CHEMICAL RXNS IN AQUEOUS SOLNS (4.4) Chem. Rxns are driven by energetic forces. Precipitation (formation of solid is the driving force). Acid-Base neutralization (formation of water is the driving force). Oxidation-Redox (redox; transfer of electrons to reduce electrical potential is the driving force).

PRECIPITATION (4.5, 4.7) The formation of a solid (precipitate) when two aqueous solutions are mixed; we say that the product cmp is insoluble or only slightly soluble in water. Sol. salt I (aq) + Sol. salt II (aq)  Solid cmp (s) Table 4.1 Solubility Rules for Salts in Water (memorize) Ion interchange or metathesis (switch cation/anion partners)

T4.1 SOLUBILITY RULES 1.Most nitrates are soluble 2.Most salts with Grp 1A ions and NH 4 + are soluble. 3.Most salts with Cl -, Br -, I - are soluble EXCEPT those with Ag +, Pb 2+, Hg Most sulfates are soluble EXCEPT those with Ba 2+, Pb 2+, Hg 2 2+, Ca Most hydroxides are slightly soluble EXCEPT the strong bases. 6.Most sulfides, carbonates, chromates and phosphates are slightly soluble.

CHEMICAL EQUATION (4.6) Identify reactants, products, states of matter [g, s, aq, ℓ]. Balance equation to conserve mass. Calculate quantitative or stoichiometric relationships between rxn participants (R or P) based on balanced chemical rxn.

CHEMICAL EQUATION (2) Formula Equation: write all reactants and products as “neutral molecules”, show state of each. Complete Ionic Equation: write strong electrolytes as ions (aq). Net Ionic Equation: cancel out spectator ions.

SOLUTION STOIOCHIOMETRY Typical stoichiometric calculation for reactions taking place in aq soln. 1.Write balanced net ionic eqn to identify the chem reaction 2.Calculate mols of known A from V A and M A 3.Calculate mols of unknown B, then V B 4. V A, M A  #mol A  #mol B  V B if M A and M B are known 5.Determine LR after Step 1 if appropriate.

STOICHIOMETRIC PROBLEMS Prob 48, 50

ACID + BASE RXNS Acids donate protons, i.e. provide H + (aq) or H 3 O + (aq, hydronium) ions in water (Arrhenius). Polyprotic acids: sulfuric, phosphoric. Bases accept protons.

ACID + BASE RXN: NEUTRALIZATION Acid + Base → Salt + Water SA + SB: HCl (aq) + NaOH(aq) → NaCl(aq) + H 2 O(ℓ) –Net ionic:H + (aq) + OH - (aq) → H 2 O(ℓ) WA + SB: HF(aq) + KOH(aq) → KF(aq) + H 2 O(l) –Net ionic: HF(aq) + OH - (aq) → F - (aq) H 2 O(ℓ) SA + WB: Problem 4.57c

ACID-BASE TITRATION (volumetric analysis) Exptal technique for determining quantity of an unknown substance (analyte in beaker) by reacting a measured volume of it with another reactant (titrant in buret) of known concentration. This method works when the rxn is 100% complete (reaches equivalence pt) and that there is an indicator (color change, pH) that signals the rxn completion (endpoint).

NEUTRALIZATION TITRATION Write the balanced acid (assume to be analyte) + base (use strong base) rxn for the titration. Use an indicator (e.g. phenolphthalein) that signals the equivalence point. The molarity and volume of the titrant (SB) must be known accurately. Fig 4.18

NEUTRALIZATION TITRATION Prob 4.66, 64

OXIDATION-REDUCTION REACTIONS A redox reaction involves the transfer of electrons between reactants Electrons gained by one species must equal electrons lost by another Oxidation states or numbers are assigned to atoms and they change in a redox rxn. Both oxidation and reduction must occur simultaneously. 2

OXIDATION STATES OR NUMBERS (OX#) Actual or imaginary charge on atom: single atom, atom in molecule or atom in polyatomic ion We use these to keep track of electrons ub redox rxns. We will study rules for assigning OX# and then use this information to balance redox equations

DETERMINING OX# (T4.2) OX# of an atom in an element is 0 [Fe, O 2 ] If the species is neutral, sum of OX# is 0 [NaCl, SnO 4 ] If the species is charged, sum of OX# is value of charge (NH 4 + ; SO 4 2- ) OX# of a monatomic ions is its charge: 1A atoms have OX# = +1; 2A atoms have OX# = +2; 7A atoms have OX# = -1, etc

OX# (2) In molecular (covalent) cmps O has OX# = -2; sometimes -1 (with metal) In molecular (covalent) cmps H has OX# = +1; sometimes -1 (peroxide) F always has OX# = -1; other halides can have other OX#s There are exceptions

OXIDATION If atom X in compound A loses electrons and becomes more positive (OX# increases), we say X (with charge) or A is oxidized. Also, we say that A is the reducing agent (RA) or is the electron donor.

REDUCTION If atom Y in compound B gains electrons and becomes more negative (OX# decreases), we say Y (with charge) or B is reduced. Also, we say that B is the oxidizing agent (OA) or is the electron acceptor.

ACTIVITY SERIES (Expt 7) Redox participants have varying capacities to gain or lose electrons. The Activity Series lists metal elements in order of decreasing strength as a reducing agent; ie. ability to lose electrons and undergo oxidation. A particular rxn in the list will cause the reduction of any rxn below it.

Activity Series of Metals in Aqueous Solution

INTERPRETATION OF ACTIVITY SERIES The strongest RA is at the top (Li) meaning that Li loses electrons and is oxidized. Therefore, the other metal must be reduced. When Lithium and Calcium react, 2Li(s) + Ca 2+ (aq)  2Li + (aq) + Ca(s) We say that Li displaces calcium ion from soln. Li(s) dissolves and Ca(s) forms

BALANCING REDOX EQNS Half-Rxn Method (acid) Write half chem eqn for reduction Write half chem eqn for oxidation Balance all atoms except H and O Balance O with H 2 O and H with H +

Half-Reaction Method (acid, 2) Add electrons to balance charge (I.e. show loss or gain of electrons) Balance the number of electrons between the two half-rxns by multipying by appropriate factor (i.e. #e- gained by by atom Y = #e- lost by atom X) Add two half-rxns and cancel identical species. Check for atom and charge balance

The Half-Reaction Method (Acidic Solution)

Half-Reaction Method (base) Follow steps for acidic solution Add OH - ions to cancel out the H + ions, thus forming water. Cancel out water molecules Check for atom and charge balance. Make sure there are no H + ions remaining.