SolutionsChapters 12 and 13

Holt Modern ChemistryLisa Allen, Stonington High School

Concepts to considerSolubility

Energy changes in dissolvingConcentrations

Precipitation reactionsNet ionic equations

Effects on vapor pressure

Solubility VocabularySolventSoluteElectrolyteSaturatedUnsaturatedSolution equilibriumSupersaturated

I put some salt in water. The water is the solvent, and the salt is a solute. The solution was dilute, because I only used a little solute. I added more, and made a more concentrated solution. Later, by heating the solution, I increased the solubility of the salt, and was able to dissolve much more. The concentration at equilibrium was much greater in hot water. By gently cooling this hot saturated solution, I was able to obtain a supersaturated solution.

How to use these words…

Reading solubility curves

Solubility curve Simple questions

What is the solubility of potassium nitrate at 60˚C?

What is the solubility of sodium chloride at 20˚C?

Which of these substances is most soluble at 80 ˚C?

What is the effect of temperature on solubility?

See pages 410 and 414. Note the differences between gases and solids!

And what is the effect of dissolving on temperature?Enthalpy of solution: “the net amount of energy

absorbed as heat by the solution when a specific amount of solute dissolves in a solvent”

HMC, p. 416

Enthalpy of solution

Endothermic processes Exothermic processes

ABSORBING energy Breaking up the solid

solvent Moving molecules of

solvent apart to make room for solute

RELEASING energy Forming solvent/solute

attractions

What is the net change?

Endothermic Exothermic

If these processes absorb MORE energy than those on the right, the net difference is that the solution TAKES IN energy.

The enthalpy of solution is POSITIVE

Solutions get COLDER

If these processes absorb MORE energy than those on the left, the net difference is that the solution GIVES OFF energy.

The enthalpy of solution is NEGATIVE.

Solutions get WARMER

Enthalpy of solution problemsAs always, solving chemistry problems is easy if you use dimensional analysis, and

know what the units of your answer should be!

How much energy is released by the dissolving of 125 grams of KOH? (p 416)

If that energy is used to heat 800 grams of 25 ˚C water, which has a specific heat capacity of 4.18 J/g ˚C, what is the final temperature of the water?

Divide A/B to find DT. Add

to original temperature

.

A. Find the joules of energy

generated or absorbed

B. Find the joules required to change

the solvent temperature 1

degree

Determine whether the

solution becomes hotter or cooler

Calculating temperature change

What is the final temperature of 240 grams of water at 25 ˚C to which 35 grams of HCl is added?

What is the final temperature of 100. grams of water at 25 ˚C in which 12 grams of KClO3 is dissolved?

What is the final temperature of 50. grams of water at 25 ˚C in which 20. grams of NaCl is dissolved?

More practice

HOMEWORKPages 426 – 427

#10-14#36, 37

ConcentrationMolarity and molality:

2 ways to define concentrations of solutions

Designated with a capital M Easiest to use of all of the ways to quantify

concentrations Allows users to obtain known number of

moles of solutes from volumes of solutions 2M means that a liter of the solution will

contain 2 moles of the solute, 500 mL will contain a mole of it.

Molarity = moles soluteliters solvent

Molarity = Moles Liters

Liters x Molarity = Moles

Liters = Moles Molarity

Algebra in action!

Determine the molarity of a solution prepared by dissolving 141.6 grams of citric acid, C3H5O(COOH)3, in water and then diluting the resultant solution to 3500.0 mL.

What mass of ammonium chloride is dissolved in 300. mL of a 0.875M solution?

What mass of magnesium bromide would be required to prepare 720. mL of a 0.0939 M aqueous solution?

Molarity math practice

Molarity x volume = number of moles This means that if you put a certain number

of moles of a solute into a solvent, adding more solvent does not change the number of moles!

The number of moles will stay constant as you alter the volume of the solution, and the molarity will change to reflect that.

Dilution math M1 xV1 = M2 xV2

Dilutions and mixing solutions

Rearrange the equation first!

Use moles/liters instead of M to facilitate canceling units.

HCl is 12M when concentrated. If I need 150 mL of 2.0M solution for a lab, how do I mix it?

125 mL of 5.0 M potassium nitrate solution is diluted to 500 mL. What is the new concentration?

Plug in the values with their labels.

BTW-Special precautions must be taken when diluting acids!

If I combine 5.00 L of 1.5M HCl with 3.5 L of 2.5M NaOH, which is the limiting reactant?

What is the molarity of the resultant salt solution?

Don’t panic! Just calculate moles of each reactant, and use

the mole ratio to find the moles of product from each. The one that makes the smallest amount of product is the limiting reactant.

Molarity and stoichiometry

More practice with molarity mathStrategy:

Write a balanced chemical equation Convert each reactant to moles

Use stoichiometry to find moles of productEstablish which reactant is limiting

Calculate grams of product

50 mL of .065 M silver nitrate is combined with 50 mL of 1.0M NaCl solution. What is the mass of solid silver chloride that precipitates?

Homework Page 427, 18-28

Aqueous cadmium chloride reacts with sodium sulfide to produce bright yellow cadmium sulfide.

How many moles of CdCl2 are in 50.00 mL of 3.91 M solution?

If this solution is reacted with excess sodium sulfide, how many moles of CdS would form?

If this solution is reacted with 35.00 mL of 4.33 M sodium sulfide solution, which is the limiting reactant?

More practice on solution stoichiometry

What is the molarity of a solution of barium nitrate made from 28.83 grams of solid in 250. mL of solution?

What is the molarity of sodium sulfate if 7.00 grams are in 35.0 mL of solution?

If I combine these solutions, what is the limiting reactant?

How many grams of solid product is produced?

What is the molarity of the sodium nitrate solution that remains?

Barium nitrate + Sodium sulfate ?

Net ionic equationsA slightly different way to look at solution

reactions.

Writing net ionic equations

Step by step

Write the dissociation reactions for each of the reactant compounds.

Write the reaction of these aqueous compounds with the dissociated ions as reactants.

Determine the solubility of the possible products.

Insoluble products are written as a compound, soluble ones are called “spectators”

NO PRECIPITATE, NO REACTION!!!

Practice from page 440

Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined? If so, write the net ionic equation for the reaction.

• Write the equation that represents the dissolution of barium nitrate in water.

• Write the equation that represents the dissolution of sodium sulfate in water.

• Write the complete ionic equation.

• Write the net ionic equation.

• Identify the spectator ions.

Will a precipitate form if solutions of potassium sulfate and barium nitrate are combined? If so, identify the spectator ions and write the net ionic equation.

Write the net ionic equation for the precipitation of nickel (II) sulfide.

Write one possible “regular” equation from which the previous equation could have come.

Note: there are several answer to this! Why?

Practice from page 440

What will form a precipitate?See page 437 of your text

Simple rules? Sodium, potassium, ammonium, nitrates, acetates, chlorates

are soluble. Anything else, look up.

Na2CO3(aq) + Pb(NO3)4 (aq) ???? Note that sodium carbonate and lead

nitrate are both soluble. (Table 14-1) Write the net ionic equation for the

reaction between solutions of lead acetate and ammonium carbonate.

Practice writing net ionic equations

Solubility rules: a more accurate viewThe simple rules are good enough to

get through the problems.

Substances are considered insoluble if less than .1 g dissolves in 100 g of solvent.

Silver chloride has a solubility of .000 089 g/100g H2O

EVERY ionic substance has some level of solubility. EVERY one!

Homework:Page 458, 1, 2, 8-13

The van t’Hoff factor is a fancy name for a simple concept. If a compound dissociates into ions, you get more than one mole of ions for a mole of solute.

NaCl Na+ + Cl- For one mole of salt, you get 2 moles of

ions. We say that the van t’Hoff factor is 2 for

NaCl, or i=2 What is the van t’Hoff factor for aluminum

nitrate?

Van t’Hoff factor

Molalitym = Moles of solute

Kg of solvent

• Convert grams of solute to moles

• Convert grams of solvent to kilograms

• Divide moles solute by Kg solvent

Calculating molality

What is the molality of a solution of 12.9 g of fructose, C6H12O6, in 31.0 g of water?

What is the molality of 71.5 grams of linoleic acid, C18H32O2 in 525 g of hexane, C6H14?

What mass of urea, NH2CONH2, must be dissolved in 2250 g of water to prepare a 1.50 m solution?

Practice

Colligative propertiesProperties of materials that change as a

result of the addition of a solute

Solutes lower the vapor pressure of solutions.Why is vapor pressure reduced?

In a solution, the molecules at the surface are no longer all solvent molecules.

Vapor pressure comes from surface molecules establishing an equilibrium concentration with the air above the liquid.

Lower concentrations of solvent molecules at the surface means lower vapor pressure.

What is the effect of changing vapor pressure?

Boiling point elevation Freezing point depression

Consider the graph on page 344

Notice that the boiling point is the temperature at which vapor pressure equals atmospheric pressure.

If solutes lower vapor pressure for a solution, the solution has to be hotter for the vapor pressure to hit atmospheric pressure.

It is easier to think of the effect of solutes on freezing point in a different way

Consider that the particles of solute get in the way of crystals of solvent that try to form.

It has to be colder to get crystals to form in a solution than in pure solvent.

This alteration in the vapor pressure is not due to the quality or the nature of the particles that make up the solute. It is only a result of the number of those particles.

Compare the dissolution of aluminum sulfate to the dissolving of sugar. What is the number of particles produced by the dissolving of 1 mole of each of these substances?

THIS is where we use the van t’Hoff factor

Important facts about colligative properties

Dt = Kim

Kf is molal freezing point constant

Kb is molal boiling point constant

Dtf = Kfim See page 448 for the

constants for various solvents

Not every solution is aqueous!

Try sample problems on page 449 and 450

Dtb = Kbim Try sample problems

on page 451 Keep in mind, the

original boiling point of each specific solvent Is unique.

These depend on molality, not molarity

Homework:Page 459, 19-28

Study for quiz