UNIT 11 Complexation and Precipitation

Question 1

What are complexation reactions used for

Answer 1

titrating cation
creating complexes that are colored or absorb
Sapringly soluble compounds that can be used in gravimetric analysis
Extracting ation from one solvent to another and dissolving insoluble precipitates

Question 2

What do most metal ion react with?

Answer 2

electron pair donors or coordination compounds to from complexes

Question 3

What is a Ligand?

Answer 3

Ion or molecule that forms a covalent bond with a bond with a cation or neutral metal atom by donateing a pair of electron which is shared

Question 4

What is a coordination number?

Answer 4

Number of covalent bonds that a cation tends to form with electron donors

Question 5

What do most metal ion exist as?

Answer 5

aquo-complexes

Question 6

What are some typical values for coordination numbers?

Answer 6

2,4,6

Question 7

What is the charge of a complex?

Answer 7

Positive, neutral or negative

Question 8

What are titration based on complex formations called?

Answer 8

Complex-metric titrations

Question 9

What are Chelates

Answer 9

Class is coordination compounds

–> metal ion coordinates with two or more donor groups of a ligand to form a 5/6 membered heterocyclic ring

Question 10

What is a Unidentate?

Answer 10

“Single-toothed”

Ligand that has a single donor group

Question 11

What is a Bidentate?

Answer 11

Ligand with two groups available for covalent bonding

Question 12

What are Macrocycle?

Answer 12

Cyclic organic compounds which contain none or more atoms and contain at least three heteroatoms (usually O, N, S)

Question 13

What are Cryptands?

Answer 13

Macrocyclic compunds that form 3D cavities that can accomodate appropriately sized ions

Question 14

How do you show a complexation equilibria?

Answer 14

M= metal ion
L= ligand
ML= complex
M+LML

Question 15

How does a complexation reaction occur?

Answer 15

Stepwise fashion

M+L ML2

Question 16

What is the complexation of unidentate ligand?

Answer 16

Invariably add in series of steps ??

Question 17

How do you satisfy the maximum coordination number ?

Answer 17

Only one or a few added ligands

Question 18

How are equilibrium constants written as?

Answer 18

Formation constant

Symbol: Beta

Question 19

Why are multidentate ligand prefereable to unidentate ligand for compleometeric titrations?

Answer 19

They form more stbale complexes than unidentate and they form a single complex with the cation which implifies their titration curves and makes end-point detectoin easier

Question 20

When would a complexation reactions be treated as a dissociation reaction of a sparingly soluble salt?

Answer 20

If there is an insoluble species

Question 21

What ligands can protonate?

Answer 21

Weak acid or conj base of weak acid
Ex. Oxalate Hox- and H2ox
Basic: oxalate dominante (stable)
Acid: oxalate protonates which causes dissociation of ferric complexes

Question 22

What do complexometeric titration curves plot?

Answer 22

pM=-logM as a function of the volume of titrant added

Question 23

What is the titrant and the analyte in a complexometric titration?

Answer 23

Ligand- titrant

Metal ion- analyte

Question 24

Why are tetradentate or hexadentrate ligands more prefereable as titrants?

Answer 24

Because unidentates can lead to low complex stability and indistinct titration end points
Whereas, polydentates are better because their reaction with cation and more complex, giving a sharper end point and form 1:1 complexes

Question 25

What are precipitation titration based on?

Answer 25

reactions that yeild ionic compounds of limited solubility

Question 26

What is the most used precipitating agent and what is it used for?

Answer 26

Silver Nitrate

–> halogens, halogen like anions, mercaptans, FA, dialent inorganic anions

Question 27

What are silver nitrtate titration called ?

Answer 27

argentometric titrations

Question 28

What is the effect of concentration on titration curves?

Answer 28

Whent the analyte and the titrant are higher concentration, there is a steeper end poitn region

Question 29

What Ksp will give a sharper end point?

Answer 29

Smaller Ksp

Question 30

What endpoint are used in argentometric titrations?

Answer 30

Chemical, potentiometirc and amperometric

Question 31

What endpoint do chemical indicators create?

Answer 31

Color change or occasionally the apperance of turbidity in the solution

Question 32

What are the requirements of an indicator?

Answer 32

The color change should occur over a limited reange in the p function of the analyte titration curve
The color change should take place within the steep portion of the titration curve of the analyte

Question 33

What is the Voldhard method?

Answer 33

Silver ion are titration with a standard solution of thiocyanate ion
Fe3+ serves as the indicator, The solution turns red with the first slight excess of thiocyanate ion due to the formation of FeSCN2+

Question 34

What is the Mohr method?

Answer 34

Indicator: Sodium Chromate
For the argentometric titration of Cl-, Br- and CN-
Ag+ react with the chromate to form a brick red silver chromates (AgCrO4) precipitae when all the Cl ions have been used up

Question 35

What is the Fajans Method?

Answer 35

Used adsorption indicator

Transfer of color from the solution to the precipitate at eq point

Question 36

What is a adsorption indicator?

Answer 36

Organic compounds that absorbs onto the surface of the solid in a precipitation titration which resutls in a color change at eq point.