Download Chemistry B11 Chapter 4 Chemical reactions

Chemistry B11
Chapter 4
Chemical reactions
Chemical reactions are classified into five groups:
A + B → AB
Synthesis reactions (Combination)
2H2 + O2 → 2H2O
AB → A + B
Decomposition reactions (Analysis)
2NaCl → 2Na +Cl2
Single replacement reactions
Fe + CuSO4 → FeSO4 +Cu
A + BC → AC + B
AB + CD → AD + CB
Double replacement reactions NaCl + AgNO3 → NaNO3+AgCl
AB + xO2 → yCO2 + zH2O
Combustion
C3H8 + 5O2 → 3CO2 + 4H2O
Reactants: starting materials.
Products: the materials that are created in a reaction.
Chemical Equation: we represent a chemical reaction in the form of a chemical equation,
using chemical formulas for the reactants and products, and an arrow to indicate the direction
in which the reaction proceeds.
Note: It is important to show the state of each reactant and product in a chemical equation
(immediately following each reactant and product). We use the symbol (g) for gas, (l) for
liquid, (s) for solid, and (aq) for a substance dissolved in water (aqueous).
6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)
Note: the term “Formula” can be used for both ionic and covalent compounds (formula of
NaCl). However; the term “Molecule” is strictly correct only when used covalent compound
(molecule of H2O).
Formula and molecular weight: formula weight (FW) of a compound is the sum of the
atomic weights in atomic mass units (amu) of all atoms in the compound’s formula (for both
ionic and covalent compounds). The molecular weight (MW) is the same as the formula
weight; however, it is only used for the covalent compounds.
MW of H2SO4:
MW or FW of AlCl3:
2(1 amu) for H + 1(32 amu) for S + 4(16 amu) for O = 98 amu
1(27 amu) for Al + 3(35.5 amu) for Cl = 133.5 amu
Mole (mol): is the amount of substance that contains as many atoms, molecules, or ions as
there are atoms in exactly 12g of carbon-12. Mole is the formula weight of a substance
expressed in grams.
FW of NaCl = 58.5 amu → 58.5g of NaCl = 1 mole of NaCl
MW or FW of AlCl3 = 133.5 amu → 133.5g of AlCl3 = 1 mole of AlCl3
Dr. Behrang Madani
Chemistry B11
Bakersfield College
Avogadro’s number (6.022×
×1023): number of formula units in a mole.
1 mole of hydrogen atoms = 6.022×1023 atoms of hydrogen
1 mole of water molecules = 6.022×1023 molecules of water
1 mole of Na+ ions = 6.022×1023 ions of Na+
Molar mass: is the mass of one mole of the substance expressed in grams. We can say that it
is the formula weight of a compound expressed in grams.
Formula weight of H2O = 18 amu → molar mass = 18 g (mass of 1 mole H2O)
Formula weight of NaCl = 58.5 amu → molar mass = 58.5 g (mass of 1 mole NaCl)
Balance a chemical equation: in a balanced equation, there must be the same number of
atoms of each element on both sides (the law of conservation of mass: atoms are neither
destroyed nor created in chemical reactions; they merely shift from one substance to another).
How to balance a chemical equation:
1. We begin with atoms that appear in only one compound on the left and only one compound
on the right. 2. If an atom occurs as a free element, we balance this element last. 3. We can
change only the coefficients in balancing an equation (we cannot change chemical formulas).
C3H8(g) + O2(g) → CO2(g) + H2O(g)
We begin with either carbon or hydrogen and we balance oxygen last (free element):
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
Stoichiometry: the study of mass relationships in chemical reactions. The coefficients in a
balanced equation refer to the relative numbers of moles, particles (atoms, molecules, ions),
and volume, not grams.
2H2O(g) → O2(g) + 2H2(g)
2
1
2
2 moles 1 mole 2moles
2 liters
1 liter 2 liters
2 molecules 1 molecule 2 molecules
2 grams
1 gram
2 grams
Note: We use the coefficients of a balanced chemical equation for the following conversions.
Therefore, there is only one step.
mole A → mole B
Example:
volume A → volume B
# of particles A → # of particles B
2H2 + O2 → 2H2O
How many moles of H2O are produced from the reaction of 56 moles of hydrogen?
Dr. Behrang Madani
Chemistry B11
Bakersfield College
56 mole ×
2 moles H2O
= 56 moles H2O
2 molesH2
Note: For other conversions, we use the following diagram:
mass
volume
volume
A
mole
B
mole
Particle
Particle
(atom)
(molecule)
(atom)
(molecule)
Example:
mass
2H2 + O2 → 2H2O
1. We have a sample of 74 grams of oxygen. Find the volume (in liter) that this sample can
occupy.
Note: at STP condition (0ºC as the standard temperature and 1 atm as the standard pressure),
one mole of any gas occupies a volume of 22.4 L (or 22.4 dm3 or 22400 cc).
74g O2 ×
1 mole O2 22.4 L O2
×
= 52 L O2
32 g O2 1 mole O2
2. A sample of 23.0 grams of hydrogen is reacted with oxygen. How many grams of H2O are
produced?
23.0 grams H2 ×
1 mole H2 2 moles H2O 18 grams H2O
×
×
= 207 grams H2O
2 grams H2 2 moles H2
1 mole H2
Limiting reagent: is the reactant that is used up first, leaving an excess of another reagent(s)
unreacted.
Note: The limiting reagent can control a reaction. Whenever, the limiting reagent is used up
the reaction will be stopped. Therefore, to determine how much product can be formed a
given mixture of reactants, we have to look for the reactant that is limiting.
How to find the limiting reagent: 1. Write and balance the equation for the reaction. 2.
Convert known masses of reactants to moles. 3. Using the numbers of moles of reactants to
determine which reactant is limiting.
Note: If the coefficients of reactants in a balanced reaction are not the same, we need to
divide the number of the moles of each reactant by its coefficient.
Dr. Behrang Madani
Chemistry B11
Bakersfield College
Note: Always use the amounts of a limiting reagent to find the amount of the product.
Example:
2H2 + O2 → 2H2O
If 39 grams oxygen reacts with 87 grams hydrogen, which reactant is limiting. How many
grams of H2O will be formed?
39g O2 ×
1 mole O2
= 1.2 moles O2
32 g O2
87g O2 ×
1 mole H2
= 44 moles H2
2 g H2
Because the coefficients of reactants (hydrogen and oxygen) are not the same, we have to
divide the number of moles of each reactant by its coefficient:
1.2 mole O2
= 1.2 moles O2
1
44 mole H2
= 22 moles H2
2
Therefore, oxygen is the limiting reagent. To find the amount of the product, we need to use
the amount of the limiting reagent (which is oxygen):
39 grams O 2 ×
1 mole O 2
2 moles H 2 O 18 grams H 2 O
×
×
= 44 grams H 2 O
32 grams O 2
1 moles O 2
1 mole H 2 O
Percent Yield:
precent yield =
actual yield
× 100
theoretical yield
Actual yield: the mass of product formed in a chemical reaction (experimental).
Theoretical yield: the mass of product that should form in a chemical reaction according to
the stoichiometry of the balance equation (it is always more than actual yield).
Aqueous solution: a solution in which the solvent is water. Many ionic compounds are
soluble in water. Water molecules separate the positive and negative ions from each other
(dissociation reactions).
H2O
NaCl(s) 
→ Na +(aq) + Cl -(aq)
H2O
AgNO3(s) 
→ Ag +(aq) + NO3 -(aq)
Dr. Behrang Madani
Chemistry B11
Bakersfield College
If we mix the two solutions together:
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) → AgCl(s) + Na+(aq) + NO3-(aq)
This equation is called “Ionic equation”. Na+ and NO3- ions do not participate in this
reaction. They are called “Spectator ions” and we can eliminate them from both sides:
Ag+(aq) + Cl-(aq) → AgCl(s)
Net ionic equation
Oxidation and reduction reactions (redox reactions):
Oxidation is the loss of electrons and Reduction is the gain of electrons.
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Zn(s) → Zn2+ + 2eCu2+ + 2e- → Cu(s)
Zn is oxidized (loses electrons) so it is a reducing agent.
Cu2+ is reduced (gains electrons) so it is an oxidizing agent.
In some reactions, it is not easy to see the electron loss and gain, so chemists developed
another definition of oxidation and reduction:
Oxidation is the gain of oxygen atoms and/or the loss of hydrogen atoms. Reduction is the
loss of oxygen atoms and/or the gain of hydrogen atoms.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
CH4 gains oxygen (loses hydrogen), therefore it is oxidized (a reducing agent). O2 gains
hydrogen, so it is reduced (an oxidizing agent).
Note: all single replacement reactions and all combustion reactions are redox reactions. All
double replacement reactions are non-redox reactions.
Exothermic reaction: a chemical reaction that gives off heat.
C(s) + O2(g) → CO2(g) + heat (energy)
Note: All combustion reactions are exothermic.
Endothermic reaction: a chemical reaction that absorbs heat (needs heat to accomplish).
2HgO(s) + heat (energy) → 2Hg(l) + O2(g)
Dr. Behrang Madani
Chemistry B11
Bakersfield College