Chapter 4 Aqueous Reactions and Solution Stoichiometry

Water• covers ______ of Earth• makes of ____% mass of body• exceptional ability to dissolve a variety of substances• A solution in which water is the dissolving mediumà _____________________ • Water is medium for most chemical reactions that take place within us and around us

4.1 General Properties of Aqueous Solutions• _________________________ are defined as homogeneous mixtures of two or more pure

substances.• The ____________________________ is present in greatest abundance.• All other substances are ___________________________.

In aqueous solutions (aq)• solvent is water• solute can be ionic compounds, aqueous acids, bases, or molecular compounds

“Like dissolves like”• A solvent such as water is needed to dissolve polar solutes such as sugar and ionic solutes

such as ______________________.• A solvent such as hexane (C6H14) is needed to dissolve nonpolar solutes such as

___________________________________.

Which of the following solutes will dissolve in water? Why?1) Na2SO4

2) gasoline3) I2

4) HCl

Dissociation• When an ionic substance dissolves in water, the solvent pulls the individual ions from the

crystal and solvates them.• This process is called ________________________________.• An __________________________is a substance that dissociates into ions when dissolved in water.

ex: NaCl

• A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does

so. ex:

Molecular compounds tend to be nonelectrolytes bc

Electrolytes

• A ____________electrolyte dissociates completely when dissolved in water.

Ex:

• A ____________ electrolyte only dissociates partially when dissolved in water.

4.2 Precipitation Reactions

Solubilityà def.

Classify these ionic compounds as soluble or insoluble in water: (a) sodium carbonate, Na2CO3

(b) lead sulfate, PbSO4

(c) barium nitrate, Ba(NO3)2 (d)ammonium phosphate, (NH4)3PO4

Metathesis (Exchange) Reaction• Metathesis comes from a Greek word that means ________________________________• One of the following is needed to drive reaction- formation of precipitate,

generation of gas, production of weak electrolyte, or production of nonelectrolyte

AgNO3(aq) + KCl(aq) ¾®

(a) Predict the identity of the precipitate that forms when aqueous solutions of BaCl2 and K2SO4 are mixed. (b) Write the balanced chemical equation for the reaction.

Molecular Equation

The molecular equation lists the reactants and products in their molecular form:

Complete Ionic Equation• In the complete ionic equation all strong electrolytes (strong acids, strong bases, and

soluble ionic salts) are dissociated into their ions.• This more accurately reflects the species that are found in the reaction mixture:

Net Ionic Equation• Spectator ionsà

• To form the net ionic equation, cross out anything that does not change from the left side of

the equation to the right (spectator ions):• The only things left in the equation are those things that change (i.e., react) during the

course of the reaction:

PRACTICE: Decide whether the following reaction occurs. If it does, write the molecular, ionic, and net ionic equations.

• KBr + MgSO4 ®

M

CIE

NIE

• NaOH + MgCl2 à

M

CIE

NIE

K3PO4 + CaCl2 ®

M

CIE

NIETypes of Chemical Reactions

1. Precipitation reactions: a solid ionic substance forms from the mixture of two solutions of ionic substances.

2. Acid–base reactions: reactions that involve the transfer of a proton (H+) between reactants.

3. Oxidation–reduction reactions: reactions that involve the transfer of electrons between reactants.

A _______________________ is an insoluble solid compound formed during a chemical reaction in solution.

Predicting Precipitation Reactions1. Predict the products (exchange of parts).2. Determine the state of each product: (s), (l), (g), (aq).3. If all products are aqueous (aq), no net reaction occurred.

Arrhenius Acid

Arrhenius Base

Strong Acid

Weak Acid

Strong Base

Weak Base

Classify the following as strong or weak acids or bases:

a. KOHb. H2Sc. CH3NH2

d. HClO4

Neutralization Reaction

Write the molecular, ionic, and net ionic equations for the neutralization of sulfurous acid, H2SO3, by potassium hydroxide, KOH.

Acid−base reactions with gas−formationSulfides, carbonates, sulfites react with acid to form a gas.

Na2S(aq) + 2HCl(aq) ®

Na2CO3(aq) + 2HCl(aq) ®

Na2SO3(aq) + 2HCl(aq) ®

Write the molecular, ionic, and net ionic equations for the reaction of copper(II) carbonate with hydrochloric acid.

Oxidation-Reduction Reactions

Oxidation Numbers- To determine if an oxidation–reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity.

Rules for Assigning Oxidation Numbers1. Elements: The oxidation number of an atom in an element is __________2. Monatomic ions: The oxidation number of an atom in a monatomic ion equals the

________________________3. Oxygen: The oxidation number of oxygen is _________ in most of its compounds. (An

exception is O in H2O2 and other peroxides, where the oxidation number is −1.)4. Hydrogen: The oxidation number of hydrogen is ________ in most of its compounds. (The

oxidation number of hydrogen is ________ in binary compounds with a metal such as CaH2.)5. Halogens: The oxidation number of fluorine is −1. Each of the other halogens (Cl, Br, I) has

an oxidation number of _____ in binary compounds, except when the other element is another halogen above it in the periodic table or the other element is oxygen.

6. Compounds and ions: The sum of the oxidation numbers of the atoms in a compound is _________. The sum of the oxidation numbers of the atoms in a polyatomic ion equals the ___________ on the ion.

Potassium permanganate, KMnO4, is a purple−colored compound; potassium manganate, K2MnO4, is a green−colored compound. Obtain the oxidation numbers of the manganese in these compounds.

What is the oxidation number of Cr in dichromate, Cr2O72−?

Displacement Reactions

Cu(s) + 2Ag+(aq) ¾® Cu2+(aq) + 2Ag(s)

Will an aqueous solution of iron(II) chloride oxidize magnesium metal? If so, write the balanced molecular and net ionic equations for the reaction.

4.5 Concentration of Solutions

• Two solutions can contain the same compounds but be quite different because the proportions of those compounds are differentà concentration def.

• Molarity is one way to measure the concentration of a solution:

Mixing a Solution• To create a solution of a known molarity, one weighs out a known mass (and,

therefore, number of moles) of the solute.• The solute is added to a volumetric flask, and solvent is added to the line on the

neck of the flask.

Expressing Concentration of an Electrolyte:• When ionic compounds dissolve, the [ ] of ions depends on chemical formula:

Dilution def.

EXAMPLE: Prepare a dilute solution from a more concentrated oneà GOAL: Prepare 250.0 mL of 0.100 M CuSO4 solution by diluting a 1.00 M CuSO4 stock

solution REMEMBER: Even when add solvent is added to solution

CALCULATION: The molarity of the new solution can be determined from the equation

• where Mc and Md are the molarity of the concentrated and dilute solutions, respectively, and Vc and Vd are the volumes of the two solutions.

METHOD: Using a pipet to deliver a volume of the solution (25.0 mL) to a new volumetric flask, and adding solvent (225 mL water) to the line on the neck of the new flask.

Give it some thought…• How is the molarity of a 0.50 M KBr solution changed when water is added to

double it volume?

HOMEWORK # 4.71- 4.77 ODD

Titration def:

• What is the molarity of an HCl solution if 27.3 ml of it neutralizes 134.5 mL of 0.0165 M Ba(OH)2?