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Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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 Reactions and Equations: Basic Concepts
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 Reactions and Equations: Basic Concepts
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 Reactions and Equations: Basic Concepts
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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 Reactions and Equations: Basic Concepts
Chemical Equations
• The following equation shows these symbols
added to the equation for the reaction of
vinegar and baking soda.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
Energy and Chemical Equations
• Reactions that release heat energy are called
exothermic reactions.
Chemical Reactions and Equations: Basic Concepts
Energy and Chemical Equations
• 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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
Balancing Chemical Equations
• Examine the equation for the formation
of sodium carbonate and water from the
reaction between sodium hydroxide and
carbon dioxide.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
A Synthesis Reaction
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
A Decomposition Reaction
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
Single Displacement
Chemical Reactions and Equations: Basic Concepts
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 or
water.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
Double Displacement
Chemical Reactions and Equations: Basic Concepts
Major Classes of Reactions
• A combustion reaction is one in which a
substance rapidly combines with oxygen
to form one or more oxides.
Chemical Reactions and Equations: Basic Concepts
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.
Chemical Reactions and Equations: Basic Concepts
Combustion
Chemical Reactions and Equations: Additional Concepts
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.
Chemical Reactions and Equations: Additional Concepts
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.
Chemical Reactions and Equations: Additional Concepts
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.
Chemical Reactions and Equations: Additional Concepts
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.
Chemical Reactions and Equations: Additional Concepts
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.
Chemical Reactions and Equations: Additional Concepts
Writing Ionic Equations
• Write the correct skeleton equation.
• Use coefficients to produce the balanced
chemical equation.
Chemical Reactions and Equations: Additional Concepts
Writing Ionic Equations
• Write the complete ionic equation.
Chemical Reactions and Equations: Additional Concepts
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.
Chemical Reactions and Equations: Additional Concepts
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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.
Chemical Reactions and Equations: Additional Concepts
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.
Chemical Reactions and Equations: Additional Concepts
Reactions that form water or a gas
• The balanced chemical equation is the same
as the skeleton equation.
Chemical Reactions and Equations: Additional Concepts
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.