Solutions 2
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state henry’s law
at a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution. The mass of gas dissolved in a given volume of liquid at a constant temperature is directly proportional to the pressure of the gas present in equilibirum with the liquid. m = Kp m mass K constant p pressure
state henry’s law in term of partial pressure
“the partial pressure of the gas in vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution” and is expressed as: p = KH x Here KH is the Henry’s law constant.
describe the characteristics of KH
• when we plot a graph of equilibrium pressure v/s mole fraction, we get a straight line whose slope is Kh • Different gases have different values of Kh at the same temperature- Kh is a function of the nature of gas.. The value of Kh differs for a same gas at different temperatures. -When kH increases with the increase in temperature. The solubility decreases with increase in Kh.
applications of henry’s law: carbonated drinks
To increase the solubility of CO2 in soft drinks and soda water, the bottle is sealed under high pressure.
applications of henry’s law: bends/ decompression sickness/ caisson disease
Scuba divers must cope with high concentrations of dissolved gases while breathing air at high pressure underwater. Increased pressure increases the solubility of atmospheric gases in blood. When the divers come towards surface, the pressure gradually decreases. This releases the dissolved gases and leads to the formation of bubbles of nitrogen in the blood. This blocks capillaries and creates a medical condition known as bends, which are painful and dangerous to life. To avoid bends, as well as, the toxic effects of high concentrations of nitrogen in the blood, the tanks used by scuba divers are filled with air diluted with helium. The helium is insoluble in blood. It serves to decrease the amount of dissolved nitrogen and hence reduce its effects.
applications of henry’s law: anoxia/ altitude sickness
At high altitudes the partial pressure of oxygen is less than that at the ground level. This leads to low concentrations of oxygen in the blood and tissues of people living at high altitudes or climbers. Low blood oxygen causes climbers to become weak and unable to think clearly, symptoms of a condition known as anoxia.
state raoult’s law
the Raoult’s law which states that for a solution of volatile liquids,the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in solution. p1 =p1o x1 where p1o is the vapour pressure of pure component 1 at the same temperature. p2 =p2o x2
what is dalton’s partial pressure
According to Dalton’s law of partial pressures, the total pressure ( ptotal ) over the solution phase in the container will be the sum of the partial pressures of the components of the solution P total = P1o + (p2o-p1o) x2
Total vapour pressure over the solution can be related to the mole fraction of any one component. These lines (I and II) pass through the points for which x1 and x2 are equal to unity. Similarly the plot (line III) of ptotal versus x2 is also linear (Fig. 2.3). The minimum value of ptotal is p1 0 and the maximum value is p2 0 , assuming that component 1 is less volatile than component 2, i.e., p1 0 < p2 0 .
why is raoul’ts law a special case of henry’s law
If we compare the equations for Raoult’s law and Henry’s law, it can be seen that the partial pressure of the volatile component or gas is directly proportional to its mole fraction in solution. Only the proportionality constant KH differs from p1 0 . Thus, Raoult’s law becomes a special case of Henry’s law in which KH becomes equal to p1 0 .
what happens when a non volatile solute is added to a volatile solution
In a pure liquid the entire surface is occupied by the molecules of the liquid. If a non-volatile solute is added to a solvent to give a solution [Fig. 2.4.(b)], the vapour pressure of the solution is solely from the solvent alone. This vapour pressure of the solution at a given temperature is found to be lower than the vapour pressure of the pure solvent at the same temperature. In the solution, the surface has both solute and solvent molecules; thereby the fraction of the surface covered by the solvent molecules gets reduced. Consequently, the number of solvent molecules escaping from the surface is correspondingly reduced, thus, the vapour pressure is also reduced.
does the decrease in vapour pressure depend upon
The decrease in the vapour pressure of solvent depends on the quantity of non-volatile solute present in the solution, irrespective of its nature. For example, decrease in the vapour pressure of water by adding 1.0 mol of sucrose to one kg of water is nearly similar to that produced by adding 1.0 mol of urea to the same quantity of water at the same temperature. why do we not refer to salt(nacl)? because nacl splits into 2 ions hence there will be twice as many particles,and hence the change will not be similar to urea/glucose which remain as one molecule.
the vapour pressure of the solvent, x1 be its mole fraction, pi 0 be its vapour pressure in the pure state. Then according to Raoult’s law p1 ∝ x1 and p1 = x1 0 1 p (2.20) The proportionality constant is equal to the vapour pressure of pure solvent, 0 1 p . A plot between the vapour pressure and the mole fraction of the solvent is linear