Precipitates • A precipitate is the solid substance that separates from solution • Precipitates can also form from reactions • Reactions that produce a precipitate are called precipitation reactions • Most ionic compounds dissolve in water • Solutes that produce ions in solution are called electrolytes because their solutions can conduct electricity
Electrolytes: An ionic compounds dissociates as it dissolves in water Ions separate from the solid and become hydrated or surrounded by water molecules. The ions move freely and the solution is able to conduct electricity. Ionic compounds that dissociate completely are strong electrolytes
Nonelectrolytes: • Most solutions of molecular compounds do not conduct electricity and are called nonelectrolytes The molecules of a nonelectrolyte separate but stay intact. The solution is nonconducting because no ions are generated. Some ionic compounds have low solubilities in water but are still strong electrolytes because what does dissolve is 100% dissociated.
Electrolytes: • The dissociation of ionic compounds may be described with chemical equations • The hydrated ions, with the symbol (aq), have been written separately • Since physical states are often omitted, you might encounter the equation as:
Know your Solubility Rules! • Ionic compounds often react when their aqueous solutions combine When a solution of Pb(NO3)2 is mixed with a solution of KI the yellow precipitate PbI2 rapidly forms.
Know your Solubility Rules! • A precipitation reaction may be represented with a molecular, ionic, or net ionic equation: Molecular: Ionic: Net Ionic: • The most compact notation is the net ionic equation which eliminates all the non-reacting spectator ions from the equation
Types of chemical equations • Here is a typical molecular equation: Cd(NO3)2(aq) + Na2S(aq) CdS(s) + 2NaNO3(aq) • We can write this as an ionic equation (all compounds that are (aq) are written as ions): Cd2+(aq) + 2NO3–(aq) + 2Na+(aq) + S2–(aq) CdS(s) + 2Na+(aq) + 2NO3–(aq) • To get the NET ionic equation we cancel out all terms that appear on both sides: Net: Cd2+(aq) + S2–(aq) CdS(s)
Criteria for balanced net ionic equations: • Material balance – the same number of each type of atom on each side of the arrow • Electrical balance – the net electrical charge on the left side of the arrow must equal the net electrical charge on the right side of the arrow Remember that the charge on an ion must be included when it is not in a compound. Adding the charges on all the ions on one side of the arrow gives the net electrical charge.
NaOH(aq) + MgCl2(aq) Mg(OH)2(s) + NaCl(aq) Next, balance the equation 2 2 First write the skeleton equation Second write the Ionic equation: 2Na+(aq) + 2OH-(aq) + Mg2+(aq) + 2Cl-(aq) Mg(OH)2(s) + 2Na+(aq) + 2Cl-(aq) Third cancel the spectator ions and write the Net ionic equation: 2OH-(aq) + Mg2+(aq) Mg(OH)2(s)
Practice Equations Write balanced net ionic equations: • CuSO4(aq) + BaCl2(aq) • Fe(NO3)3(aq) + LiOH(aq) • Na3PO4(aq) + CaCl2(aq) • Na2S(aq) + AgC2H3O2(aq)
Net Ionic Equation Solutions Net: SO42–(aq) + Ba2+(aq) BaSO4(s) Net: Fe3+(aq) + 3OH–(aq) Fe(OH)3(s) Net: 2PO43–(aq) + 3Ca2+(aq) Ca3(PO4)2(s) Net: S2–(aq) + 2Ag+(aq) Ag2S(s)