pHYSICAL cHEMISTRY - v

Experiment 6

Aim of the Experiment

Determine the heat of solution of a given substance (e.g, oxalic acid,benzoic acid,etc.) by solubility method.

Principle

When a sufficiently large amount of solute is in contact with the limited amount of solvent,dissolving of the solute starts taking place. After a certain period of time , the reverse process,precipitation,starts to occur.When dissolving and precipitation of the solute take place at equal rate (equilibrium has been established), the solution is said to be saturated.After this point the concentration of the solute in the solution remaining unchanged.The concentration of the saturated solution is defined as the solubility of the solute in the given solvent. Solubility is generally a function of temperature.

Heat of Solution:- is the amount of heat that absorbed when one mole of the solid is dissolved in a solution that is already practically saturated.

The dissolution of a solid into a liquid is usually accompanied with a heat effect; heat is absorbed or evolved. The heat evolved or absorbed when 1 mole of the solid is dissolved into a solution which is already practically saturated differs from that at infinite dilution by an amount equal to the heat of dilution from saturation to infinite dilution.

The influence of temperature on solubility is best expressed by Van’t Hoff equation,i.e.;

d ln S/ dT = Δ H / RT² …………..(1)

where S is the solubility of the salt in moles per 1000 g of the solvent and Δ H is the heat of solution per mole.

On integration of the above expression, we have

Δ H is assumed to be independent of temperature .A plot of log S values (ordinates)against corresponding values of 1/T (abscissas) will practically be a straight line or a smooth curve.The slope of the straight line (or the slope of the tangent drawn at a convenient temperature ) is - Δ H / 2.303 R .Thus, Δ H can be calculated (R= 8.3143 Joules or 1.9872 calories).

Alternatively, integrating above equation between two temperatures T₁ and T₂, we get

It is thus seen that if the solubilities at two temperatures are determined,the mean heat of solution between the two temperature limits can be calculated.

Materials required

Apparatus: A large beaker containing water as thermostat, a thermometer graduated in 0.1 ⁰C , 10 Cm³ pipette, burette, a small beaker.

Chemicals: 0.05 M NaOH solution, pure benzoic acid, phenolphthalein as indicator.

Procedure

(1)Place about 60-70 Cm³ of distilled water in a small beaker suspended in a large beaker containing water at a temperature about 13-15 ⁰C above the room temperature. Add increasing amounts of benzoic acid to the beaker with efficient stirring until a small amount of the solid remains undissolved .Caution, above 70 °C ,Benzoic acid may be decomposed.

(2) As the solution cools to the studied temperature, crystallization takes place. After stir frequently for 20 minutes, a 10 ml of solution is removed using a pipette, drained into an Erlenmeyer flask which is previously weighed. (Remember to weigh the Erlenmeyer flasks before experiment). To prevent drawing small crystals into the pipette along with the solution, you can heat the pipette previously passing through a the flame of Bunsen burner before pipetting in the solution so as to prevent the separation of the solid in the pipette at lower temperature.

(3) Drain the pipette into a weighed weighing tube or flask. Reweigh the weighing tube or flask.

(4) Titrate it with 0.05 M NaOH solution using phenolphthalein as the indicator. In this way determine the weight of the acid dissolved in a known weight of the solution and hence in a known weight of the solvent .

(5) Make duplicate determination at the same temperature.

(6) Repeat steps (1)-(4) for four more temperatures with described temperature range.

Observation Table

Result

The heat of the solution of a given substance is ______________ J/mol or Cal/ mol

Questions

1. What are the factors that affected on the solubility?

2. Give the difference between endothermic and exothermic reaction.

Reference Material

1. Practical Physical Chemistry by J.B. YADAV

2. P. W. Atkins and J. de Paula, "Physical Chemistry," 9th ed., pp. 172-173, Oxford University Press, U.S.A (2010).

3. F. Daniels and others, "Experimental Physical Chemistry," 6th ed., pp. 124-128, Europe-Asia book company, Taiwan (1956).

4. O. F. Steinbach and C. V. King, "Experiments in Physical Chemistry," pp. 89-93, American book company, U.S.A. (1950)

5. https://www.academia.edu/39084776/Exp5_Determination_of_Heat_of_Solution_From_Solubility