Solution Chemistry

Solution Chemistry Honors Chemistry

Types of Solutions • Unsaturated • Less than the maximum amount of solute for that temperature is dissolved in the solvent.

Types of Solutions • Saturated • Solvent holds as much solute as is possible at that temperature. • Dissolved solute is in dynamic equilibrium with solid solute particles.

Types of Solutions • Supersaturated • Solvent holds more solute than is normally possible at that temperature. • These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.

Adding Solute to various Solutions unsaturated saturated supersaturated Tro's Introductory Chemistry, Chapter 13

Supersaturation

Solubility Rules • We use a list of rules to help us determine which compounds will dissolve in water and which will remain a solid. • You must memorize these rules!

Predicting the Formation of a Precipitate • A precipitate is a solid that forms out of solution. • Ex: the mixing of two clear solutions: silver nitrate (AgNO3) and sodium chloride (NaCl):

Practice Solubility #1 Use the solubility rules to determine if the following compounds are soluble when placed in water. Write the formula followed by “(s)” or “(aq)”: • Barium sulfate • Plumbic chloride • Potassium chlorate • Sodium nitride • Beryllium chromate • Lithium chloride • Magnesium carbonate • Calcium sulfate • Ammonium phosphate

Once you know it will dissolve, how can you make it dissolve faster?? • Increasing the contact will increase the speed at which something dissolves • Stirring/Agitation • Temperature • Surface Area • Pressure (gases)

1. Stirring/Agitation • Stirring or shaking will help a solid dissolve faster in a liquid. • Moving the solvent (liquid) around will increase the contact between solute and solvent.

2. Surface Area • Breaking the solute up into smaller pieces increases the surface area • This allows more solute-solvent particles direct contact • Increases the solubility

3. Solubility and Temperature • the solubility of the solute in the solvent depends on the temperature • Higher temp = higher solubility of solid in liquid • Higher temp = lower solubility of gas in liquid • As the temperature increases, the motion of the particles increases, increasing contact between solute and solvent.

Temperature (Solids in Liquid) Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

Temperature (Gases in Liquid) • The opposite is true of gases: • Carbonated soft drinks are more “bubbly” if stored in the refrigerator. • Warm lakes have less O2 dissolved in them than cool lakes.

Solubility and Temperature Warm soda pop fizzes more than cold soda pop because the solubility of CO2 in water decreases as temperature increases. Tro's Introductory Chemistry, Chapter 13

Gases in Solution • The solubility of liquids and solids does not change appreciably with pressure. • The solubility of a gas in a liquid is directly proportional to its pressure.

Solubility and Pressure • the solubility of gases in water depends on the pressure of the gas • higher pressure = higher solubility Tro's Introductory Chemistry, Chapter 13

Solubility and Pressure When soda pop is sealed, the CO2 is under pressure. Opening the container lowers the pressure, which decreases the solubility of CO2 and causes bubbles to form. Tro's Introductory Chemistry, Chapter 13

Henry’s Law Sg = kPg where • Sg is the solubility of the gas; • k is the Henry’s law constant for that gas in that solvent; • Pg is the partial pressure of the gas above the liquid. • OR……comparing two solutions S1/P1 = S2/P2

Practice Solubility #2 During the industrial revolution, factories were built on rivers so that the river water could be used as a coolant for the machinery. The hot water was dumped back into the river and cool water recirculated. After some time, the rivers began to darken and many fish died. The water was not found to be contaminated by the machinery. What was the cause of the mysterious fish kills?

Practice Solubility #3 The solubility of oxygen gas is 2.2 x 10-4 M at 0oC and 0.10 atm. Calculate the solubility of oxygen gas at 0oC and 35.46 kPa.

Student, Beware! Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved.

Student, Beware! • Dissolution is a physical change—you can get back the original solute by evaporating the solvent. • If you can’t, the substance didn’t dissolve, it reacted.

Concentration • A measure of the amount of solute that is dissolved in a given amount of solvent. • Solute: what goes into the solution • Solvent: the dissolving medium in a solution

Solution Concentration Descriptions • dilute solutions have low solute concentrations • concentrated solutions have high solute concentrations Tro's Introductory Chemistry, Chapter 13

Concentrations – Quantitative Descriptions of Solutions • 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 Tro's Introductory Chemistry, Chapter 13

Ways of Expressing Concentrations of Solutions

Expressing Concentration

Temperature Extremes • Molarity changes with temperature • Molality, Weight Percentages, and Mole Fractions do not!

Mass Percent • parts of solute in every 100 parts solution • if a solution is 0.9% by mass, then there are 0.9 grams of solute in every 100 grams of solution • or 10 kg solute in every 100 kg solution • since masses are additive, the mass of the solution is the sum of the masses of solute and solvent Tro's Introductory Chemistry, Chapter 13

mass of A in solution total mass of solution Mass Percentage Mass % of A =  100

Percent: By mass and volume

Mole Fraction (X) In some applications, one needs the mole fraction of solvent, not solute—make sure you find the quantity you need! moles of A total moles in solution XA =

where XAand XB represent mole fractions of each substance. A = solute B = solvent N = number of moles Mole fractionis the ratio of the number of moles of solute in solution to the total number of moles of solute and solvent.

Expressing Concentration (cont.) Each mole fraction can be thought of as a percent. Ex: The mole fraction of water is 0.78, which is equivalent to 78%.

Practice Concentration #1 What is the mole fraction of NaOH in an aqueous solution that contains 22.8% NaOH by mass?

Practice Concentration #1 What is the mole fraction of NaOH in an aqueous solution that contains 22.8% NaOH by mass? Convert mass to moles and solve for mole fraction.

Practice Concentration #2 Calculate the mass percent of a solution containing 27.5 g of ethanol (C2H6O) and 175 mL of H2O (assume the density of H2O is 1.00 g/mL) Tro's Introductory Chemistry, Chapter 13

mass of A in solution total mass of solution mass of A in solution total mass of solution Parts per Million andParts per Billion ppm = Parts per Million (ppm)  106 Parts per Billion (ppb)  109 ppb =

Practice Concentration #3 • What is concentration, in parts per million (ppm), of a solution in which 0.80 grams of sodium hydroxide, NaOH, is dissolved in 1 liter of solution?

moles of solute liters of solution molarity = Solution ConcentrationMolarity • 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 Tro's Introductory Chemistry, Chapter 13

mol of solute L of solution M = Molarity (M) • Because volume is temperature dependent, molarity can change with temperature: as temperature increases the volume of the solvent also increases slightly, changing the concentration.

Molarity Symbol = M, read as “molar” This means that if you have a 3.5 M solution of some compound, it contains 3.5 moles of the solute per liter of solution.

Practice Concentration #4 What is the molarity of a 5.00 liter solution that was made with 11.2 moles of KBr?

Solution Chemistry