Expt2 Partially Miscible Liquids

Group Leader: Jean Criste T. Cainila August 25, 2009 Group No.3 2, 2009 4ChE-B Marcelo,Ph.D.

Date

Performed:

Date Submitted:

September

Professor: Philipina A.

Experiment 2: Partially Miscible Liquids: Determination of Mutual Solubility I. Introduction The objective of the experiment is to determine the solubility of two partially miscible liquids, construct the mutual solubility curve for the pair and determine their critical solution temperature. Miscibility means how completely two or more liquids dissolve in each other. It is a qualitative rather than quantitative observation—miscible, partially miscible, not miscible. Like any other solubility phenomenon, miscibility depends on the forces of attraction between the molecules of the different liquids. The rule of thumb "like dissolves like" means that liquids with similar molecular structures, in particular similar polarity, will likely dissolve in each other. (Polarity means the extent to which partial positive and negative charges appear on a molecule, because of the type and arrangement of its component atoms.) However there are some substances with varying polarity like water which will play a vital issue on how solute will dissolve in it.

Volume vs Solute The above diagram shows the solubility trend mostly occuring in solid-liquid solutions. The level of concentration of the dissolved solute can be classified as saturated, supersaturated and unsaturated solution. Saturated solution is attained when the maximum amount of solute a solvent can dissolve has already been reached. The saturation point of a solution is represented graphically by points along the straight line. On the other hand, unsaturated solutions are those when the maximum amount of solute dissolved is not yet reached by the solvent. The unsaturated region is represented by the lower region of the graph(below the line).Lastly, a supersaturated solution is attained when the maximum amount of solute is exceeded and precipitation are evident. The supersaturated solution takes the section above the saturation line. This however can be altered depending on the temperature on which dissolving takes place. For solubility depends highly on temperature. For a liquid-liquid solution, it is but normal for components to be immiscible at the moment they are introduced to one another however they will be miscible once stirred or mixed. In some cases some liquiid-liquid solutions can be completely immiscible even if

stirring and mixing and other alteration has already been made. Liquid systems that can break up into two liquid phases are the simplest of two-component phase diagrams. Such systems are usually treated at some constant pressure high enough to ensure that no vapor can occur in equilibrium with the liquid phases and over a range of temperatures high enough to ensure that no solid phase appear. The region of the components signifies that there are regions of pure component A and B but there is also a region where in a part of A is dissolved in B and vice-versa. This is now characterized as the partial miscibility of a certain liquid-liquid solution. Such solutions are heated and cooled to attain at constant pressure to attain the complete homogeneity of the solution or complete miscibility. The graph below shows the trend for partially immiscible liquids that are either heated or cooled.

Partial Miscibility Plot From the figure above it is noticeable that a peak or maximum temperature is reached by the solution but it the maximum temperature of the solution differs into some extent to the critical temperature of a solution. The critical temperature of a solution marks the point wherein the solution will be completely miscible either if the temperature is lowered or increased. More of critical temperature will be discussed on the latter part of the analysis As been stated ealier miscibility is a solubility phenomenon, thus, it can also be affected by factors which plays an important role in a substance solubility. The factors are as follows: (1) nature of solute/solvent, (2) temperature and (3) pressure. For liquid and solid solution pressure does not affect much of their solubility. Temperature on the other hand will depend whether the reaction results to an endothermic or exothermic process, if endothermic the solubility of the solution will increase with an increase in temperature. For an exothermic process, solubility will increase with a decrease in temperature. The nature of solvent and solute also take part in its solubility, such nature involves polarity and molecular size. Polarity was discussed earlier stating that “likes dissolves like”. While larger molecules are less soluble than that of the smaller ones. If the phase rule is applied in this two-phase mixture, two variables are needed to be specified to describe the system completely. The phase rule calculation is shown below. P=c–p+2 =2–2+2 =2 At constant pressure, the remaining significant variables are temperature and composition. The composition is uttered in percent by weight. Usually, both liquids become more soluble as the temperature is increased, and eventually a critical solution temperature is reached above which the two liquids are completely miscible. An upper critical solution temperature will be observable, the highest temperature at which phase separation occurs. On the other hand, some systems show a lower critical solution temperature below which the components mix in all proportions and above which the components form two phases.

II. Methodology

Preliminary procedures were done before the actual experiment. The density of phenol was obtained from literature and the amount of water required to prepare 5% to 95% phenol at 5% increment, starting with 10 mL phenol in each proportion, was calculated. 95% phenol-5% water by weight mixture was prepared based on 10 mL phenol. The mixture was heated with mild stirring until the cloudiness disappeared, the temperature of which was recorded. The mixture was then cooled with mild stirring until the cloudiness reappeared, the temperature of which was recorded. The process was repeated until fairly constant readings were observed. Water was added to make 90% phenol and 10% water. The procedure was repeated until 55% phenol. For 5% to 50%, 5 mL phenol was initially used.

III.Discussion of Results Different changes were observed and noted in the experiment. After the 95% phenol5% water by weight solution was prepared, the mixture was heated in a water bath with mild stirring and recorded its constant temperature until the cloudiness of the solution disappeared and cooled instantly until the cloudiness reappeared. It is observed that the turbidity, which signifies the partial immiscibility of the liquids, is highly dependent on concentration of mixture and temperature. If the mixture is highly concentrated to either water or phenol, after the mild stirring in room temperature there would be no visible cloudiness observed. It is seen in the 5% and 95% phenol-water solution that no cloudiness appeared because both exhibits almost a pure compound. In the 80, 85, and 90% it is noted that there was cloudiness before heating the mixture and required a certain heating temperature to make the mixture miscible. In the cooling process, however, it is observed that no cloudiness reappeared. The reason for this is that just a certain small amount of heat can make the solute and solvent miscible but once the equilibrium is established, the mixture is hard to separate into two components. An account to the said results is seen on Table I. Table I: Values of the temperature for each increment % by Weight Phenol-Water Solution 5 10 15 20 25 30 35 40 45 50 55 60 65

HEATING Temperat ure (ºC) No change 40 58 65 66 66 66 66 66 66 64 63 55

COOLING Temperat ure (ºC) No change 38 56 63 64 65 65 65 66 64 58 58 54

70 75 80 85 90 95

45 41 34 30 29 No change

No No No No

42 30 change change change change

The heating and cooling temperatures gradually increase but as it reached a certain constant high temperature, it gradually decreases. It is seen on the 45% mixture that the heating and cooling temperature is the same. It is where, after heating the solution and putting in the ice bath, the cloudiness immediately reappears. The temperature versus percentage by weight phenol mixture was plotted in order to determine the critical solution temperature of the mixture and is seen in Graph 1.

Graph 1 Critical solution temperature is the temperature at which a mixture of two liquids immiscible at ordinary temperatures, cease to separate into two distinct phases. The red line symbolizes the temperature reading of phenol mixture in the hot water bath while the blue line is the temperature reading of phenol mixture in the cold water bath. To clearly see the single and double-phase regions, the lines were set to polynomial trend line forming smooth curves. The dotted lines represent the temperature at which the cloudiness of the mixture disappeared in the heating step and said to be the state where the mixture is single-phase. The dashed lines represent the temperature at which the cloudiness reappeared in the cooling step and said to be the point in which the mixture began to separate and became double-phase. It is seen in the graph that there would be a region created in the intersection of the dotted and dashed line. This region is a single-phase region in which the phenol and water are miscible. Beyond this region would be a doublephase in which the mixture is said to be partially miscible or immiscible. Critical solution temperature is the temperature which will lead to separation of the mixture into two distinct liquid phases. This is denoted by the intersection of the polynomial trend line of the heating and cooling temperature. The critical temperatures at which the lines intersect are 63ºC and 66ºC.

IV.Answers to Questions 1. Two types of liquid-liquid critical points are the upper critical solution temperature, or, which denotes the warmest point at which cooling will induce phase separation, and the lower critical solution temperature, which denotes the coolest point at which heating will induce phase separation. Based on Graph 1, the maximum temperature is 66ºC and the minimum critical solution temperature is 63ºC. 2. The different temperature readings have very small differences. Thus, the temperature readings were constant.

3. The high phenol concentration of percentage of phenol-water mixture solidifies during cooling process mainly because of phenol’s freezing point. At 41˚C, phenol forms into a solid that can be liquefied by a very small amount of water. Thus, a high concentration solution of phenol with water would solidify on lower temperatures.

4. Given an unknown solution of the components, its concentration will be known by the use of the mutual solubility plot/graph. The composition of a layer can be determined by getting the critical solution temperature experimentally and plotting the point on the graph. The weight percentage could be obtained at the point where in the temperature intersects the curve. I.

Conclusions In this experiment, it is concluded that the solubility of two partially miscible liquids could be determined by constructing a mutual solubility curve for the pair of liquids. Turbidity of the mixture signified two-phase mixture and the cloudiness could disappear and reappear depending on the heating and cooling temperature at which the mixture became one-phase. The heating and cooling temperatures gradually increased but as it reached a certain constant high temperature, it gradually decreased. It is also concluded that in the plot of the mutual solubility curve, a region was enclosed and signified the single-phase stage of the mixture. Beyond that region, the mixture is of double-phase. The intersection of the trend lines for heating and cooling would be the critical solution temperature of the mixture and was found to be approximately 63ºC and 66ºC.

II. References [1] Atkins, Peter & De Paula, Julio. Atkins’ Physical Chemistry, 7th ed. (Oxford University Press Inc., New York, 2002.) [2] Barrow, Gordon M. Physical Chemistry, 6th ed. (The McGraw-Hill Companies Inc., USA, 1995) pp. 327-330 [3] Daniels, Farrigton and Alberty, Robert. Physical Chemistry, 2nd ed. (John Wiley & Sons, Inc., USA, 1961) pp. 241-243 [5] http://science.jrank.org/pages/4382/Miscibility.html [4] http://www.citycollegiate.com/chapter3d.htm [6] http://www.sciencebyjones.com/solubility.htm