chem. 1b – 11/12 lecture. announcements i mastering chemistry –chapter 18 assignment is due...

Chem. 1B – 11/12 Lecture

Announcements I

• Mastering Chemistry– Chapter 18 Assignment is due 11/17

• Lab– Experiment 9 Report due next week– Quiz on Electrochemistry

• Today’s Lecture– Electrochemistry (Ch. 18)

• Electrolytic Cells + Corrosion

Announcements II

• Today’s Lecture – cont.– Transition Elements (Ch. 24)

• Absorption of visible light• Electron configuration and charactestics of

transition metals• Coordination Compounds (if time)

Chapter 18 ElectrochemistryElectrolytic Cells

• Differences with Voltaic Cells– Uses External voltage to drive

unfavorable reaction– Charge at electrodes is reversed

anode (note: oxidation driven by voltage, but now + charge)

cathode (reduction, - charge)

Power Supply

+

-


• Example Reactions1. Electrolysis of water (opposite of fuel

cell example)• Anode: H2O – oxygen is oxidized to O2(g)

• Cathode: H2O – hydrogen is reduced to H2(g)

2. Industrial Use – Electroplating (Chrome, nickel, silver plating possible) – using external potential to deposit metal to electrode


• Example Reactions3. Electrolysis of Mixtures – e.g. analysis

• External potential will work on easiest to oxidize/reduce pair

• For example, if we have a mixture of NaI and NaCl in water, electrolysis will cause the following reactions:

– Na+(aq) + e- ↔ Na(s) Eº = -2.71 V

– H2O(l) + 2e- ↔ H2(g) + 2OH-(aq) Eº = -0.83 V

– 2Cl-(aq) ↔ Cl2(g) + 2e- Eº = +1.36 V

– 2I-(aq) ↔ I2(aq) + 2e- Eº = +0.54 V

– 2H2O(l) ↔ O2(g) + 4H+(aq) + 4e- Eº = 1.23 V

Chapter 18 ElectrochemistryElectrolytic Cells - Questions

1. Which of the following changes in switching from a voltaic to an electrolytic cell?

a) Charge on anode/cathodeb) Which electrode (e.g. anode) does

oxidation/reductionc) Ion migration to electrode

2. An anode in an electrolytic cell is used to measure oxalate (Eº = -0.49V) in the presence of pyruvate (Eº = -0.70V). Can this be done? What if one is interested in pyruvate?

Chapter 18 ElectrochemistryCorrosion

• Most metals are more stable as oxides, so oxidation of metals is common

• One would think that oxidation is primarily dependent upon Eº values, but oxides like Al2O3 can protect further oxidation of Al metal

• Iron in particular is prone to rusting• Galvanized metal uses a more readily

oxidized metal (e.g. Zn) to protect iron

Chapter 18 ElectrochemistryCorrosion - Questions

• Using the table below, which metals can be added as a sacrificial agent to prevent iron oxidation?

Reaction Eº (V)

Ag+(aq) + e- ↔ Ag(s) +0.799

Cu2+(aq) + 2e- ↔ Cu(s) +0.337

Co2+(aq) + 2e- ↔ Co(s) -0.277

Fe2+(aq) + 2e- ↔ Fe(s) -0.45

Cr3+(aq) + 3e- ↔ Cr(s) -0.73

Mn2+(aq) + 2e- ↔ Mn(s) -1.18

Chapter 24 Transition Metals

• Overview– Compared with the main group elements,

differences in transition metals are smaller– Variation is in how full d-shell orbitals are– Much of the interesting chemistry is from

Coordination Compounds (metal – ligand complexes)

– Focus will be on types of compounds and their relationship to the electron configurations


• Color– A variety of compounds are colored

because they absorb visible light– Most organic compounds have strong

bonds and a large energy gap between ground and excited states

– Transition metals, in coordination complexes, tend to have weaker bonds and smaller energy gaps, so that they often absorb visible light


• Color – Cobalt Chloride Compounds– In Quantitative Analysis Lab, we analyze an

aqueous mixture of Co2+ and Cr3+

– Students tend to think that Co2+ is the blue solution (it is the red/purple solution)

– Why? Co in inorganic compounds (anhydrous CoCl2, CoO) is blue, but in a coordination complex with water it turns red/purple

– This is the basis for indicator Drierite (show samples)


• Properties– D-Block Elements (show on periodic table)– Electron Configuration

• nS and (n-1)d shells are similar in energy (depends on several factors)

• transition metals start on the 4th row because only 3rd row (n = 3) capable of having d shell

• Filling goes 4s → 3d → 4p (with a few exceptions) or 5s → 4d → 5p (for 5th row)

• Filling for 6th row is more complicated: 6s → 4f (lanthanides) → 5d → 6p


• Properties – cont.– Filling exceptions – 1st row

• Cr (4s13d5 instead of 4s23d4) and Cu (4s13d10 instead of 4s23d9) due to extra stability of half- and completely-filled d orbitals

– Electron Configuration – for ions• electron removal in oxidation is different: first

lost are ns electrons and then (n-1)d electrons• reason is because outside a cation, (n-1) d

electrons are more strongly attracted to the nucleus than the ns electrons


• Properties – cont.– Size

• decreases slightly across a row• so right hand transition metals (e.g. silver) are

more dense than left hand metals (titanium)

– Oxidation State• All elements but Cu column will lose 2 ns

electrons (Cu column is stabilized in +1 state due to full d orbital)

• Left hand side elements tend to lose additional d orbitals (up to complete emptying)


• Questions1. Give the electron configurations for:

V, Fe, Ni, Cu, Fe3+ and Ni2+

2. Explain why Fe3+ is a stable ion while Mn3+ is not very stable.

3. Why are only the elements Cu and Ag able to form stable +1 oxidation states?

4. What is the maximum oxidation state expected for V?


• Coordination Complex– Covered previously (to some degree as

complex ions) in Chapter 16– Coordination complexes consist of:

• metal ion (typically same charge as will exist in water although stability of different oxidation states – such as Fe2+ vs. Fe3+ can change)

• ligand(s)• counter ions (not part of complex, but

associated with complex ion)


• Coordination Complex – cont.– Both the metal (covered more later in

the chapter) and ligand affect the type of coordination complex formed

– Types of ligands:• monodentate (one metal – ligand bond per

ligand)• bidentate (two metal – ligand bonds per

ligand – so requires to parts of ligand capable of acting as Lewis bases)


• Coordination Complex – cont.– Examples: Ag(NH3)2

+ = [Ag(NH3)2]+

– Ni(C2O4)2-

H3N-Ag-NH3 linear structure – uncharged monodentate ligand

O-

O

O-

O

O

O-

O

O-

Ni2+oxalate is a bidentate

ligand and forms a “square planar” complex (view from above)

chem. 1b – 11/12 lecture. announcements i mastering chemistry –chapter 18 assignment is due...

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