Balancing chemical equations

Chemical Reactions and

Equations

• What is evidence of chemical change?
• How are chemical reactions represented?
• Why do chemical equations need to be balanced and how is this accomplished?
• How are chemical reactions classified?
• What are the characteristics of different classes of chemical reactions?

Reactions and Equations
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Essential Questions

Vocabulary

Review

• Chemical change

New

•Chemical reaction

•Reactant

•Product

•Chemical equation

•Coefficient

•Synthesis reaction

•Combustion reaction

•Decomposition
reaction

•Single-replacement
reaction

•Double-replacement
reaction

•Precipitate

• The process by which the atoms of one or more substances are rearranged to

form different substances is called a chemical reaction.

• A chemical reaction is another name for chemical change.

• Chemical reactions affect every part of your life. They break down your

food. Chemical reactions in the engines of cars and buses provide the
energy to power the vehicles. They produce natural fibers, such as cotton
and wool, in plants, and animals. In factories, they produce synthetic fibers
such as nylon.

Reactions and Equations
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Chemical Reactions

Evidence that a chemical reaction may have
occurred:

Change in temperature

Change in color

Odor

Gas bubbles

The appearance of a solid

Representing Chemical Reactions

Chemists use statements called equations to represent chemical reactions.

• Reactants are the starting substances.

• Productsare substances formed in the reaction.

Reactions and Equations
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Chemical Reactions

Word

Equation

Skeleton
Equation

You can use
statements called
word equations to
indicate the
reactants and
products of
chemical reactions.

Skeleton equations
use symbols and
formulas rather than
words to represent
the reactants and
products.

• The word equation below describes the reaction between aluminum (Al) and

bromine (Br).

• Aluminum is a solid, bromine is a liquid, and the aluminum bromide produced is a

solid precipitate.

• In word equations, aluminum(s) + bromine(l) → aluminum bromide(s) reads as

“aluminum and bromine react to produce aluminum bromide”.

Reactions and Equations
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Representing Chemical Reactions

Skeleton Equations

Word Equations

The skeleton equation for the reaction between aluminum and bromine uses the
formulas for aluminum, bromine, and aluminum bromide in place of words.

Al(s) + Br(l) → AlBr3(s)

Both word and skeleton equations lack information about

how many atoms are involved in the reaction.

• Like word equations, skeleton equations lack some information about

reactions. Recall that the law of conservation of mass states that in a
chemical change, matter is neither created nor destroyed.

• Chemical equations must show that matter is conserved during a reaction.

Skeleton equations lack that information.

• To accurately represent a chemical reaction by an equation, the equation

must show equal numbers of atoms of each reactant and each product on
both sides of the arrow.

• Such an equation is called a balanced chemical equation.

Reactions and Equations
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Representing Chemical Reactions

• A chemical equation is a statement that uses chemical formulas to show

the identities and relative amounts of the substances involved in a chemical
reaction.

Reactions and Equations
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Representing Chemical Reactions

•To balance an equation, you must find the correct coefficients for the
chemical formulas in the skeleton equation.

•A coefficientin a chemical equation is the number written in front of a
reactant or product.

•Coefficients are usually whole numbers and are not written if the value
is one. The coefficients in a balanced equation describe the lowest whole
number ratio of the amounts of all of the reactants and products.

Law of Conservation of Mass

States that mass is neither created nor destroyed during a chemical reaction but is
conserved.

Reactions and Equations
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Chemical Reactions

Balancing Chemical Equations

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Balancing Chemical Equations

In a balanced chemical equation, the number of particles on the reactant side
of the equation equals the number of particles on the product side of the
equation. In this case, two aluminum atoms and six bromine atoms are
needed on both sides of the equation.

Reactions and Equations
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Chemical Reactions

Problem
Write the balanced chemical equation for
the reaction in which aqueous sodium
hydroxide and aqueous calcium bromide
react to produce solid calcium hydroxide
and aqueous sodium bromide.

Write the skeleton equation for the chemical reaction.

NaOH(aq) + CaBr2(aq) → Ca(OH)2(s) + NaBr(aq)

•Count the atoms of each element in the
reactants.

1 Na, 1 O, 1 H, 1 Ca, 2 Br

•Count the atoms of each element in the
products.

1 Na, 2 O, 2 H, 1 Ca, 1 Br

Reactions and Equations
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•Insert the coefficient 2 in front of NaOH to balance the hydroxide ions.

2NaOH + CaBr2 → Ca(OH)2 + NaBr

•Insert the coefficient 2 in front of NaBr to balance the Na and Br atoms.

2NaOH + CaBr2 → Ca(OH)2 + 2NaBr

•Write the coefficients in their lowest-possible ratio.

The ratio of the coefficients is 2:1:1:2.

•Check to make sure that the number of atoms of each element is equal on both sides
of the equation.

Reactants: 2 Na, 2 OH, 1 Ca, 2 Br

Products: 2 Na, 2 OH, 1 Ca, 2 Br.

Chemical Reactions

2NaOH(aq) + CaBr2(aq) → Ca(OH)2(s) + 2NaBr(aq)

• The most fundamental law in chemistry is the law of conservation of mass.

Reactions and Equations
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Chemical Reactions

Types of Chemical Reactions

Synthesis
reactions

Combustion

reactions

Decomposition

reactions

Replacement

reactions

Multiple Choice

Which Equation is balanced?

• PbO₂ + 2H₂

• SO₂ + H₂0 --> H₂SO₄

• 2Na + 2H₂O --> 2NaOH + H₂

• Al + O₂ → 2Al₂O₃

Multiple Choice

Balance this equation by choosing the corresponding coefficient

Al + HCl --> H₂ + AlCl₃

• 2,6,3,2

• 2,1,4,5

• 4,12,3,4

• it's already balanced

Multiple Choice

Is this equation balanced:

FeS + 2HCl → FeCl2​ + H2​S

• Yes

• No

Types of Chemical Reactions

Synthesis
reactions

Combustion

reactions

Decomposition

reactions

Replacement

reactions

A reaction in
which two or

more substances
react to produce a

single product

Oxygen combines
with a substance

and releases

energy in the form
of heat and light.

One important
combustion reaction
occurs when coal is
burned to produce energy.

A single compound
breaks down into

two or more

elements or new

compounds.

The opposite of

synthesis
reactions

In contrast to

synthesis,

combustion, and
decomposition
reactions, many

chemical reactions

are replacement

reactions and involve
the replacement of an

element in a

compound. These
reactions are also
called displacement

reactions.

Combustion Reactions

• Heated hydrogen reacts with oxygen to produce heat and water in a

combustion reaction, seen below. This is also a synthesis reaction.

Note that the combustion reaction just
mentioned is also a synthesis reaction.

However, not all combustion reactions are synthesis
reactions. For example, the reaction involving Methane gas
(𝐶𝐻4) and oxygen where methane burns in oxygen to yield
carbon dioxide and water.

Decomposition Reactions

• Decomposition reactions often require an energy source, such as heat,

light, or electricity, to occur.

• For example, ammonium nitrate breaks down into dinitrogen monoxide

and water when the reactant is heated at a high temperature.

•

• Notice that this decomposition reaction involves one reactant breaking

down into more than one product.

Replacement Reactions

Single

replacement

reaction

Double

replacement

reaction

A reaction in which the atoms of one
element replace the atoms of another
element in a compound is called a
single replacement reaction.

Replacement Reaction

Single

replacement

Double

replacement

Double replacement reactions occur
when ions exchange between two
compounds.

The solid product produced during a
chemical reaction in a solution is
called a precipitate.

All double-replacement reactions
produce either water, a precipitate, or
a gas.

Single Replacement Reactions

Metal replaces hydrogen or another metal

The reaction between lithium and water is one type of single-replacement
reaction, in which a metal replaces a hydrogen atom in a water molecule.

Another type of single-replacement reaction occurs when one metal replaces another
metal in a compound dissolved in water.

• A metal will not always replace a metal in a compound dissolved in water because of

differing reactivities.

• An activity series can be used to predict if reactions will occur. This series orders metals

by reactivity with other metals.

• The most active metals are at the top of the list and the least active metals are at the

bottom.

• Halogens frequently replace other halogens in replacement reactions. Halogens also have

different reactivities and do not always replace each other.

• You can use the activity series to predict whether or not

certain reactions will occur.

• A specific metal can replace any metal listed below it

that is in a compound. It cannot replace any metal
listed above it.

• For example, copper atoms replace silver atoms in a

solution of silver nitrate. However, if you place a silver
wire in aqueous copper (II) nitrate, the silver atoms will
not replace the copper. Silver is listed below copper in
the activity series, so no reaction occurs. The letters
NR ( no reaction) are commonly used to indicate that a
reaction will not occur.

Classifying Chemical Reactions
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Replacement Reactions

Classifying Chemical Reactions
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Replacement Reactions

Nonmetal replaces nonmetal

The third type of single-replacement reaction involves the replacement of a
nonmetal in a compound by another nonmetal.
Halogens are frequently involved in these types of reactions.
The most active halogen is fluorine, and the least active is iodine.
A more reactive halogen replaces a less reactive halogen that is part of a
compound dissolved in water.

Classifying Chemical Reactions
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SINGLE-REPLACEMENT REACTIONS

Problem
Predict the products that will result when
these reactants combine and write a balanced
chemical equation for each reaction.

a. Fe(s) + CuSO4(aq) →

b. Br2(l) + MgCl2(aq) →

c. Mg(s) + AlCl3(aq) →

a. Iron is listed above copper in the activity
series. Therefore, the first reaction will occur
because iron is more reactive than copper. In
this case, iron will replace copper. The
skeleton equation for this reaction is

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

This equation is balanced.

b. In the second reaction, chlorine is more
reactive than bromine because bromine is
listed below chlorine in the activity series.
Therefore, the reaction will not occur. The
skeleton equation for this situation is

Br(l) + MgCl2(aq) → NR

c. Magnesium is listed above aluminum in
the activity series. Therefore, the third
reaction will occur because magnesium is
more reactive than aluminum. In this case,
magnesium will replace aluminum. The
skeleton equation for this reaction is

Mg(s) + AlCl3(aq) → Al(s) + MgCl2(aq)

This equation is not balanced. The balanced equation
is:
3Mg(s) + 2AlCl3(aq) → 2Al(s) + 3MgCl2(aq)

• This table shows the steps to write double replacement reactions.

Classifying Chemical Reactions
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Replacement Reactions

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Chemical Reactions and Equations Grade 11

Classwork

True/False
1. In a chemical reaction, the products are found on the left side of the arrow.
2. All chemical reactions release energy, although some release it in the form of light and others in the form

of heat.

3. A coefficient tells the lowest whole number ratio of compounds in a given chemical reaction.
4. The coefficient of each element should be the same on both sides of the equation.
5. The arrow in a chemical equation means “is equivalent to”.
6. A skeleton equation shows the formulas of the compounds participating in a chemical reaction and their

states of matter but not the ratio in which they react.

7. Some single replacement reactions are impossible depending on the activity of the different elements

participating in the reaction.

Answer the Following Questions (pick 5)
1.Explain what is wrong with this chemical equation.
2NaCl(s) = 2Na(s) + Cl2(g)
2. Explain why a precipitate forms in some reactions.
3. Use the activity series to determine if this reaction will take place. Explain how you can tell.

4. Predict the products of this reaction. Show the state of matter of each product.

Compounds

Soluble in Water

Insoluble in Water

NaCl

AgCl

Mg (NO3)2

CuCl

NaNO3

BaSO4

Na2CO3

SrSO4

MgSO4

MgCO3

CaSO4

CaCO3

AgNO3

PbF2

Chemical Reactions and Equations Grade 11

Classwork

Page 2 of 3

5. Write the balanced chemical equation for this reaction.

6. Explain what is wrong with this balanced chemical equation.

7. Add the correct coefficients to properly balance this equation.

8. Add the correct coefficients to properly balance this equation.

9. Write the correct skeleton equation for the reaction: solid mercury (II) oxide yields liquid mercury plus

oxygen gas.

10. Predict the products and write the correct balanced chemical equation for the reaction: aqueous

magnesium chloride plus aqueous ammonium nitrate yields...

Matching (pick 1 group)
1-Match the symbol with its function in a chemical equation.
a. →
b. (aq)
c. +
d. (hydro)
e. (g)
f. (l)
g. (fl)
h. (sol)
i. (s)
j. (gas)
k. ↔
____

1.

separates two or more reactants or products

2.

separates reactants from products

3.

separates reactants from products in a reversible reaction

4.

Identifies a solid state

5.

identifies a liquid state

6.

Identifies a gaseous state

7.

Identifies a water solution

Chemical Reactions and Equations Grade 11

Classwork

Page 3 of 3

2- Match the type of chemical reaction with its symbolic equation.
a. single-replacement
b. decomposition
c. synthesis
d. double-replacement
e. combustion
____

1.

A + B → AB

2.

A + O2 → AO

3.

AB → A + B

4.

A + BX → AB + B

5.

AX + BY → AY + BX