The Properties of Mixtures: Solutions REVIEW

Solution – any substance that is evenly dispersed or distributed throughout another substance. A. homogeneous mixture – mixture in which the components are uniformly mixed and cannot be visually distinguished. B. heterogeneous mixture – mixture characterized by observable segregation of component substances. C. A solution contains both a solvent (dissolving medium) and solute (substance dissolved in the solvent). D. Solid solutions with a metal as the solvent are called alloys. For example, solid brass is a mixture of Cu and Zn.

The major types of intermolecular forces in solutions.

LIKE DISSOLVES LIKE Substances with similar types of intermolecular forces dissolve in each other. When a solute dissolves in a solvent, solute-solute interactions and solvent- solvent interactions are being replaced with solute-solvent interactions. The forces must be comparable in strength in order to have a solution occur.

Like dissolves like: solubility of methanol in water. water methanol A solution of methanol in water

SOLUTIONS AND CONCENTRATION Qualitative: dilute vs concentrated dilute = relatively small amount of solute in solvent concentrated = relatively large amount of solute in solvent Quantitative solubility = amount of solute needed to form a saturated solution in a given solvent saturated = a solution in equilibrium with undissolved solute. Additional solute will not dissolve unsaturated = more solute needs to be added before reaching equilibrium saturation supersaturated = solution containing more solute than saturated solution

Expressing Solution Concentration

A. parts per billion (for even MORE dilute solutions!) ppb of component = B. mole fraction (denoted with the symbol  ) Mole fraction of component = C. molality (denoted with the lowercase letter m) molality = The molality of a given solution does NOT vary with temperature because masses do not vary with temperature. Molarity, however, changes with temperature because the expansion or contraction of the solution changes its volume. Thus, molality is often the concentration unit of choice when a solution is to be used over a range of temperatures.

MOLARITY & DILUTION M 1 V 1 = M 2 V 2 The act of diluting a solution is to simply add more water (the solvent) thus leaving the amount of solute unchanged. Since the amount or moles of solute before dilution (n b ) and the moles of solute after the dilution (n a ) are the same: n b = n a And the moles for any solution can be calculated by n=MV A relationship can be established such that M b V b = n b = n a = M a V a Or simply : M b V b = M a V a

Acid-Base Titrations Acid-base titrations are an example of volumetric analysis, a technique in which one solution is used to analyze another. The solution used to carry out the analysis is called the titrant and is delivered from a device called a buret, which measures the volume accurately. The point in the titration at which enough titrant has been added to react exactly with the substance being determined is called the equivalence point (or stoichiometric point). This point is often marked by the change in color of a chemical called an indicator. The titration set-up is illustrated in the schematic shown above.

Acid-Base Titrations The following requirements must be met in order for a titration to be successful: 1.The concentration of the titrant must be known (called the standard solution). 2.The exact reaction between the titrant and reacted substance must be known. 3.The equivalence point must be known. An indicator that changes color at, or very near, the equivalence point is often used. 4.The point at which the indicator changes color is called the end point. The goal is to choose an indicator whose end point coincides with the equivalence point. NOTE: Equivalence Point  End Point! WHY??? 5. The volume of titrant required to reach the equivalence point must be known (measured) as accurately as possible.

Acid-Base Titrations When a substance being analyzed contains an acid, the amount of acid present is usually determined by titration with a standard solution containing hydroxide ions. The pH at certain points in the titration can be taken using different indicators, or alternatively, a pH meter can be used to give a readout of the exact pH. pH = - Log [H 3 O + ] pH > 7 is referred to as a base pH < 7 is referred to as an acid