Chapter4

CHEM 103
Chemical ReactionsChapter 4
• Molecular and formula weight
• Moles
• Balancing equations
• Calculating mass relationships
• Percent Yield
• Predicting interactions between ions in solutions
• Oxidation/ Reduction reactions
• Heat of Reaction

Formula weightFormula weight
is the sum of the atomic weights in amu units of all atoms in a
compound’s formula. It can be used for both ionic and
molecular compounds. The term molecular weight means the
same thing but it is used only for covalent compounds and is
expressed as g/mol.

THE MOLE
The ratio of grams to molecular mass is so useful that it was
given a name “the molethe mole”
1 mol H atoms = 1 g of Hydrogen
1 mol O atoms = 16.00 g of Oxygen

1 mol of “something” always has 6.022 x 1023
“things”
1 mol of Cl2 molecules = 6.022 x 1023
molecules of Cl2
1 mol of Cl atoms = 6.022 x 1023
atoms of Cl
Therefore there are:
6.022 x 1023
Hatoms in 1 g H ; atomic mass H = 1 amu
6.022 x 1023
Oatoms in 16 g O; atomic mass O = 16 amu
Note: 6.022 x 1023
is known as Avogadro’s number

How many atoms of copper are in a 1.5 mole
sample of copper?
1. 2.
3. 4.

The molar mass, MM, in grams per mole, is numerically
equal to the sum of the masses (in amu) of the atoms in the
formula.
Formula Sum of Atomic Masses Molar Mass, MM
O 16.00 amu 16.00 g/mol
O2 2(16.00 amu) = 32.00 amu 32.00 g/mol
H2O 2(1.008 amu) + 16.00 amu = 18.02 amu 18.02 g/mol
NaCl 22.99 amu + 35.45 amu = 58.44 amu 58.44 g/mol

What is the molar mass of SO3?
1. 48.07 amu
2. 48.07 g/mol
3. 80.07 amu
4. 80.07 g/mol

The subscripts in a formula represent the atom ratio and/or
mole ratio in which the different elements are combined.
1 mol of H20 1 mol of C9H8O4
Contains: Contains:
2 mol of H 9 mol of C
1 mol of O 8 mol of H
4 mol of O

Calculate the number of moles of water in 36.0 g of water
36.0 g H2O
1.00 mol H2O
18.0 g H2O
= 2.00 mol H 2Ox

Calculate the number of moles of sodium ions, Na+
, in 5.63 g
of sodium sulfate, Na2SO4
formula weight = 2(23.0) + 32.1 + 4(16.0) = 142.1 amu
therefore, 1 mol of Na2SO4 = 142.1 g Na2SO4
there are two moles of Na+
ions per mole of Na2SO40.0396 mol Na2SO 4
2 mol Na
+
1 mol Na2SO 4
=x 0.0792 mol Na
+
5.63 g Na2SO4 x
1.00 mol Na2SO4
142.1 g Na2SO4
= 0.0396 mol Na2SO4

A tablet of aspirin, C9H8O4, contains 0.360 g of aspirin. How
many molecules of aspirin are present?
0.360 g aspirin x
1 mol aspirin
180.0 g aspirin
= 0.00200 mol aspirin
0.00200 mole x 6.02 x 1023 molecules
mole
= 1.20 x 1021
molecules

1. 2.
3. 4.
How many molecules of Cl2 are in a 54 g sample?

Balancing Equations
– begin with atoms that appear only in one compound on both sides;
in this case carbon (C)
– now balance hydrogens,
– if an atom occurs as a free element, as for example O2, balance it
last:

1. 1
2. 2
3. 3
4. 4
What is the coefficient in front of O2 when the
following equation is balanced?
CH4 (g) + O2 (g) → CO2 (g) + H2O (g)

Stoichiometry
StoichiometryStoichiometry:: the study of mass relationships in chemical
reactions

How many grams of nitrogen, N2, are required to produce
7.50 g of ammonia, NH3
x
28.0 g N2
1 mol N2
= 6.18 g N2
7.50 g NH 3 x
1 mol NH3
17.0 g NH3
x
1 mol N2
2 mol NH3

1. 0.0043 g
2. 2.3 g
3. 15 g
4. 7800 g
How many grams of NaCl can be produced when 5.8
g Na reacts with excess chlorine?
2 Na (s) + Cl2 (g) → 2 NaCl (s)

How many grams each of CO22 and NH33 are produced from
0.83 mol of urea?

IDENTIFYING LIMITING REACTANTS
AND THEORETICAL YIELD
a) Calculate the amount of product that would form if the first
reactant were completely consumed.
b) Do this for the second reactant.
c) The smaller number produced from the two calculations
above is the theoretical yield of product, and the reactant
that produced it is the limiting reactant.

Determine the limiting reactant and theoretical yield for
2Sb(s) + 3I2(g) 2SbI3 (s)
a) When 1.2 mol of Sb and 2.4 mol of I2 are mixed
n SbI3 from Sb = 1.20 mol Sb x 2 mol SbI3 = 1.20 mol SbI3
2 mol Sb
n SbI3 from I2 = 2.40 mol I2 x 2 mol SbI3 = 1.60 mol SbI3
3 mol I2
Sb is the limiting reactant
Theoretical yield = 1.20 mol SbI3

Determine the limiting reactant and theoretical yield for
2Sb(s) + 3I2(g) 2SbI3 (s)
b) When 1.20 g of Sb and 2.4 g of I2 are mixed
mass of SbI3 from Sb = 1.20 g Sb x 1 mol Sb x 2 mol SbI 3 x 502.5 g SbI 3 = 4.95 g SbI3
121.8 g Sb 2 mol Sb 1 mol SbI3
mass of SbI3 from I2 = 2.40 g I2 x 1 mol I2 x 2 mol SbI3 x 502.5 g SbI3 = 3.17 g SbI3
253.8 g I2 3 mol I2 1 mol SbI3
Therefore I2 is the limiting reactant and 3.17 g is the theoretical yield

1. 9.6 g
2. 11.6 g
3. 15 g
4. 24 g
If 5.8 g Na react with 5.8 g Cl2, how much NaCl can be produced?
2 Na (s) + Cl2 (g) → 2 NaCl (s)

Percent YieldPercent Yield
Experimental yield:Experimental yield: the actual mass of product formed in a
chemical reaction.
Theoretical yield:Theoretical yield: the mass of product that should form
according to the stoichiometry of the balanced equation.
Percent yieldPercent yield:: experimental yield divided by theoretical yield
times 100.
Percent yield =
Actual yield
Theoretical yield
x 100

A reaction produces 28.4 g of product. If the theoretical
yield for this experiment was 35.0 g, what is the percent
yield?
1. 0.811%
2. 1.23%
3. 81.1%
4. 123%

In general, ions in solution react with each other when one of the
following can happen:
• Two ions form a compound that is insoluble in water.
• Two ions react to form a gas that escapes from the reaction mixture as
bubbles (sodium bicarbonate and hydrochloric acid).
• An acid neutralizes a base (Chapter 9).
• One of the ions can oxidize another (Section 5.7).

SOLUBILITY
These are some generalizations :
– All compounds containing Na+
, K+
, and NH4
+
are soluble in water.
– All nitrates (NO3
-
) and acetates (CH3COO-
) are soluble in water.
– Most chlorides (Cl-
) and sulfates (SO4
2-
) are soluble in water;
exceptions are AgCl, PbCl2, BaSO4, and PbSO4.
– Most carbonates (CO3
2-
), phosphates (PO4
3-
), sulfides (S2-
), and
hydroxides (OH-
) are insoluble in water; exceptions are LiOH, NaOH,
KOH, and NH4OH, which are soluble in water.

We can simplify by omitting all ions that do not participate in
the reaction:
The equation is called a net ionic equationnet ionic equation. Ions that do not
participate in a reaction are called spectator ionsspectator ions..

OXIDATION-REDUCTION REACTIONS
A common type of reaction in aqueous solution involves transfer of
electrons between two species. Such reactions are called redox
reaction.
In a redox reactions one species loses (donates) electrons and is
said to be oxidized. This species is called the reducing agent.
The other species gains electrons (receives) and is reduced. This
species is named the oxidizing agent.

The net ionic equation is
This equation can be split into two half-equations:
Zn(s) Zn2+
(aq) + 2e-
Zn is oxidized
Cu2+
(aq) + 2e-
Cu(s) Cu2+
is reduced
Note that oxidation and reduction occur together and that there is no net change in the number of
electrons in the reaction.
Reducing
agent
Oxidizing
agent

Oxidation:Oxidation: gain of oxygen and/or loss of hydrogen.
Reduction:Reduction: loss of oxygen and/or gain of hydrogen.

1. oxidized; gaining
2. oxidized; losing
3. reduced; gaining
4. reduced; losing
Aluminum is _______ because it is _______ electrons.
Al(s) → Al 3+
(aq) + 3 e-

CombustionCombustion:: burning in air. The products of complete
combustion of carbon compounds are CO2 and H2O.
2 C8H18 (l) + 25 O2 (g) 16 CO2 (g) + 18 H2O (l) + Heat
Five important types of redox reactionsFive important types of redox reactions

Respiration:Respiration: the process by which living organisms
oxidize carbon-containing compounds for energy.

RustingRusting:: the oxidation of iron to iron oxides

BleachingBleaching:: the oxidation of colored compounds to
colorless products.

BatteriesBatteries:: in all cases, the reaction taking place in
a battery is a redox-reaction.

Heat of Reaction
In almost all chemical reactions, heat is either given off or
absorbed
plus 1263.1 kcal of heat

Heat of reaction:Heat of reaction: the heat given off or absorbed in a
chemical reaction.
exothermic reaction:exothermic reaction: one that gives off heat.
endothermic reaction:endothermic reaction: one that absorbs heat.

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