LECTURE Ten CHM 151 ©slg TOPICS: 1. Ions in Aqueous Solutions: Double Replacement Reactions 2. Electrolytes 3. Precipitation Reactions

Chapter Five Double Replacement Reactions in Aqueous Solutions: AB(aq) + CD(aq)  AD + CB A) Precipitation reactions B) Acid/ Base reactions C) Gas Formation reactions NOTE: All these reactions are often represented by a balanced “net ionic equation” which we will meet as we look at these types!

“The Game Players” Generally, two aqueous solutions, each containing a 100% ionized solute, a “strong electrolyte,” are required to commence action in any of these three types of reactions. In order to appreciate what is happening, we must examine all terms... AB (aq)  A + (aq) + B - (aq) CD (aq)  C + (aq) + D - (aq)

“AQUEOUS SOLUTIONS” “AQUEOUS SOLUTION” = homogeneous mixture of some solute in a specific solvent, water HOMOGENEOUS: uniform composition throughout, one phase. Liquid solution: transparent, no boundaries, layers, bubbles or solid particles visible.

“Strong Electrolytes” Double Replacement reactions in aqueous solutions occur when both reactants are “100% ionized” in aqueous solutions. H 2 O AB > A + (aq) + B - (aq) H 2 O CD > C + (aq) + D - (aq ) The solutes described above are called “strong electrolytes”. To better understand this concept, let’s consider the following demonstration and video clip.

SOLUTES TO BE BE TESTED AS ELECTROLYTES: salt (NaCl), sugar (C 12 H 22 O 12 ), hydrochloric acid (HCl), acetic acid (vinegar, HC 2 H 3 O 2 ), ammonia (NH 3 ), alcohol (CH 3 CH 2 OH), sodium hydroxide (NaOH) We are going to see what happens to the demo light bulb when the electrodes are immersed in first pure water, and then into water containing these solutes. DEMONSTRATION!!! Compounds which form ions in water solution are considered “electrolytes” because their presence allows the solution to conduct electric current.

If there are NO ions present in solution, the liquid or solution will NOT conduct a current and the light bulb will not “light up” If the solute present in the solution is completely ionized, the solution will readily conduct a current and the light bulb will “light up brightly” If the solute is ionized to a small extent (“mostly molecular”), then the light bulb may “glow faintly”

Summary, Results:

We can sort out our results into three categories, based on our observations: Strong electrolytes: allow current to flow through solution: NaCl, HCl, NaOH, all 100% ionized in solution Weak Electrolytes: allow a small amount of current to flow through solution: HC 2 H 3 O 2 and NH 3 (aq) as “NH 4 OH”, small amount of ionic presence, mostly “molecular” in nature Non Electrolytes: No ions, no current: molecular in nature: water, alcohol (CH 3 CH 2 OH), sugar (C 12 H 22 O 12 ).

Strong Bases: Soluble Metal hydroxides: NaOH, KOH STRONG ELECTROLYTES: REACTANTS, DOUBLE REPLACEMENT REACTIONS Salts: metal or ammonium cation, monatomic or polyatomic anion: NaCl K 2 SO 4 AgNO 3 NH 4 BrO 3 CuI 2 Strong Acids: H written first in the formula, Strong: HCl, HBr, HI, H 2 SO 4, HNO 3

Salts in water: Cation, not H +, Anion, not OH -

BASES IN WATER: Cation not H + ; Anion: OH -

ACIDS IN WATER: H + Cation; Anion, not OH -

Double Replacement Reactions “go to completion” because collisions between some of the mixed anions and cations causes precipitates, molecules or gases to form, removing ions from solution. Our first type of reaction between aqueous solutions containing electrolytes involves forming a precipitate when the solutions are mixed. This type of reaction goes to completion because ions are removed from solution as an insoluble precipitate.

AB (aq) + CD (aq)  AD (s) + CB (aq) [A + (aq) + B - (aq) ] + [C + (aq) + D - (aq) ]  AD (s) + [C + (aq) + B - (aq) ] Reactions in Aqueous Solutions #1: Precipitation reactions “salt A (aq) + salt B (aq)  salt C (s) + salt D (aq) ”

NaCl(aq) + AgNO 3 (aq)  AgCl (s) + NaNO 3 (aq)

Net Ionic Equations When we finish writing and balancing a double replacement reaction, we go several steps further: After we have completed an equation representing all the ions present in water, we delete all ions present on both sides of the equation. The final result is called the “net ionic equation”. We will practice this procedure as we go through the various types of double replacement reactions.

AB (aq) + CD (aq)  AD (s) + CB (aq) [A + (aq) + B - (aq) ] + [C + (aq) + D - (aq) ]  AD (s) + [C + (aq) + B - (aq) ] A + (aq) + D - (aq)  AD (s) Total balanced equation: Total Ionic Equation: Net Ionic Equation:

NaCl(aq) + AgNO 3 (aq)  AgCl (s) + NaNO 3 (aq) Na + (aq) + Cl - (aq) + Ag + (aq) + NO 3 - (aq)  AgCl (s) + Na + (aq) + NO 3 - (aq) Ag + (aq) + Cl - (aq)  AgCl (s) Total equation: Total Ionic Equation: Net Ionic Equation:

Acids: Mostly water soluble, commercially available in water solution Salts and Bases: if both cation and anion are large in size and small in charge, ( +1,- 1), it is probably soluble in H 2 O. Checkout following tables... SOLUBILITY OF “Strong Electrolytes” IN WATER To predict when a precipitate will form, we need to know some solubility guidelines:

The Electrolyte is Usually Water Soluble if: THE CATION IS: Na + K + NH 4 + OR THE ANION IS*: Cl -, Br -, I - ClO 4 -, ClO 3 - NO 3 - SO 4 2- C 2 H 3 O 2 -

THE COMPOUND IS PROBABLY INSOLUBLE IF: THE CATION IS NOT: Na + K + NH 4 + THE ANION IS: CO 3 2- PO 4 3- S 2- O 2- OH -

There are a few notable exceptions to the solubility guide on the last slide, principally the ones noted below, which you should be aware of: Insoluble in Water: AgCl, AgBr, AgI PbCl 2, PbBr 2, PbI 2 BaSO 4, PbSO 4

SOLUBILITY WORKSHEET #1

SOLUBILITY WORKSHEET #2

Bismuth(III) Perchlorate Yes, Perchlorate Silver Bromide No, Exception Cadmium Sulfate Yes, Sulfate Zinc Nitrate, Yes, Nitrate

Nickel(II) Nitrate Yes (nitrate) Potassium Permanganate Yes (K + ) Barium Sulfate No (exception) Aluminum Oxide No (oxide) Magnesium Nitride No (Nitride)