Chapter 11
Connecting
chemical reactions
and equations
Chemical Changes
• A chemical change is a reaction in
which one or more substances are
transformed into one or more new
substances.
• Physical changes do not produce new
substances – change of state.
Chemical Changes
• When chemical changes are taking place,
we may observe one or more of the
following:
–
–
–
–
–
–
–
A substance disappears
A gas is given off (effervescence)
A solid is precipitated
A colour change takes place
The temperature changes
A new odour is released
Light is given out
Chemical Equations
• A chemical equation is a way of summarising a
chemical change.
• It shows the formulae of the original reactants on
the left and the formulae of the new substances
(products) on the right.
• It also shows the physical state of each
substance involved, and it may indicate
conditions necessary for the reaction to occur.
Chemical Equations
• In a chemical reaction, the way
in which the atoms are joined
together is changed.
• Bonds are broken and new
ones formed as the reactants
are changed into products.
• The same atoms (both number
and type) are present before
and after the reaction – they
are just arranged differently
Chemical Equations
• When Na is heated and plunged into green
chlorine gas, the white ionic solid sodium
chloride is formed.
– Na + Cl2 ----> NaCl
– Name the reactants, name the products
• The equation above is called a ‘skeleton’
equation, the physical states need to be
added and it needs to be balanced
– 2Na(s) + Cl2(g) ----> 2NaCl(s)
Chemical Equations
•
•
•
•
(s) solid
(l) liquid
(g) gas
(aq) solution of water



In some chemical reactions, a substance called a
catalyst is used.
A catalyst speeds up a reaction without being
used up
Because a catalyst is not used up, it is neither a
reactant or a product and is written above the
arrow in the equation.

MnO2
2H2O2(aq) ----> 2H2O(l) + O2(g)
Chemical Equations – dissolving
• When a substance such as copper sulphate,
CuSO4, is dissolved, water merely causes
the ions in the lattice to come apart.
• Since the water is not being changed, it is
written above the arrow.
H2O
• CuSO4(s) ----> Cu2+(aq) + SO42-(aq)
Chemical Equations - heating
• When a reaction requires heat, the word
‘heat’ is written above the arrow to denote
that heat has been applied.
Heat
– H2O(s) ---> H2O(l)
Review
• Complete the sample problems 11.1 & 11.2
page 258
• Complete the revision questions 1, 2 page 258
Balancing Chemical Equations
• To represent chemical equations correctly,
equations must be balanced.
• The number of atoms on both sides of the
equation must be the same
• Law of conservation of mass – the total mass
of the reactants in a chemical reaction is equal
to the total mass of the products.
• Atoms are not created or destroyed, but are
rearranged to form new substances
Balancing Chemical Equations
• In order to balance an equation, numbers
called coefficients are placed in front of the
whole formulas.





1. Write the reactants and products using formula and state for
each substance
2. Count the number of atoms of each element on the left-hand
side of the equation. Do the same for the right-hand side and
compare for each element. If any of these numbers do not
match, the equation is not balanced and you will need proceed
to the following steps
3. Balance by placing coefficients in front of the formulae. Do
not change the actual formula. If the substance is present as an
element, leave the balancing of it to last
4. Check all atoms of ions to ensure that they are balanced
5. Make sure that the coefficients are in the lowest possible ratio

Complete the sample problem 11.3 page 260

Complete revision questions 3 – 5 page 260
Types of Chemical
Reaction
• Chemists have determined several
main groups of chemical reactions
that help us predict the products of
these reactions.
• Keep in mind, though, that when we
write equations they should be
based on experimental data for
complete certainty
Types of Chemical
Reaction
• Main types of chemical reactions
– Precipitation
– Acid/base
– Combustion
– Acid molecules
– Acid Base reaction animation
Precipitation
• Precipitation occurs when ions in solution
combine to form a new compound of low
solubility in water
• This low-solubility compound forms as solid
particles that eventually settle.
• It is called a precipitate
• In order to predict whether a precipitate will
form, we must know which substances are
soluble in water and which substances are
insoluble
Precipitation
• From table 11.2 (page 262), we can predict that
when a solution of NaCl is mixed with a solution of
AgNO3, a precipitate of AgCl will form
• NaCl(aq) + AgNO3 ----> NaNO3(aq) + AgCl(s)
• The Ag+ ions combine with the Cl- ions to form
solid AgCl while the Na+ and NO3- ions remain in
solution.
• Precipitation animations

Complete the sample problem 11.4 page 262

Complete the revision questions 6 – 8 page 263
ACID/BASE AND NEUTRALISATION
REACTIONS

When an acid reacts with a base to form a salt
and water, a neutralisation reaction occurs


Acid + Base ----> salt + water
HCl(aq) +NaOH(aq) ----> NaCl(aq) + H2O(l)
ACID/BASE AND NEUTRALISATION
REACTIONS

1.
Other common reactions involving acids are:
Acid + Metal ----> salt + hydrogen

2.
3.
Acid + Metal carbonate ----> salt + water +
carbon dioxide

2HCl(aq) + Na2CO3(aq) ----> 2NaCl(aq) +
HCl(l) + CO2(g)
Acid + metal oxide ----> salt + water

4.
This reaction does not occur with Cu, Hg, or Ag
2HCl(aq) + CuO(s) ----> CuCl2(aq) + H2O(l)
Acid + metal hydroxide ----> salt + water

H2SO4(aq) + 2NaOH(aq) ----> Na2SO4(aq) +
2H2O(l)
ACID/BASE AND NEUTRALISATION
REACTIONS
Since a neutralisation reaction is often difficult to
detect, a chemical indicator can be used.
 Indicators are compounds that change colour
when an acid or a base have completely reacted


Complete the sample problem 11.5 page 263

Complete the revision questions 9, 10 page 264
Combustion reactions
• When hydrocarbons burn in a plentiful
supply of oxygen, they give off heat to
their surroundings and produce carbon
dioxide and water
• When limited air is available for a
combustion reaction, carbon monoxide
may be formed in preference to carbon
dioxide.
• Burning Hydrocarbons
Combustion reactions
• Octane burnt in air
– 2C8H18(l) + 25O2(g) ----> 16CO2(g) + 18H2O(g)
• Octane burnt in an engine (limited O2)
– 2C8H18(l) + 17O2(g) ----> 16CO(g) + 18H2O(g)
• The products of hydrocarbon combustion are
in the gaseous state
Review
• Complete the sample problem 11.6 page
264
• Complete the revision questions 11, 12
page 265
Ionic Equations
• In all the preceding equations, the elements
and compounds have been written in their
molecular or formula unit forms.
• In aqueous solutions, the reactions are best
represented by simpler equations called
ionic equations.
Ionic Equations
• Ionic equations are equations that show
only the species that are formed or changed
in a reaction
• Any ions that remain unchanged in a
reaction are included in an ionic equation.
• Ions that are present in a reaction but do not
react are called spectator ions.
Ionic Equations – The Rules
1.
2.
3.
4.
5.
6.
•
•
Write the balanced chemical equation
Decide, from the solubility table, which substances are soluble
and which will form precipitates
Expand the equation by dissociating all the soluble compounds
into their free ions
Check for any molecular substances such as acids and certain
bases that react with water to produce ions (hydrolyse).
Replace the formulae of these substances by the ions that they
form
Cancel all free ions that are unchanged on both sides of the
equation (the spectators)
Write the net ionic equation
Note – the equation must be balanced in charge as well as in
the number of atoms
Precipitation and neutralisation reactions are often represented
by ionic equations
Writing ionic equations for precipitation reactions
• When a salt is dissolve in water it breaks up, or
dissociates, into its constituent ions:
H2O
• NaCl(s) ----> Na+(aq) + Cl-(aq)
• The symbol (aq) indicates that each ion has
become surrounded by water molecules
• If a second soluble salt, silver nitrate, is added,
it will also dissociate
H2O
• AgNO3(aq) ----> Ag+(aq) + NO3-(aq)
• When the ions of both solutions come into
contact a white precipitate or AgCl is formed
Writing ionic equations for precipitation reactions
Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) ---->
AgCl(s) + Na+(aq) + NO3-(aq)
Precipitates are insoluble salts and so are not
dissociated in solution
The net ionic equation does not include the
spectator ions, Na+ and NO3Ag+(aq) + Cl-(aq) ----> AgCl(s)
Ionic Equation Practice


Follow the steps and complete the sample
problem 11.7 page 266
Complete the revision questions 13 – 17 pages
266, 267
Chemical Reactions – by
patterns
• Prediction of reactions by
patterns
• Only a prediction – exceptions
can occur
Chemical Reactions – by patterns
• A list of common patterns
–
–
–
–
Acid + metal hydroxide (base) ---> salt + water
Acid + basic oxide ---> salt + water
Acidic oxide + base ---> salt + water
Acid + metal ---> salt + hydrogen (no reaction
for Ag, Cu, Pt or Au)
– Acid + metal carbonate ---> salt + carbon
dioxide + water
– Metal carbonate ---> metal oxide + carbon
dioxide
Heat
– Hydrocarbon + (plentiful) oxygen ---> carbon
dioxide + water