11.1 Chemical Reactions >
Chapter 11
Matter and Change
11.1 Describing Chemical
Reactions
11.2 Types of Chemical Reactions
11.3 Reactions in Aqueous Solution
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11.1 Chemical Reactions > Introduction to Chemical
Equations
Do Now:
Work with your table mates to come up
with a list of 10 chemical reactions that
you have seen or heard of.
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11.1 Chemical Reactions > Introduction to Chemical
Equations
How do you know if a chemical reaction is taking
place?
https://www.youtube.com/watch?v=FofPjj7v414
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11.1 Chemical Reactions > Introduction to Chemical
Equations
Signs of a Chemical Reaction
• Transfer of energy
• Change in color
• Production of a gas
• Formation of a precipitate
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11.1 Chemical Reactions > Introduction to Chemical
Equations
How do you describe what is happening in a
chemical reaction?
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11.1 Chemical Reactions > Introduction to Chemical
Equations
Word Equations
Reactants → Products
• Reactants are on the left, products on the right.
• An arrow separates them.
• You read the arrow as yields, gives, or reacts to
produce.
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11.1 Chemical Reactions > Introduction to Chemical
Equations
Word Equations
Iron + oxygen → iron(III) oxide
• Rusting of Iron
• You could say, “Iron reacts
with oxygen to produce
iron(III) oxide (rust).”
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11.1 Chemical Reactions > Introduction to Chemical
Equations
Skeleton Equations
Equations that show just the formulas of
the reactants and products.
Fe + O2 → Fe2O3
The first step in writing a complete chemical
equation is to write the skeleton equation.
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11.1 Chemical Reactions >
Interpret Data
Symbols Used in Chemical Equations
Symbol
+
→
Explanation
Separates two reactants or two products
“Yields,” separates reactants from products
Use in place of → for reversible reactions
(s), (l), (g)
(aq)
Designates a reactant or product in the solid state, liquid
state, or gaseous state; placed after the formula
Designates an aqueous solution; the substance is
dissolved in water; placed after the formula
Δ
heat
Pt
9
Indicates that heat is supplied to the reaction
A formula written above or below the yields sign indicates
its use as a catalyst (in this example, platinum).
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11.1 Chemical Reactions > Introduction to Chemical
Equations
Chemical Equations
Indicate the physical states of substances
by putting a symbol after each formula.
•
•
•
•
(s) for solid
(l) for liquid,
(g) for gas,
(aq) for a substance in an aqueous solution
(a substance dissolved in water).
Fe(s) + O2(g) → Fe2O3(s)
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11.1 Chemical Reactions > Introduction to Chemical
Equations
Chemical Equations
Catalyst: substance that speeds up the
reaction but is not used up in the reaction.
• A catalyst is neither a reactant nor a product,
so its formula is written above the arrow in a
chemical equation.
H2O2(aq)
11
MnO2
H2O(l) + O2(g)
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11.1 Chemical Reactions >
Sample Problem 11.1
Write a Skeleton Equation
Hydrochloric acid reacts with solid
sodium hydrogen carbonate. The
products formed are aqueous sodium
chloride, water, and carbon dioxide gas.
Write a skeleton equation for this
chemical reaction.
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11.1 Chemical Reactions >
Sample Problem 11.1
Write a Skeleton Equation
Hydrochloric acid reacts with solid
sodium hydrogen carbonate. The
products formed are aqueous sodium
chloride, water, and carbon dioxide gas.
Write a skeleton equation for this
chemical reaction.
NaHCO3(s) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(g)
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11.1 Chemical Reactions > Balancing Chemical
Equations
Law of Conservation of Mass
• During any chemical reaction, the mass
of the products always equals the mass
of the reactants.
• Atoms are neither created nor destroyed
in ordinary chemical reactions.
• The same number of atoms of each
element must be present on both sides
of the equation.
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11.1 Chemical Reactions > Balancing Chemical
Equations
What does that mean for you?
• The same number of atoms of each
element must be present on both sides
of the equation.
• Add coefficients to balance the reactants
and products. Do not add subscripts!
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11.1 Chemical Reactions > Balancing Chemical
Equations
Coefficients— small whole numbers that
are placed in front of the formulas in an
equation in order to balance it.
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11.1 Chemical Reactions > Balancing Chemical
Equations
Balanced equation: each side of the
equation has the same number of atoms of
each element and mass is conserved.
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11.1 Chemical Reactions > Balancing Chemical
Equations
To write a balanced chemical equation
1. Write the skeleton equation.
2. Use coefficients to balance the
equation so that it obeys the law of
conservation of mass.
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11.1 Chemical Reactions > Balancing Chemical
Equations
Carbon burns in the presence of oxygen to
produce carbon dioxide.
C(s)
Carbon
+
O2(g)
Oxygen
Reactants
1 carbon atom, 2 oxygen atoms
CO2(g)
Carbon dioxide
Product
1 carbon atom, 2 oxygen atoms
• This equation is balanced.
• You do not need to change the coefficients.
– They are all understood to be 1.
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11.1 Chemical Reactions > Balancing Chemical
Equations
When hydrogen and oxygen are mixed, the
product is water.
• The formulas for all the reactants and the product
are correct, but this equation is not balanced.
– As written, the equation does not obey the law of
conservation of mass.
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11.1 Chemical Reactions > Balancing Chemical
Equations
When hydrogen and oxygen are mixed, the
product is water.
• If you put the coefficient 2 in front of H2O, oxygen
will be balanced.
– Now twice as many hydrogen atoms are in the
product as are in the reactants.
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11.1 Chemical Reactions > Balancing Chemical
Equations
When hydrogen and oxygen are mixed, the
product is water.
• To correct this equation, put the coefficient 2 in
front of H2.
– The equation is now balanced.
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11.1 Chemical Reactions >
CHEMISTRY
& YOU
The reaction between oxygen and
hydrogen in fuel cells produces the
energy to power a car. What are the
products of the reaction in a fuel cell that
make the fuel-cell car a zero-emission
car?
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11.1 Chemical Reactions >
CHEMISTRY
& YOU
The reaction between oxygen and
hydrogen in fuel cells produces the
energy to power a car. What are the
products of the reaction in a fuel cell that
make the fuel-cell car a zero-emission
car?
Water is the product of the reaction between
oxygen and hydrogen in a fuel cell. Water is not a
pollutant, and so fuel-cell cars are considered
“zero-emission” cars.
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11.1 Chemical Reactions >
Interpret Data
Rules for Writing and Balancing Equations
1. Determine the correct formulas for
all the reactants and products.
4. Balance the elements one at a time
by using coefficients. When no
coefficient is written, it is assumed
2. Write the skeleton equation by
to be 1. Begin by balancing
placing the formulas for the
elements that appear only once on
reactants on the left and the
each side of the equation. Never
formulas for the products on the
balance an equation by changing
right with a yields sign (→) in
the subscripts in a chemical formula.
between. If two or more reactants or
Each substance only has one
products are involved, separate their
correct formula.
formulas with plus signs.
5. Check each atom or polyatomic ion
3. Determine the number of atoms of
to be sure that the number is equal
each element in the reactants and
on both sides of the equation.
products. Count a polyatomic ion as
a single unit if it appears unchanged 6. Make sure all the coefficients are in
on both sides of the equation.
the lowest possible ratio.
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11.1 Chemical Reactions >
Sample Problem 11.2
Balancing a Chemical Equation
Students suspended copper wire in an
aqueous solution of silver nitrate. They
noticed a deposit of silver crystals on the
copper wire when the copper reacted with
the silver nitrate. They recorded the
equation for this reaction but didn’t balance
it. Balance their equation:
AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + Ag(s)
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11.1 Chemical Reactions >
Sample Problem 11.2
2 Solve Apply concepts to this situation.
Balance the nitrate ion.
• Put a coefficient 2 in front of AgNO3(aq).
2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + Ag(s)
Remember that a coefficient
must always go in front of a
compound’s formula, not in
the middle of it.
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11.1 Chemical Reactions >
Sample Problem 11.2
2 Solve Apply concepts to this situation.
Balance the silver.
• Put a coefficient 2 in front of Ag(s).
2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s)
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11.1 Chemical Reactions >
Sample Problem 11.3
Balancing a Chemical Equation
Aluminum is a good choice for
outdoor furniture because it reacts
with oxygen in the air to form a thin
protective coat of aluminum oxide.
Balance the equation for this
reaction.
Al(s) + O2(s) → Al2O3(s)
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11.1 Chemical Reactions >
Sample Problem 11.3
1 Analyze Identify the relevant concepts.
Apply the rules for balancing equations.
• Notice the odd number of oxygen atoms in
the product.
Al(s) + O2(s) → Al2O3(s)
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11.1 Chemical Reactions >
Sample Problem 11.3
2 Solve Apply concepts to this situation.
First balance the aluminum by placing
the coefficient 2 in front of Al(s).
2Al(s) + O2(s) → Al2O3(s)
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11.1 Chemical Reactions >
Sample Problem 11.3
2 Solve Apply concepts to this situation.
Multiply the formula with the odd
number of oxygen atoms (on the right)
by 2 to get an even number of oxygen
atoms on the right.
2Al(s) + O2(s) → 2Al2O3(s)
Any whole-number coefficient
placed in front of O2 will
always give an even number
of oxygen atoms on the left.
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11.1 Chemical Reactions >
Sample Problem 11.3
2 Solve Apply concepts to this situation.
Balance the oxygens on the left by
placing a 3 in front of O2.
2Al(s) + 3O2(s) → 2Al2O3(s)
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11.1 Chemical Reactions >
Sample Problem 11.3
2 Solve Apply concepts to this situation.
Then rebalance the aluminum by changing
the coefficient of Al(s) from 2 to 4.
4Al(s) + 3O2(s) → 2Al2O3(s)
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11.1 Chemical Reactions >
https://www.youtube.com/watch?v=2S6e11NB
wiw
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11.1 Chemical Reactions >
Balance the following equation.
C3H8(g) + O2(g) → CO2(g) + H2O(l)
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11.1 Chemical Reactions >
Balance the following equation.
C3H8(g) + O2(g) → CO2(g) + H2O(l)
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l)
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11.1 Chemical Reactions >
END OF 11.1
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