Question

1. Explain the procedure for a thermogravimetric analysis. Provide an example, including reactions and/or graphs to illustrate your answer.

2. Argentometric titrations are typically used for the determination of halides, although they can be used for the determination of a variety of anions which form insoluble salts with silver. Name the three argentometric methods discussed in class, and describe one of the methods in detail (including titration reactions and description of how the end point is determined).

3. List and discuss three factors that will affect the solubility of a sparingly soluble salt. Include relevant examples, including equilibrium/reaction equations, and indicate if each factor will increase or decrease solubility.

4. Identify three factors that affect the value of the activity coefficient. Briefly discuss how each factor relates to the activity of ions in ionic equilibria, and how the activity coefficient changes with each factor.

5. Direct titrations, indirect titrations, and back titrations. Discuss the details related to each procedure, and illustrate each with a relevant example.

6. Discuss the experimental criteria for a spectrophotometric titration, and provide a graph that would be used to determine the end point for the following titration conditions:

            (as an example!)   ε_titrant = 0,   ε_analyte = 0,    ε_product > 0.

Answer 1

1. Thermo gravimetric analysis (TGA) is a method of thermal analysis in which we measures the mass change of a sample as a function of temperature (in the scanning mode or as a function of time (in the isothermal mode). TGA is conducted on an instrument known as Thermogravimetric analyzer which continuously measures mass while the temperature of a sample is changed over time. Thermogravimetric analyzer consists of a precision balance with a sample pan located inside a furnace with a programmable control temperature. The temperature is generally increased at constant rate to induce a thermal reaction which occur under a variety of atmospheres including: air, vacuum, inert gas, oxidizing/reducing gases, corrosive gases, vapors of liquids or "self-generated atmosphere"; as well as a variety of pressure including: a high vacuum, high pressure, constant pressure, or a controlled pressure. The data collected from a thermal reaction is compiled into a plot of mass or percentage of initial mass on the y axis versus either temperature or time on the x-axis which is referred to as a TGA curve.

Let us take the example of silver nitrate (AgNO₃), As we know, AgNO₃ is thermally stable upto a temperature of 473˚C, so the weight of AgNO₃ remains constant upto the temperature of 473˚C (as shown by curve AB). Above this temperature, it starts decomposing into NO₂, O₂, and Ag and thus losing its weight. The loss in weight continues to 608˚C. Beyond this temperature, weight of sample remains constant (as shown by curve CD).The portion between BC represents the decomposition of AgNO₃. The decomposition is complete at 608 ˚C leaving metallic silver as stable residue. So the horizontal portion of the weight i.e. AB and CD represents that there is no change in mass and the portion BC represents the change in mass.

2. the three argentometric methods are

(a) Mohr Method

(b) Fazan Method,

(c) Volhard Method

(a) Mohr Method: Mohr method of determination of chlorides and other halides by titration with silver nitrate in the presence of chromate anions. End point is indicated by the appearance of the red silver chromate.

Reaction taking place during titration is

Ag+ + Cl- → AgCl(s)

Before titration small amount of sodium or potassium chromate is added to the solution which makes it slightly yellow in color. During titration, as long as chlorides are present, concentration of Ag+ is too low for silver chromate formation. Near equivalence point concentration of silver cations rapidly grows, allowing precipitation of intensively red silver chromate which indicates the end point

3. Facors affecting solubility of a sparingly soluble salt

(a) common ion effect

The effect of adding an ion common to one already in equilibrium in a solubility reaction is to lower the solubility of the salt. In the case of AgBr solubility:

AgBr(s) Ag⁺(aq) + Br^–(aq)

The addition of either Ag⁺ or Br^– ions will shift the equilibrium to the left, in accord with Le Chatelier's principle, thus decreasing the amount of AgBr dissolved. Ag⁺ ions can be added by pouring in a solution of AgNO₃. Recall that AgNO₃ is very soluble, and is a strong electrolyte.

AgNO₃(s) Ag⁺(aq) + (aq)

The nitrate ion will not interfere with AgBr solubility because it is not a common ion.

Additional Br^– ions could be supplied by adding NaBr, for instance. Sodium bromide is very soluble and is a strong electrolyte.

NaBr(s) Na⁺(aq) + Br^–(aq)

The sodium ion will not affect the solubility of AgBr because it is not a common ion.

Here we see that the addition of an ion that is common to one already in the solubility equilibrium shifts the equilibrium to the left, which decreases the solubility. This is equivalent to the case of a weak acid, where the presence of a common ion decreases the percent ionization.

(b) Formation of complex ion:

Formation of complex ion will increas the solubility of salt.

(c) amphoterism:

It is the ability of a molecule to act as both an acid and base. Amphoteric metal oxides and hydroxides are soluble in strong acid or base, as they can act either as acids or bases.