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Chemical Reactions
Chapter 7
Describing Chemical
Reactions
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What type of
change is
happening in the
picture to the left?
Chemical Reaction
New Products
being formed
Heat and Light
being produced
Chemical Equations
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What is present before and after a chemical
change takes place?
Reactants – the substances undergoing the
chemical change
Products – the new substances formed as a
results of the chemical change
In the case of the burning charcoal
Reactants are carbon and oxygen and the
product is carbon dioxide
C + O2 → CO2
Using Equations to
Represent Reactions
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Reactants → Products
Chemical Equation - a representation
of a chemical reaction in which the
reactants and products are expressed
as formulas
Conservation of Mass
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The mass of the products is always
equal to the mass of the reactants
Called the Conservation of Mass
 Principal was establish by Antoine
Lavoisier
 Mass is neither created nor
destroyed in a chemical reaction
Conservation of Mass
Balancing Equations
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In rockets
 Hydrazine + Oxygen yields
Nitrogen + Water
 N H + O → N + H O
2 4
2
2
2
This equation is not in balance
Balancing Equations
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We balance equations by adding
coefficients (a whole number)
before the formula itself
 We never change subscripts
when balancing an equations
N2H4 + O2 → N2 + 2H2O
Some Practice Balancing
Equations
 H + O → H O
2
2
2
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HgO → Hg + O2
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Zn + HCl → ZnCl2 + H2
Counting with Moles
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Chemists use a counting unit called the
mole
1 mole = 6.02 X 1023 particles of that
substance
 Particles might be atoms, ions, molecules
Molar Mass
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Molar mass – the mass of one mole of a
substance
 the atomic mass expressed in grams
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1 mole of Na = 23 grams
1 mole of Cl = 35 grams
1 mole of NaCl = 58 grams
1 Mole of Hydrogen = 1 gram
1 Mole of Oxygen = 16 grams
1 Mole of H2O = 18 grams
Mole-Mass Conversions
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Express molar mass as a conversion
factor
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ie: CO2 has a molar mass of 44 grams
1
mole of Carbon = 12 grams
 2 moles of Oxygen = 32 grams (16 g x 2)
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44.0g CO2/1 mole of CO2
1 mole of CO2/44.0 g CO2
How many moles of CO2 do I have if I
have 55 grams of CO2?
The Calculation
55.0 g CO2 X 1 mole of CO2/44 g CO2
= 1.25 moles of CO2
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If you know how many grams of a
substance you have ~divide by the
molar mass to determine how many
moles you have
Types of Reactions
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Synthesis Reactions
Decomposition Reactions
Single-Replacement Reactions
Double-Replacement Reactions
Combustion Reactions
Synthesis Reactions
A synthesis reaction is a reaction in which
two or more substances react to form a single
substance – the reactants may be either
elements or compounds
A + B → AB

2 Na + Cl2 → 2 NaCl
2 H2 + O2 → 2 H2O
This reaction generates electricity
for satellites and spacecraft
Decomposition Reactions
A decomposition reaction is a reaction in which
a compound breaks down into two or more
simpler substances – The reactant in a
decomposition reaction must be a compound –
the products may be elements or compounds
AB → A + B
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2 H2O → 2H2 + O2
CaCO3 → CaO + CO2
Making cement
2NaN3 → 2 Na + 3 N2 (Air bags)
Single-Replacement
Reactions
A Single-replacement reaction is a
reaction in which one element takes the
place of another element in a compound
A + BC → B + AC (A and B have switched
places)
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Cu + 2Ag NO3 → Cu(NO3)2 + 2Ag
2K + 2H2O → H2 + 2 KOH
Double-Replacement
Reactions
A double-replacement reaction is one in
which two different compound exchange
positive ions and form two new compounds
AB + CD → AD + CB
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Pb(NO3)2 + 2 KI → PbI2 + 2KNO3
CaCO3 + 2 HCl → CaCl2 + H2CO3
Combustion Reactions
A combustion reaction is one in which
a substance reacts rapidly with
oxygen, often (not always) producing
heat and light.
 One of the reactants must be oxygen
CH4 + 2 O2 → CO2 + H2O
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2H2 + O2 → 2H2O
Reactions as Electron
Transfers (Redox Reactions)
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Oxidation – A loss of electrons during a
chemical reaction
Reduction – a gain of electrons during a
chemical reaction
2 Ca + O2 → 2 CaO
 Calcium is oxidized
 Oxygen is reduced
Na + Cl →NaCl
 Sodium is oxidized
 Chlorine is reduced
Energy Changes in
Reactions
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C3H8 + 5O2 → 3CO2 + 4 H2O
+ Heat
Chemical energy is the
energy stored in the chemical
bonds of a substance
Chemical reactions involve
the breaking of chemical
bonds in the reactants and
the formation of chemical
bonds in the products
Breaking Bonds
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Breaking bonds requires energy
 This is why we need a spark to light
the propane because it doesn’t
happen spontaneously
Forming bonds requires energy
During a chemical reaction energy is
either released or absorbed
Exothermic Reactions
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A chemical reaction in which energy is
released to the surroundings
Combustion reactions are extreme
exothermic reactions
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Ex. 1 mole of Propane produces 2220
kilojoules of heat
In an exothermic reaction the kilojoules
would be written to the right of the arrow
Endothermic Reactions
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A chemical reaction that absorbs
energy from its surroundings is called
an endothermic reaction
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2HgO + Heat → 2 Hg + O2
In an endothermic reaction heat appears
on the left side of the arrow
Conservation of Energy
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The total amount of energy before and
after a reaction must always be the
same
Reaction Rates
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The progress of a chemical reaction can be
measured over time
Different reactions have different durations
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TNT
Leaves turning color
Any reaction that takes places over time
can be expressed as a rate
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A reaction rate is the rate at which reactants
change into products over time – in other
words how fast the reaction is going
Factors affecting Reactions
Rates
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Temperature
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Surface area
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Generally an increase in temperature
will increase the reaction rate
Increasing the surface area of a reactant
tends to increase the reaction rate
Stirring
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Stirring the reactants will generally
increase the reaction rate
Factors affecting Reactions
Rates
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Concentration
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Increasing the concentration generally
increases the reaction rate
Catalysts
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A substance that affects the reaction rate
without being used up in the reaction
 Generally
speed up a reaction
Equilibrium
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Equilibrium is a state in which the
forward and reverse paths of a change
take place at the same rate
Physical Equilibrium
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H2O (l) ↔ H2O (g)
When physical change does not go to
completion, a physical equilibrium is
established between the forward and
reverse changes
Equilibrium
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Most chemical reactions are reversible to some
extent
A reversible reaction is a reaction in which the
conversion of reactants into products and the
conversion of products into reactants can
happen simultaneously
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2 SO2 (g) + O2 (g) ↔ 2 SO3 (g)
When a chemical reaction does not go to
completion, a chemical equilibrium is established
between the forward and reverse reactions
Factors Affecting Chemical
Equilibrium
When a change is introduced to a system in equilibrium, the
equilibrium shifts in the direction that relieves the change
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Discovered by Henri Le Chatlier
N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat
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→ This reaction is exothermic
← This reaction would be endothermic
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Temperature
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If you add heat to the system the equilibrium would shift
in the direction that removes heat from the system (In the
above reaction you would decrease the amount of
ammonia produced
Factors Affecting Chemical
Equilibrium
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Pressure
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Increasing the pressure would shift the
equilibrium in the direction that reduces
the pressure
 More
gas molecules = more pressure
 Increasing the pressure in the ammonia
production would produce more ammonia
because the right side of the equation has
fewer ( 2 vs. 4 ) gas molecules
N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat
Factors Affecting Chemical
Equilibrium
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Concentration
 If we removed ammonia from the
system the reaction would favor
producing more ammonia
 If we removed either of the
reactants the reaction would favor
breaking down the ammonia and
producing more of the reactants
N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat