Physical Chemistry LD Chemical Equilibrium Chemistry Complex Equilibrium Leaflets C4.2.3.1

Physical chemistry LD Chemical equilibrium Chemistry Complex equilibrium Leaflets 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. Principles Here,

Complex compounds consist of a central atom, usually a metal, KC = 1/KD around which ligands are coordinated. These ligands are molecules with an additional pair of electrons that are available as The law of mass action with regard to the complex formation a Lewis base to the central atom (a Lewis acid). The charge of constant KC states the following: a complex corresponds to the sum of charges of the central c(Ag(NH ) ) K = 3 2 atom and its ligands. C + 2 푐(Ag ) ∙ 푐 (NH3) In aqueous solution, ligands remain coordinated around the The greater the complex formation constant, the further toward central atom when the bond with the ligands is stronger than the complex side equilibrium lies, and the more stable the com- with water. Cations that are dissolved in water, such as silver plex. (Ag+), generally exist as complex compounds with water, as they are hydrated (aq). The complex formation constant can be determined experi- mentally using voltage measurements and the Nernst equa- The diamine-silver complex arises when ammonia is added to tion. A voltage measurement is established between a meas- a silver salt solution. Then, the water ligands are replaced by urement half-cell and a reference half-cell. In this case, the ref- ammonia ligands at the hydrated silver ion because the ammo- erence half-cell has a defined silver ion concentration (here: nia ligands have a stronger bond. 0.1 mol/l). In the measurement half-cell, ammonia is pre- + + Ag (aq) + 2 NH3 (aq)  Ag(NH3)2 sented. When silver ions are added in drops, the silver-diamine complex forms. In addition to the complex, there is a defined Complex formations are equilibrium reactions. Therefore, both amount of silver ions in the solution in accordance with the forms exist in equilibrium. The equilibrium constant for the for- complex formation constant. A potential difference arises due mation of complexes is called the complex formation constant to the different concentration of silver ions in the two half-cells. (or stability constant) Kc, and for the decay of complexes it is The concentration of the silver ions in the silver-diamine solution can be calculated from this potential difference. In turn, the

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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 P304+P340 If inhaled: Bring person to for silver chloride can be derived therefrom. fresh air and make sure airways are not blocked. Risk assessment P305+P351+P338 IF IN EYES: During the experiment, wear the necessary protective equip- Rinse carefully with water for several ment (goggles, gloves) as a few of the solutions being used minutes. Remove contact lenses if are corrosive. present and easy to do so. Continue Ammonia solution has an intense odour. Therefore, the dilution rinsing. in particular should be performed under the fume cupboard. P309+P310 IF exposed or you feel ill: Silver nitrate causes permanent black spots on the skin. Call the Poison Centre or a physician. P403+P233 Store containers in a well Silver nitrate solution, 0.1 mol/l ventilated place and keep tightly Hazard statements closed. H272 May intensify fire; oxidiser P501 Take contents/containers to an agent. approved disposal company or take to a community collection point. H314 Causes severe skin burns and severe eye damage. H410 Very toxic to aquatic life with Equipment and chemicals long-lasting effects. 1 Universal chemical measuring instrument ..... 531 836 Safety statements 1 UIP sensor S ...... 524 0621 P210 Keep away from heat sources. 2 Beaker, Boro 3.3, 100 ml, tall ...... 664 137 1 Measuring cylinder 100 ml, with plastic base 665 754 P221 Take every precaution to avoid 1 Salt bridge, reaction tube 90 x 90 mm...... 667 455 mixing with combustibles 1 Rubber stopper, solid, 16...21 mm diam...... 667 255 P273 Avoid release to the environ- 1 Beaker, Boro 3.3, 250 ml, squat ...... 664 130 ment. 1 Plate electrode, silver, 55x40 mm, set of 2 .... 664 421 P280 Wear protective gloves/protec- 1 Set of 6 crocodile-clips, polished ...... 501 861 tive clothing/eye protection/face pro- 1 Connecting lead 19 A, 25 cm, pair ...... 501 44 tection. 1 Electronic balance 440-3N, 200 g : 0.01 g .... 667 7977 1 Saddle base ...... 300 11 P301+330+331 If swallowed: Rinse Signal word: 1 Stand rod 47 cm, 12 mm diam...... 300 42 mouth. Do not induce vomiting. 1 Bosshead S ...... 301 09 Hazard P305+P351+P338 IF IN EYES: Rinse 1 Universal clamp 0...80 mm ...... 666 555 carefully with water for several 1 Dropping pipette, 7 x 150 mm, 10 pcs...... 665 953 minutes. Remove contact lenses if 1 Rubber bulbs, 10 pcs...... 665 954 present and easy to do so. Continue 1 Funnel, PP. 75 mm diam...... 665 009 rinsing. 1 Silver nitrate solution 0.1 mol/l, 250 ml ...... 674 8800 1 Ammonia solution, 25 %, 250 ml ...... 670 3600 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. H314 Causes severe skin burns and 2. Fix the stand rod in the saddle base to do so. severe eye damage. 3. Fasten the universal clamp to the stand rod using a boss- H335 May cause respiratory irritation. head S clamp. H400 Very poisonous to aquatic or- 4. The salt bridge reaction tube is fastened in the universal ganisms. clamp. Safety statements 5. The Universal Chemistry Measuring Instrument (UMI C) is connected to the power and the UIP S sensor is plugged into P102 Keep out of reach of children. the interface at the UMI C. P273 Avoid release to the environ- 6. The connecting leads are provided with crocodile clips on ment. one side and are inserted into the voltage V measurement in- P280 Wear protective gloves /protec- puts of the UIP S sensor. tive clothing/eye protection /face pro- Preparation of the experiment tection. 1. In addition to the prepared silver nitrate solution, ammonia P273 Avoid release to the environ- solution and potassium nitrate solution are used for the exper- ment. iment, both of which must first be prepared. P301+330+331 If swallowed: 2. To prepare the solutions, the sample weights or volumes Signal word: Rinse mouth. Do not induce vomiting. must first be calculated. Hazard

2 LD Chemistry Leaflets C4.2.3.1

a. 2.4 molar ammonia solution (25 % NH3) Where,

M(NH3) = 17.03 g/mol, ρ(25% NH3) = 0.91 g/l c1: Numerical value of the silver ion concentration in the refer-

n(NH3)desired = 2.4 mol ence half element (the higher concentration) in mol/l.

Calculation of the mass of NH3 in 1 litre of 25 % ammonia c2: Numerical value of the silver ion concentration in the meas- solution: urement half element (the lower concentration) in mol/l.

m(NH3)/l = 1000 ml ∙ 0.91 g/l ∙ 0.25 For monovalent ions such as silver (n = 1), the following ap-

m(NH3)/l = 227.5 g plies: R∙T Calculation of the moles and concentration of NH3 in 1 litre = 0,0059 of 25 % ammonia solution: n∙M∙F

n(NH3) = m(NH3) / M(NH3) To determine the dissociation constant KD, the concentration + of the silver ions in the measurement half element (c2(Ag )) is n(NH3) = 227.5 g / 17.03 g/mol required. This can be calculated using the law of mass action. n(NH3) = 13.4 mol The law of mass action for the formation of the complex is as c(NH3) = 13.4 mol/l follows: Calculation of the required volume of 25% ammonia solu- + c([Ag(NH3)2] ) tion: KK = 푐 (Ag+) ∙ 푐2(NH ) V(NH3)Initial solution = 2 3 After mixing and prior to reaction, the concentrations are c(NH3)desired / c(NH3)initial ∙ V(NH3)desired + V(NH3)Initial = 2.4 mol / 13.2 mol ∙ 50 ml c(Ag ) = 0.1 mol/l and c(NH3) = 1.2 mol/l. + V(NH3)Initial = 9 ml As a result of the almost complete reaction, 0.1 moles of Ag Therefore, start with 41 ml of water in a small beaker and ions react with 0.2 moles of NH3 molecules. The concentrations after the reaction are therefore: add 9 ml of ammonia solution (25 %). + c([Ag(NH3)2] ) = 0.1 mol/l and c(NH3) = 1 mol/l b. Saturated potassium solution So For a saturated potassium nitrate solution, dissolve 32 g of + potassium nitrate in 100 ml of water in the large beaker (250 0,1 mol/l 푐2(Ag ) KC = + , or KD = ml) at a temperature of 20 °C. 푐2(Ag ) 0,1mol/l 3. Fill the saturated potassium nitrate solution into the salt and bridge reaction tube using a funnel and seal it with a stopper. c2(Ag+) = 0.1 mol/l ∙ KD 4. Connect each of the two silver plate electrodes to one croc- So, the following applies for the Nernst equation: odile clip, respectively. + c1(Ag ) + + ∆E = 0,059 lg + = 0.059 lg c1(Ag ) – 0.059 lg c2(Ag ) Performing the experiment c2(Ag ) 1. Fill a small beaker with 50 ml of silver nitrate solution. If the initial concentration c = 0.1 mol/l is now used as the silver 2. Turn on the Universal Chemistry Measuring Instrument concentration c1 and the law of mass action is used for the sil- (UMI C). ver concentration c2, the formula reduces to the following. 3. Dip one silver plate electrode into the silver nitrate solution ΔE = -0.059 – 0.059 ∙ lg 0.1 mol/l ∙ KD and one into the ammonia solution. The electrode in the silver and nitrate solution forms the positive pole. ∆E+0,059 0,059 4. Add 2 - 3 drops of the 0.1 mol/l silver chloride solution to the KD= 10 ammonia solution. This allows the change in potential to be converted directly to 5. Now, position the beakers containing the two solutions such the dissociation constant. At a measured potential difference that one shoulder of the salt bridge reaction tube dips into one of 0.353 V, the dissociation constant KD is calculated as of the solutions and one dips into the other. As soon as a con- 10.4 ∙ 10-8 mol2/l2. stant value is shown at the UMI C, record it. Results Observation Based on the potential difference, the dissociation constant of When silver chloride is pipetted into the ammonia solution, the silver-diamine complex is 10.4 ∙ 10-8 mol2/l2. This is com- there is no recognisable change. After dipping the salt bridge, parable to the literature value (8 ∙ 10-8 mol2/l2). Deviations arise a potential difference of 0.353 V can be read at the UMI C. mostly due to the simplifications assumed. Evaluation Cleaning and disposal Calculating the concentration of silver ions in the silver Silver nitrate in aqueous solution is reduced to silver by adding complex solution iron chips or by heating with glucose. Then, the solution can The concentration of silver ions in the silver complex solution be disposed of down the drain. can be calculated from the potential difference measured using Potassium nitrate should be disposed of separately as nitrate the Nernst equation. waste. This must be kept alkaline in order to prevent formation In this case, the Nernst equation is as follows: of hydrogen cyanide. Small amounts may be disposed of down + the drain as well. Current regional disposal rules must be fol- R∙T c1(Ag ) ∆E = lg + lowed. n∙M∙F c2(Ag )