4.5-4.7 Electrolytes, Precipitation Reactions, and Aqueous Reaction

The Process of Solvation

• The reason that certain substances dissolve in water, is due to the fact that the sovent-solute attraction of certain mixtures, overcome the solute-solute attraction.

• For example, when placing NaCl(s) in H2O(l), the attraction between water and the ions that make up table salt, is stronger than the attraction between the solute ions.

Electrolyte and Nonelectrolyte Solutions

• When a substance dissolves, either ions separate is the solvent (conducts electricity) or molecules separate to weak intermolecular forces (IMF’s) (does not conduct electricity).

• Ionic compounds that dissolve in water, and acids and bases whose ions dissociate, are considered electrolytes.– Strong electrolytes are made up of compounds whose ions completely

dissociate in solution. If these compounds are acids or bases, they are considered strong.

– Weak electrolytes are compounds whose ions do not completely dissociate into solution. If these compounds were acids or bases, they would be considered weak.

• Other molecular compounds are non-electrolytes.

The Solubility Rules of Ionic Compounds

• 2014 AP Exam Revision: Students no longer have to memorize solubility rules, however, you still need to be able to apply them.

• All sodium, potassium, ammonium and nitrate salts are soluble in water. college board.

Use the following Solubility Rules in the Regents Reference Table to Answer the Questions that Follow.

Predict whether each compound is soluble or insoluble:

a.) NiSb.) Mg3(PO4)2

c.) Li2CO3

d.) NH4Cl

a.) insolubleb.) insolublec.) solubled.) soluble

“Insoluble” compounds are still somewhat soluble, just to a much smaller magnitude. So “insoluble” ionic compounds may still be considered very weak electrolytes.

Precipitation Reactions

• When two solutions are mixed, a solid forms, this is known as a precipitation reaction.

• Solubility rules can be used to predict precipitates. Only insoluble compounds form precipitates.

• When two solutions are mixed and no precipitate forms, a reaction has not occurred.

Sample Problem # 1

Write an equation for the precipitate reaction that occurs (if any) when solutions of ammonium chloride and iron(III) nitrate are mixed.

NH4Cl(aq) + Fe(NO3)3(aq) No reaction occurs!

Sample Problem # 2

Write an equation for the precipitation reaction that occurs (if any) when solutions of sodium hydroxide and copper(II) bromide are mixed.

2NaOH(aq) + CuBr2(aq) 2NaBr(aq) + Cu(OH)2(s)

Cu(OH)2(s) = precipitate!

Representing Aqueous SolutionsTypes of chemical equations:

Molecular equation: showing complete neutral formulas.

2NaOH(aq) + CuBr2(aq) 2NaBr(aq) + Cu(OH)2(s)

Complete ionic equation: showing all of the species as they exist in solution.

2Na+(aq) + 2OH-(aq) + Cu2+(aq) + 2Br-(aq) 2Na+(aq) + 2Br-(aq) + Cu(OH)2(s)

Ions that show up on both sides of the equation are called: spectator ions.When writing the net ionic equation, we ignore these.

Net ionic equation: showing only the species that actually change during the reaction.

Cu2+(aq) + 2OH-(aq) Cu(OH)2(s)

Sample Problem

Consider the following reaction in aqueous solution: 2HI(aq) + Ba(OH)2(aq) 2H2O(l) + BaI2(aq)

Write the complete ionic and net ionic equation for this reaction.

2H+(aq) + 2I-(aq) + Ba2+(aq) + 2OH-(aq) 2H2O(l) + Ba2+(aq) + 2I-(aq)

2H+(aq) + 2OH-(aq) 2H2O(l)

Chapter 4 pgs. 188-189 #’s 72, 74, 76, 78, 80 & 82 (just the a’s)Read 4.8-49 pgs. 168-182