Reactions in Aqueous Solution A review of types of reactions

A bit of review Solutions are made up of solvents (substance in larger quantity) and solutes (substance in smaller quantities). Aqueous solutions are those that have water as the solvent. Ionic compounds that dissolve in water are called electrolytes because the separated ions conduct electricity. The more ions a solution contains, the better it is able to conduct electricity.

Molarity and Molality Molarity is a measure of concentration. Molarity = amount in moles / Liters Molality = amount in moles / mass of solvent Overhead problems (only a few….dozen….) Dilution time!!!!!!!!

Dilutions I have to dilute stock solutions (comes with the job). If I want to make a 500 mL solution of 3.5 molar HCl, how much 12 M HCl will I need? Slow way. Derive the fast way.

Ionic Compounds in Polar Solvents For example, NaCl dissolves in water. NaCl(s)  Na+(aq) + Cl-(aq)

Compounds in water Most non-electrolyte compounds (molecular compounds) do not form ions when dissolved in water and do not conduct electricity (sugar, methanol, ethanol). Weak electrolytes (weak acids and bases) dissociate a little and weakly conduct electricity (acetic acid). Strong electrolytes completely dissociate in water and are highly conductive (strong acids and strong bases).

Like Dissolves Like Polar substances dissolve in polar solvents. Due to hydrogen bonding (remember this??) Non-polar substances dissolve in non-polar solvents.

Types of Reactions Single-Replacement Decompositions Precipitation Reactions Acid/Base Reactions Redox

Precipitation Reactions In precipitation reactions the attraction between oppositely charged ions is greater than the attraction between the H2O molecules and the ions. The result is the formation of a solid. Remember net ionic equations???

Balanced Chemical Equation: Pb(NO3)2(aq) + 2NaI(aq)  PbI2(s) + 2NaNO3(aq) “Complete Ionic” Equation: Pb2+(aq) + 2NO3-(aq) + 2Na+(aq)+ 2I-(aq)  PbI2(s) + 2Na+(aq) + 2NO3- (aq) Cancel the “spectator ions” that appear on both sides of the arrow “Net Ionic” Equation: Pb2+(aq) + 2I-(aq)  PbI2(s)

Solubility Chart 1. All compounds containing alkali metal cations and the ammonium ion are soluble. 2. All compounds containing NO3-, ClO4-, ClO3-, and C2H3O2 anions are soluble. 3. All chlorides, bromides, and iodides are soluble except those containing Ag+, Pb2+, or Hg2+. 4. All sulfates are soluble except those containing Hg2+, Pb2+, Sr2+, Ca2+, or Ba2+.

Cont’d 5. All hydroxides are insoluble except compounds of the alkali metals, Ca2+, Sr2+, and Ba2+. 6. All compounds containing PO4- , S2- , CO32-, and SO32- ions are insoluble except those that also contain alkali metals or NH4+ . Examples

Common Precipitates Precipitate Formed Color PbI2 Bright Yellow CdS Dark Yellow PbS Black Ag2S Ni(OH)2 Green Al(OH)3 White PbSO4 BaSO4 Ca3(PO4)2 AgCl Copper (II) hydroxide Blue

Get Ready For This!!

Reduction – Oxidation Reactions In these reactions electrons are transferred between elements. One substance is oxidized and loses electrons (also called the reducing agent). The other substance is reduced and gains electrons (also called the oxidizing agent). LEO goes GER

Example A piece of iron is immersed in a solution of copper (II) sulfate. Draw what this experiment will look like.

What is happening? Fe(s) + Cu2+(aq)  Fe2+(aq) + Cu(s) Example Cont’d Reduction Cu2+ is the oxidizing agent Fe(s) + Cu2+(aq)  Fe2+(aq) + Cu(s) Fe is the reducing agent Oxidation What is happening?

Rules for oxidation numbers See if you can recognize the rule Each substance is shown with its oxidation number. Mg(s) = 0, Fe(s) = 0, H2(g) = 0, O2(g) = 0 What is the rule?? Elements in their elemental forms have an oxidation state of 0.

How about this one See if you can guess the rule!!! Mg2+ = +2, Cl- = -1, Li+ = +1 For monatomic elements, their oxidation state is equal to their charge.

Got to give you this one Oxygen is -2 when in compounds (such as H2O). There is an exception When oxygen is a peroxide (H2O2) it has a -1 oxidation state.

Hydrogen Hydrogen has an oxidation state of +1 when bound to chlorine (HCl). Hydrogen has an oxidation state of -1 when it is bound to magnesium (MgH2). So what is the rule for hydrogen?

Okay, another freebie For compounds that do not contain Hydrogen or Oxygen, the most electronegative element has an oxidation state equal to its charge. BF3 (fluorine has a charge of -1 and an oxidation state of -1).

This is a good one that you already know. The sum of the oxidation states of all elements in a given compound must equal………can you think of it……. …..Zero!!!!! If the compound is a polyatomic ion, the sum of oxidation states must equal the charge on the ion. Example

Practice Find the oxidation numbers for the elements in PCl5.

How can you tell what gets oxidized and what gets reduced? 2Ca + O2  2CaO OS = +2 OS = -2 OS = 0 OS = 0 The answer is oxidation numbers

Practice KClO3 + 3 HNO2  KCl + 3 HNO3