Transcript Chemical Reactions

Chemical Reactions
• Elements and compounds frequently undergo
chemical reactions to form new substances
• In a chemical reaction, chemical bonds are
frequently broken and new chemical bonds
are formed
• Atoms are neither created nor destroyed in an
ordinary chemical change
Chemical Reactions
• A balanced chemical reaction is used to describe
the process that occurs in a chemical change.
• For example: Zinc reacts with hydrochloric acid
to produce zinc chloride and hydrogen gas.
• This chemical reaction could be written as
Zn + 2 HCl  ZnCl2 + H2
Reactants and Products
• In the chemical reaction
Zn + 2 HCl  ZnCl2 + H2
Reactants
Products
• This shorthand way of describing a chemical
reaction is known as a chemical equation
• The starting materials are shown on the left and are
known as reactants
• The substances formed are shown on the right and
are known as the products
Balancing a Chemical Reaction
• A proper chemical reaction must be balanced
Zn + 2 HCl  ZnCl2 + H2
Reactants
Products
• Each element must appear on both sides of
the arrow and equal number of times
• Chemical reactions can be balanced by
inserting numbers in front of formulas.
• These numbers are called coefficients
Balancing Chemical Reactions
•
•
Most simple equations can be balanced by inspection
Example: Balance the following equation
BaCl2 + K3PO4  Ba3 (PO4)2 + KCl
•
•
•
There are 3 Ba on the right so we need coefficient of 3 in
front of BaCl2
There are 2 PO4 on the right so we need a coefficient of 2 in
front of K3PO4.
This leaves 6 K on the left so we need a coefficient of 6 in
front of the KCl on the right
The balanced equation is
3 BaCl2 + 2 K3PO4  Ba3 (PO4)2 + 6 KCl
Balancing Chemical Reactions
•
An equation is balanced when there are the same
number and kind of atoms on both sides of the arrow
3 BaCl2 + 2 K3PO4  Ba3(PO4)2 + 6 KCl
Reactants (Left) Products (Right)
Ba
3
Ba
3
Cl
3x2=6
Cl
6
K
2x3= 6
K
6
P
2
P
2
O
2x4=8
O
2x4 =8
State Symbols
• State symbols are often added to chemical equations.
CaCO3 (s) + 2 HCl (aq)  CaCl2 (aq) + CO2 (g) + H2O (l)
Symbols
(s)
Solid
(l)
Liquid
(g)
Gas
(aq)
Aqueous (Water Solution)
Types of Reactions
• There are many kinds of chemical reactions that
occur. Some are very simple while others are
very complex and may occur in multiple steps.
• A number of reactions conform to some
relatively simple patterns.
• Understanding and identifying these patterns
can be helpful in predicting the products of
similar reactions
Combustion
• In a direct combustion, an element or compound(s)
(hydrocarbons) combine with Oxygen (O2) to form
Water and CO2 (under complete combustion).
• Examples:
CH4 + 2 O2  2 H2O + CO2
C4H8 + 6 O2  4 H2O + 4 CO2
(“exception”): 2 H2 + O2  2 H2O
Limited O2: Reaction gives CO + H2O as products.
Extremely limited O2: Reaction gives C + H2O as
products.
Direct Combination/Composition
• In a direct combination, two elements or
compounds combine to form a more
complicated product
• Examples
CaO + CO2  CaCO3
2 H2 + O2  2 H2O
FeCl2 + Cl2  FeCl3
N2 + O2
 2 NO
Decomposition
• In a decomposition, a single compound is
broken down into two or more simpler
substances
• Examples
2 KClO3  2 KCl + 3 O2
ZnCO3  ZnO + CO2
Cu(OH)2  CuO + H2O
Single Replacement
• In a single replacement, one substance
(usually an element) takes the place of
another in a compound
• Examples
Zn + H2SO4  ZnSO4 + H2
Cl2 + 2 KBr  2 KCl + Br2
Mg + CuCl2  MgCl2 + Cu
Double Replacement
• In a double replacement, two substances
exchange places in their respective
compounds
• Examples
AgNO3 + NaCl  AgCl + NaNO3
3 CaCl2 + 2 K3PO4  Ca3(PO4)2 + 6KCl BaCl2 +
Na2SO4  BaSO4 + 2NaCl