The Solutions Lecture Note

The solutions Solutions are a homogeneous mixture of one or more substances contained in a liquid or solid or gaseous phase. In a solution the component present in greater quantity is called a solvent, while the others are called solutes. Each portion of the solution, being homogeneous, must have the same number of solute and solvent particles. Example: in 10 cubic meters of water, each cubic meter has the same composition and each solute particle is surrounded by solvent particles. A liquid solution has a liquid solvent in which at least one other liquid or gas or solid is solubilised. A gaseous solution consists of a solvent gas containing other soluble gases. A solid solution is formed by a major solid substance which forms the crystalline lattice together with another, the solute. A solution is called true or false: A solution is defined as true if each solute particle has a diameter of less than 1nm, otherwise it is defined as false or colloidal if the solute particles have diameters up to 1000 nm. In ion compounds, the solute is solved by the solvent in each of its ions, as if the solvent cages it. Obviously the negative charged part of the solvent molecule is oriented towards the cation and the positive part is oriented towards the anion. Usually aqueous ion solutions conduct current. The solutions are characterized by a solubility. Solubility is the specific quantity of solute that can dissolve in a fixed quantity of solvent at a given temperature. Most solutions, having a liquid as a solvent, with a liquid or solid solute have a solubility proportional to temperature while gaseous solutes often behave in the opposite way. Example: we take a solvent and add solute, making sure that the two compounds are soluble and maintaining a constant temperature, we will obtain an unsaturated solution as long as the solvent is able to solubilize the solute. As soon as the solubility limit is reached, the solute will separate from the solution. We will obtain a precipitating solid or a phase separation between two liquids or a whirl, which will indicate that we have obtained a saturated solution. By varying the temperature of a saturated solution, it is possible to obtain a precipitation given the change in solubility value. This process can be used to make crystals. There are some special conditions in which an over-solute solution can be obtained but, after a small mechanical stimulus or after the addition of a small solute crystal, an instantaneous precipitation of the excess solute itself will be obtained. A similar case can be obtained using a sodium acetate over-saturated solution which, under a mechanical stimulus, tends to solidify in a very short time, with an exothermic process used in hand heaters. Konowaloff's Law The composition of the vapours of a solution is different from that of the solution from which they come. It is richer, in molar fraction, than the more volatile component of the solution. The only exception are azeotropes. The partial pressures of a vapour mixture are bound by Dalton's law: P TOT= pA*yA - pa=particle steam pressure a; - ya = molar fraction of component a in the steam mixture = steam moles a/(steam moles a + steam moles b); - P TOT = total vapour pressure Distillation Distillation is a technique used to separate two or more substances present in a mixture, which exploits the difference in the boiling points of these substances. Here are the most common types of distillation: Simple distillation: allows to separate liquid substances in solution with different boiling temperature of at least 80°C. Fractional distillation or rectification: it can be imagined as a series of simple distillations in a row, where the distilled vapours are recondensed and then re- evaporated in a rectification column. Steam distillation: is a particular type of distillation used to extract thermolabile substances from a solution. There are cases where two liquid substances in solution evaporate together, in a fixed molar fraction ratio and at a constant boiling temperature, and the vapours themselves have the same composition as the solution to be distilled. The solution behaves almost as if it were a pure substance and, therefore, it becomes much more complex to separate the various components, even with fractional distillation! This is the case with azeotropic mixtures. Solid Solutions We have a solid solution where in the crystalline lattice of a solid solvent we have particles of a solid solute instead of a node in the crystalline lattice (solution with "substitution" solute) or between the solvent particles themselves inserting themselves in the space between the solvent particles (solution with "interstitial" solute). The result of these solutions are alterations in the physical properties of the solution with respect to its components, an example being metal alloys. It is not obvious to obtain solid solutions, as conditions of "compatibility" between solvent and solute must be present: similar atomic rays same crystalline structure similar electronegativity similar valence Buffer Solutions Buffer Solutions: Maintaining pH Stability A buffer solution is a mixture of a weak acid/base and its conjugate pair that resists changes in pH. Buffer solutions consist of a weak acid and its conjugate base or a weak base and its conjugate acid, typically in comparable concentrations. This composition allows them to effectively resist pH changes when small amounts of acid or base are added. Key property: pH stabilization against both acidic and basic additions Titration Volumetric analysis or volumetric analysis is an analytical technique consisting of reacting a solution of a known titre of a reagent (titrant) with a known volume of a solution of unknown titre containing the analyte (titrant). Knowing the volume of the titrant used and the stoichiometry of the reaction can be traced back to the concentration of the analyte by simple calculations. The analytical procedure is called titration.