Download Chemical Equations

Chemical
Reaction
Equations
Evidence of Chemical Reactions
• A gas is produced.
• A permanent color change is observed.
• An energy change occurs.
• A precipitate is formed.
– Solid produced from mixture of liquids
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Chemical Reaction
A process in which one or more substances is
changed into one or more new substances is a
chemical reaction (rxn)
reactants
products
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A chemical equation uses chemical symbols to
show what happens during a chemical reaction
3 ways of representing the reaction of H2 with O2 to form H2O
2 H2
+
O2

2 H2O
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Chemical Equations: Direction
The direction of equation arrow can vary
Mg + O2
MgO
reactants
products
Mg + O2
MgO
products
reactants
Mg + O2
MgO
equilibrium
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Chemical Equations: States
The state of each substance may be denoted
Mg (s) + O2(g)
MgO(s)
State Designations:
(g) gas
(l) liquid
(s) solid
(aq) aqueous (dissolved in water)
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The Law of Conservation of Mass
• Matter can be neither created
nor destroyed
• In any chemical rxn, the total
mass of reactants = total mass
of products
• In 1789, first demonstrated
experimentally by the Lavoisiers
Antoine Lavoisier
(1743 – 1794)
Marie-Anne Pierrette Paulze
(1743 – 1794)
• During a chemical rxn atoms do not change,
they merely rearrange
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Chemical Equations: Reading
2 Mg + O2
2 MgO
2 atoms Mg + 1 molecule O2 makes 2 formula units MgO
2 moles Mg + 1 mole O2 makes 2 moles MgO
48.6 grams Mg + 32.0 grams O2 makes 80.6 g MgO
NOT
2 grams Mg + 1 gram O2 makes 2 g MgO
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Chemical Equations: Balancing
Equations must obey the Law of
Conservation of Mass
Mg (s) + O2(g)
1 Mg + 2 O
MgO(s)
1 Mg + 1 O
Whole number coefficients are added to
‘balance’ equations
2 Mg (s) + O2(g)
2 MgO(s)
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Balancing Chemical Equations
1. Write the correct formula(s) for the reactants on
the left side and the correct formula(s) for the
product(s) on the right side of the equation.
Ethane reacts with oxygen to form carbon dioxide and water
C2H6 + O2
CO2 + H2O
2. Change the numbers in front of the formulas
(coefficients) to make the number of atoms of
each element the same on both sides of the
equation. Do not change the subscripts.
2 C2H6
NOT
C4H12
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Balancing Chemical Equations
3. Start by balancing those elements that appear in
only one reactant and one product.
C2H6 + O2
2 carbon
on left
C2H6 + O2
6 hydrogen
on left
C2H6 + O2
CO2 + H2O
start with C or H but not O
1 carbon
on right
multiply CO2 by 2
2CO2 + H2O
2 hydrogen
on right
2CO2 + 3H2O
multiply H2O by 3
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Balancing Chemical Equations
4. Balance those elements that appear in two or
more reactants or products.
C2H6 + O2
2 oxygen
on left
2 CO2 + 3 H2O
multiply O2 by 7
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Want
Have
4 oxygen+ 3 oxygen = 7 oxygen
(3x1)
(2x2)
on right
C2H6 + 7 O2
2
2CO2 + 3H2O
2C2H6 + 7O2
4CO2 + 6H2O
remove fraction
multiply both sides by 2
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Balancing Chemical Equations
5. Check to make sure that you have the same
number of each type of atom on both sides of the
equation.
2C2H6 + 7O2
4CO2 + 6H2O
4 C (2 x 2)
4C
12 H (2 x 6)
12 H (6 x 2)
14 O (7 x 2)
14 O (4 x 2 + 6)
Reactants
4C
12 H
14 O
Products
4C
12 H
14 O
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Balance the following rxn equations:
H2 
1.
N2 +
2.
HCl +
CaCO3

CaCl2 +
3.
4.
NH3
H2O +
CO2
C3H4O + O2  CO2 +
H2O
Na2S2O3 +

Fe(NO3)3
NaNO3 +
Fe2(S2O3)3
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Balance the following rxn equations:
1.
2.
N2 + 3 H2 
2 HCl +

2 NH3
CaCO3
CaCl2 +
H2O +
CO2
3.
2 C3H4O + 7 O2  6 CO2 + 4 H2O
4.
3 Na2S2O3 + 2 Fe(NO3)3
 6 NaNO3 +
Fe2(S2O3)3
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Evidence of Chemical Reactions
• A gas is produced.
• A permanent color change is observed.
• An energy change occurs.
• A precipitate is formed.
– Solid produced from mixture of liquids
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Rxn Types
1) Synthesis
2) Decomposition
3) Single Replacement
4) Double Replacement
5) Combustion
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Chemical Equations: Translating from English
English
Symbol
“and” , “with”, “mixed”, “dissolve”
+
“produce”, “decompose”, “combust”,
“burned”, “electrolyzed”, “form”
“precipitate”, “suspension”, “turnings”,
“pellet”, “ribbon”
(s)
“molten”
(l)
“solution”, “acid”
(aq)
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Synthesis
Combination of two or more substances
to form one compound
Ex: “Chlorine gas is passed over a piece of sodium metal,
producing solid sodium chloride.”
Cl2(g)+ 2 Na(s)  2 NaCl(s)
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Decomposition
A compound is broken down into simpler
substances (which usually requires energy)
Ex: “Powdered ammonium dichromate is strongly heated,
producing heat, nitrogen gas, water and chromium(III) oxide.”
(NH4)2Cr2O7(s)  N2(g) + 4 H2O(g) + Cr2O3(s)
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Single Replacement
(single displacement)
One element is replaced by another in a compound.
(compound is usually an ionic compound)
Ex: “A piece of magnesium ribbon is dropped into hydrochloric
acid generating hydrogen gas and aqueous magnesium
chloride.”
Mg(s) + 2 HCl(aq)  H2(g) + MgCl2(aq)
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Double Replacement
(double displacement)
The elements (or ions) in two compounds are exchanged
(compounds are usually an ionic compounds)
Ex: “Sodium iodide and lead(II) nitrate solutions are mixed to
produce lead(II) iodide precipitate.”
2 NaI(aq) + Pb(NO3)2(aq)  PbI2(s)+ 2 NaNO3(aq)
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Combustion
Molecular oxygen combines with a hydrocarbon
to form carbon dioxide and water
(hydrocarbon = molecule consisting
mainly of carbon and hydrogen)
Ex: “Liquid ethanol is burned in air.”
C2H5OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(g)
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