This will most likely be the case for all problems you encounter related to freezing point depression and boiling point elevation in this course, but it is a good idea to keep an eye out for ions. It is worth mentioning that these equations work for both volatile and nonvolatile solutions.
This means that for the sake of determining freezing point depression or boiling point elevation, the vapor pressure does not effect the change in temperature. Also, remember that a pure solvent is a solution that has had nothing extra added to it or dissolved in it. We will be comparing the properties of that pure solvent with its new properties when added to a solution. Because of this, the newly altered solution's chemical and physical properties will also change.
The properties that undergo changes due to the addition of solutes to a solvent are known as colligative properties. These properties are dependent on the number of solutes added, not on their identity. Two examples of colligative properties are boiling point and freezing point: due to the addition of solutes, the boiling point tends to increase, and freezing point tends to decrease.
The freezing point and boiling point of a pure solvent can be changed when added to a solution. When this occurs, the freezing point of the pure solvent may become lower, and the boiling point may become higher.
The extent to which these changes occur can be found using the formulas:. If solving for the proportionality constant is not the ultimate goal of the problem, these values will most likely be given. Molality is defined as the number of moles of solute per kilogram solvent. Be careful not to use the mass of the entire solution. Often, the problem will give you the change in temperature and the proportionality constant, and you must find the molality first in order to get your final answer.
The solute, in order for it to exert any change on colligative properties, must fulfill two conditions. Colligative properties of solutions are properties that depend upon the concentration of solute molecules or ions, but not upon the identity of the solute.
Colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. Let's first remind ourselves about what vapor pressure is and what affects it chemically and physically.
In a closed container, a liquid will evaporate until an equal amount of molecules are returning to the liquid state as there are escaping into the gas phase. The pressure of the vapor phase above the liquid at this point is called the equilibrium vapor pressure. This vapor pressure is dependent on a number of factors, including the temperature of the system kinetic energy is required to help the molecules escape into the gas phase , the pressure of the system high pressure can keep the gas contained in the liquid e.
What happens to the system if we add a solute to the solvent in question? Well, in the simplest terms, since some of the solute molecules will take up spaces at the surface of the liquid, this will limit the number of solvent molecules at the surface. Since only solvent molecules located at the surface can escape evaporate , the sheer presence of the solute lowers the number of solvent molecules coming and going and therefore lowers the equilibrium vapor pressure.
In order to figure out how much the solute has affected the vapor pressure, we need to approximate how many fewer molecules are able to reach the surface of the liquid, correct? A good approximation of how many molecules there are of solvent versus solute is the mole fraction, X solvent.
If we simply multiply the new mole fraction of solvent by the standard vapor pressure P o solvent of the pure solvent, this will give us a good approximation of the new vapor pressure of the solvent.
This equation is called Raoult's Law. Note that the pressure you obtain from this equation is the new equilibrium vapor pressure of the system with the solute included. For almost all non-volatile solvents, this change will be a negative value. Raoult's law only works for low concentration solutions.
Well, in order for our approximation to work, the interactions between the solute and solvent molecules must be nearly identical. If the interactions are stronger, then the heat of vaporization of the solvent will change and thus our whole approximation falls apart. Since we know that intermolecular forces vary greatly between molecules, the affect of those forces must be kept at a minimum by keeping the solute concentration real low.
You can think of it as having one stinky guy at a party in a room of people, there is a very good chance that you won't have to smell him, but if there were stinky guys in the room, it would ruin the whole party, right? Freezing point depression is the phenomena that describes why adding a solute to a solvent results in the lowering of the freezing point of the solvent.
When a substance starts to freeze, the molecules slow down due to the decreases in temperature, and the intermolecular forces start to take over. The molecules will then arrange themselves in a pattern, and thus turn into a solid. In order to achieve a solid, the solution must be cooled to an even lower temperature. The freezing point depression can also be explained in terms of vapor pressure.
Considering the fact that the vapor pressure of the solid and liquid forms must be the same at freezing point, because otherwise the system would not be at equilibrium, the lowering of the vapor pressure leads to the lowering of the temperature at which the vapor pressures of the liquid and frozen forms of the solution will be equal.
What is the freezing point of an aqueous solution when enough NaCl has been added to create a 0. The K f value for water is 1.
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