Phase Diagram for Two Partially-miscible Liquids

Name: Eghan Kojo Index N o : 6138811 Experiment N 0 : P 2.2.3 Graduate Assistant: Adolf Oti Bakye Date: 26 th March, 2013

TITLE: PHASE DIAGRAM FOR TWO PARTIALLY-MISCIBLE LIQUIDS AIMS 1. To construct a phase diagram for the aniline-heptane system. 2. To be able to use a phase diagram and show how it can be used to predict various properties of the system.

INTRODUCTION Phase diagram is a graphical representation of the physical states of a substance under different conditions of temperature and pressure. A typical phase diagram has pressure on the y-axis and temperature on the x-axis. As we cross the lines or curves on the phase diagram, a phase change occurs. In addition, two states of the substance coexist in equilibrium on the lines or curves. A phase diagram in physical chemistry and engineering is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium. Phase diagrams illustrate the variations between the states of matter of elements or compounds as they relate to pressure and temperatures. The following is an example of a phase diagram for a generic singlecomponent system:

Triple point – the point on a phase diagram at which the three states of matter: gas, liquid, and solid coexist. Critical point – the point on a phase diagram at which the substance is indistinguishable between liquid and gaseous states. Fusion(melting) (or freezing) curve – the curve on a phase diagram which represents the transition between liquid and solid states. Vaporization (or condensation) curve – the curve on a phase diagram which represents the transition between gaseous and liquid states. Sublimation (or deposition) curve – the curve on a phase diagram which represents the transition between gaseous and solid states Phase diagrams plot pressure (typically in atmospheres) versus temperature (typically in degrees Celcius or Kelvin). The labels on the graph represent the stable states of a system in equilibrium. The lines represent the combinations of pressures and temperatures at which two phases can exist in equilibrium. In other words, these lines define phase change points. The red line divides the solid and gas phases, represents sublimation (solid to gas) and deposition (gas to solid). The green line divides the solid and liquid phases and represents melting (solid to liquid) and freezing (liquid to solid). The blue divides the liquid and gas phases, represents vaporization (liquid to gas) and condensation (gas to liquid). There are also two important points on the diagram, the triple point and the critical point. The triple point represents the combination of pressure and temperature that facilitates all phases of matter at equilibrium. The critical point terminates the liquid/gas phase line and relates to the critical pressure, the pressure above which a supercritical fluid forms. A phase transition, the spontaneous conversion of one phase into another phase, occurs at a characteristic temperature for a given pressure. The transition temperature is the temperature at which the two phases are in equilibrium and the Gibbs energy is minimized at the prevailing pressure. The triple point is a point on a phase diagram at which the three phase boundaries meet and all three phases are in mutual equilibrium. A point in the two-phase region of a phase diagram indicates not only qualitatively that both liquid (aniline and heptane) are present, but represents quantitatively the relative amounts of each. Aniline and heptane are partial miscible, this means that they are liquids that do not mix in all proportions at all temperatures. However their transition temperature is to be determined and the phase diagram drawn using their transition temperature against the weight fraction of aniline. Thus, it can be seen that no matter how immiscible a solute is in a solvent, there exists a temperature where they form a uniform solution.

With most substances, the temperature and pressure related to the triple point lie below standard temperature and pressure and the pressure for the critical point lies above standard pressure. Therefore at standard pressure as temperature increases, most substances change from solid to liquid to gas, and at standard temperature as pressure increases, most substances change from gas to liquid to solid.

CHEMICALS 1. Aniline 2. Heptane 3. Distilled water

APPARATUS 1. 2. 3. 4. 5. 6. 7.

Water bath Test tubes Tongs Electronic thermometer Cramp Funnel Burette

PROCEDURE 1. 10ml of aniline was measured into a test tube and then 1ml of heptane added to it. This formed a twophase solution with the aniline (brown) at the bottom and the heptane (colorless) at the top. 2. The two phase solution was then heated in a water bath until a single phase solution (brown) was formed. 3. The temperature at which the single phase formed was measured and recorded. 4. The volume of the aniline was kept constant whiles that of the heptane was varied from 2ml to 10ml, each time repeating steps 1-3.

TABLE OF RESULTS Volume of Aniline in ml 10 10 10 10 10 10 10 10 10 10

Volume of Heptane in ml 1 2 3 4 5 6 7 8 9 10

Temperature in 0C 36.6 73.9 78.0 83.2 85.2 91.4 93.7 96.0 97.3 97.7

CALCULATIONS

DISCUSSION Phase diagrams illustrate the variations between the states of matter of elements or compounds as they relate to pressure and temperatures From the values obtained, it was probable that each of the recorded temperature and corresponding weight fraction would make up a barrier beyond which a single phase solution would exist and below which a two phase solution would form. This is however true for some of the points but for others it does not occur. This can be attributed to experimental errors which are both determinate and indeterminate. From the graph, most of the points that formed the barrier are those which contained less volumes of the solute. With high volumes of the solute, it was difficult to determine whether a complete dissolution has occurred. From the graph, the upper critical solution temperature which is the highest temperature at which phase separation occurs is 97.7oC. Heptane was the solvent at volumes where that of aniline is less i.e. from 0-0.5 whiles from 0.5 upwards, aniline becomes the solvent whiles heptane become the solute. This is so because the more dissolves the less however at equilibrium, aniline is saturated with heptane and heptane is saturated with aniline. With most substances, the temperature related to the triple point lie below standard temperature. Therefore at standard pressure as temperature increases, most substances change from solid to liquid to gas, and at standard temperature as pressure increases, most substances change from gas to liquid to solid.

PRECAUTIONS 1. Since aniline is toxic and can be absorbed by the skin, care was taken not to spill. 2. Accurate measurements were ensured. 3. The solution was heated in a water bath because aniline is explosive CONCLUSION A phase diagram for the aniline-heptane system was constructed and the upper critical solution temperature found to be 97.7oC

REFERENCES 1. Mark W. Zemansky, Richard H. Dittman, Heat and Thermodynamics, McGraw-Hill, 6th ed., 1981, Pages 261 – 265 2. Petrucci, Ralph, and William Harwood. F. Geoffrey Herring. Jeffry Madura. General Chemistry: Principles and Modern Applications. 9th ed. Upper Saddle River, NJ: Pearson, 2007. Page 521 3. Kotz, John C., and Paul Jr. Treichel. Chemistry & Chemical Reactivity. N.p.: Saunders College Publishing, 1999. Page 356

POST LABORATORY QUESTIONS 1

The Lever Rule equation it is called because the lever also uses the same principle in the determination of masses.

α

nα

nβ

lα

lβ

β

To prove the lever rule let n = nα + nβ where nα= the amount of phase α

nβ= the amount of phase β The overall amount of A as nzA nzA = nαxA + nβyA since also

nzA = nαzA + nβzA by equating these two expressions it follows that

nα(x A - zA) = nβ(zA - YA) this implies that nαlα=nβlβ 2

The compositions of the two phases are 0.13 and 0.875 using the lever rule, the relative amount is given by

nα/nβ= lβ/lα= (0.875-0.6)/(0.6-0.13) = 0.585 3

A) The solubility of aniline in heptane at 60oC is 0.44 b) The solubility of heptane in aniline at 60oC is 0.53