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Chemical Reaction
and Equations
Evidence of a Reaction
Chemical Equations
Balancing Chemical
Equations
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Writing and Balancing Chemical
Reactions and Equations
• 1. List four observations that suggest that a
chemical reaction has taken place.
• 2. List three requirements for a correctly
written chemical equation.
• 3. Write a word equation and a formula
equation for a given chemical reaction.
• 4. Write a chemical equation by balancing a
formula equation by inspection.
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Evidence of a Chemical Reaction
1. The release of
light
or
heat .
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More Light and Heat Evidence
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Evidence of a Chemical Reaction
2. The production of
a gas
(bubbles) .
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More Gas Production Evidence
O2
2H2 O2 O2 2
H2
H2O
Zn
H2 HCl
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Evidence of a Chemical Reaction
3. The formation of
a precipitate
(solid)
when two aqueous
solutions are
mixed together.
Pb(NO3)2
KI
PbI2
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When gases form a Precipitate
NH4Cl(s)
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Evidence of a Chemical Reaction
4. A color change is often evidence of a
chemical reaction.
PbI2
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Characteristics of Chemical
Equations
1. The equation represents known facts.
2. The equation must contain correct formulas!
Reactant + Reactant Product + Product
3. The equation must obey The Law of Conservation of Mass.
Mass of Reactants = Mass of Products
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Word Equations
A chemical equation in which the reactants and products are represented by words (names).
Methane + Oxygen Carbon dioxide + Water
The means yields or produces
The + means “and”
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Formula Equation
• Reactants and products are described by using their symbols and/or formulas.
CH4 + O2 CO2 + H2O
• A formula equation does not obey “The Law of Conservation of Mass.”
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Chemical Equations
1. Use symbols and formulas to represent the reactants and products.
2. Adjust the coefficients in front of each reactant or product to identify the number of each reactant or product.
CH4 + 2 O2 CO2 + 2 H2O
Ones are not written but they are still there!
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Chemical Equations
3. Use special symbols in parenthesis to indicate
the state of the reactants or products.
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous (dissolved in water)
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Chemical Equations
4. Use symbols above the yield arrow to
represent catalysts or special conditions.
heat Δ atm pressure MnO2
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Reversible Reactions and Equations
Some chemical reactions occur in both directions.
N2 (g) + 3 H2 (g) 2 NH3 (g)
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Balanced Chemical Equations
The coefficients represent the relative
amounts of reactants and products.
1 CH4 + 2 O2 1 CO2 + 2 H2O
1 molecule 2 molecules 1 molecule 2 molecules
1 mole 2 moles 1 mole 2 moles
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The Importance of a Balanced
Chemical Equation cont. Coefficients can be used to determine the relative
masses of reactants and products
1 CH4 + 2 O2 1 CO2 + 2 H2O
1 mole 2 moles 1 mole 2 moles
1(16.05g) + 2(32.00g) = 1(44.01g) + 2(18.02g)
80.05 g = 80.05 g
Law of Conservation of Mass
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A Chemical Reaction
Evidence = ?
CH4 2 O2
Reactants
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Light from a Big Bang!
CH4 2 O2 CO2 2 H2O
ENERGY
Reactants Products
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Balancing Chemical Equations
1.Write the Word (Name) Equation.
Methane + Oxygen Carbon dioxide + Water
2. Write the Formula Equation:
CH4 + O2 CO2 + H2O
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Balancing Chemical Equations
3. Change the Coefficients as needed.
4. NEVER change a subscript !
5. Balance different types of atoms one
at a time.
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9. Elements
Use the symbol from the periodic table for an
element.
Na Au S C Sn P
The following elements always appear as
diatomic molecules in chemical equations.
H2 N2 O2 F2 Cl2 Br2 I2
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Balancing Chemical Equations
6. Balance elements that only appear once
on each side of the equation first.
7. Balance polyatomic ions that appear on
both sides of the equation as one unit.
8. Balance “H” and “O” last.
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Checking Your Work
Count atoms to be sure that the equation
is balanced.
Counting = Coefficient x subscript
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Example One
• Sodium metal (solid) combines with
chlorine gas to produce solid sodium
chloride.
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Example One
• Sodium metal (solid) combines with chlorine gas
to produce solid sodium chloride.
Na (s) + Cl2 (g) NaCl (s)
• Remember that chlorine is diatomic!
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Example One
• Sodium metal (Solid) combines with
chlorine gas to produce solid sodium
chloride.
Na (s) + Cl2 (g) NaCl (s)
2 Na (s) + Cl2 (g) 2 NaCl (s)
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Example Two
• When copper metal (solid) reacts with
aqueous silver nitrate, the products formed
are aqueous copper (II) nitrate and silver
metal (solid).
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Example Two
• When copper metal (solid) reacts with
aqueous silver nitrate, the products formed
are aqueous copper (II) nitrate and silver
metal (solid).
Cu (s) + AgNO3 (aq) Cu(NO3)2 (aq) + Ag(s)
Cu
Ag
N
O
1 1
1 1
1 2
2
2
6 6
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Example Two
• When copper metal (solid) reacts with
aqueous silver nitrate, the products formed
are aqueous copper (II) nitrate and silver
metal (solid).
Cu (s) + AgNO3 (aq) Cu(NO3)2 (aq) + Ag(s)
Cu (s) + 2 AgNO3 (aq) Cu(NO3)2 (aq) + 2 Ag(s)
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Example Three
• Solid Iron (III) oxide and carbon monoxide
gas react to form solid iron and carbon
dioxide gas.
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Example Three
• Solid Iron (III) oxide and carbon monoxide
gas react to form solid iron and carbon
dioxide gas.
Fe2O3 (s) + CO(g) Fe(s) + CO2(g)
Fe
C
O
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Example Three
• Solid Iron (III) oxide and carbon monoxide
gas react to form solid iron and carbon
dioxide gas.
Fe2O3 (s) + CO(g) Fe(s) + CO2(g)
Fe2O3 (s) + 3 CO(g) 2 Fe(s) + 3 CO2(g)
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THE END