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CHEM1612 - Pharmacy Week 7: Oxidation Numbers. Dr. Siegbert Schmid School of Chemistry, Rm 223 Phone: 9351 4196 E-mail: siegbert.schmid@sydney.edu.au. Unless otherwise stated, all images in this file have been reproduced from:

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chem1612 pharmacy week 7 oxidation numbers

CHEM1612 - PharmacyWeek 7: Oxidation Numbers

Dr. SiegbertSchmid

School of Chemistry, Rm 223

Phone: 9351 4196

E-mail: siegbert.schmid@sydney.edu.au

slide2

Unless otherwise stated, all images in this file have been reproduced from:

Blackman, Bottle, Schmid, Mocerino and Wille,Chemistry, John Wiley & Sons Australia, Ltd. 2008      ISBN: 9 78047081 0866

oxidation numbers
Oxidation numbers

Textbook: Blackman, Bottle, Schmid, Mocerino & Wille, “Chemistry”,John Wiley & Sons Australia, Ltd., 2008.

Today’s lecture is in

  • Section 4.6, 4.8
  • Section 12.1
  • Section 13.1, 13.2

Potassium atom, K

19 protons, 19 neutrons

19 electrons

oxidation numbers definition
Oxidation numbers: definition
  • Each atom in a molecule is assigned an OXIDATION NUMBER (O.N.).
  • The oxidation number is the charge the atom would have if the electrons in a bond were not shared but transferred completely to the more electronegative atom.

Electrons shared equally as both Cl atoms in Cl2 have the same electronegativity.

Oxidation number = 0.

Unequal sharing of electrons, F has higher electronegativity than H. Therefore oxidation number of H will be positive (+I), and F will be negative (-I).

oxidation numbers states
Oxidation numbers (states)
  • USE OF OXIDATION NUMBERS
    • Naming compounds
    • Properties of compounds
    • Identifying redox reactions
  • In a binary ionic compound O.N.= its ionic charge.
  • In a covalent compound O.N. ≠ a charge.
  • O.N. is written as
    • a roman numeral (I, II, III, etc.)
    • a number preceded by the sign (+2)
  • Ionic charge has the sign after the number (2+).

Figure from Silberberg, “Chemistry”, McGraw Hill, 2006.

electronegativity
Electronegativity
  • Definition: Ability of a bonded atom to attract the shared electrons.

(Different from electron affinity, which refers to the ability of an isolated atom in the gas phase to gain an electron and form a gaseous anion).

  • Electronegativity is inversely related to atomic size.
  • Atomic size: increases down group (electrons in outer shells)

decreases across period (electrons in same shell)

  • Electronegativity is directly related to ionization energy (energy required to remove an electron from atom).
electronegativity and the periodic table
Electronegativity and the Periodic Table
  • Linus Pauling defined electronegativity in arbitrary units 0.7 to 4.0
  • smallest at lower left Periodic Table - Cs cesium
  • greatest at upper right - F fluorine

Blackman Figure 5.5

rules for assigning o n
Rules for assigning O.N.
  • The oxidation number for any free element (eg. K, Al, O in O2) is zero.
  • The oxidation number for a simple, monatomic ion is equal to the charge on that ion (eg. Na+ has oxidation number +I)
  • The sum of all the oxidation numbers of the atoms in a neutral compound must equal zero (e.g. NaCl). The sum of all the oxidation numbers of all the atoms in a polyatomic ion must equal the charge on that ion (e.g. SO42-).
  • In all its compounds fluorine has oxidation number –I.
  • In most of its compounds hydrogen has oxidation number +I.
  • In most of its compounds oxygen has oxidation number -II.

Blackman pg. 464

oxidation numbers1

H

H-C-H

H

Oxidation numbers
  • Molecules and polyatomic ions: shared electrons are assigned to the more electronegative atom.
  • Examples: HF F-I H I

CO2 O-II C+IV O=C=O

CH4 H +I C-IV

NO3- -1 charge on anion = 3 x O-II + NV

  • Determining an atom’s oxidation number:
  • The more electronegative atom in a bond is assigned all the shared electrons; the less electronegative atom is assigned none.
  • Each atom in a bond is assigned all of its unshared electrons.
  • The oxidation number is give by:
  • O.N. = no. of valence e- - (no. of shared e- + no. of unshared e-)

For F, O.N. = 7 – (2 + 6) = -1

pop quiz
Pop Quiz

What is the oxidation number of Cr in the following?

[Cr2O7]2

2(x) + 7(-2) = -2, x = +6, Cr(VI)

CrO3

x + 3(-2) = 0, x = +6, Cr(VI)

Cr2O3

2(x) + 3(-2) = 0, x = +3, Cr(III)

pop quiz1
Pop Quiz
  • Examples

I2 O.N.=0 (elemental form)

Zn in ZnCl2 O.N.=+2 (Cl=-1, sum of O.N.s =0)

Al3+ O.N.=+3 (ON of monatomic ion=charge)

N in HNO3 O.N.=+5 (O=-2, H=+1, sum of ONs=0)

S in SO42- O.N.=+6 (O=-2, sum of O.N.s=charge on ion)

N in NH3 O.N.= -3 (H=+1, sum of O.N.s = 0)

N in NH4+ O.N.= -3 (H=+1, sum of O.N.s =charge on ion)

demo oxidation states of v
Demo: Oxidation states of V
  • Zn (s) + 2 VO3-(aq) + 8H+ (aq) → 2VO2+ (aq) + Zn2+ (aq) + 4 H2O

+5, vanadate, yellow +4, vanadyl, green

  • Zn (s) + 2 VO2+ (aq) + 4 H+ → 2 V3+(aq) + Zn2+(aq) + 2 H2O

+4, vanadyl, green +3, blue

  • Zn (s) + 2 V3+(aq) → 2 V2+ (aq) + Zn2+ (aq)

blue +2, violet

transition metals
Transition Metals

Multiple oxidation numbers – ns and (n-1)d electrons are used for bonds.

transition metals1
Transition Metals

Multiple oxidation numbers – ns and (n-1)d electrons are used for bonds.

filling of atomic orbitals aufbau
Filling of Atomic Orbitals (Aufbau)

In general,

the (n-1)d orbitals are filled between the ns and np orbitals.

Blackman Figure 4.29

transition metals ion formation
Transition Metals – Ion Formation
  • Period 4 Transition Metals: as the d orbitals fill, the 3d orbital becomes more stable than the 4s.
  • In the formation of Period 4 transition metal ions, the 4s electrons are lost before the 3d electrons.
  • The 4s orbital and the 3d orbitals have very similar energies

 variable oxidation states.

3d electrons
3d electrons

Common

O.N.

+III +IV +V +VI +VII +III +III +II +II +II

+IV +III +IV +II +II

+II +II

slide19

Mn = [Ar]4s23d5

7 valence electrons

Figure from Silberberg, “Chemistry”, McGraw Hill, 2006.

Orbital Occupancy

influence of oxidation state
Influence of Oxidation State

Hexavalent Chromium

  • Cr(VI) is classified as “carcinogenic to humans”
  • Cr(VI) compounds are soluble in water & may have a harmful effect on the environment.
  • Cr(VI) is readily reduced by Fe2+ and dissolved sulfides.

Trivalent Chromium

  • Cr(III) is considered an essential nutrient.
  • Most naturally occuring Cr(III) compounds are insoluble and it is generally believed that Cr(III) does not constitute a danger to health.
  • Cr(III) is rapidly oxidised by excess MnO2, or slowly by O2 in alkaline solutions.
properties of n compounds
Properties of N-compounds
  • Some non-metals like sulphur or nitrogen or chlorine also have a very wide range of oxidation states in their compounds.
  • N-compounds have a very wide range of properties.
  • N has an intermediate electronegativity and has an odd number (5) of valence electrons. N has one of the widest ranges of common oxidation states of any element.
properties of n compounds1
Properties of N-compounds

HIGHLY VARIED!

Incredibly stable: N2

Extremely explosive:

trinitrotoluene (TNT)

Strong acid HNO3

Weak base NH3

Photochemical smog: NO2

Biologically important: NO + amino acids

nitroglycerine

nitrogen oxides
Nitrogen Oxides

Table from Silberberg, “Chemistry”, McGraw Hill, 2006.

air pollution
Air pollution

Picture from www.consumercide.com

Picture from http://pdphoto.org

Sydney

The brown haze is largely NO2

 Los Angeles

summary
Summary
  • Rules for assigning oxidation numbers
  • Trends in electronegativity
  • Electron configuration of elements and ions
  • Aufbau – rule for filling atomic orbitals
  • Electron configuration of transitions metals