|Chapter Six, Section Two|
I. Water as a Solvent
Water dissolves ionic compounds by glomping onto the ions and dragging them away from the ionic crystals. With me? Once the ions are separated from the crystals, the ions are surrounded by water molecules stabilizing the ions in solution. The ions are said to be hydrated when the solvent is water and solvated in general.
Some crystals participate in this process excessively so that they will drag water molecules out of the air and incorporate them in the crystal structure -- kinda scary, isn't it? Such crystals are said to be hygroscopic and the water molecules are actually chemically combined with the ionic compounds. Such a crystal without the water molecules is said to be anhydrous. The crystals with the water molecules strongly a part of their structure are called hydrates, such as CuSO4*2H2O -- gypsum.
If a positive electrode and a negative electrode are inserted into a water solution of ions, the negative ions will migrate toward the positive electrode and the positive ions will migrate toward the negative electrode. Such a movement of ions is an electric current. The positive electrode is called the anode (anions flow toward it) and the negative electrode is called the cathode (cations flow toward it). Substances that conduct electricity in a solution or in the molten state are called electrolytes and those that don't are called nonelectolytes. Compounds that dissociate completely in solution are called strong electrolytes and those that only dissociate partly are called weak electrolytes.
Covalent compounds very rarely form ions in solution. Important examples are HCl and SO3. Most organic compounds dissolve in water because they form hydrogen bonds with the water molecules. You will remember that means that the covalent compounds must contain O, N, or F atoms arranged in such a way as to allow hydrogen bonding with the water molecules. Each O atom must be chemically attached to a hydrogen atom in the covalent compound. Then the oxygen atom will have a partial negative charge and the hydrogen atom will have a partial positive charge. A hydrogen atom in the water molecule can then form a hydrogen bond with the oxygen atom in the covalent compound and similarly the oxygen atom in the water molecule can form a hydrogen bond with a hydrogen atom chemically bonded to a O, N, or F atom in the covalent molecule.
What differentiates solvents from colloids is the size of the solute particles. If the diameter of the particles is > 1 nm the solotion is a colloid. The range of particle size for colloids is 1nm to 1000 nm. Such size particles have a significant surface area, which gives rise to some unusual properties. Although homogeneously dispersed throughout the solution, the particles will scatter light and thus appear cloudy, turbid, or milky. Your text shows several examples of colloids in solid, liquid, and gas phases. Colloids are stable. Mayonnaise is a colloid.
When the particles are > 1000 nm, the suspension becomes unstable. Muddy ponds are not colloids because the suspension is not stable. So why are suspensions unstable and colloids stable? Remember that the particles in solution are not just sitting there being cool. They are moving around quite rapidly in what is called Brownian motion. If the particles are between 1 nm and 1000 nm in size when they collide they will not stick together, but continue to move around as mostly individual particles. If the particles are > 1000 nm in size when they collide they will stick together forming larger particles which will settle out of solution. Why do the smaller particles not stick together? Basically for two reasons:
If one desires to precipitate the particles out of a colloid, then these two factors have to be countered. Blood is a colloidal suspension with protein particles in the solution. If we desire to separate the proteins from the blood, this can be done by either removing the solvent sphere (add something like ethanol which has a high affinity for water) or by removing the charged sphere (add an electrolyte like NaCl to destroy the charged sphere).
III. Colligative Properties
Some properties of a system depend only upon the number of particles in solution rather than the type of particles. These properties are called colligative properties and consist of freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure. We will only cover freezing point depression and osmotic pressure.
Freezing point depression
The addition of a solute to a solvent will result in a solution that has a lower freezing point than the freezing point of the orginal solvent. The effect is dependent upon the number of particles of the solute and not upon the chemical nature of the individual particles. Remember that ionic compounds dissociate into their ions in an appropriate solvent. So the number of particles to cause a change in the freezing point is two for NaCl, three for BaCl2, etc. However for a covalent compound, there is only one particle in solution. The equation to calculate the freezing point depression is
T = m*Kfn
where T is the lowering of the freezing point from the value of the pure solvent, m is the molality (moles solute/kg of solvent) of the solution, Kf is the freezing point depression constant for the solvent (1.860 kgoK/mole for water), and n is the number of particles of solute in the solution.
Remember that T is numerically the same whether we use oC or Kelvin.
So let's look at an example. Suppose that we put 264 g of Na2SO3 in 500 g of water. What is the freezing point of the solution?
264g/126.05g/mol = 2.095 moles of Na2SO3. The molality is then
2.095moles * 2 = 4.19 moles/kgm. (Remember that molality is moles/kg and we have 2.095 moles in 1/2 kg (500 g))
So, T = 3 particles * 4.19 moles/kg * (1.860 kgK/mole) = 23.38 K = 23.38 oC. So the new freezing point is -23.38oC.
Osmotic Pressure, or the principle of the prune
Surely someone out there has either bought dried prunes and put them in hot water, or seen their mom or dad do such a thing. What happens? The prones take up water and expand -- sometimes they will burst, and that is not a pleasant sight! Why does pure water flow into the prune? The inside of the pruned has some water and sugar so that it is a solution. The skin of the prune serves as a semipermeable membrane so that the pure water will flow into the prune but the sugar molecules will not flow out (unless the skin breaks). This process is referred to as osmosis. The direction of flow is always from the region of high concentration of molecules that can pass the barrier to low concentration. The pressure that is established because of the flow is called the osmotic pressure. It is important to understand that the osmotic pressure is the result of the flow and not the cause of the flow. In science it is always critical to understand what is the cause and what is the effect.
To make the osmotic flow occur in the direction opposite to that which would naturally occur, one must apply pressure to the low concentration side. Suppose that we have a container of salt water and we wish to separate the water from the salt. If we insert a semipermeable membrane over the end of the container (one that will allow the water to pass, but not the salt), and if we then apply pressure to the salt solutionside, the pure water will flow out of the salt water. Voila, we have a process for producing pure water from salt water.
After you have studied this material and practiced some problems, take quiz two. If you score at least 80 on the test then you are ready to continue to the next section.
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