Transcript CHEMICAL REACTIONS AND EQUATIONS

CHEMICAL REACTIONS AND
EQUATIONS
CHEMISTRY
MS. WACK
CHEMICAL REACTIONS
In a chemical reaction, bonds are broken,
atoms are rearranged, and new bonds are
formed.
• Something “new” is produced.
H2 + O2  H2O
Is it a chemical reaction?
Clues that a chemical reaction has
occurred:
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Odor
Formation of a gas (may see bubbles)
Precipitate formation
Color Change (not always)
New substances formed
Energy is absorbed or released (not
always)
General Description of a Chemical Reaction
Reactants  Products
The starting
substances in a
chemical reaction
Word Equations :
The substances
formed in a
chemical reaction
Use words and symbols to
represent chemical reactions.
•Uses words and symbols to describe a chemical reaction
•Common symbols used:
+ separates reactants or separates products
(s)
solid
(l) liquid
(g) gas
 heat is added to the reaction
catalyst
a catalyst is added to the reaction
yields/produces
(aq) aqueous—dissolved in water
reversible reaction
Word Equations
Write the word equations for the following
chemical reactions:
• Hydrogen gas reacts with chlorine gas to
produce hydrogen chloride, a gas with a
sharp odor.
HYDROGEN + CHLORINE  HYDROGEN CHLORIDE
• Calcium oxide is produced when a piece
of calcium is burned in oxygen.
CALCIUM + OXYGEN  CALCIUM OXIDE
CHEMICAL EQUATIONS
• Formula Equations: Use chemical formulas
and symbols to represent a chemical reaction.
– Keys to writing formula equations:
• Write the word equation first—then, replace the words with
chemical formulas.
• Make sure your formulas are correct!!!!!
• Make sure that each element in the reactants is also in the
products and vice versa.
• The arrow must always go to the right.
CHEMICAL EQUATIONS
Write the formula equation for the
reaction of calcium burning in oxygen to
produce calcium oxide.
LAW OF CONSERVATION OF
MASS
• How does this law affect chemical equations?
– The mass of the reactants must equal the mass of the products
– The # of atoms of each element must be equivalent on both
sides of the reaction
– Chemical reactions must be balanced
What are the #’s called?
Mg+2
superscript
2H2O  2H2 + O2
subscript coefficient
BALANCING EQUATIONS
Cd
+
HCl 
CdCl2
+
H2
BALANCING EQUATIONS
MnSO4  MnO
+
SO3
BALANCING EQUATIONS
H2SO4
+
+
NaOH  Na2SO4
+
+
H2O
BALANCING EQUATIONS
Co
+
O2 
Co2O3
BALANCING EQUATIONS
Write a balanced chemical equation for the
reaction in which magnesium reacts with nitrogen
to produce magnesium nitride.
BALANCING EQUATIONS
Write a balanced chemical equation for the
combustion of methane (CH4) in oxygen to
produce carbon dioxide and water.
BALANCING EQUATIONS
Sodium phosphate is used to cut grease. Write a balanced
equation for the reaction in which iron(II) chloride reacts
with sodium phosphate to produce sodium chloride and
iron(II) phosphate.
BALANCING EQUATIONS
Magnesium metal and water combine to form solid
magnesium hydroxide and hydrogen gas.
HEAT IN CHEMICAL REACTIONS
• Most chemical reactions involve changes in
energy.
– The unit for energy is the Joule (J)
• This is because bond breaking requires energy
and bond forming releases energy.
• Almost all chemical reactions either release or
absorb energy
• This energy flow results in heat, either being
absorbed or released.
EXOTHERMIC REATIONS
• To the touch an exothermic reaction would
feel HOT because heat is being released
to the surroundings
• In a chemical reaction that is exothermic,
the energy would be represented in the
chemical equation on the product side of
the reaction (heat is formed/released).
– C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) + 2043 kJ
ENDOTHERMIC REACTIONS
• To the touch an endothermic reaction
would feel cold because heat is being
absorbed from the surroundings.
• In a chemical reaction that is endothermic,
the energy would be represented in the
chemical equation on the reactants side of
the reaction (since heat is being
absorbed/taken in).
– C(s)+H2O(g) +113 kJ  CO(g)+H2(g)
Identify each of the following
reactions as either an endothermic
reaction or an exothermic reaction.
a ) C2H4  2C + 2H2 + 52.3 kJ
b) B2H6 + 6H2O  2H3BO3 + 6H2 + 493.4 kJ
c) 2Fe + 3CO2 + 26.8 kJ  Fe2O3 + 3CO
d) Br2 + Cl2 + 29.4 kJ  2BrCl
5 MAIN CLASSIFICATIONS
OF CHEMICAL REACTIONS
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Synthesis Reactions
Decomposition Reactions
Combustion Reactions
Single-Replacement Reactions
Double-Replacement Reactions
Synthesis Reactions
• Always Have: 1 Product
• General Format: A + B  AB
• Predicting the Product:
– The reactants come together to form one product
• If the reactants are a metal & a nonmetal then they will form
an ionic compound—get the charges & crisscross
• If they are not, the product will be given
Examples of Synthesis Reactions
• Na + Cl2
Examples of Synthesis Reactions
• H2 + O2 
• Al + S 
Decomposition Reactions
• Always Have: 1 Reactant
• General Format: AB  A + B
• Predicting the Product: Separate the
reactant into its elements
• Example of Decomposition:
C6H12O6  C + H2O
Examples of Decomposition
• MgS 
• Li2O 
Combustion Reactions
• Always have: Oxygen (O2) as a reactant
• General Formats:
– CxHy + O2  CO2 + H2O
– A + O2  AxOy
• Predicting the product:
– If its combustion between a hydrocarbon & oxygen
the reaction will produce CO2 & H2O
– If its combustion between an element and oxygen it is
a synthesis reaction—get your charges and
crisscross
COMBUSTION REACTIONS
• Always has elemental oxygen (O2) as one
of the reactants
• Example:
– Burning of methane:
• CH4 + O2
CO2 + H2O
– Burning of propane
• C3H8 + O2  CO2 + H2O
– Synthesis of sodium oxide
• Na +
O2 
Na2O
Examples
• C2H2 + O2 
• Na + O2 
Single Replacement Reactions
• Always have: 1 element and 1 compound as the
reactants and as the products
• General Format: A + BC  B + AC
• Activity Series: Lists the elements in order of their
reactivity. The more reactive elements can replace the
less reactive elements
– Elements higher on the list can replace elements lower on the
list
Which element can replace the
other in a chemical reaction?
• Tin or Barium
• Iodine or Bromine
Single Replacement Reactions
• General Format: A + BC  B + AC
• Predicting the Product: If the single
element is more reactive than the similar
element in the compound, those two
elements will switch spots. This will form
an element and a new ionic compound.
Examples of Single Replacement
Reactions
• Zn + Cu(NO3)2 
• Cu + Zn(NO3)2 
• Cu + AgNO3 
More examples of Single
Replacement Reactions
• Zn(s) + H2SO4(aq) 
• Na(s) + H2O(l) 
• Sn(s) + NaNO3(aq) 
• Cl2(g) + NaBr(aq) 
Double Replacement Reactions
+

+
• Always have: 2 compounds in the reactants (typically
ionic compounds) and 2 compounds in the products
• General Format: AX + BY  BX + AY
• Double replacement reactions occur if one of the
products will be a molecular compound (such as H2O), a
precipitate or a gas.
• Predicting the product: The metals will switch places—
get the charges and crisscross
– Types of products: water, gas, or precipitate
Double Replacement Reactions
+

+
• Precipitate: An insoluble solid formed
from two aqueous solutions.
SOLUBILITY RULES
Solubility Rules: On the solubility chart on back of your
periodic table:
“s” = precipitate, “aq” = aqueous (not a precipitate)
GENERAL
SOLUBILITY
RULES
Examples of
Double Replacement Reactions
• BaCl2(aq) + K2CO3(aq) 
• FeS(s) + HCl(aq) 
• CaCO3 +
HCl 
DISSOCIATION
• Double replacement reactions occur
between 2 ionic compounds in aqueous
solution.
– When ionic compounds dissolve in water, they
dissociate into the cation and anion of the
compound.
• DISSOCATION: The separation of ions that
occurs when an ionic compound dissolves
Complete Ionic Equation
An equation that shows dissolved ionic compounds
as dissociated free ions.
Chemical Equation:
AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
Complete Ionic Equation:
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq)  AgCl(s) + Na+(aq) + NO3-(aq)
Net Ionic Equation
An equation for a reaction in solution that shows
only those particles that are directly involved in the
chemical change.
A net ionic equation shows only the particles involved in the
reaction and is balanced with respect to both mass and
charge.
Spectator Ion: An ion that appears on both sides of an
equation and is not directly involved in the reaction.
Chemical Equation:
AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
Complete Ionic Equation:
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq)  AgCl(s) + Na+(aq) + NO3-(aq)
Any ions that appear on both sides, cross out, they are spectator
ions and are not involved in the chemical reaction.
Net Ionic Equation: The ionic equation rewritten without the
spectator ions.
Ag+(aq) + Cl-(aq)  AgCl(s)
Once the net ionic equation is written, make sure all atoms
are balanced and all charges are balanced.
Example 1
Chemical Equation:
Pb(s) + AgNO3(aq)  Ag(s) + Pb(NO3)2(aq)
Complete Ionic Equation:
Net Ionic Equation:
Once the net ionic equation is written, make sure all atoms
are balanced and all charges are balanced.
Example 2
Chemical Equation:
FeCl3(aq) + KOH(aq)  Fe(OH)3(s) + KCl(aq)
Complete Ionic Equation:
Net Ionic Equation:
Once the net ionic equation is written, make sure all atoms
are balanced and all charges are balanced.
Example 3
Word Equation:
sodium carbonate + barium nitrate 
Chemical Equation:
Complete Ionic Equation:
Net Ionic Equation:
Once the net ionic equation is written, make sure all atoms
are balanced and all charges are balanced.
Exceptions to the Rules
• Many synthesis reactions are also
combustion reactions but not all
combustion reactions are synthesis
reactions
• There are other reactions that do not fit
neatly into 1 of the 5 categories.
5 GENERAL FORMATS
• Synthesis: A + B  AB
– 1 product
• Decomposition: AB  A + B
– 1 reactant
• Combustion: CxHy + O2  CO2 + H2O
X + O2  XyO
– O2 is one of the reactants
• Single Replacement: A + BC  B + AC
– 1 element + 1 compound on both sides of the rxn
• Double Replacement: AB + CD  CB + AD
– 2 compounds on both sides of the rxn