Download Chapter 7. CHEMICAL REACTIONS

Chapter 7.
CHEMICAL REACTIONS
Chemical reactions occur when a change leads to the formation of at least one NEW
substance. The starting substances in a reaction are called reactants. The substances
formed during a reaction are called products. During a chemical reaction, atoms are
rearranged and energy is absorbed or released in the process.
A reaction has occurred when one or more of the following can be observed:
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•
•
•
•
evolution of a gas (not the vapour of a reactant)
formation of a precipitate (a solid formed from two solutions)
a permanent colour change
a noticeable temperature change (heat) or other energies released/absorbed
disappearance of a solid that is known not to be soluble.
A chemical equation is a shorthand way of describing the chemical change that has
occurred.
Reactants
Products
In this module we will use word equations to describe the reactions observed. This
concept will then be extended to include balanced chemical equations and use them to
calculate quantities of reactants and products involved.
Chemical reactions are occurring inside us and around us every day.
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Our body’s metabolism involves hundreds of thousands of different chemical
reactions for the production of energy and growth.
Combustion is an important chemical reaction that is also used for energy production
(light, heat) and transport amongst other uses.
Photosynthesis is an essential process for the existence of life on earth. Essential
sugars and oxygen are produced when carbon dioxide and water react in the
presence of sunlight within the green leaves of plants.
Most metal objects have been produced through industrial chemical processes.
They would otherwise remain existing mostly as an Earth based compounds
distributed in rich crustal deposits called ores.
We can look more closely at specific chemical changes:
Energy and Chemical Processes
When a spark ignites a mixture of hydrogen gas and oxygen gas, an explosion occurs.
A large amount of energy is released. The product formed is water.
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Hydrogen (g)
+
Oxygen (g)
Water (l)
+
energy
The subscripts used in this equation indicate the states of reactants and products.
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous (dissolved in water)
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Chemical reactions which release energy are called exothermic, hence “Energy is lost
to the surrounding environment”. Reactions which absorb energy are called
endothermic, hence “energy is absorbed from the surrounding environment”.
Photosynthesis is an example of an endothermic chemical process. Plants absorb
energy in the form of sunlight to change carbon dioxide gas and water to complex
organic molecules such as glucose, releasing oxygen gas. This process is
endothermic.
Carbon +
Dioxide (g)
Water(l)
+
Energy
Glucose (aq)
+
Oxygen (g)
Many chemical reactions require some energy to start the reaction. However
endothermic reactions require energy to be available continuously, so that the reaction
can continue.
Written Exercise
Chemical Reactions
1Do each of the following involve chemical reactions? State what you would
observe in each case to justify this conclusion.
(a)
A piece of wood burns leaving grey ashes
(b)
A piece of sodium metal disappears after fizzing as it zips and releases
sparks over the surface of the water.
(c)
A tablespoon of salt dissolves when mixed in water.
(d)
A headache tablet fizzes as it dissolves in the water.
Writing Chemical Equations - Chemical Shorthand
A chemical equation is a shorthand method of communicating information about
reactants and products, quantities involved and sometimes how much energy is needed
or released. It accounts for all the atoms involved in the re-arrangement of bonds.
Reactants
(Ingredients)
(form)
Products
(new substances)
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During a chemical reaction, bonds are broken in the reactants and new bonds are
formed making the products. The atoms are re-arranged during a reaction, but the
numbers of the different types of atoms remain constant.
The law of Conservation of Matter (or mass) states that during a chemical reaction
the total mass of reactants is the same as the total mass of products formed. During a
chemical reaction, matter is neither created nor destroyed, it is only changed
from one form into another.
Law of Conservation of Mass (Matter)
Total
Mass of reactants
=
Total
Mass of products
Note: The total number of atoms of each element is unchanged!
Example 1: When a spark ignites a mixture of hydrogen gas and oxygen gas, an
explosion occurs - a large amount of energy is released. The product formed is water.
Chemical reactions which release energy are called exothermic.
Hydrogen(g) +
Word
Equation :
Symbol
Equation:
(unbalanced)
H2(g)
+
Oxygen(g)
Water(l) + energy
O2(g)
H2O(l)
+ energy
The correct chemical formula is written under each reactant and product. The extra
subscripts indicate the state of that substance.
The equation above is said to be unbalanced because the numbers of various atoms
represented on the reactants’ side (left of the arrow) is not the same as those on the
products’ side (right of the arrow). This would mean that mass is not conserved!
Reactants ( H2 + O2 )
Products ( H2O )
H=2
H=2
O=2
O=1
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The numbers of oxygen atoms are not balanced!
We must adjust the numbers of molecules of each type present to reach a balance! This
involves placing numbers where required in front of each formula. The number will
multiply each atom in the formula. This was referred to previously as a coefficient
number. A “2” in front of the H2O product will provide the required 2 oxygen atoms on
the right.
H2(g)
+
O2(g)
2 H2O(l)
+
energy
Reactants ( H2 + O2 )
Products ( 2 H2O )
H=2
H=4
O=2
O=2
Whilst oxygen has now been balanced, Hydrogen is now unbalanced!
In order to balance the hydrogen atoms it is necessary to place a “2” in front of the H2
on the left. This provides 4 hydrogen atoms on the left also.
Balanced Equation:
2 H2(g) +
O2(g)
2 H2O(l) +
energy
Reactants ( 2 H2 + O2 )
Products ( 2 H2O )
H=4
H=4
O=2
O=2
Example 2. When an emergency flare lights up, magnesium (or aluminium) metal
combines with oxygen gas releasing a bright light. This is also an exothermic reaction.
The substance formed is a metal oxide.
Word
equation:
Symbols:
Magnesium(s)
Mg(s)
+
Oxygen(g)
+
O2(g)
Magnesium + energy
oxide(s)
MgO(s)
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+
energy
Reactants ( Mg + O2 )
Products ( MgO )
Mg = 1
Mg = 1
O=2
O=1
The numbers of oxygen atoms are not balanced!
Balance the oxygen atoms by placing a coefficient “2” in front of MgO. This will also
produce 2 magnesium atoms.
Mg(s)
+
O2(g )
2MgO(s)
+
energy
Reactants ( Mg + O2 )
Products ( 2 MgO )
Mg = 1
Mg = 2
O=2
O=2
Then balance the magnesium atoms by placing a coefficient “2” in front of Mg.
Balanced
equation:
2 Mg(s)
+
O2(g)
2 MgO(s)
+
energy
Reactants ( 2 Mg + O2 )
Products ( 2 MgO )
Mg = 2
Mg = 2
O=2
O=2
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Written Exercises
1.
Balance the following chemical equations.
(a)
CH4(g) +
O2(g)
CO2(g)
+
H2O(l)
(b)
Na(s) +
H2O(l )
NaOH(aq)
+
H2(g)
(c)
Zn(s)
+
HCl(aq )
(d)
P
+
O2
(e)
NH3
+
H2SO4
(f)
CuO +
(g)
H2O2
H2O
+
O2
(h)
H2CO3
H2O
+
CO2
(i)
Fe
+
O2
Fe2O3
(j)
C8H18 +
O2
CO2
ZnCl2(aq)
+
H2(g)
P2O5
(NH4)2SO4
HCl
+
CuCl2
+
H2O
H2O
EXPRESSING BALANCED CHEMICAL REACTIONS:
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A reaction occurs because atoms are REARRANGED. Atoms are not formed or
destroyed. Mass of Reactants and products remains the SAME.
1. Identify Reactants of Chemical Reaction, for example;
Sodium Hydroxide + Sulfuric Acid  ?
2. Try to identify the Nature of reactants. Then correspond with a typical reaction
type, ie
Sodium Hydroxide + Sulfuric Acid  ?
BASE
+
ACID
 SALT + WATER
3. Write down a WORD EQUATION:
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Sodium Hydroxide + Sulfuric Acid  Sodium Sulfate + Water
4. Write Symbols for compounds
NaOH + H2SO4  NaSO4 + H2O.
5. Balance atoms in compounds according to Valency:
1+
2-
NaOH + H2SO4  Na2SO4 + H2O.
6. Reactant Atoms = Product Atoms
Balance atoms either side of arrow.
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Balance Na (2NaOH)
2NaOH + H2SO4  Na2SO4 + H2O.
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Balance O (2H2O)
2NaOH + H2SO4  Na2SO4 + 2H2O.
Elements in equation now BALANCED.
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Some Typical Chemical Reactions
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Atoms can be rearranged in a variety of ways….
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Write a word and balanced chemical equation of each reaction type listed below in
the corresponding space at the right.
REACTION TYPE
EXAMPLES
ACID + BASE  SALT + WATER
NEUTRALISATION
An ACID mixed with a BASE
(Alkali) produces a Neutral
Salt and Water.
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ACID + METAL
An ACID added to a METAL
will produce a Metal Salt and
Hydrogen Gas, H2
ACID + METAL  SALT + HYDROGEN
ACID + CARBONATE  SALT + CARBON DIOXIDE + WATER
ACID + CARBONATE
An ACID added to a Metal
CARBONATE compound will
produce a Salt, Carbon
Dioxide, CO2 and Water,
H2O.
ORGANIC FUEL + OXYGEN  CARBON DIOXIDE + WATER
ORGANIC COMBUSTION
Organic Fuels BURNT in the
presence of OXYGEN
produce Carbon Dioxide and
Water **
** assuming complete combustion.
REACTION TYPE
EXAMPLES
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ELEMENT + OXYGEN  ELEMENT OXIDE
INORGANIC COMBUSTION
Some elements will readily
burn in the presence of
oxygen to form an oxide
compound.
AB (aq) + CD (aq)  AD (s) + BC (aq)
PRECIPITATION
Two SOLUBLE Compounds
may React and Rearrange to
produce an INSOLUBLE
(solid) Compound, ie
Precipitate.
A  B+C+...
DECOMPOSITION
A single compound can
break down to form 2 or more
smaller compounds or
elements.
HYDROLYSIS
Reaction of any chemical
with WATER
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CHEMICAL EQUATIONS – Exercise 1.
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Carefully read the following chemical reaction scenarios. Firstly decide which
substances are reactants and which are products, then:
i) Write down a word equation of reactants and products.
ii) Write a balanced chemical equation.
1. Aqueous Magnesium Chloride and Hydrogen gas are produced when Magnesium
ribbon is placed into a solution of Hydrochloric acid.
2. Aluminium metal reacts with Nitric acid to produce an aqueous solution of Aluminium
Nitrate and Hydrogen gas.
3. Iron metal rusts in the presence of Oxygen to produce the compound Iron III Oxide.
4. Lithium metal, a reactive group 1 alkali metal, reacts violently when placed in water
to produce a strongly alkaline solution of Lithium Hydroxide and Hydrogen gas.
5. Solid Silver I Carbonate decomposes when heated to produce solid Silver Oxide and
Carbon Dioxide
6. Solid Silver I Chloride and an aqueous solution of Sodium Nitrate are produced
when solutions of Silver I Nitrate and Sodium Chloride are combined.
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7. A neutral solution of Sodium Sulfate and water are produced at equivalence point
when Sulfuric Acid is fully neutralised by the addition of Sodium Hydroxide.
8. A bright yellow precipitate of Lead II Iodide and aqueous solution of Potassium
Nitrate are produced when aqueous solutions of Lead Nitrate and Potassium Iodide
are combined.
9. When Hexane, C6H14 (a colourless liquid hydrocarbon) combusts in a plentiful
supply of oxygen, the compounds of carbon dioxide and water are formed. Heat is
given off indicating that the process is exothermic.
CHEMICAL EQUATIONS - Exercise 2
•
Use your knowledge of Chemical Formulae, Valencies and Reactions to
complete the following Exercises. Complete the WORD and BALANCED
CHEMICAL EQUATIONS below.
1. Magnesium + Nitric Acid 
2. Copper + Sulfuric Acid 
3. Potassium + Water 
4. Zinc (II) Carbonate + Nitric Acid 
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5. Copper II Carbonate (s) + HEAT 
6. Aluminium + Sulfuric Acid 
7. Sodium Hydroxide (aq) + Copper II Sulfate (aq) 
8. Sodium Hydroxide + Sulfuric Acid 
Ionic Equations
When solutions are involved in a reaction, only some of the ions present are usually
involved. Other ions may be present, but they are still in the solution at the end of the
reaction, unchanged by the chemical process. These ions are called spectator ions
and are best left out of the balanced equation. When spectator ions are left out of an
equation an ionic equation results. Ionic equations are the best representation of the
chemical reaction, since they show only those species which have undergone a change.
Example 1. When a solution of lead (II) nitrate is mixed with a solution of potassium
iodide, a brightly coloured precipitate forms. This precipitate is solid lead(II) iodide. Write
an ionic equation for this chemical change.
General Equation (balanced):
Pb(NO3)2(aq)
+
2KI(aq)
PbI2(s)
+ 2KNO3(aq)
Aqueous compounds split into their respective ions as they are dissolved in water. We
can expand out the general equation as follows:
Expanded Equation
Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I-(aq)
PbI2(s) + 2K+(aq) + 2NO3-(aq)
Note: PbI2 (s) on the right does not form ions, as it is in the solid state.
In this “expanded” form, the equation shows the “whole picture” of the chemical
process.
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If we look a bit closer at the reactants and products however, we can see that some of
the chemical components have remained unchanged in the process. Can you see this
???
If you can, you will have identified 2NO3-(aq) and 2K+(aq) as remaining identical, hence
unchanged on the left and right side of the equation. They are merely “spectator ions”
but can be mistaken as being part of the chemical process when referring back to the
balanced general equation. If they are deleted from the left and right of the equation, we
will arrive at the ionic equation, ie:
Pb2+(aq)
Ionic Equation:
+
2I-(aq)
PbI2(s)
Example 2. When magnesium metal reacts with dilute hydrochloric acid, a solution of
magnesium chloride remains and hydrogen gas is evolved. Write a balanced, general
chemical equation and an ionic equation for this chemical change.
Magnesium(s) +
hydrochloric
acid (aq)
magnesium +
chloride (aq)
hydrogen (g)
MgCl2(aq)
H2(g)
General Equation (balanced):
Mg(s)
+
2HCl(aq)
+
Expanded Equation
Mg(s)
+
2H+(aq) +
2Cl-(aq)
Mg2+(aq) +
2Cl-(aq) +
H2(g)
Note: Mg (s) on the left and H2 (g) on the right do not form ions. They are in the solid and
gaseous state respectively.
The spectator ions in this case are the chloride ions, which are present in the dilute
hydrochloric acid as Cl-(aq) and remain unchanged on the right as a component of
aqueous magnesium chloride. When Cl-(aq) are deleted from the left and right of the
expanded equation, we arrive at the net ionic equation:
Ionic Equation:
Mg(s) +
2H+(aq)
Mg2+(aq)
+
H2(g)
An Ionic Equation should also appear as a balanced equation. If not, a mistake has
been made and you will need to go back and check where something went wrong.
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Ionic equations must also be balanced in terms of their overall charge on the left and
right. This can be done as a simple calculation
Reactants ( Mg(s) + 2H +(aq) )
Products (Mg2+(aq) + H2 (g) )
2 x 1+ = 2+
1 x 2 + = 2+
Total charge on left
hand side of equation
= 2+
Total charge on right
hand side of equation
= 2+
You will find it useful to recognise certain common types of chemical reactions
so that you can correctly predict the products which will form. Ionic equations
will be used where appropriate in the examples following. More work on ionic
equations will be done in another module!
Written Exercises: Write out the net ionic equation of each full equation below.
1.
AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
2.
Na2CO3 (aq) + H2SO4 (aq)  Na2SO4 (aq) + CO2 (g) + H
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