CHEMICAL REACTIONS
Ch. 10
Recognizing Chemical Reactions
• When a substance
undergoes a chemical
change, it takes part in
a chemical reaction.
• After it reacts, it no
longer has the same
chemical identity.
Recognizing Chemical Reactions
• While it may seem amazing that a substance
can undergo a change and become part of a
different substance, chemical reactions occur
around you all the time.
• Many important clues indicate when chemical
reactions occur.
• None of them alone proves that such a change
occurs because some physical changes involve
one or more of these signs.
Writing Chemical Equations
• In order to completely understand a chemical
reaction, you must be able to describe any
changes that take place.
• Part of that description involves recognizing
what substances react and what substances
form.
Writing Chemical Equations
• A substance that undergoes a reaction is called
a reactant.
• When reactants undergo a chemical change,
each new substance formed is called a
product.
Writing Chemical Equations
• For example, a familiar chemical reaction
involves the reaction between iron and oxygen
(the reactants) that produces rust, which is
iron(III) oxide (the product).
• The simplest reactions involve a single
reactant or a single product, but some
reactions involve many reactants and many
products.
Word Equations
• The simplest way to represent a reaction is by
using words to describe all the reactants and
products, with an arrow placed between them
to represent change.
• Reactants are placed to the left of the arrow,
and products are placed to the right.
• Plus signs are used to separate reactants and
also to separate products.
Word Equations
• Vinegar and baking soda are common names.
• The compound in vinegar that is involved in
the reaction is acetic acid, and baking soda is
sodium hydrogen carbonate.
• These scientific names can also be used in a word equation.
Chemical Equations
• Word equations describe reactants and
products, but they are long and awkward and
do not adequately identify the substances
involved.
• Word equations can be converted into
chemical equations by substituting chemical
formulas for the names of compounds and
elements.
Chemical Equations
• The equation for the reaction of vinegar and
baking soda can be written using the
chemical formulas of the reactants and
products.
• By examining a chemical equation, you can
determine exactly what elements make up
the substances that react and form.
Chemical Equations
• It may also be important to know the
physical state of each reactant and product.
• How can we indicate the bubbles we see
during this reaction are CO2?
• Symbols in the parentheses are put after
formulas to indicate the state of the substance.
• Solids, liquids, gases, and water (aqueous)
solutions are indicated by the symbols (s), (l),
(g), and (aq).
Chemical Equations
• The following equation shows these symbols
added to the equation for the reaction of
vinegar and baking soda.
Chemical Equations
• Now the equation tells us that mixing an
aqueous solution of acetic acid (vinegar) with
solid sodium hydrogen carbonate (baking soda)
results in the formation of an aqueous solution
of sodium acetate, liquid water, and carbon
dioxide gas.
Energy and Chemical Equations
• Noticeable amounts of energy are often
released or absorbed during a chemical
reaction.
• Some reactions absorb energy. If energy is
absorbed, the reaction is known as an
endothermic reaction.
• For a reaction that absorbs energy, the word
energy is sometimes written along with the
reactants in the chemical equation.
Energy and Chemical Equations
• For example, the equation for the reaction in
which water breaks down into hydrogen and
oxygen shows that energy must be added to the
reaction.
Energy and Chemical Equations
• Reactions that release heat energy are called
exothermic reactions.
• When writing a chemical equation for a reaction
that produces energy, the word energy is
sometimes written along with the products.
• Some of this energy is in the form of light.
Energy and Chemical Equations
• You may have also noticed that the word energy
is not always written in the equation.
• It is used only if it is important to know whether
energy is released or absorbed.
Balancing Chemical Equations
• The mass of the products is always the same
as the mass of the reactants that react to form
them.
• The law of conservation of mass summarizes
these findings.
• Matter is neither created nor destroyed during
a chemical reaction.
Balancing Chemical Equations
• Remember that atoms don’t change in a
chemical reaction; they just rearrange.
• The number and kinds of atoms present in the
reactants of a chemical reaction are the same as
those present in the products.
• When stated this way, it becomes the law of
conservation of atoms.
Balancing Chemical Equations
• For a chemical equation to accurately represent
a reaction, the same number of each kind of
atom must be on the left side of the arrow as
are on the right side.
• If an equation follows the law of
conservation of atoms, it is said to be
balanced.
Balancing Chemical Equations
• The easiest way to count atoms is to practice—
first with a simple reaction and then with some
that are more complex.
• For example, consider the equation that
represents breaking down carbonic acid into
water and carbon dioxide.
Balancing Chemical Equations
• Because a subscript after the symbol for an
element represents how many atoms of that
element are found in a compound, you can see
that there are two hydrogen, one carbon, and
three oxygen.
• All of the atoms in the reactants are the same as
those found in the products.
Balancing Chemical Equations
• Examine the equation for the formation of
sodium carbonate and water from the
reaction between sodium hydroxide and
carbon dioxide.
Balancing Chemical Equations
• One carbon atom is on each side of the arrow,
but the sodium, oxygen, and hydrogen atoms
are not balanced.
• The equation, as written, does not truly
represent the reaction because it does not show
conservation of atoms.
Balancing an Equation
• To indicate more than one unit taking part or
being formed in a reaction, a number called
a coefficient is placed in front of it to indicate
how many units are involved.
• Look at the previous equation with a coefficient
of 2 in front of the sodium hydroxide formula.
Balancing an Equation
• Is the equation balanced now?
• Two sodium atoms are on each side. How
many oxygen atoms are on each side?
• You should be able to find four on each side.
How about hydrogen atoms?
• Now two are on each side.
• Because one carbon atom is still on each side,
the entire equation is balanced; it now
represents what happens when sodium
hydroxide and carbon dioxide react.
Balancing an Equation
• The balanced equation tells us that when sodium
hydroxide and carbon dioxide react, two units of
sodium hydroxide react with each molecule of
carbon dioxide to form one unit of sodium
carbonate and one molecule of water.
Major Classes of Reactions
• If you can classify a reaction into one of
five major categories by recognizing
patterns that occur, you already know a lot
about the reaction.
Major Classes of Reactions
• In one type of reaction, two substances—either
elements or compounds—combine to form a
compound.
• Whenever two or more substances combine to
form a single product, the reaction is called a
synthesis reaction.
A Synthesis Reaction
• When iron rusts, iron
metal and oxygen gas
combine to form one
new substance, iron(III)
oxide.
• The balanced equation for this synthesis reaction
shows that there is more than one reactant but
only one product.
A Synthesis Reaction
Major Classes of Reactions
• In a decomposition reaction, a compound
breaks down into two or more simpler
substances.
• The compound may break down into
individual elements, such as when mercury(II)
oxide decomposes into mercury and oxygen.
Major Classes of Reactions
• The products may be an element and a
compound, such as when hydrogen peroxide
decomposes into water and oxygen.
• The compound may break down into simpler
compounds.
A Decomposition Reaction
• When ammonium nitrate is heated to a high
temperature, it explosively breaks down into
dinitrogen monoxide and water.
• The decomposition reaction taking place is
represented by a balanced equation that shows
one reactant and more than one product.
A Decomposition Reaction
Major Classes of Reactions
• In a single-displacement reaction, one element
takes the place of another in a compound.
• The element can
replace the first
part of a
compound, or it
can replace the
last part of a
compound.
Single Displacement
• If an iron nail is placed into an aqueous solution
of copper(II) sulfate, the iron displaces the
copper ions in solution, and copper metal forms
on the nail.
Single Displacement
Major Classes of Reactions
• In double-displacement
reactions, the positive
portions of two ionic
compounds are
interchanged.
• For a double-displacement
reaction to take place, at
least one of the products
must be a precipitate,
water, or a gas.
Double Displacement
• When clear aqueous solutions of lead(II)
nitrate and potassium iodine are mixed, a
double-displacement reaction takes place and
a yellow solid appears in the mixture.
• This solid is lead(II) iodine, and it
precipitates out because it is insoluble in
water, unlike the two reactants and the other
product.
Double Displacement
Major Classes of Reactions
• A combustion reaction is one in which a
substance rapidly combines with oxygen to
form one or more oxides.
Combustion
• When welding is done
with an acetylene
torch, acetylene
combines with oxygen
to form carbon dioxide
and water.
• This combustion reaction is exothermic, and
enough energy is released to melt metal.
Combustion
Write a balanced equation for the
following reactions:
Solid lithium reacts with chlorine
gas to produce solid lithium
chloride.
Nitrogen gas reacts with oxygen gas to
produce nitrogen dioxide gas.
Solid potassium reacts with liquid
water to produce hydrogen gas and
a water solution of potassium
hydroxide.
Calcium chloride and sodium
carbonate in water solution produce
solid calcium carbonate and a water
solution of sodium chloride.
Identify each of the following
reactions as representing a synthesis
reaction, a combustion reaction,
both synthesis and combustion, or
decomposition reaction. Balance
each equation if necessary.
Synthesis reaction
decomposition; already
balanced
Reactions in Aqueous Solutions
• When aqueous solutions that contain ions are
mixed, the ions may react in a doublereplacement reaction.
• The product is typically a solid precipitate,
water, or a gas.
Reactions in Aqueous Solutions
• An example of a double-replacement reaction
that produces a precipitate occurs when
aqueous solutions of sodium chloride and
silver nitrate are mixed to form a precipitate of
solid silver chloride.
Reactions in Aqueous Solutions
• To show all of the particles in solution as they
really exist, a complete ionic equation can be
written.
• The sodium and nitrate ions are on both sides
of the equation.
• Such ions that do not participate in a reaction
are called spectator ions.
Reactions in Aqueous Solutions
• An ionic equation that does not show
spectator ions but only the particles that
participate in a reaction is called a net ionic
equation.
• In the case of the reaction above, the net ionic
equation from which the sodium and nitrate
ions have been removed is as follows.
Writing Ionic Equations
• Write the balanced chemical equation for the
reaction between aqueous solutions of
strontium nitrate and potassium sulfate, which
forms the precipitate strontium sulfate.
• Then write the complete ionic and net ionic
equations.
Writing Ionic Equations
• Write the correct skeleton equation.
• Use coefficients to produce the balanced
chemical equation.
Writing Ionic Equations
• Write the complete ionic equation.
Writing Ionic Equations
• Cross out the spectator ions, which are those
that are on both sides of the equation.
• That leaves the net ionic equation.
Reactions that form water or a gas
• Some double-replacement reactions in
aqueous solution produce water or a gas (or
both) rather than a precipitate.
• In such cases, the water or gas is shown as a
product in the net ionic equation, as are the
ions that produced it.
• The remaining ions are eliminated as
spectator ions.
• The following example problem illustrates
this concept.
Reactions that form water or a gas
• When hydrochloric acid and potassium
hydroxide solutions are mixed, water results,
together with an aqueous solution of
potassium chloride.
• Write the balanced chemical equation, a
complete ionic equation, and a net ionic
equation for this reaction.
Reactions that form water or a gas
• The balanced chemical equation is the same as
the skeleton equation.
Reactions that form water or a gas
• Write the complete ionic equation, which
includes all of the ions.
• Remove the spectator ions to produce the net
ionic equation.
Practice: Question 1
Write a balanced chemical, complete ionic, and
net ionic equations for the following reaction:
Aqueous solutions of lead(II) nitrate and
ammonium chloride are mixed, forming a
precipitate of lead(II) chloride.
Answer
Chemical:
Complete Ionic:
Net ionic:
Question 2
Write a balanced chemical, complete
ionic, and net ionic equations for the
reaction between the following
substances, which produce water:
nitric acid (HNO3) and aqueous barium
hydroxide
Answer
Chemical:
Complete ionic:
Net ionic:
or, with coefficients reduced to lowest terms,
Question 3
Write a balanced chemical, complete ionic, and
net ionic equations for the reaction between the
following substances, which produce a gas:
hydrochloric acid and aqueous sodium
cyanide, with production of hydrogen
cyanide gas (HCN)
Answer
Chemical:
Complete ionic:
Net ionic: