Chemistry 141 Friday, September 29, 2017 Lecture 11 Solution Chemistry

Objectives for today Begin to gain chemical intuition about reactions in solution Learn the terminology we use to describe solutions Perform stoichiometric calculations using solution concentrations and volumes Understand the factors that determine solubility and solution conductivity (electrolytes) Predict whether a precipitate will form in a reaction

The balanced reaction for the combustion of methanol is:  2 CH3OH + 3 O2 → 2 CO2 +  4 H2O  If 0.098 mol of O2 reacts with excess CH3OH, what is the theoretical yield of water vapor from this reaction (in g) if the formula weight of water is 18.02 g/mol?

If the actual (experimental) yield of water was 1 If the actual (experimental) yield of water was 1.9 grams, what was this reaction's percent yield?  (Theoretical yield = 2.4 g)

Chemistry in solution Many reactions take place in solution (i.e. homogeneous mixtures usually in the liquid phase), particularly aqueous solution (dissolved in water) Solution terminology: solution: mixture of solute and solvent solute: substance dissolved in liquid solvent: liquid itself (most abundant) solvent does not have to be water (and a solution does not have to be liquid!) Examples: coffee, salt water, vinegar, ethanol/n-propanol, fat-soluble vitamins (such as Vitamin A) in body fat

Units of concentration Molarity [M], moles solute per liter of solution Molality: moles solute per kg of solvent (rarely used!) Parts per million: (usually by mass, but can be volume)

Using concentrations How would you prepare 2.00 L of a 0.50 M solution of NiCl2 from the salt NiCl26H2O?

Dilutions How much water must be added to 15.5 ml of a 7.16 M solution of H3PO4 to reduce its concentration to 1.00 M? 1. 15.5 ml of 7.16 M solution 2. dilute with water 3. final 1.00 M solution

Solubility What makes one substance soluble in another?

Water, the universal solvent

Dissolving a covalent compound Covalent (molecular) compounds: Consist of neutral atoms (usually non-metals) bonded together in discrete molecules Dissolve in ‘like’ solvents (polar ethanol molecule in polar water) Dissociate very little

Dissolving an ionic compound (salt) Consist of a lattice of + (metal) and – (non-metal) ions Dissociate in water into ions Conduct electricity (electrolyte) Cl– Na+ NaCl (s) Na+ (aq) + Cl– (aq)

Electrolytes A strong electrolyte dissociates completely when dissolved in water; solution conducts electricity (e.g. soluble salts, strong acids and bases) A weak electrolyte only dissociates partially when dissolved in water; conducts weakly (e.g., weak acids and bases) A nonelectrolyte does NOT dissociate in water

Solubility rules Compounds containing the following ions are soluble: - Li+, Na+, K+, Cs+, and NH4+ - nitrates (NO3–), chlorates (ClO3–), and acetates (C2H3O2–) - chlorides (Cl–), bromides (Br–), and iodides (I–) (except those of Ag+, Hg22+, Tl+, and Pb2+) - sulfates (SO42–) (except those of Ca2+, Ba2+, Pb2+, Ag+, and Hg22+) Compounds containing the following ions are insoluble: - sulfides (S2–) (except those of alkali metals, NH4+, Ca2+, and Ba2+) - carbonates (CO32–), phosphates (PO43–), and sulfites (SO32–) (except those of alkali metals and NH4+) - hydroxides (OH–)

Solubility trends Most compounds containing +1 and –1 ions are soluble Most compounds containing +2 and –2 or more highly charged ions are insoluble Why? The coulombic (charge) forces holding these ions together is greater +1 –1 +2 –2 more force less force

Compound types and solubility Ionic (salt) Covalent (molecule) - Made up of + and – ions - Made up of neutral atoms - Consist of metal + nonmetal - Consist of nonmetal + nonmetal - Exist as a continuous lattice of alternating + and – ions - Exist as discrete molecules - Dissociate in water - Dissociate very little in water Solutions conduct electricity (electrolytes) Solutions do not conduct electricity (non-electrolytes) Ionic Polar covalent Non-polar covalent (+2)(-2) (+1)(-1) ethanol hexane dissolves in water? NO YES increasing charge separation (use solubility rules for each case)

Precipitation What happens when aqueous solutions of KCl and AgNO3 are mixed?