Chapter 4
Chemical Reactions and Stoichiometry

Representing Chemical Reactions
Reactants on Left Products on Right
I ndicates a chemical reaction

Balancing Chemical Reactions
• 1st Balance elements that occur in only one compound on each side
• Balance free elements last
• (Balance unchanged polyatomics as groups)
• Clear fractional coefficients by multiplication
• Never introduce extraneous atoms
• Never change chemical formulas

Equations and the Mole Concept
Na + Cl
1 atom Na
1 dozen Na
1 mol Na
+
+
+
1 atom Cl
1 dozen Cl
1 mol Cl
NaCl
1 NaCl
1 dozen NaCl
1 mol NaCl

Reaction Types
Combination Reaction
A + B C
Decomposition Reaction
Combustion Reaction
X
C x
H y
O z
+ O
2
Y + Z
CO
2
+ H
2
O

What mass of water is produced when 33.2 g ethanol is burned completely?

You are a biologist studying the mating habits of the Asian
Civet cat.
You manage to isolate 62 mg of the Civet cat sex pheromone
Combustion analysis of the pheromone produces:
138.0 mg CO
2 and 49.4 mg H
2
O

Combustion Analysis

The Sex Pheromone For The Asian Civet Cat

Limiting Reagents
We are making bagel pizzas (pepperoni, of course).
For each pizza, we need 1 bagel, 1 oz. tomato sauce,
2 slices of cheese and 5 slices of pepperoni:

Limiting Reagents
We are making bagel pizzas (pepperoni, of course).
For each pizza, we need 1 bagel, 1 oz. tomato sauce,
2 slices of cheese and 5 slices of pepperoni:
1 bagel + 1 oz. tomato sauce + 2 cheese + 5 pepperoni 1 pizza

Limiting Reagents
We are making bagel pizzas (pepperoni, of course).
For each pizza, we need 1 bagel, 1 oz. tomato sauce,
2 slices of cheese and 5 slices of pepperoni:
1 bagel + 1 oz. tomato sauce + 2 cheese + 5 pepperoni 1 pizza
If we have 13 bagels, 20 oz. of tomato sauce, 26 slices of cheese and 60 slices of pepperoni, how many pizzas can we make?

If we react 85.0 g of Zn metal with 35.2 g of HCl, what is the mass of H
2 gas that is produced?

Actual & Theoretical Yield
Theoretical Yield: The maximum amount of product that can be produced (usually reported in mol or g)
Actual Yield: Real (measured) amount of product that was produced/obtained (usually reported in mol or g)
Percent Yield: The ratio of actual yield to theoretical yield
(reported as a percent, ALWAYS 100% or less)
Percent Yield =
Actual Yield
Theoretical Yield
X
100%

We reacted 12.8 g of Al metal with excess HBr and obtained 77.9 g of
AlBr
3
. What is our percent yield?
Al
(s)
+ HBr
(aq)
H
2 (g)
+ AlBr
3 (s)

Units of Concentration: Molarity
Molarity ( M ) =
Amount of solute ( mol solute )
Volume of solution (L)
Solute - lesser component of a mixture
Solvent - greater component of a mixture
Aqueous - dissolved in water

How many moles of HCl are in 1.00 L of muriatic acid (a.k.a. concentrated aqueous HCl: 37% HCl by mass, density = 1.200 g/mL)?

How many grams of AgNO
3 ml of a 1.0 M AgNO
3 do we need to make 500 solution?

What volume of a 3.0 M HCl solution is needed to react with 38.4 g of Zn metal?

Compounds in Aqueous Solution
When NaCl dissolves in water – what happens to the NaCl?
It separates into ions

Compounds in Aqueous Solution
A Strong electrolyte dissociates completely.
Soluble ionic compounds – Good electrical conduction.
A Weak electrolyte partially dissociates.
Weak (molecular) acids & bases – Fair conductor of electricity.
A Non-electrolyte does not dissociate.
Molecular/Covalent Compounds – Poor (non-) conductor of electricity.

If we dissolve 8.0 g of potassium carbonate in water and the total solution volume is 100 mL, what is the concentration of potassium ions in solution? The concentration of carbonate? Is potassium carbonate a strong electrolyte?

Precipitation Reactions
A solid (the precipitate ) forms when two solutions are mixed

Ion Exchange (Metathesis)
Why does a precipitate form when the following solutions are mixed?
AgNO
3 (aq)
+ NaI
(aq)

AgI
(s)
+ NaNO
3 (aq)

What Ionic Compounds Are Soluble In Water?
Alkali metal ion and ammonium ion salts
Li + , Na + , K + , Rb + , Cs + , NH
4
+
Nitrates, perchlorates and acetates
NO
3
, ClO
4
, CH
3
CO
2
-
Chlorides, bromides and iodides
Cl , Br , I Except those of Pb +2 , Ag + , and Hg
2
+2
Sulfates
SO
4
–2 Except those of Sr +2 , Ba +2 , Pb +2 and Hg
2
+2
Ca(SO
4
) is slightly soluble.

What Ionic Compounds Are Insoluble In Water?
Hydroxides and sulfides
HO – , S –2
Except alkali metal (group 1) and ammonium salts
Sulfides of alkaline earth metals (group 2) are soluble
Hydroxides of Sr 2+ and Ca 2+ are slightly soluble
Carbonates and phosphates
CO
3
–2 , PO
4
–3
Except alkali metal and ammonium salts

Write The Net Ionic Equation For:
Ca(NO
3
)
2 (aq)
+ Na
3
PO
4 (aq)

The reaction of lithium carbonate with magnesium sulfide?

Net Ionic Equations
Overall Equation:
AgNO
3 (aq)
+NaI
(aq)

AgI
(s)
+ NaNO
3 (aq)
A net ionic equation contains only the ions undergoing a change during the reaction

Net Ionic Equations
Net Ionic Equation:
Ag +
(aq)
+ I
–
(aq)

AgI
(s)
A net ionic equation contains only the ions undergoing a change during the reaction

Acids and Bases
• Acids provide H + (protons) in aqueous solution (Arrhenius 1884)
• Bases provide OH in aqueous solution (Arrhenius 1884)
• Bases react to accept or consume H + (Bronstead 1923)
HCl
(aq)

H +
(aq)
+ Cl
–
(aq)
H
2
SO
4 (aq)
 2
H +
(aq)
+ SO
4
–2
(aq)
NaOH
(aq)

Na +
(aq)
+ OH
–
(aq)
NH
3
+ H
2
O

NH
4
+
(aq)
+ OH
–
(aq)

Acids and Bases
• Acids provide H + (protons) in aqueous solution (Arrhenius 1884)
• Bases provide OH in aqueous solution (Arrhenius 1884)
• Bases react to accept or consume H + (Bronstead 1923)
Is water an acid, a base, or neither?
H
2
O  ?

Autoionization of Water and pH
In pure water at 25 ° C, the concentration of H + ions is always 1X10 -7 M
H
2
O

H +
(aq)
+ OH
–
(aq) pH = -log[H + ] = -log( 1X10 -7 ) = 7

Acids and Bases
Strong acids/bases – Acids and bases that dissociate/react completely in water to yield ions (including H + or HO – ) [ Strong bases: hydroxides of group 1 & “ heavy ” group 2 metals (Ca, Sr, Ba) ]
Weak acids/bases - Acids and bases that only partially dissociate/react in water to yield ions (including H + or HO – )

Acids and Bases
Strong acids/bases – Acids and bases that dissociate/react completely in water to yield ions (including H + or HO – ) [ Strong bases: hydroxides of group 1 & “ heavy ” group 2 metals (Ca, Sr, Ba) ]
Weak acids/bases - Acids and bases that only partially dissociate/react in water to yield ions (including H + or HO – )
Strong acids/bases:
HCl
(aq)
 H +
(aq)
+ Cl
–
(aq)
NaOH
(aq)
 Na +
(aq)
+ OH
–
(aq)
Weak acids/bases: acetic acid
CH
3
CO
2
H
(aq)
 H +
(aq)
+ CH
3
CO
2
–
(aq) ammonia
NH
3
+ H
2
O  NH
4
+
(aq)
+ OH
–
(aq)

Acid/Base Neutralization
The reaction of H + with HO
– yields water:
H +
(aq)
+ OH
–
(aq)

H
2
O
Therefore:
H + and HO
– cannot
“ co-exist
” beyond the amount of water autoionization
Acids and bases will react to consume each other

Acids and Bases
Monoprotic acids yield 1 H + ion per formula unit: HCl, HClO
4
, HNO
3
Diprotic acids yield 2 H + ion per formula unit: H
2
SO
4
Triprotic acids yield 3 H + ion per formula unit: H
3
PO
4

Acids and Bases
Monoprotic acids yield 1 H + ion per formula unit: HCl, HClO
4
, HNO
3
Diprotic acids yield 2 H + ion per formula unit: H
2
SO
4
Triprotic acids yield 3 H + ion per formula unit: H
3
PO
4
200 mL of aqueous 0.1 M H
2
SO
4 is allowed to react completely with 7.4 g of solid NaOH. What ions remain in solution at the end of the reaction? If we assume that the total volume remains constant at 200 mL, what are the concentrations of the remaining ions?

Acids and Bases
A student carefully adds 0.05 M potassium hydroxide to 50 mL of a hydrobromic acid solution with an unknown concentration. If it takes 17.8 mL of the potassium hydroxide solution to turn the indicator (phenolphthalein) slightly pink, what is the concentration of the hydrobromic acid solution?
• The above process is known as a titration – the careful addition of one solution to another until one component has exactly consumed another (at the Equivalence Point )
• An indicator is a substance that undergoes an observable
(color) change near or at an equivalence point

Acids and Bases
A student carefully adds 0.05 M potassium hydroxide to 50 mL of a hydrobromic acid solution with an unknown concentration. If it takes 17.8 mL of the potassium hydroxide solution to turn the indicator (phenolphthalein) slightly pink, what is the concentration of the hydrobromic acid solution?

Oxidation/Reduction Reactions (Redox)
In an oxidation-reduction (redox) reaction, some atoms undergo changes in oxidation state : charges change due to the exchange of electrons

Oxidation State
Oxidation state is a term used by chemists to give a general idea of how electron rich/poor an atom is
• Oxidation state describes the charge an atom would possess if all bonds in the compound were ionic
• Oxidation states refer to real charges for ions
• Oxidation states do not refer to real charges in covalent (molecular) compounds

Determination of Oxidation States
1) All free elements = 0
2) The sum of all atomic oxidation states in a compound = the total charge on the compound
3) For monoatomic ions, the oxidation state = charge on the ion
(Group 1 elements = +1, Group 2 elements = +2, etc.)
4) Fluorine = –1
5) Hydrogen = +1
6) Oxygen = –2
7) Halogens = –1, Group 16 = –2, Group 15 = –3

Determination of Oxidation States
1. H
2
S
2. S
8
3. ClO
4
–
4. Na
2
S
5. O
3
6. SO
4
–2
7. Fe
2
S
3
8. ClO
2

Redox Vocabulary
During an Oxidation
The oxidation state of a species increases (becomes more positive)
Electrons are lost by the atom
During a Reduction
The oxidation state of a species decreases (becomes more negative)
Electrons are gained by the atom
Oxidation and reduction always occur together.
Zn + HCl

H
2
Ag +
(aq)
+ Cu
(s)
+ ZnCl

Ag
(s)
2
+ Cu 2+
(aq)

Redox Vocabulary
An oxidizing agent ( oxidant ) oxidizes something it reacts with
Contains an element whose oxidation state decreases (is reduced) during the redox reaction
A reducing agent ( reductant ) reduces something it reacts with
Contains an element whose oxidation state increases (is oxidized) during the redox reaction

In which of the following reactions do atoms undergo oxidation state changes?
CaCO
3
+ 2 HCl
C
2
H
6
O + 3 O
2
2 Al + 3 Cl
2
2 HI + Ca(OH)
2
CaCl
2
+ H
2
O + CO
2
2 CO
2
+ 3 H
2
O
2 AlCl
3
CaI
2
+ 2 H
2
O

Identifying Oxidizing and Reducing Agents
H
2
O
2
+ Fe +2
(aq)

H
2
O + Fe +3
(aq)
H
2
O
2
+ MnO
4
–
(aq)

Mn +2
(aq)
+ O
2
2 H
2
O
2 (aq)

2 H
2
O
(l)
+ O
2 (g)
Hydrogen peroxide, H
2
O
2
, is a versatile chemical. Its uses include bleaching wood pulp and fabrics and substituting for chlorine in water purification. One reason for its versatility is that it can be either an oxidizing or a reducing agent.

Writing and Balancing Redox Reactions
Cu
(s)
+ Ag +
(aq)

Cu +2
(aq)
+ Ag
(s)
Is this reaction balanced?

Balancing Redox Reactions
Na
2
SO
3 (aq)
+ KMnO
4 (aq)

Na
2
SO
4 (aq)
+ K +
(aq)
+ Mn +2
(aq)
CN
–
(aq)
+ MnO
4
–
(aq)

MnO
2 (s)
+ OCN
–
(aq)

Balancing Redox Reactions: Half-Reactions
• Write the equations for the half-reactions.
– Balance all atoms except H and O (balance H and O also if they undergo redox)
– Add e based on oxidation state changes
– Balance oxygen atoms using H
2
O
– Balance hydrogen atoms using H +
• Equalize the number of electrons.
• Add the half reactions.
• If in base, add hydroxide to neutralize H + and cancel water molecules that appear on both sides of the equation.
• Check the balance.

The Activity Series
We can measure (electrochemically) how easily an element will accept electrons (reduce) or release electrons (oxidize).
By putting these measurements in order of reduction potential, we generate an activity series