Chemical Reactions
Chapter 7
A time to Review
• What are Physical Properties of
Matter?
• What are Chemical Properties of
Matter?
• What does a Chemical Formula tell
us?
A Time to Review
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What is an atom?
What does the term mass mean?
What is atomic mass?
What is a chemical change?
What is the evidence for a chemical
change?
A Time to Review
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What is a precipitate?
What is an element?
What is a compound?
What is an electron?
What is an ion?
What are the two main types of
chemical bonds?
– How do they form?
A Time to Review
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What is temperature?
What is surface area?
What is pressure?
What is concentration?
Think About It
• What happens to a piece of charcoal when
it burns?
• How do air bags inflate?
• Why does a propane stove need a spark in
order for the propane to ignite?
• How does a refrigerator keep food fresh?
• What changes take place inside a closed
container of water?
Describing Chemical Reactions
• 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
• 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
• Reactants → Products
• Chemical Equation - a representation
of a chemical reaction in which the
reactants and products are expressed
as formulas
Conservation of Mass
• What happens to the charcoal as it burns?
– Get smaller until it is actually just a pile of ash
• If you measured the amount of carbon
dioxide produced it would equal the
amount of carbon and oxygen consumed in
the reaction
• 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
• In rockets
– Hydrazine + Oxygen yields Nitrogen +
Water
– N2H4 + O2 → N2 + H2O
– Does the number of atoms of each
element on the left side of the arrow
equal the number of atoms on the right
side of the arrow?
• The equation is not in balance
Balancing Equations
• 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
• H2 + O2 → H2O
• HgO → Hg + O2
• Zn + HCl → ZnCl2 + H2
Inquiry Activity
•
Procedure
1. Pour 100 mL of water into a resealable plastic
bag. Flatten the bag and remove all of the air
and then seal the bag. Measure and record
the mass of the bag and the water
2. Measure and record the mass of 1 Alka
Seltzer tablet.
3. Add the masses of the bag with water and the
Alka Seltzer tablet and record the number
4. Slightly open one side of the bag and drop
the tablet into the water. Quickly reseal the
bag. After the bubbling has stopped, measure
and record the mass of the bag and its
contents
Answer these questions
• Does a chemical change take place?
• How do you know?
• What happened to the mass of the
plastic bag and its contents after the
bubbling stopped?
• What might this information tell you
about a chemical change?
Counting with Moles
• Because chemical reactions often
involve large numbers of small
particles, chemists use a counting unit
called the mole to measure amounts
of a substance
• 1 mole = 6.02 X 1023 particles of that
substance
– Particles might be atoms, ions,
molecules
Molar Mass
• Molar mass – the mass of one mole
of a substance
– the atomic mass expressed in grams
– 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
• Express molar mass as a conversion
factor
– 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)
– 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
• If you know how many grams of a
substance you have divide by the
molar mass to determine how many
mole 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
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)
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
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
2H2 + O2 → 2H2O
Reactions as Electron
Transfers (Redox Reactions)
• 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
Lab: Using Single-Replacement
Reactions to Compare Reactivities
1. Read the entire lab and answer the PreLab Discussion Questions before you do
anything. Be sure you read the
background information (This should take
AT LEAST 15 minutes)
2. No group begins without showing Mr. W
that you have answered the pre-lab
discussion questions and understand the
lab
3. YOU MUST WEAR GOGGLES TODAY!
4. ARRANGE IN 8 LAB GROUPS – NONE
MORE THAN 3
Lab Today
• Metal + HCl → Metal Chloride (An
Ionic Compound soluble in water) +
H2 (Hydrogen Gas)
• Some of the reactions may not take
place!
• You need to do the go further in the
lab today!
Energy Changes in Reactions
• 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
• 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
• A chemical reaction in which energy
is released to the surroundings
• Combustion reactions are extreme
exothermic reactions
– 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
• A chemical reaction that absorbs
energy from its surroundings is called
an endothermic reaction
– 2HgO + Heat → 2 Hg + O2
– In an endothermic reaction heat appears
on the left side of the arrow
Conservation of Energy
• The total amount of energy before
and after a reaction must always be
the same
Reaction Rates
• The progress of a chemical reaction can be
measured over time
• Different reactions have different durations
– TNT
– Leaves turning color
• Any reaction that takes places over time
can be expressed as a rate
– 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
• Temperature
– Generally an increase in temperature will
increase the reaction rate
• Surface area
– Increasing the surface area of a reactant
tends to increase the reaction rate
• Stirring
– Stirring the reactants will generally
increase the reaction rate
Factors affecting Reactions
Rates
• Concentration
– Increasing the concentration generally
increases the reaction rate
• Catalysts
– A substance that affects the reaction
rate without being used up in the
reaction
• Generally speed up a reaction
Equilibrium
• Equilibrium is a state in which the
forward and reverse paths of a
change take place at the same rate
• Physical Equilibrium
– H2O (l) ↔ H2O (g)
– When physical change does not go to
completion, a physical equilibrium is
established between the forward and
reverse changes
Equilibrium
• 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
– 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
– Discovered by Henri Le Chatlier
N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat
→ This reaction is exothermic
← This reaction would be endothermic
• Temperature
– 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
• Pressure
– 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
• 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