Chem. 31 – 6/22 Lecture

Announcements I Laboratory Stuff –Cl Lab Report – Due today –AA Lab: I have posted information about your tap water that can make dilutions of it more successful –IC Lab: Supplemental Information - Posted (We are switching columns – new column is faster but poorer for F - ) –Spectroscopic Lab: Working on Supplemental Information (how to use new spectrometers

Announcements II Tomorrow: –Additional Problem 2.1 due –Quiz on Chapter 6 (6-2 to what is covered today) Today’s Lecture –Chapter 6 (Basic Equilibrium) Complex Ions (mostly done) Acid, Bases and Salts –Chapter 7 (Advanced Equilibrium – correcting errors in Basic Equilibrium) Introduction (normally done by demonstration – may do on Thursday

Some Questions 1.In the reaction: Ca 2+ + Y 4- ↔ CaY 2- (where Y 4- = EDTA), which species is the Lewis acid? 2.List two applications in which the formation of a complex ion would be useful for analytical chemists. 3.List two applications in the lab in which you used or are using complex ions. 4.AgCN is a sparingly soluble salt. However, a student observed that adding a little of a NaCN solution to a saturated solution of AgCN did not result in more precipitation of solid. Addition of more NaCN solution resulted in total dissolution of the AgCN. Explain what is happening.

One More Question 1.Cu 2+ reacts with thiosulfate (S 2 O 3 2- ) to form a complex which is most stable when two moles of thiosulfate to one mole of Cu 2+ are present. The  2 value is found to be 2.00 x If a solution containing both Cu 2+ and S 2 O 3 2- is prepared and found to contain 1.7 x M free (uncomplexed) S 2 O 3 2- at equilibrium, what is the ratio of complexed to free Cu? Assume that little CuS 2 O 3 forms.

Acids, Bases and Salts Definitions of Acids and Bases - Lewis Acids/Bases (defined before, most general category) - Br ø nsted-Lowry Acids/Bases: acid = proton donor base = proton acceptor (must have free electron pair so also is a Lewis base) - definitions are relative

Br ø nsted-Lowry Acids - examples HCO 2 H(aq) + H 2 O(l) ↔ HCO H 3 O + acid base conjugate conjugate base acid CH 3 NH 2 (aq) + H 2 O(l) ↔ CH 3 NH OH - base acid conjugate conjugate acid base H 2 SO 4 + CH 3 CO 2 H(l) ↔ HSO CH 3 CO 2 H 2 + acid base conjugate conjugate base acid

Br ø nsted-Lowry Acids Note: for most acids, the reaction with water is simplified: Example: HNO 2 (nitrous acid) HNO 2 ↔ H + + NO 2 -

Autoprotolysis and the pH Scale Autoprotolysis refers to proton transfer in protic solvents like water: H 2 O(l) ↔ H + + OH - K = K w = [H + ][OH - ] = 1.0 x (T = 25°C) In pure water [H + ] = [OH - ] = K w 0.5 = 1.0 x M pH = -log[H + ] = 7.0 Acidic is pH 7

Strong Acids Strong acids completely dissociate in water ( except at very high concentrations ) –HX(aq) → H + + X - (no HX(aq) exists) K a > 1 Major strong acids: HCl, HNO 3, H 2 SO 4 Note: –For H 2 SO 4, 1 st dissociation is that of a strong acid, but 2 nd dissociation is that of a weak acid (K a ~ 0.01)

Weak Acids Partially dissociate in water Most have H that can dissociate HX(aq) ↔ H + + X - (HX(aq) exists) Example: HNO 2 ↔ H + + NO 2 - Degree of dissociation given by K a value K a = [H + ][NO 2 - ]/[HNO 2 ] Metal cations can be acids through the reaction: M n+ + H 2 O(l) ↔ MOH (n-1)+ + H + (although for +1 and some +2 metals the above reactions favor reactants so strongly the metals can be considered “neutral”)

Bases Strong Bases: completely dissociate to give OH - in water –Examples: KOH (s) → K + + OH - (No KOH(aq)) Ca(OH) 2 (s) → Ca OH - Weak Bases: react partially in water to give OH - - NH 3 (aq) + H 2 O (l) ↔ NH OH - - strength of weak base given by K b for above reaction

Ionic Compounds in Water First step should be dissociation to respective ions: example: NaCl(s) → Na + + Cl - In subsequent steps, determine how anion/cation react: - anions usually only react as bases - cations may react as acids - see if ions are recognizable conjugate acids or bases - polyprotic acids are somewhat different

Ionic Compounds in Water Conjugate bases of weak acids are basic. NO H 2 O(l) ↔ HNO 2 (aq) + OH - Conjugate bases of weaker weak acids are stronger bases. K b = K w /K a CN - is a stronger base than NO 2 - because K a (HCN) = 6.2 x and K a (HNO 2 ) = 7.1 x 10 -3

Acidity of Ionic Compounds Determine if the ionic compounds are acidic or basic in the following examples: 1.NaCl 2.NH 4 Cl 3.NaCH 3 CO 2 4.Fe(NO 3 ) 3 5.NH 4 CN

Chapter 7 “Adjustments” to Equilibrium Theory There are two areas where the general chemistry equilibrium theory can give wrong results: –When the solution has high concentrations of ions –When multiple, interacting equilibria occur –I had planned a demonstration, which I can do on Thursday if time

Demonstration – Slide 1 Summary of Observation: –Two saturated solutions of MgCO 3 are prepared. –One is prepared in water and the other is prepared in ~0.1 M NaCl. –5.0 mL of each solution was transferred (and filtered) into a beaker. –3.5 mL of M HCl needed for saturated MgCO 3 and 6.0 mL needed in 0.1 M NaCl Saturated MgCO 3 Saturated MgCO 3 in NaCl(aq)

Demonstration – Slide 2 Did the moles of HCl used match expectations? and Why did the solution containing NaCl need more HCl? –First Question: How many mL of HCl were expected? MgCO 3 (s)  Mg 2+ + CO 3 2- K sp = 3.5 x T = 25°C K sp = 3.5 x = [Mg 2+ ][CO 3 2- ] since [Mg 2+ ] = [CO 3 2- ] (assuming no other reactions), [CO 3 2- ] = (3.5 x ) 0.5 = 1.87 x M n(HCl) = (2 mol HCl/mol CO 3 2- )(1.87 x mmol/mL)(5.0 mL) = mmol HCl Calculate V(HCl) = mmol HCl/[HCl] = mmol HCl/0.002 mmol/mL = mL Actual V(HCl) > 1 mL Conclusions It takes more HCl than expected, so more CO 3 2- dissolved than expected. Also, the NaCl increased the solubility of MgCO 3

Demonstration – Slide 3 What was the affect of the NaCl? –More CO 3 2- (and Mg 2+ ) was found to dissolve in the 0.10 M NaCl Why? –The Na + and Cl - ions stabilize CO 3 2- and Mg 2+ ions

Ionic Strength Effects Spheres Surrounding Ions Mg 2+ Low Ionic Strength CO 3 2- HO H  HO H HO H Ion – dipole interaction HO H HO H HO H Mg 2+ CO 3 2- HO H HO H HO H  High Ionic Strength Na + Stronger ion – ion interaction replaces ion - dipole Cl - HO H