Download Chemical Equations and Reactions

Chemical Equations
and Reactions
Some Definitions
Chemical reaction = The process by
which one or more substances are
changed into one or more different
substances
Chemical equation = Represents a
reaction (with symbols and formulas)
the identities and relative molecular or
molar amounts of the reactants and
products in a reaction
Indications of a Chemical Reaction
1. Evolution of energy as heat and light
2. Production of a gas
Indications of a Chemical Reaction
3. Formation of a precipitate
A solid that is produced as a result of a
chemical reaction in solution and that
separates from the solution
4. Color Change
Characteristics of
Chemical Equations
1. The equation must represent known
facts: all reactants and products must be
identified
2. The equation must contain the correct
formulas for the reactants and products:
use knowledge of writing formulas with
oxidation states
Assigning unknown oxidation numbers to
multi atom elements
For example: Cr2O72- is Cr = +6 O = -2
Characteristics of
Chemical Equations
3. The law of the conservation of mass
must be satisfied


Balancing equations --- YAY!!
Same # of atoms of each element must appear
on each side of a correct chemical equation
Word Equation
An equation which the reactants and
products in a chemical reaction are
represented by words
ONLY Qualitative Information Given
Ethane + Oxygen  Carbon dioxide + Water
Time to practice writing word equations
from a word problem!
Formula Equation
An equation representing the reactants
and products of a chemical reaction by
their symbols or formulas (also
including state symbols)
ONLY Qualitative Information Given
(Not usually balanced!!)
C2H6 (g) + O2(g)  CO2(g) + H2O(l)
Word and Formula
Word equation:
Aluminum + Hydrochloric acid 
Aluminum chloride + Hydrogen
Formula equation (not balanced):
Al(s) + HCl(aq)  AlCl3(s) + H2(g)
Correct Chemical Equation
Begin with formula equation
Balance using coefficients
 Now it is correctly written!!
Wrong:
Al(s) + HCl(aq)  AlCl3(s) + H2(g)
Right:
2Al(s) + 6HCl(aq)  2AlCl3(s) + 3H2(g)
Balancing Chemical Equations
1. Balance the different types of atoms 1 at a
time
2. 1st balance the atoms of elements that are
combined and that appear only once on
each side of the equation
3. Balance polyatomic ions that appear on
both sides of the equation as single units
4. Balance H and O atoms after atoms of all
other elements have been balanced
5. Check by final count
Balancing from Word Problem
1. 1st figure out the word equation
2. Then write the formula equation from the
word equation
3. Balance the equation following the rules
from previous slide
Practice: Nitrogen dioxide gas reacts with
water to form aqueous nitric acid and
nitrogen monoxide gas.
Significance of a Chemical Equation
1. The coefficient of a chemical reaction indicate
relative, not absolute, amount of reactants and
products
 Smallest possible amounts
2. The relative masses of the reactants and
products of a chemical reaction can be
determined from the reaction’s coefficients
 Figure out grams from moles to get total of product
3. The reverse reaction for a chemical equation
has the same relative amounts of substances
as the forward reaction
5 Types of Chemical Reactions
1. Synthesis (or Composition)
2. Decomposition
3. Single-displacement (or
Replacement)
4. Double-displacement
5. Combustion
Synthesis Reactions
2 or more substances combine
A(element or compound) + X (element
or compound)  AX (Compound)
 Elements with oxygen and sulfur
(forms oxides and sulfides)
CaO(s) + H2O(l)  Ca(OH)2(s)
8Ba(s) + S8(s)  8BaS(s)
Synthesis Reactions
 Metals with Halogens (usually
forming ionic compounds)
2K(s) + Cl2(g)  2KCl(s)
 Reactions with Oxides
Oxides of active metals + water = metal
hydroxides
Oxides of nonmetals + water = oxyacids
Synthesis Reactions
Reactants: Zn + I2
Product: Zn I2
Decomposition Reactions
Single compound produces 2 or
more simpler substances
AX (Compound)  A(element or
compound) + X (element or compound)
Decomposition Reactions
Binary Compounds
 Electrolysis: decomposition by
electric current
 2H2O(l)  2H2(g) + O2(g) = LAB!
Carbonates
 Forming a metal oxide and CO2
Hydroxides
 Pb(OH)2(s) ----> PbO(s) + H2O(g)
Single-Displacement
Reactions
In the case of a positive ion being replaced:
A + BC  B + AC
OR
In the case of a negative ion being replaced:
A + BC  C + BA
For either case we have:
element + compound  element + compound
Double-Displacement
Reactions
AB + CD  CB + AD
Exchange Partners!
Basically:
Compound + Compound  Compound
+ Compound
Double-Displacement
Reactions
Formation of a Precipitate
AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
Formation of a Gas
FeS(s) + 2HCl(aq)  H2S(g) + FeCl2(aq)
Formation of Water
H2SO4(aq) + 2NaOH(aq)  Na2SO4(aq) + 2H2O(l)
Double-Displacement
Reactions
Formation of a Precipitate
AgNO3(aq) + KCl(aq)� � AgCl(s) + KNO3(aq)
Combustion Reactions
Usually:
hydrocarbon + oxygen 
carbon dioxide and water
Example:
CH4(g) + 2O2(g)  2H2O(g) + CO2(g)
Activity Series
Whether or not a chemical reaction
will really occur!
See Handout and/or Page 286 in
your textbook (same thing!)
Activity Series Practice
1st can they occur, 2nd if so write
product, then balance:
MgCl2(aq) + Zn(s)  NO
Al(s) + H2O(g)  Yes (Al2O3(s) + H2(g))