Equations & Reactions
8.1 Describing Chemical Reactions
A. Chemical Changes and Reactions
1. New substances are produced.
2. Chemical reaction – chemical bonds between atoms or
ions break, and new bonds form between atoms or
ions.
B. Evidence of a Chemical Reaction
1. color change
2. formation of a precipitate
3. temperature change
4. formation of a gas
C. Mechanics of a Chemical Reaction
1. Starting Materials – reactants
2. Ending Materials - products
3. reactants → products
Arrow = yields or produces
4. Many reactions occur to complete a set of valence electrons.
5. Symbols above the yield sign represent conditions necessary for a
reaction to proceed.
Ex)
Δ

= delta = heat
elec = electrolysis

6. Some reactions occur spontaneously.
7. Symbols represent the state of the reactants and products.
Liquid = l Gas = g Solid = s Crystal = cr
Aqueous = aq (solids in water solution)
DEMO
Ex) 2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)
silver
blue
gray
red
8. Complete chemical equations include the
subscript to indicate the physical state of
each substance.
9. Diatomic molecules – certain elements
exist in nature as diatomic molecules (X2)
List them: N2 O2 F2 Cl2 Br2 I2 H2
Natural States of the Elements
• Diatomic Molecules
Nitrogen gas contains
N2 molecules.
Oxygen gas contains
O2 molecules.
Highlight your Periodic Table
Natural States of the Diatomic Elements
8.2 Balancing Equations
A. Equations in Chemistry
1. Chemical equation: an expression that uses
symbols and formulas to describe a chemical
reaction.
2. + means “reacts with.”
3. → means produces (called the yield sign.)
B. Balancing Chemical Equations
1. Conservation of mass leads to balancing equations – the
number of atoms of each element must be the same
before & after the reaction.
2. The Law of Conservation of Mass also states that the total
mass before and after the reaction must be the same.
You cannot lose or gain mass.
3. Therefore the MASS OF THE PRODUCTS =
MASS OF REACTANTS
4. Subscript – indicates number of atoms of an element
present in a compound.
5. Coefficient – indicates the number of atoms or
molecules involved in the reaction.
6. Steps to Balance Equations:
A. Write equation with symbols.
B. Count # of atoms on each side of the
reaction.
C. Balance atoms using coefficients.
D. General Rule: Balance all elements first.
Then, balance C, H, and O.
E. NEVER CHANGE SUBSCRIPTS!!!!
3 H2 + N2
→ 2 NH3
3 Na2SO4 + Ca3(PO4)2 → 2 CaSO4 + 2 Na3PO4
2 NaNO3 →
2 NaNO2 + O2
2 C8H18 + 25 O2 → 16 CO2 + 18 H2O
8.3 Classifying Chemical Reactions
A. Synthesis Reactions (direct combination)
1. Two or more elements or compounds combine
to form a more complex product.
A + B → AB
2. Ex. Fe + S → FeS
CaO + H2O → Ca(OH)2
sodium reacts with chlorine
2 Na + Cl2 → 2 NaCl
Synthesis Reaction
Sodium Metal plus
Chlorine Gas Video
2 Na + Cl2  2 NaCl
Synthesis Reaction
B. Decomposition Reactions (analysis)
1. A single reactant breaks down into simpler
compounds or elements.
AB → A + B
2. The opposite of a synthesis reaction.
3. Carbonates (compounds ending in CO3) break
down into metal oxides and carbon dioxide.
4. Ex.
2 HgO
CaCO3
→
→
2 Hg + O2
CaO + CO2
Decomposition Reaction
CuCO3(s)  CO2(g) + CuO(s)
flame goes out
copper (II) carbonate
metal oxide
What is one of the products?
2HgO → 2Hg + O2
mercury (II) oxide
mercury (II) oxide
C. Single Replacement Reactions
1. Atoms of an uncombined element replace atoms of
another element in a compound.
A + BX → AX + B
2. A more active element will replace a less active
element. (See activity series.)
3CuCl2 + 2Al  2AlCl3 + 3Cu
3. An Activity Series is a way of ranking elements
(usually metals) in order from greatest to least
reactivity. It can be used to predict whether a reaction
will occur or not.
2 Al + 3 CuSO4
Fe + MgCl2
PbSO4 + Au
AgCl2 + Cu
→ 3 Cu + Al2(SO4)3
→ No Reaction
→ No Reaction
→ CuCl2 + Ag
D. Double-Replacement Reactions
1. Atoms or ions from two different compounds
replace each other.
AX + BY →
AY + BX
2. These types of reactions will
(A) form precipitates (↓)
(B) form gases (↑)
(C) are acid-base neutralizations
Double Replacement Examples:
A. Pb(NO3)2 + 2 KI → PbI2 ↓ + 2 KNO3
B. CaCO3 + 2 HCl → CaCl2 + H2CO3
C. NaOH + HCl → NaCl + HOH
3. In letter B above, carbonic acid, H2CO3, is
unstable and will immediately decompose into
carbon dioxide and water.
CaCO3 + 2 HCl → CaCl2 + CO2↑ + H2O
E. Combustion Reactions
1. One substance reacts with oxygen, O2 to
produce oxide compounds.
2. Occurs during burning or oxidation
(rusting.)
3. The reactions that only add oxygen are
classified as synthesis reactions.
Ex) S + O2 → SO2
Hydrogen Burning Video
H2 + 2 O2  2 H2O
Synthesis Reaction
4. Combustion reactions are exothermic, releasing a large
amount of energy as light, heat, or sound.
5. A true Combustion reaction occurs when a hydrocarbon
(compound containing H & C ) reacts to form
carbon dioxide and water that are always the
products.
CxHx + O2 → CO2 + H2O
2 2O + 803 kJ
2 2 → __CO2 + __H
6. Ex. __CH4 + __O
C6H12 O6 + 6 O2
→ 6 CO2 + 6 H2O + heat
Combustion Reaction
Combustion Reaction
5 Types of Chemical
Reactions Video