CHEMICAL REACTIONS
7-1 DESCRIBING REACTIONS
CHEMICAL EQUATIONS
To describe a chemical change, describe what is
present before and after the change.
Reactants: substances that undergo
a change
Products: new substances that form
• Using Equations to Represent Reactions
• Word equations:
• Reactants → Products
• Carbon + Oxygen → Carbon dioxide
• Chemical formulas simplify word equations:
• C + O2 → CO2
• Read “The reaction of C and O yields carbon
dioxide.
• Conservation of Mass Reviewed
• The mass of the reactants equals the mass of the
products
• Number of atoms in the reactants equals the
number of atoms in the products
• Mass is neither created nor destroyed
BALANCING EQUATIONS
When hydrazine
burns in the presence
of oxygen, the
reaction produces
nitrogen, water
vapor and heat.
This chemical
equation is at
illustrated in the table
to the right in an
unbalanced
equation and in a
balanced equation.
Study the table
carefully.
PRACTICE BALANCING CHEMICAL
EQUATIONS
ANOTHER EXAMPLE…
FOUR STEPS TO REMEMBER
LET’S TRY SOME!
3
3
____ AlBr3 + ____ K  ____ KBr + ____ Al
Count the atoms:
1 Aluminum atom
3 Bromine atoms
1 Potassium atom
1 Potassium atom
1 Bromine atom
1 Aluminum atom
Is it balanced?
No!
Add coefficients to balance atoms:
1 Aluminum atom
3 Bromine atoms
3 Potassium atoms
Is it balanced?
YES!
3 Potassium atoms
3 Bromine atoms
1 Aluminum atom
LET’S TRY ANOTHER!
____ FeO + ____ PdF2  ____ FeF2 + ____ PdO
Count the atoms:
1 Iron
1 Oxygen
1 Palladium
2 Flourine
Is it balanced?
YES!
Do we need to add coefficients?
NO!
1 Iron
1 Palladium
2 Flourine
1 Oxygen
NEED ANOTHER? OKAY!
6
4
____ P4 + ____ Br2  ____ PBr3
Count the atoms:
4 Phosphorus
2 Bromines
1 Phosphorus
3 Bromines
Is it balanced?
No!
Add coefficients to balance atoms:
4 Phosphorus
Is it balanced?
YES!
12 Bromines
4 Phosphorus
12 Bromines
LET’S TRY A HARDER ONE!
2
3
3
____ CoBr3 + ____ CaSO4  ____ CaBr2 + ____ Co2(SO4)3
Count the atoms:
1 Co
3 Br
1 Ca
1S
4O
1 Ca
2 Br
2 Co
3S
12 O
Is it balanced?
No!
Add coefficients to balance atoms:
2 Co
6 Br
Is it balanced?
3 Ca
3S
12 O
3 Ca
6 Br
YES!
2 Co
3S
12 O
NOW TRY BALANCING CHEMICAL
EQUATIONS ON YOUR OWN
• More practice:
1. Na+H2O→NaOH+H2
Show your work.
2. HCl+CaCO3→CaCl2+CO2+H2O
Show your work
3. Al+Cl2→AlCl3
Show your work.
COUNTING WITH MOLES
Counting things in terms of
atoms and molecules is not
always practical for chemists
because chemical reactions
involve large number so
these particles.
This is why chemists use a
counting unit called a mole.
1 mole = 6.02X1023
particles
Known as Avogadro’s
number
Just as the mass of 12 eggs
does not equal the mass of
12 oranges, a mole of
carbon has a different mass
than a mole of sulfur.
MOLES CONTINUED
• Molar mass for an element is the same as its atomic
mass expressed in grams.
Carbon’s atomic mass is 12.0 amu so its molar mass is 12.0 grams.
• Molar mass for a compound can be calculated by
adding up the atomic masses of its atoms and
expressing the result in grams.
MOLAR MASS CONVERSIONS
CHEMICAL CALCULATIONS
• Chemical equations can be read as recipes for making new
substances.
• In chemical reactions, the mass of a reactant or product can be
calculated by using a balanced chemical equations and molar
masses of the reactants and products.
• The chemical equation tells you how to relate amounts of
reactants to amounts of products.
7-2 TYPES OF REACTIONS
Different reactants can produce the same product.
Consider that CO2 is produced in the following
multiple ways:
dropping hydrochloric acid on limestone
heating limestone
gasoline
CLASSIFYING REACTIONS:
1. SYNTHESIS
& 2. DECOMPOSITION
Synthesis reaction: Two or more substances
react to form a single substance A + B → AB
• Sodium and Chlorine 2Na + Cl2 → 2NaCl
• Hydrogen and Oxygen 2H2 + O2 → 2H2O
Decomposition reaction: The opposite of synthesis and
one in which a compound breaks down (the reactant
must be a compound but the products can be elements
or compounds) AB → A + B
• Water decomposes into hydrogen gas and oxygen gas
2H2O → 2H2 + O2
• Calcium carbonate in limestone decomposes into
lime, CaO, and carbon dioxide CaCO3 → CaO + CO2
CLASSIFYING REACTIONS CONTINUED:
3. SINGLE REPLACEMENT
4. DOUBLE REPLACEMENT
CLASSIFYING REACTIONS CONTINUED:
5. COMBUSTION
A combustion reaction is one in which a
substance reacts rapidly with oxygen often
producing heat and light.
Example:
• natural gas contains methane (CH4)
• combined with an unlimited supply of
oxygen
• the products are carbon dioxide, water,
heat and light
REACTIONS AS ELECTRON TRANSFERS:
OXIDATION AND REDUCTION
The discovery of subatomic particles
enabled scientists to classify certain
chemical reactions as transfers of electrons
between atoms.
A reaction where electrons are transferred from
one reactant to another is called an oxidationreduction reaction, or redox reaction.
7-3 ENERGY CHANGES IN REACTIONS
CHEMICAL BONDS AND ENERGY
• Chemical energy is the energy stored in the chemical bonds of
a substance.
• Chemical reactions involve there breaking of chemical bonds I
the reactants ad the formation of chemical bonds in the
products.
• Propane grill = site of a chemical reaction
•
•
•
•
Reactants = propane and oxygen
Products = carbon dioxide and water
Since heat is produced (energy), heat can be included in the equation
indicating heat is released in the reaction from the reactants
In the combustion of propane, the bonds in propane and oxygen molecules
are broken while bonds in carbon dioxide and water molecules re formed.
• Oxygen, carbon dioxide, and water molecules all have energy stored in
their bonds too.
Breaking Bonds
Forming Bonds
REVIEWING ENDOTHERMIC
REACTIONS = ENERGY ABSORBED
• Chemical reactions occur when bonds break and new bonds
form. They usually absorb heat or liberate heat(release it).
• Endothermic reactions absorb energy when the bonds break
• Energy is absorbed from nearby matter
• Often temperature changes
• Baking soda combined with vinegar absorbs heat, making it
feel colder
• When heat is added constantly (egg frying)
• Mercury(II) oxide is heated to a temperature of about 450ºC,
it breaks down into mercury and oxygen (decomposition)
REVIEWING EXOTHERMIC REACTIONS =
ENERGY RELEASED
• The making or breaking of chemical bonds results in
a net release of energy
• Example: combustion
• Energy typically releases as heat
Fuel and oxygen in the plane’s engine release heat causing the
gases in the engine to expand. This moves the gases out of the
plane which exerts a force that moves the plane forward.
EXOTHERMIC AND ENDOTHERMIC REACTIONS
In any reaction, the chemical energy reaches a peak before the
reactants change into products. The peak represents the amount
of energy require to break the chemical bonds of the reactants
(activation energy).
CONSERVATION OF ENERGY
Exothermic reaction = chemical energy of the
reactants is converted into heat + the chemical
energy of the products
Endothermic reaction = heat + the chemical
energy of the reactants is converted into chemical
energy of the products
The total amount of energy is the same before and
after both types of reactions…the law of
conservation of energy
7-4 REACTION RATES
REACTIONS OVER TIME
• A reaction rate is the rate at which reactants
change into products over time.
• Reaction rates tell you how fast a reaction is going.
• How fast the reactants are being consumed
• How fast the products are being formed
• How fast energy is being absorbed or released
• Five Factors Affect Reaction Rates
• Temperature, surface area, stirring, concentration, catalysts
(inhibitors)
FACTORS AFFECTING REACTION RATES
•
•
•
•
•
•
Temperature: High = faster
Surface Area: More = faster
Stirring: Speeds up, Washing clothes
Concentration: More = faster
Catalysts: Speeds up, Enzymes
Bonus: Inhibitors: Slows reaction, preservatives
7-5 EQUILIBRIUM
TYPES OF EQUILIBRIA
Equilibrium is a state in which the forward and
reverse path of a change take place at the
same rate.
• Physical Equilibrium in physical changes:
• When a physical change does not go to completion,
a physical equilibrium is established between the
forward and reverse changes.
• Chemical Equilibrium in chemical changes:
• In a reversible reaction, the conversion of reactants
into products and the conversion of products into
reactants can happen simultaneously.
• When a chemical reaction does not go to
completion, a chemical equilibrium is established
between the forward and reverse reactions.
• Synthesis of sulfur trioxide from sulfur dioxide and
oxygen
FACTORS AFFECTING CHEMICAL EQUILIBRIUM
When a change is introduced to a system in
equilibrium, the equilibrium shifts in the direction that
relieves the change.
Class Participation Opportunity: Research the life of
Louis Le Chatelier (1850-1936)
Example: Synthesis of Ammonia (system containing
nitrogen, hydrogen and ammonia in equilibrium)
Temperature:
Increase temp…system removes heat, decreasing the
amount of ammonia
Pressure
Increase pressure…produces fewer gas molecules, more
ammonia is produced
Concentration
Remove ammonia, system would favor the forward
reaction, producing more ammonia