate and aqueous barium nitrate.

Ionization The formation of ions from solute molecules by the action of the solvent. Different from the term dissociation. When a molecular compound dissolves and ionizes in a polar solvent, ions are formed where none existed in the undissolved compound.

Hydronium Ion H2O(l) + HCl(l)  H30+ (aq) + Cl- (aq) The H30+ ion is known as the hydronium ion. Hydration of H+ to form H30+ is a highly exothermic reaction. Energy released makes large contribution to the energy needed to ionize a molecular solute.

Sources Davis, Raymond, et al. Modern Chemistry. Austin: Holt, Rineheart, and Winston, 2002. Smoot, Robert C., Richard G. Smith, and Jack Price. Merrill Chemistry. Lake Forest: Glencoe, 1993.