Physical chemistry
LD
Chemistry
Leaflets
Chemical equilibrium
Complex equilibrium
C4.2.3.1
Determination of the complex
decay constant of the silver-diamine complex
Aims of the experiment
 To investigate a complex compound.
 To recognise that many complexes remain intact even when solved.
 To learn about complex decay and complex formation constants.
 To apply the law of mass action.
 To use the Nernst equation to determine the dissociation constant.
called the complex decay constant KD. KD values are tabulated.
Here,
Principles
Complex compounds consist of a central atom, usually a metal,
around which ligands are coordinated. These ligands are molecules with an additional pair of electrons that are available as
a Lewis base to the central atom (a Lewis acid). The charge of
a complex corresponds to the sum of charges of the central
atom and its ligands.
KC = 1/KD
The law of mass action with regard to the complex formation
constant KC states the following:
KC =
In aqueous solution, ligands remain coordinated around the
central atom when the bond with the ligands is stronger than
with water. Cations that are dissolved in water, such as silver
(Ag+), generally exist as complex compounds with water, as
they are hydrated (aq).
c(Ag(NH3 )2 )
𝑐(Ag+ ) ∙ 𝑐 2 (NH3 )
The greater the complex formation constant, the further toward
the complex side equilibrium lies, and the more stable the complex.
The complex formation constant can be determined experimentally using voltage measurements and the Nernst equation. A voltage measurement is established between a measurement half-cell and a reference half-cell. In this case, the reference half-cell has a defined silver ion concentration (here:
0.1 mol/l). In the measurement half-cell, ammonia is presented. When silver ions are added in drops, the silver-diamine
complex forms. In addition to the complex, there is a defined
amount of silver ions in the solution in accordance with the
complex formation constant. A potential difference arises due
to the different concentration of silver ions in the two half-cells.
The concentration of the silver ions in the silver-diamine solution can be calculated from this potential difference. In turn, the
The diamine-silver complex arises when ammonia is added to
a silver salt solution. Then, the water ligands are replaced by
ammonia ligands at the hydrated silver ion because the ammonia ligands have a stronger bond.
Ag+(aq) + 2 NH3 (aq)  Ag(NH3)2+
SW-2016-03
Complex formations are equilibrium reactions. Therefore, both
forms exist in equilibrium. The equilibrium constant for the formation of complexes is called the complex formation constant
(or stability constant) Kc, and for the decay of complexes it is
Fig. 1: Set-up of the experiment. Only 2 beakers are used in this experiment.
1
C4.2.3.1
LD Chemistry Leaflets
complex formation constant and the complex decay constant
for silver chloride can be derived therefrom.
P304+P340 If inhaled: Bring person to
fresh air and make sure airways are
not blocked.
Risk assessment
P305+P351+P338 IF IN EYES:
During the experiment, wear the necessary protective equipment (goggles, gloves) as a few of the solutions being used
are corrosive.
Rinse carefully with water for several
minutes. Remove contact lenses if
present and easy to do so. Continue
rinsing.
Ammonia solution has an intense odour. Therefore, the dilution
in particular should be performed under the fume cupboard.
P309+P310 IF exposed or you feel ill:
Call the Poison Centre or a physician.
Silver nitrate causes permanent black spots on the skin.
P403+P233 Store containers in a well
ventilated place and keep tightly
closed.
Silver nitrate solution, 0.1 mol/l
Hazard statements
P501 Take contents/containers to an
approved disposal company or take to
a community collection point.
H272 May intensify fire; oxidiser
agent.
H314 Causes severe skin burns and
severe eye damage.
H410 Very toxic to aquatic life with
long-lasting effects.
Equipment and chemicals
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2
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Safety statements
P210 Keep away from heat sources.
P221 Take every precaution to avoid
mixing with combustibles
P273 Avoid release to the environment.
P280 Wear protective gloves/protective clothing/eye protection/face protection.
Signal word:
Hazard
P301+330+331 If swallowed: Rinse
mouth. Do not induce vomiting.
P305+P351+P338 IF IN EYES: Rinse
carefully with water for several
minutes. Remove contact lenses if
present and easy to do so. Continue
rinsing.
P310+P310 If exposed or affected,
call a Poison Centre or a physician.
Set-up and preparation of the experiment
Ammonia solution, 25 %
Set-up of the apparatus
Hazard statements
1. The apparatus is set up as can be seen in Fig. 1.
2. Fix the stand rod in the saddle base to do so.
3. Fasten the universal clamp to the stand rod using a bosshead S clamp.
4. The salt bridge reaction tube is fastened in the universal
clamp.
5. The Universal Chemistry Measuring Instrument (UMI C) is
connected to the power and the UIP S sensor is plugged into
the interface at the UMI C.
6. The connecting leads are provided with crocodile clips on
one side and are inserted into the voltage V measurement inputs of the UIP S sensor.
Preparation of the experiment
H314 Causes severe skin burns and
severe eye damage.
H335 May cause respiratory irritation.
H400 Very poisonous to aquatic organisms.
Safety statements
P102 Keep out of reach of children.
P273 Avoid release to the environment.
P280 Wear protective gloves /protective clothing/eye protection /face protection.
1. In addition to the prepared silver nitrate solution, ammonia
solution and potassium nitrate solution are used for the experiment, both of which must first be prepared.
2. To prepare the solutions, the sample weights or volumes
must first be calculated.
P273 Avoid release to the environment.
P301+330+331 If swallowed:
Signal word:
Universal chemical measuring instrument ..... 531 836
UIP sensor S ................................................. 524 0621
Beaker, Boro 3.3, 100 ml, tall ........................ 664 137
Measuring cylinder 100 ml, with plastic base 665 754
Salt bridge, reaction tube 90 x 90 mm. .......... 667 455
Rubber stopper, solid, 16...21 mm diam. ....... 667 255
Beaker, Boro 3.3, 250 ml, squat .................... 664 130
Plate electrode, silver, 55x40 mm, set of 2 .... 664 421
Set of 6 crocodile-clips, polished ................... 501 861
Connecting lead 19 A, 25 cm, pair ................ 501 44
Electronic balance 440-3N, 200 g : 0.01 g .... 667 7977
Saddle base .................................................. 300 11
Stand rod 47 cm, 12 mm diam. ..................... 300 42
Bosshead S ................................................... 301 09
Universal clamp 0...80 mm ............................ 666 555
Dropping pipette, 7 x 150 mm, 10 pcs. .......... 665 953
Rubber bulbs, 10 pcs. ................................... 665 954
Funnel, PP. 75 mm diam. .............................. 665 009
Silver nitrate solution 0.1 mol/l, 250 ml .......... 674 8800
Ammonia solution, 25 %, 250 ml ................... 670 3600
Rinse mouth. Do not induce vomiting.
Hazard
2
C4.2.3.1
LD Chemistry Leaflets
a. 2.4 molar ammonia solution (25 % NH3)
M(NH3) = 17.03 g/mol, ρ(25% NH3) = 0.91 g/l
n(NH3)desired = 2.4 mol
Calculation of the mass of NH3 in 1 litre of 25 % ammonia
solution:
m(NH3)/l = 1000 ml ∙ 0.91 g/l ∙ 0.25
m(NH3)/l = 227.5 g
Calculation of the moles and concentration of NH3 in 1 litre
of 25 % ammonia solution:
n(NH3) = m(NH3) / M(NH3)
n(NH3) = 227.5 g / 17.03 g/mol
n(NH3) = 13.4 mol
c(NH3) = 13.4 mol/l
Calculation of the required volume of 25% ammonia solution:
V(NH3)Initial solution =
c(NH3)desired / c(NH3)initial ∙ V(NH3)desired
V(NH3)Initial = 2.4 mol / 13.2 mol ∙ 50 ml
V(NH3)Initial = 9 ml
Therefore, start with 41 ml of water in a small beaker and
add 9 ml of ammonia solution (25 %).
b. Saturated potassium solution
For a saturated potassium nitrate solution, dissolve 32 g of
potassium nitrate in 100 ml of water in the large beaker (250
ml) at a temperature of 20 °C.
3. Fill the saturated potassium nitrate solution into the salt
bridge reaction tube using a funnel and seal it with a stopper.
4. Connect each of the two silver plate electrodes to one crocodile clip, respectively.
Where,
c1: Numerical value of the silver ion concentration in the reference half element (the higher concentration) in mol/l.
c2: Numerical value of the silver ion concentration in the measurement half element (the lower concentration) in mol/l.
For monovalent ions such as silver (n = 1), the following applies:
R∙T
= 0,0059
n∙M∙F
To determine the dissociation constant KD, the concentration
of the silver ions in the measurement half element (c2(Ag+)) is
required. This can be calculated using the law of mass action.
The law of mass action for the formation of the complex is as
follows:
KK =
c([Ag(NH3 )2 ]+ )
𝑐2 (Ag+ ) ∙ 𝑐 2 (NH3 )
After mixing and prior to reaction, the concentrations are
c(Ag+) = 0.1 mol/l and c(NH3) = 1.2 mol/l.
As a result of the almost complete reaction, 0.1 moles of Ag+
ions react with 0.2 moles of NH3 molecules. The concentrations after the reaction are therefore:
c([Ag(NH3)2]+) = 0.1 mol/l and c(NH3) = 1 mol/l
So
KC =
0,1 mol/l
+
𝑐2 (Ag )
, or KD =
𝑐2 (Ag+ )
0,1mol/l
and
c2(Ag+) = 0.1 mol/l ∙ KD
So, the following applies for the Nernst equation:
c1 (Ag+ )
Performing the experiment
∆E = 0,059 lg
1. Fill a small beaker with 50 ml of silver nitrate solution.
2. Turn on the Universal Chemistry Measuring Instrument
(UMI C).
3. Dip one silver plate electrode into the silver nitrate solution
and one into the ammonia solution. The electrode in the silver
nitrate solution forms the positive pole.
4. Add 2 - 3 drops of the 0.1 mol/l silver chloride solution to the
ammonia solution.
5. Now, position the beakers containing the two solutions such
that one shoulder of the salt bridge reaction tube dips into one
of the solutions and one dips into the other. As soon as a constant value is shown at the UMI C, record it.
If the initial concentration c = 0.1 mol/l is now used as the silver
concentration c1 and the law of mass action is used for the silver concentration c2, the formula reduces to the following.
c2 (Ag+ )
= 0.059 lg c1(Ag+) – 0.059 lg c2(Ag+)
ΔE = -0.059 – 0.059 ∙ lg 0.1 mol/l ∙ KD
and
KD = 10
∆E+0,059
0,059
This allows the change in potential to be converted directly to
the dissociation constant. At a measured potential difference
of 0.353 V, the dissociation constant KD is calculated as
10.4 ∙ 10-8 mol2/l2.
Results
Observation
When silver chloride is pipetted into the ammonia solution,
there is no recognisable change. After dipping the salt bridge,
a potential difference of 0.353 V can be read at the UMI C.
Based on the potential difference, the dissociation constant of
the silver-diamine complex is 10.4 ∙ 10-8 mol2/l2. This is comparable to the literature value (8 ∙ 10-8 mol2/l2). Deviations arise
mostly due to the simplifications assumed.
Evaluation
Cleaning and disposal
Calculating the concentration of silver ions in the silver
complex solution
Silver nitrate in aqueous solution is reduced to silver by adding
iron chips or by heating with glucose. Then, the solution can
be disposed of down the drain.
The concentration of silver ions in the silver complex solution
can be calculated from the potential difference measured using
the Nernst equation.
In this case, the Nernst equation is as follows:
∆E =
R∙T
c1 (Ag+ )
lg
n∙M∙F c2 (Ag+ )
Potassium nitrate should be disposed of separately as nitrate
waste. This must be kept alkaline in order to prevent formation
of hydrogen cyanide. Small amounts may be disposed of down
the drain as well. Current regional disposal rules must be followed.
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