Chapter 4

Chapter 4

Aqueous Reactions and Solution Stoichiometry

Properties of Aqueous Solutions

• Solutions are homogeneous mixtures of 2 or more substances


• Aqueous Solutions are solutions in which water is the dissolving medium


• A solvent is the substance that is present in larger quantities.


• A solute is the substance being dissolved into the solvent.


• Electrolytes conduct electricity.


• Aqueous solutions made of ionic compounds are electrolytes due to the presence of ions.


An aqueous solution of a molecular compound is called a nonelectrolyte


Strong electrolytes exist in solution completely as ions (soluble ionic compounds)


Weak electrolytes exist mostly as molecules


Nonelectrolytes exist only as molecules

Precipitation Reactions

• Precipitation reactions are those that result in the formation of an insoluble product


• Precipitation reactions occur when certain pairs of oppositely charged ions attract to each other so strongly that they form an insoluble ionic solid


• Solubility of a substance is the amount of that substance that can be dissolved in a given quantity of solvent


• Any substance with a solubility less than 0.01mol/L will be referred to as insoluble


• The solubility guidelines for common ionic compounds in water is organized by anions

Practice 4.2 Soluble or Insoluble?

• Sodium carbonate

Lead (II) sulfate

Barium nitrate

Cobalt(II) hydroxide

Ammonium phosphate


• To predict whether a precipitate will form when we mix aqueous solutions of electrolytes …


• (1)note the ions present in the reactants

• (2) consider combinations of anions and cations

• (3) use the table to predict if any of the combinations are insoluble


• Will a precipitate form when Mg(NO3)2 and NaOH are mixed?


• This is called a double replacement reaction.

• AX + BY AY + BX

Practice 4.3

• (a) Write the balanced rxn and identify the ppt that forms when the following solutions are mixed:

• (a) BaCl2 and K2SO4

• (b) Ba(NO3)2 and KOH

• (c) Fe2(SO4)3 and LiOH


• A molecular equation shows the complete chemical formulas of the reactants and the products


• A complete ionic equation shows all strong electrolytes as ions rather than as compounds


• Spectator ions are ions that appear as identical forms on both sides of the equation; they are present but don’t play a direct role in the reaction


• Net ionic equations do not show spectator ions

Writing Net Ionic Equations

• Write a balanced mol. E.q

• Rewrite to show ions that form in solution

• Cancel spectator ions

Practice 4.4

• Write the net ionic equation for the following rxns.

(1) aqueous solutions of calcium chloride and sodium carbonate are mixed.

(2) aqueous solutions of silver nitrate and potassium phosphate are mixed.

Acid-Base Reactions

• Acids produce H+ when dissolved in water

• Acids are proton donors

Acid-Base Reactions

• Bases accept H+ions

• Bases produce OH- when

they dissolve in water

Acid-Base Reactions

• Strong acids and bases are strong electrolytes – ionize completely

Acid-Base Reactions

• Weak acids and bases are weak electrolytes – partially ionize ( do not write in ionized form)

Acid-Base Reactions

• List Strong Acids and Bases

Practice Exercise 4.5

The following diagrams represent aqueous solutions of three acids (HX, HY, and HZ) with water molecules omitted for clarity. Rank them from strongest to weakest.

Identifying Strong, Weak, and Non Electrolytes Chart

Strong Weak Non

IonicSoluble N/A Insoluble

Molecular Strong AcidsWeak Acids

Weak Bases (NH3)

All other compounds

Classifying SE, WE, or NE • 1. Is it ionic or molecular? • 2. If ionic – is it soluble? • 3. If molecular – is it an acid?• 4. If an acid – strong or weak?• 5. Is it weak base NH3?• 6. Everything else is a non

electrolyte

Practice 4.6

SE, WE, NE?CaCl2

HNO3

C2H5OH HCOOHKOHNH3

Practice 4.6

Rank the following solutions in order of least to most electrical conductivity.

Ca(NO3)2

C6H12O6 CH3COONa or NaC2H3O2

CH3COOH or HC2H3O2

Acid-Base Reactions

• A neutralization reaction occurs when an acid and base are mixed producing water and salt

• Salt means any ionic compound whose cation comes from the base and anion from the acid

Acid-Base Reactions

• Example:

• Aq. solutions of hydrobromic acid and sodium hydroxide mix

Acid-Base Reactions

Aq. hydrochloric acid reacts with solid magnesium hydroxide.

Acid-Base Reactions

• Reactions with the S2- ion or CO3

2- ion with acids will form gases with low solubility in water

Acid-Base Reactions

Aq. HCl

reacts with

Baking soda

Acid-Base Reactions

Aq. Hydrochloric acid reacts with Aq. sodium sulfide

Practice 4.7

Write a balanced molecular equation and net ionic equation for the following rxns:(a) aqueous solutions of acetic acid and barium hydroxide(b) aq solns of carbonic acid and potassium hydroxide

Oxidation Numbers

Oxidation numbers of an atom in a substance are a hypothetical charge based on a set of rules.

1. For an atom in its elemental form the oxidation number is always zero

2. For any monatomic ion the oxidation number equals the charge on the ion

Oxidation Numbers

• 3. Nonmetals usually have negative oxidation numbers.

a. O is –2 except in peroxides where O22-

giving each O –1.

b. H is +1 with nonmetals and –1 with

metals .

c. F is –1 in all compounds. Other halogens have –1

except when combined with oxygen they have

positive oxidation states.

•

Oxidation Numbers

• 4. The sum of oxidation numbers of all atoms in a neutral compound is 0.

• The sum of oxidation numbers in a polyatomic ion is equal to the charge of the ion.

Practice 4.8

• Deterimine the ox#’s in the following substances • (a) H2S (b) S8

• (c) SCl2 (d) Na2SO3

• (e) SO42– (f) P2O5

• (g) NaH (h) Cr2O72–

• (i) SnBr4 (j) BaO2

Oxidation-Reduction Reactions

• Reactions where electrons are transferred between reactants


• Oxidized – when an atom, ion, or molecule becomes more positively charged (lost e-)

• Reducing Agent = the substance that is oxidized


• Reduced – when an atom, ion, or molecule has become more negative (gain e-)

• Oxidizing agent = the substance that is reduced

What does Leo the Lion say?


• Determine oxidation numbers, then identify the oxidizing agent and reducing agent in the rxn below.


• A single replacement rxn is when an ion in solution is replaced through oxidation of an element

Practice 4.9

• Write the molecular and net eq for the following:

(a)Aluminum and hydrobromic acid(b)Magnesium and cobalt(II)sulfate


• Activity series is a list of metals arranged in order of decreasing ease of oxidation

• Any metal on the list can be oxidized by the ions of elements below it.

Practice 4.10

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

2. Which of the following metals will be oxidized by Pb(NO3)2: Zn, Cu, Fe?

Concentration of Solutions


• Molarity (M)

= moles of solute / L soln

Practice 4.11

• Calculate the molarity of a solution made by dissolving 23.4 g of sodium sulfate (Na2SO4) in enough water to form 125 mL of solution

Making a Solution:How do you make a 250.0mL soln of 0.1M CuSO4?


• Electrolytes and Concentration: When an ionic compound dissolves, the

relative concentration of ion depends on the chemical formula

Practice 4.12

• What are the molar concentrations of each of the ions present in a 0.025 M aqueous solution of calcium nitrate?


• A dilution can be made to concentrated stock solutions by adding water to the solutions, and therefore making them less

concentrated.

M1V1 = M2V2

Practice 4.14

How many milliliters of 3.0 M H2SO4 are needed to make 450 mL of 0.10 M H2SO4?

Solution Stoichiometry

• Use mole conversions (molarity, molar mass, etc.) and mole ratios to solve stoichiometry problems through DA

Solution Stoichiometry

Practice 4.15

• How many grams of Ca(OH)2 are needed to neutralize 25.0 mL of 0.100 M HNO3?

Practice 4.16

• 45.7 mL of 0.500M H2SO4 is required to neutralize 20.0mL of NAOH soln. What is the concentration of NaOH?

Practice 4.17

• How many grams of chloride ion are in a sample of water if 20.2mL of 0.100M Ag+

is needed to react with all the chloride in the sample. If the sample has a mass of 10.0g what percentage of chloride does it contain?

Putting it all together

A sample of 70.5 mg of potassium phosphate is added to 15.0 mL of 0.050 M silver nitrate.

(a) Write the molecular equation for the reaction.

(b) What is the limiting reactant in the reaction?

(c) Calculate the theoretical yield, in grams, of the precipitate that forms.

Putting it all together

A solution of 100.0mL of 0.200M KOH is mixed with a solution of 200.0mL of 0.150M NiSO4.

(a) Write the molecular equation for the reaction. (b) What is the limiting reactant in the reaction? (c) Calculate the theoretical yield, in grams, of the

precipitate that forms.(d) What is the molarity of each ion that remains in

the solution.

Chapter 4

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