Introductory Chemistry: A Foundation FIFTH EDITION by Steven S. Zumdahl University of Illinois

Solutions Chapter 14

The colors of soap bubbles depend on the thickness of their walls. Source: Graeme Richard White

Solutions Solutions are homogeneous mixtures Solute is the dissolved substance –Seems to “disappear” or “Takes on the state” of the solvent Solvent is the substance the solute dissolves in –Does not appear to change state –When both solute and solvent have the same state, the solvent is the component present in the highest percentage Solutions in which the solvent is water are called aqueous solutions –Water is often called the universal solvent Solutions that contain metal solutes and solvent are called alloys

Brass, a solid solution of copper and zinc, is used to make musical instruments and many other objects. Source: Lawrence Migdale/Stock Boston

The Solution Process - Ionic Compounds When ionic compounds dissolve in water they dissociate into ions –ions become surrounded by water molecules - hydrated When solute particles are surrounded by solvent molecules we say they are solvated

Figure 14.1: When solid sodium chloride dissolves, the ions are dispersed randomly throughout the solution.

Figure 14.2: Polar water molecules interact with the positive and negative ions of a salt.

The Solution Process Covalent Molecules Covalent molecules that are small and have “polar” groups tend to be soluble in water The ability to H-bond with water enhances solubility O H H C O H H H H O H H

Solubility When one substance (solute) dissolves in another (solvent) it is said to be soluble –Salt is soluble in Water, –Bromine is soluble in methylene chloride When one substance does not dissolve in another they are said to be insoluble –Oil is insoluble in Water There is usually a limit to the solubility of one substance in another –Gases are always soluble in each other –Some liquids are always mutually soluble

Solutions & Solubility Molecules that are similar in structure tend to form solutions –Like dissolves like The solubility of the solute in the solvent depends on the temperature –Higher Temp = Larger solubility of solid in liquid –Lower Temp =Larger solubility of gas in liquid The solubility of gases depends on the pressure –Higher pressure = Larger solubility

Figure 14.3: (a) The ethanol molecule contains a polar O—H bond similar to those in the water molecule. (b) The polar water molecule interacts strongly with the polar O— H bond in ethanol.

Figure 14.5: A molecule typical of those found in petroleum. The bonds are not polar.

The drycleaning agent PERC is a health concern for workers in the drycleaning industry. Source: Michael Newman/PhotoEdit

Figure 14.6: An oil layer floating on water.

Describing Solutions - Qualitatively A concentrated solution has a high proportion of solute to solution A dilute solution has a low proportion of solute to solution A saturated solution has the maximum amount of solute that will dissolve in the solvent –Depends on temp An unsaturated solution has less than the saturation limit A supersaturated solution has more than the saturation limit –Unstable

A solution of cobalt (II) nitrate. Source: Tom Pantages

Describing Solutions Quantitatively Solutions have variable composition To describe a solution accurately, you need to describe the components and their relative amounts Concentration = amount of solute in a given amount of solution –Occasionally amount of solvent

Solution Concentration Percentage Parts Per Hundred % = grams of solute per 100 g of solution –Mass Percent or Percent by Mass –5.0% NaCl has 5.0 g of NaCl in every 100 g of solution Mass of Solution = Mass of Solute + Mass of Solvent Divide the mass of solute by the mass of solution and multiply by 100%

What is the percent by mass of CaCl 2 solution if 2.50 grams is added to 50.0 g of water

Solution Concentration Molarity moles of solute per 1 liter of solution used because it describes how many molecules of solute in each liter of solution If a sugar solution concentration is 2.0 M, 1 liter of solution contains 2.0 moles of sugar, 2 liters = 4.0 moles sugar, 0.5 liters = 1.0 mole sugar, etc. molarity = moles of solute liters of solution

What is the molarity of a solution made by dissolving 25 grams of NaCl in 500. mL of water

Molarity & Dissociation the molarity of the ionic compound allows you to determine the molarity of the dissolved ions CaCl 2 (aq) = Ca +2 (aq) + 2 Cl -1 (aq)‏ What is the concentration of Ca +2 and Cl -1 ions in a 0.15 M solution of CaCl 2.

Dilution Dilution is adding solvent to decrease the concentration of a solution The amount of solute stays the same, but the concentration decreases Dilution Formula M 1 x V 1 = M 2 x V 2 Concentrations and Volumes can be most units as long as consistent

Figure 14.7: Steps involved in the preparation of a standard aqueous solution.

Figure 14.8: (a) 28.6 mL of 17.5 M acetic acid solution is transferred to a volumetric flask that already contains some water. (b) Water is added to the flask (with swirling) to bring the volume to the calibration mark, and the solution is mixed by inverting the flask several times. (c) The resulting solution is 1.00 M acetic acid.

Approximate dilutions can be carried out using a calibrated beaker. Here concentrated sulfuric acid is being added to water to make a dilute solution. Source: Tom Pantages

When aqueous sodium chloride is added to a solution of silver nitrate, a white silver chloride precipitate forms.

Solution Stoichiometry Many reactions occur in solution, therefore you need to be able to predict amounts of reactants and products in terms of concentrations and volumes as well as masses Basic strategy is the same  Balance the Equation  Change Given Amounts to Moles  Determine Limiting Reactant  Calculate Moles of Required Substance °Convert Moles of the Required Substance into the Desired Unit

Example Calculate the Mass of Solid NaCl required to Precipitate all the Ag +1 ions from 1.50 L of a M AgNO 3 Solution

Neutralization Reactions Acid-Base reactions are also called Neutralization Reactions Often we use neutralization reactions to determine the concentration of an unknown acid or base The procedure is called a titration. With this procedure we can add just enough acid solution to neutralize a known volume of a base solution –Or visa versa

Solutions

Solution Formation Solutions are homogeneous mixtures that may be solid, liquid, or gaseous. Properties of solutions change when the number of particles dissolved in them is changed.

Factors affecting solubility are the following…

–Stirring(agitation): speeds up the rate that a crystal will dissolve because the dissolving process is a surface phenomenon. –Temperature: Increase the rate at which solutes dissolve because the kinetic energy of the solute is greater at higher temperature –Particle Size: a fine powder dissolves quicker than larger crystals because finer particles expose a greater surface area to the colliding water molecules.

Solubility Solubility: is the amount that dissolves in a given quantity of a solvent at a given temperature to produce a saturated solution.

Solubility A solution that contains less solute than a saturated solution is unsaturated. Two liquids are miscible if they dissolve in each other. Liquids that are insoluble in each other are immiscible.

Factors Affecting Solubility The solubility of most solid substances increase as the temperature of the solvent increases. Gas solubility increases as the partial pressure of the gas above the solution increases.

Factors Affecting Solubility Henrys Law:at a given temperature the solubility (S) of a gas in a liquid is directly proportional to the pressure(p) of the gas above the liquid S 1 = S 2 P 1 P 2 Supersaturated solution : a solution that contains more solute than it should theoretically can continue to hold at a given temperature.

Molarity Molarity is the number of moles of a solute dissolved per liter of solution. –Molarity = Moles of solute Liters of solution The concentration of a solution is a measure of the amount of solute that is dissolved in a given quantity of solvent.

Molarity Dilute solution is one that contains only a low concentration of solute. Concentrated solution contains a high concentration of solute.

Boiling-Point Elevation The boiling point elevation: the difference in temperature between the boiling point of a solution and that of the pure solvent. The boiling point of a substance is the temperature at which the vapor pressure of the liquid phase equals the atmospheric pressure.

Boiling-Point Elevation Attractive forces exist between the solvent and solute particles. The presence of a solute raises the boiling point in the solvent. Boiling point elevation is a colligative property, it depends on the concentration of particles, not their identity

Freezing-Point Depression The freezing-point depression: the difference in temperature between the freezing point of a solution and that of the pure solvent. (freezing- point depression is also a colligative property)‏

Freezing-Point Depression When a substance freezes, the particles of the solid take on a pattern. The presence of a solute in water disrupts the formation of this pattern because of the shells of water of solvation.

Thermal Pollution Gas in a liquid  O 2 dissolved in water  relate to “thermal pollution” Temp changes the Solubility in a liquid Ecosystem: O 2 decreases, Fish population decreases, Algae increase -Oxygen dissolves into water because gas is the solute -Decrease the temp of water, it will dissolve faster -Decrease the rate of dissolving and Oxygen goes down

Reducing Thermal Pollution