Download Chemical Reactions and Equations

Chemical Reactions and
Equations
Chapter 8
Chemical Changes and
Reactions
 New
substances are produced.
reaction – chemical bonds
between atoms or ions break, and new
bonds form between atoms or ions.
 Chemical
Evidence of a Chemical Change
 Color
Change
 Precipitate
 Release
of Energy (Temperature change
or release of light)
 Gas
formation (Bubbles, Fizzing, Smoke,
odor)
Mechanics of a Chemical Reaction
REACTANTS
CH4 + O2  H2O + CO2
reactants
 Written
products
on the left side of the arrow
 Starting
material in a chemical reaction
Mechanics of a Chemical Reaction
PRODUCTS
CH4 + O2  H2O + CO2
reactants
 Written
products
on the right side of the arrow.
 Newly
formed substances that are
produced in a chemical reaction.
 Properties
reactants.
are different than those of the
Mechanics of a Chemical Reaction
ARROW
CH4 + O2  H2O + CO2
reactants
 Yield
products
sign
 Means
yields or produces
 Separates
products.
the reactants from the
Mechanics of a Chemical Reaction
PLUS SIGN
CH4 + O2  H2O + CO2
reactants
 Used
products
to separate reactants and to
separate products
Mechanics of Chemical
Reactions
 Most
reactions occur in order to complete a set
of valance electrons.
 Chemical
equations are equivalent to
mathematical equation.
 Symbols
above the yield sign represent
conditions necessary for a reaction to proceed.
 Some
reactions occur spontaneously.
Mechanics (cont.)
 Symbols
represent the state of the reactants
and products.




Liquid = l
Solid = s
Gas = g
Aqueous = aq (solids in water or solution)
molecules – certain elements exist in
nature as diatomic molecules
List them: N2, O2, F2, Cl2, Br2, I2, H2
 Diatomic
Equations in Chemistry
 Chemical
equation: an expression that
uses symbols and formulas to describe a
chemical reaction.
+
means reacts with
→
means produces (called the yield sign)
Example Reaction
H2 (g) + Cl2 (g) → HCl (g)
 What
are the reactants?
H2 and Cl2
 What
are the products?
HCl
 What
is the physical state of all the
substances?
gas
Balancing Chemical Equations
 Conservation
of Mass leads to balancing
equations – the number of atoms of each
element must be the same before and after the
reaction.
– indicates number of atoms of an
element present in a compound.
 Subscript
– indicates the number of atoms or
molecules involved in the reaction.
 Coefficient
Balancing Equations
H2 (g) + Cl2 (g) → HCl (g)
Reactant side
# of H atoms = 2
# of Cl atoms = 2

Product side
# of H atoms = 1
# of Cl atoms = 1
Does this equation have the same number of atoms
of each element on both sides of the equation?
 NO !!!
 Therefore, it does not follow the Law of
Conservation of Mass.
Balancing Equations (cont.)
H2 (g) + Cl2 (g) → HCl(g)
 This
equation is called a skeleton
equation.
 This
equation must be balanced with
coefficients (the number in front of a
formula for a substance).
a BALANCED Equation…
H2 (g) + Cl2 (g) → 2HCl(g)
Reactant side
# of H atoms = 2
# of Cl atoms = 2
Product side
# of H atoms = 2
# of Cl atoms = 2
A coefficient of 1(one) is understood and is not necessary in
the balanced chemical equation.
Steps to Balance Equations
 1)
Write equation with symbols
 2) Count # of atoms on each side of the
reaction.
 3) Balance atoms using coefficients.
 4) General Rule: Balance all elements
firsts. Then, balance C, H, and O, in that
order.
 5) NEVER CHANGE SUBSCRIPTS!!!
Synthesis Reactions
 Two
or more substances combine to form a
more complex product.
A + B → AB (only ONE PRODUCT)
 A.K.A.
Direct Combination Reactions, or
composition reactions
 Ex.
 Ex.
Fe + S → FeS
CaO + H2O → Ca(OH)2
Synthesis Reaction
Sodium Metal plus
Chlorine Gas Video
2 Na + Cl2  2 NaCl
Synthesis Reaction
Decomposition Reactions
 Single
Reactant breaks down to into a simpler
substance.
AB → A + B (only ONE REACTANT)
 The
opposite of a synthesis reaction.
 Ex.
2HgO → 2Hg + O2
 Ex.
CaCO3 → CaO + CO2
Single Replacement Reaction
 Atoms
of one element replace atoms of another
element in a compound.
A + BX → AX + B
 A more
active element will replace a less active
element. (See activity series)
Fe + CuSO4 → FeSO4 + Cu
 Ex. Mg + CuSO4 → MgSO4 + Cu
 Ex.
Reactivity or Activity of Metals
The reactivity of a metal is based on its
ability to replace another in a compound. The
metal is oxidized.
 If a single replacement reaction occurs, the
metal that “cuts in” is MORE reactive than
the one that was removed or replaced.
 An activity series of metals is a listing that
ranks metals according to their reactivity.



The most active metal is at the TOP of the list
The least active metal is at the BOTTOM of the list
The ACTIVITY SERIES is listed below:
lithium
potassium
barium
strontium
calcium
sodium
magnesium
aluminum
manganese
zinc
iron
cadmium
cobalt
nickel
tin
lead
hydrogen
copper
silver
mercury
gold
The most active metal is LITHIUM
The least active metal is GOLD
Which is more active nickel or iron?
IRON
3CuCl2 + 2Al  2AlCl3 + 3Cu
Double-Replacement Reactions
 Atoms
or ions from 2 different compounds
replace each other.
AX + BY → AY + BX
 Ex.
CaCO3 + 2HCl → CaCl2 + H2CO3
Combustion Reactions
 One
substance reacts with oxygen to produce
oxide compounds.
 Occurs
when burning.
 Combustion
reactions are often classified as
synthesis reactions.
 These
reactions are usually exothermic,
releasing a large amount of energy as light,
heat, or sound.
Combustion Reactions (cont.)
 When
a hydrocarbon is involved in a
combustion reaction, H2O and CO2 are the
products.
 Ex.
 Ex.
CH4 + 2O2 → CO2 + 2H2O + 803 kJ
S + O2 → SO2
Combustion Reaction
5 Types of Chemical
Reactions Video