1

1

Ch 4. Aqueous Reactions and Solution Stoichiometry

• Understand the nature of ions in solutions• Recognize and write common chemical reactions• Use the activity series to predict products of

reactions• Write net-ionic equations• Understand solution stoichiometry using molarity,

solution preparation and dilutions of solutions

2

A Note on phase symbols, solubility and soluble compounds

• In a balanced equation we will now begin to use phase symbols to indicate the physical state of the reactants/products. There are 4 phase symbols:

1. (s): solid2. (l): liquid3. (g): gas4. (aq): aqueous. This means dissolved in water to form a

homogeneous solution. By definition any soluble compound will have the (aq) phase symbol in a reaction where water is present. Soluble = (aq), Insoluble = (s).

3

Solutions:

• Homogeneous mixtures of two or more pure substances.

• The solvent is present in greatest abundance.

• All other substances are solutes.

4

Dissociation

• When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them.

• This process is called dissociation.

2

5

Electrolytes

• Substances that dissociate into ions when dissolved in water.

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

6

Electrolytes and Nonelectrolytes

Soluble ionic compounds tend to be electrolytes.

7

Electrolytes and Nonelectrolytes

Molecular compounds tend to be nonelectrolytes, except for acids and bases.

8

Non electrolyte Weak electrolyte Strong electrolyte

All Soluble Compounds have Electrolytic Properties !

3

9

Strong Electrolytes Are…

• Strong acids, Strong bases, Soluble ionic salts

10

11

Reaction Types in this Chapter

1.Precipitation2.Acid-Base Ionization/Dissociation

Reactions3.Acid-Base Chemical Reactions4.Gas Formation5.Oxidation-Reduction

You must be familiar with these and be able to predict the products (if possible) for each of these types of reactions!

12

Precipitation Reactions

When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate is formed. These reactions are often metathesis reactions.

PbCrO4 ppt rxn.MOV

4

13

1. Precipitation or Exchange (Metathesis) Reactions

Predict the products of the following exchange reactions:

a) BaCl2(aq) + K2SO4(aq) —>

b)Fe(NO3)3(aq) + NaOH(aq) —>

c) (NH4)2S(aq) + Pb(NO3)2(aq) —>

14

Solution Chemistry

• It is helpful to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution)I.e look at the phase labels.

• If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction.

15

Acids:

• Substances that increase the concentration of H+

when dissolved in water (Arrhenius).

• Proton donors (Brønsted–Lowry).

16

Acids

There are only seven strong acids:• Hydrochloric (HCl)• Hydrobromic (HBr)• Hydroiodic (HI)• Nitric (HNO3)• Sulfuric (H2SO4)• Chloric (HClO3)• Perchloric (HClO4)

5

17

Bases:

• Substances that increase the concentration of OH−

when dissolved in water (Arrhenius).

• Proton acceptors (Brønsted–Lowry).

18

Bases

The strong bases are the soluble salts of hydroxide ion:• Alkali metals• Calcium• Strontium• Barium

19

2. Acid-Base Reactions

In an acid-base reaction, the acid donates a proton (H+) to the base.

20

2. Acid-Base Ionization ReactionsAcids: Proton (H+) Donors

• Acids: compounds that form hydrogen ions in water.

Examples of ACID IONIZATION EQUATIONS:

a) HCl(aq) —> H+(aq) + Cl–(aq)• 100% ionized since HCl is a strong acid/electrolyte

b) CH3COOH(aq) <<—> H+(aq) + CH3COO–(aq)HC2H3O2(aq) <<—> H+(aq) + C2H3O2

–(aq)• ≈ 1% ionized since CH3COOH is a weak acid/electrolyte

c) H2SO4(aq) —> H+(aq) + HSO4–(aq)

• 100% ionized since H2SO4 is a strong acid/electrolyte

d) HSO4–(aq) <<—> H+(aq) + SO4

2–(aq)• ≈ 1% ionized since HSO4

– is a weak acid/electrolyte

6

21

2. Strong Acid Ionization - 100% ions formed

• We write H+(aq) to show the proton is ionized from the acid. HCl(aq) → H+(aq) + Cl–(aq) 100% ionization for strong acids!

• In reality that proton is always attached to water forming the hydronium ion, H3O+(aq).

QuickTime™ and aAnimation decompressor

are needed to see this picture. Strong acid.MOV

22

2. Weak acid ionization, only a few % of the acid molecules ionize

Weak acid ionization is written with a double arrow to indicate less than 100% ionization.

Examples:Oxalic acid: H2C2O4(aq) ⇄ H+(aq) + HC2O4

–(aq)

Acetic acid: CH3CO2H(aq) ⇄ H+(aq) + CH3CO2–(aq)

23

Weak Base Hydrolysis ≈ 1% ions formed

2. Strong Base Dissociation ≈ 100% ions formed

QuickTime™ and aGraphics decompressor

are needed to see this picture. Strong base.MOV

24

Types of Reactions: 3. Acid-BaseOften called Neutralization Reactions

Acids: compounds that can donate a proton, H+, to a base.Bases: compounds that can accept a proton, H+, from an acid.Reaction: Transfer the proton(s) to the base to make the products.

– Example reaction:Ba(OH)2(aq) + 2 HNO3(aq) —> Ba(NO3)2(aq) + 2 H2O(l)

• The insoluble hydroxide salts can also act as bases when in the presence of an acid:– Al(OH)3, Fe(OH)2, Mg(OH)2, etc.

– Example reaction:Al(OH)3(s) + 3 HCl(aq) —> AlCl3(aq) + 3 H2O(l)

The weak base NH3 also accepts a proton from an acid:– NH3(aq) + HNO3(aq) —> NH4

+(aq) + NO3–(aq)

7

25

3. Acid-Base (Neutralization)

• Predict the products of the following reactions. React all acidic protons on the acid, complete neutralization.a) HI(aq) + LiOH(aq) —>

b) H2SO4(aq) + Fe(OH)3(s) —>

c) CH3COOH(aq) + NH3(aq) —>

26

Types of Reactions: 4. Gas-Forming

• There are several reactions that produce a gas as the product. We call these Acid-base Reactions with Gas Formation.

QuickTime™ and aCinepak decompressor

are needed to see this picture. Gummy Bear rxn.MOV

27

4. Gas-Forming Reactions

• Carbonates + acid —> CO2(g) + H2O(l) + salt• Sulfides + acid —> H2S(g) + salt

• Predict the products of the following reactions:

a) HI(aq) + NaHCO3(aq) —>

b) H2SO4(aq) + CaS(s) —>

c) CH3COOH(aq) +Al2(CO3)3(s) —>

28

Writing Net-ionic Equations (for all reaction types)

• Net-ionic equations are used to describe the actual chemical speciesinvolved in an aqueous reaction. To write net-ionic equations you must identify ALL the STRONG ELECTROLYTES!

• Steps:1. Write the correct, balanced molecular equation2. Add phase symbols: (s), (l), (g), (aq).3. Identify all the STRONG ELECTROLYTES.

1. Search only the (aq) species for the strong electrolytes!4. Rewrite the strong electrolytes as ions wherever they occur. All

other species are rewritten without modification.5. Cancel any ions that appear on both sides of the reaction.6. Balance the final net-ionic equation.

8

29

Writing Net-Ionic Equations

Na2CO3(aq) + HCl(aq) —> NaCl + H2O + CO2

AgNO3 + KCl⎯→ AgCl + KNO3

30

Reaction Types: 5. Oxidation and Reduction (redox)

Oxidation and reduction involve the transfer of electronsfrom one species to another.

• Oxidation: The lose of electrons• Reduction: The gain of electronsOne cannot occur without the other!

31

5. First step: Assigning Oxidation Numbers to Atoms

I. For an atom in its elemental form the oxidation number is zero.II. For any monatomic ion the oxidation number equals the charge of the

ion.III. For nonmetals the oxidation number is usually negative.

a) Oxygen is usually -2 in all compounds.b) Fluorine is -1 in all compounds.c) Hydrogen is +1 when bonded to nonmetals and -1 when bonded

to metals (metal hydrides).IV. The sum of the oxidation numbers for all atoms is zero for neutral

compounds or equals the charge for polyatomic ions.

Assign oxidation numbers to each atom in the following:

(a) NaCl (b) H2O (c) S8 (d) SO42- (e) Na2SO4 (f) Mn(NO3)2

32

5. Oxidation of Metals by Acids and Salts

• Elemental metals can be oxidized in the presence of a salt or acid. The general form is:

M + AX —> MX + A where M is a metal and AX is a salt or acid.

Examples:

Cu(s) + 2 AgNO3(aq)—> Cu(NO3)2(aq) + 2 Ag(s)

Net Ionic: Cu(s) + 2 Ag+(aq) —> Cu2+(aq) + 2 Ag(s)

Mg(s) + 2 HCl(aq) —> MgCl2(aq) + H2(g)

Net ionic: Mg(s) + 2 H+(aq) —> Mg2+(aq) + H2(g)

9

33

5. Activity Series of Metals/Ions(to predict if a redox reaction will take place)

Any metal on the left will be oxidized (will react) with any ion on the right that is below the metal in the table.

Noble Metals(don’t react with acids!

Rea

ctiv

ity o

f Met

al In

crea

ses

Reactivity of M

etal Ion Increases

34

5. Oxidation Reduction Reactions

• Predict the products (if any) of the following reactions:

a) Mg(s) + FeCl2(aq) —>

b) HNO3(aq) + Ag(s) —>

c) Zn(s) + CuSO4(aq) —>

35

Definition of Molarity• Molarity is the most common unit of concentration

in chemistry.• Molarity is defined as: (moles solute)/(1 L of

solution)• Solutions of precise molarity are made using

volumetric flasks.QuickTime™ and a

Graphics decompressorare needed to see this picture.

36

Calculation of Molarity

1. Calculate the molarity of the of a CaCl2 solution prepared by dissolving 0.435 g of CaCl2 into a 250-mL volumetric flask.a) What are the concentrations of each ion in solution?b) How many grams of CaCl2 are contained in 75.0 mL

of a 0.334 M CaCl2 solution?

10

37

Calculation of Molarity

1. How many grams of Sr(OH)2 are needed to prepare a solution that is 0.12 M in OH– ion?

a) What volume of this solution (mL) is needed to obtain 1.0 g of Sr(OH)2?

b) How many moles of Sr(OH)2 are contained in 25 mLof a 0.12 M solution?

38

Dilution of Known Concentrations

The formula for dilution is:

C1V1 = C2V2 = moles soluteWhere:C1 is the concentration of the stock solution

(before dilution)V1 is the volume of the stock solution to be

dilutedC2 is the concentration after dilutionV2 is the volume of the diluted solution

QuickTime™ and aGraphics decompressor

are needed to see this picture.

Often solutions must be diluted before they can be used.

39

Dilution of Known Concentrations

• Often solutions must be diluted before they can be used.

In lab you are asked to prepare a 0.020 M NaOHsolution from a 6.2 M stock solution. You need 300 mL of the diluted NaOH. How would you prepare this solution?

40

Using Molarity with Stoichiometry

11

41

Stoichiometry of Aqueous Solutions

• Since M x V = moles solute, we can use M and volume of a reactant to find moles of reactant. With moles we can do reaction stoichiometry!

Problem: Help! An acid spill! A student spills 500-mL of 3.4 M sulfuric acid on the floor. Your instructor quickly dumps 250 g of sodiumbicarbonate on the acid to neutralize it. Was all the acid neutralized?

This is a limiting reactant problem…

42

Titrations (Covered in lab as well)

• In a titration you are trying to determine the concentration of a solute in a solution of unknown concentration.

• A standard solution is used to react with the solute of interest.• The equivalence point is when stoichiometrically equivalent quantities

are brought together (balanced eqn. needed)• The reaction can be of any form: acid-base, gas-formation or

precipitation.

HCl(standard) + NaOH(unknown conc.) —>

HNO3(standard) + 2 Na2CO3(unknown conc.) —>

2 Al2(SO4)3 (standard) + 3 BaCl2(unknown conc.) —>

43

Acid-Base Titration Example

• What is the molarity of a NaOH solution that requires 37.5 mL of 0.224 M HCl to titrate 50.0 mL of the NaOH?

44

Precipitation Examples

• What is the molarity of a BaCl2 solution that requires 14.5 mL of 0.412 M Al2(SO4)3 to titrate 20.0 mL of the BaCl2?

• What is the concentration of chloride ions (g/L) in solution if a 2.75 L sample of water requires 35.7 mL of a 0.100 M AgNO3 to precipitate all the chloride?

12

45

Ionic Salts - Solubility Rules

46

HCl in Solution, 100% Ionized

H3O+(aq)

Cl–(aq)

47

Acetic Acid, a Weak Acid and Electrolyte

48

“Driving Forces” for These Rxn’s

• Why do these reactions take place?• These reactions are said to be “product-favored”.

– Precipitation: the formation of the precipitate “drives” the reaction forward.

– Acid-base: the transfer of a proton “drives” the reaction forward.– Gas-formation: the formation of a gaseous product “drives” the

reaction forward.

• Reactions that do not take place or take place very little are said to be “reactant-favored”– NaCl(aq) + HF(aq) <— NaF(aq) + HCl(aq)

13

49

pH of Solutions

• To show the acid content of solutions we use the pH scale.pH = -log[H+]

Where [H+] is the molarity of hydrogen ions in solution.(From now on [] represent concentration in molarity.)

What is the pH of a 1.0 M HCl solution?[H+] in 1.0 M HCl is 1.0 M since HCl is a strong electrolyte.

pH = -log(1.0) = -(0.0) = 0.0

What is the pH of a 6.0 M HCl solution? a 0.00444 M solution?

Answers: -0.78 and 2.35350

pH Scale

• Acidic solutions have pH values below 7• Basic solutions have pH values above 7• Neutral solutions have a pH = 7

Acidic BasicNeutral

51

Determining [H+] from pH

• If the pH is known the [H+] can be determined.

[H+] = 10–pH

• A 200-mL solution of acetic acid has a pH of 2.74.– What is the [H+]?– How many moles of H+ are in solution?

[H+] = 10-2.74 = 0.0018 MMoles H+ = (0.0018 M H+)(0.200 L solution) = 3.6x10-4 mol H+

52

Common Strong Acids and Bases

14

53

More on Titrations in Lab….