CHEM 1220 Unit 3

Ligands

are Lewis bases

Metal center

Lewis acid

Tetrahedral complexes have ligand-metal-ligand bond angles of _____

109.5^o

square planar complexes have bond angles of ___ and _____.

90^o and 180^o

Stereoisomers:

same formula, same connectivity, different spatial arrangement

Geometric isomers:

ligands of the same kind are arranged with different bond angles.  Figure 19.21 shows great examples of cis-trans isomerism in octahedral complexes.  This kind of isomerism also is possible for square planar complexes. 

Optical isomers:

These are molecules that are nonsuperimposable mirror images of each other, like your two hands. Skip this bit on optical isomers entirely.  If you go on to take organic chemistry, you'll learn a lot about optical isomers involving carbons bound to four different groups.  Optical isomerism happens in coordination complexes, too, but the examples are either trivial or esoteric or both.

Structural isomers:

same formula, different connectivity

Coordination isomers:

different molecules/ions are connected directly to the metal center. 

Linkage isomers…

are different in the atom that forms the bond to the metal.

Octahedral

Coordination number = 6

Square Planar

CN = 4

octahedral splitting energy (Δ_o)

The difference in energy between the lower and upper sets of orbitals

• This energy difference accounts for the colors of most octahedral coordination complexes

Strong-field ligands

large Δ_o values

weak-field ligands

small Δ_o values

relative ligand field strengths

I^– < Br^– < S^2– < Cl^– < NO₃^– < F^– < OH^– < C₂O₄^2– < O^2– < H₂O < NH₃ < en < CN^– < CO

Everything from NH₃ to the right is generally considered to be strong-field; everything to the left of NH₃ is generally considered to be weak-field.

Strong-field ligands lead to ________ complexes, where we have the minimum number of unpaired electrons. 

low-spin; This is because Δ_o is greater than the electron pairing energy.

Weak-field ligands lead to _________ complexes, where we have the maximum number of unpaired electrons.

high-spin; This is because Δ_o is smaller than the electron pairing energy.

When the complex Ti(H₂O)₆³⁺ absorbs light at ~499 nm, the single electron in the lower set of d-orbitals transitions to the upper set of d-orbitals, and we can use ______ to figure out the value of Δ_o.

E = hc/λ **actually doing this won’t be on exam

Degenerate

Same energy level

spontaneous process

is one that happens without the input of energy from outside the system

As the state gets more disordered (entropy increases), ΔS is ________

positive

0th Law of Thermodynamics

If A is in equilibrium with B, and B is in equilibrium with C, then A is in equilibrium with C

1st law of Thermodynamics

In an isolated system, the total amount of energy is constant. ΔE = q + w

2nd Law of Thermodynamics

For every spontaneous reaction or process, the total entropy of the universe increases

3rd Law of Thermodynamics

The entropy of a perfect crystal lattice at 0 Kelvin is 0

What is the ΔS for the reaction?: 3H₂ (g) + N₂ (g) —> 2NH₃ (g)

ΔS = negative

This is because there are 4 moles on the reactant side and 2 on the product side; the number of moles decreased

Find ΔS^o of the system: 3H₂ (g) + N₂ (g) —> 2NH₃ (g)

(3H₂ (g) @191.6, N₂ (g) @130.7 & 2NH₃ (g) @192.8)

ΔS = 2(NH₃) - [1(N₂) + 3(H₂)]

ΔS = 2(192.8) - [1(130.7) + 3(191.6)]

ΔS^o_sys = -198.1 J/mol*K

ΔG^o = ΔH^o − TΔS^o

analyze reaction spontaneity

When ΔG < 0

the reaction is spontaneous in the forward direction

When ΔG > 0

the reaction is nonspontaneous in the forward direction

When ΔG = 0

the reaction is at equilibrium

The three ways to calculate the standard free energy change for a reaction

1. ΔG^o = ΔH^o − TΔS^o, where we first calculate the standard enthalpy change and the standard entropy change using a table of thermodynamic data

2. Use the equation [equation with sigmas], where ΔG_f^o is the free energy of formation of the particular substance.  This equation can only be used when T = 298 K

3. combine other reactions with known standard free energy changes to get an overall reaction.  Here, the rules are the same as for manipulating reactions and their ΔH^o and/or ΔS^o values

Expanded on the third way to calculate the standard free energy (combining reactions)

• If a chemical equation is multiplied by some factor, ΔG^o is also multiplied by the same factor.

• If a chemical equation is reversed, ΔG^o changes sign.

• If a chemical equation can be expressed as the sum of a series of steps, ΔG^o for the overall equation is the sum of the free energies of reactions for each step.

The free energy change for a process may be viewed as a measure of its driving force

ΔG

the free energy change under any conditions

ΔG^o

the free energy change under standard conditions

ΔG = ΔG^o + RT·ln Q

calculates the non-standard Gibbs free energy change of a chemical reaction, allowing you to determine its spontaneity under any given set of conditions

• ΔG^o = −RT·ln K

• Q = K

ΔG = RT*ln(Q/K)

is an alternative to ΔG = ΔG^o + RT·ln Q

Oxidation

loss of electrons, increase in oxidation state

Reduction

gain of electrons, decrease in oxidation state

spontaneous redox reaction

Electrons are spontaneously flowing from the neutral Cu atoms to the Ag⁺ ions, resulting in neutral Ag atoms and Cu²⁺ ions

calculate the standard potential for a redox reaction

E^o = E^o_cat – E^o_an

where E^o_cat is the standard reduction potential for the cathode half-cell and E^o_an is the standard reduction potential for the anode half-cell.

the standard reduction potential for the reduction actually taking place and the standard oxidation potential for the oxidation

E^o = E^o_red + E^o_ox

Anode

oxidation

cathode

reduction

ΔG = −nFE

where n is the number of moles of electrons transferred and F is Faraday's constant

Faraday's constant (F)

can be written as 96,485 C/mol or 96,485 J/mol·V.  (F is just Avogadro's number multiplied by the charge of 1 electron in coulombs.)

−nFE^o =

ΔG^o = −RT·ln K

ΔG = ΔG^o + RT·ln Q

free-energy change under non-standard conditions

n =

the number of electrons transferred