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CHAPTER 13 - CHEMICAL REACTIONS
Balancing Chemical Equations
Physical properties of a substance describe characteristics that
are not involved in chemical changes. Density, hardness, phase,
color, melting point, electrical conductivity and specific heat
are examples of physical properties.
Chemical properties describe how matter can undergo changes
in composition either by itself or as it interacts with other
matter.
A chemical property of wood or paper is that it undergoes the
chemical reaction of burning. A chemical property of water is
that it does not. Iron rusts but gold does not.
Physical changes in a substance do not alter its chemical
composition, just its appearance. Water turns to ice. A pencil
may be sharpened. Alcohol can evaporate. Sugar can dissolve
in water.
A chemical change occurs when a substance with a different
chemical composition is produced. This is called a chemical
reaction. Rusting iron, burning paper, and cooking biscuits
that rise are all examples of chemical changes.
A chemical reaction is simply a rearrangement of atoms during
which some of the original chemical bonds are broken and new
bonds are formed to give new chemical structures.
During a chemical reaction, the substances present at the
beginning are called reactants. After the reaction is finished,
the substances that are present are called products.
The general form of a chemical reaction is:
A+B→C+D
Reactants → Products
There are three things that happen during a chemical reaction.
1. The reactants disappear or their quantities are reduced.
2. New substances appear as products that have different
chemical and physical properties from the reactants.
3. Energy (heat, light, electricity, or sound) is either released
or absorbed. Sometimes the energy change is very small and
not easily detected.
A balanced chemical equation can be written to describe the
chemical reaction.
A word equation simply names the products and reactants.
Methane + Oxygen → Carbon Dioxide + Water
This is the starting point for writing a balanced equation.
The next step is to write the correct chemical formulas for the
reactants and products.
CH4 + O2 → CO2 + H2O
Next count the number of atoms of each element on both sides
of the yields arrow. If the number of atoms of each element on
the left side of the arrow equals the number on the right, the
equation is balanced and no further changes are needed. If
they are not the same, then we balance the equation by
changing the coefficients of corresponding substances on both
sides of the equation. Once the correct formulas are written,
we cannot change them without changing the names of the
reactants or products.
In this case, carbon is balanced, but oxygen and hydrogen are
not.
We can balance the hydrogen by placing a 2 in front of H2O in
the products.
CH4 + O2 → CO2 + 2H2O
This gives us 4 hydrogen atoms on both sides, but gives us 4
oxygen atoms on the right and 2 oxygen atoms on the left.
We can remedy this by placing a 2 in front of the oxygen on the
left.
CH4 + 2O2 → CO2 + 2H2O
When we count our atoms again we have 1 carbon atom on
both sides, 4 hydrogen atoms on both sides, and 4 oxygen
atoms on both sides and the equation is balanced.
This tells us that one formula unit of methane reacts with two
formula units of oxygen to produce one formula unit of carbon
dioxide and two formula units of water.
Some reactions involve polyatomic ions (groups of atoms that
remain together during the reaction). Treat these polyatomic
ions as though they were single atoms.
Example
Sodium Hydroxide + Sulfuric Acid → Sodium Sulfate + Water
NaOH + H2SO4 → Na2SO4 + HOH
2NaOH + H2SO4 → Na2SO4 + 2HOH
We first placed a 2 in front of NaOH on the left to balance Na.
This gave us 2 OH ions which we balanced with a 2 in front of
HOH on the right. We already had 2 H atoms on the left and 1
SO4 on both sides.
There are five general categories of chemical equations. Many
reactions fit into one of these.
1. Combination (Composition) - two or more reactants
combine to produce one substance as a product.
Hydrogen + Oxygen → Water
2H2 + O2 → 2H2O
2. Decomposition - One reactant is broken down into two or
more substances as products.
Mercury (II) Oxide → Mercury + Oxygen
2HgO → 2Hg + O2
3. Single Replacement - One element replaces one other
element in a compound to form a new compound and a
different separate element.
Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen
Zn + 2HCl → ZnCl2 + H2
4. Double Replacement - The positive ion from one compound
changes places with the positive ion from another compound.
Sulfuric Acid +Sodium Hydroxide → Sodium Sulfate + Water
H2SO4 + 2NaOH → Na2SO4 + 2HOH
AB
+
CD

AD
+
CB
5. Hydrocarbon Combustion- a special type of combination
reaction which involves rapid oxidation. It is always
exothermic.
Energy and Rate of Reaction
All chemical reactions involve a change in energy. Energy must
be absorbed to break bonds in the reactants and energy is
released when bonds are formed in the products.
If more energy is released than absorbed, the reaction is called
an exothermic reaction.
Examples of exothermic reactions include burning of anything
that will burn, rusting of iron and corrosion of other metals.
When more energy is absorbed than produced, the reaction is
called an endothermic reaction. The electrolysis of water to
produce hydrogen and oxygen is an example of an endothermic
reaction.
The energy absorbed to break chemical bonds in the reactants
and give them enough kinetic energy to collide and form new
bonds is called activation energy. Striking a match is an
example of putting energy into a substance to start the
reaction.
Combustion reactions occur when a substance combines with
oxygen to form an oxide while it releases energy.
If the energy is released rapidly enough, the rapid expansion of
gasses associated with combustion creates a shock wave that
we call an explosion.
During complete combustion of hydrocarbons, the only
products are carbon dioxide and water. If there is insufficient
oxygen for complete combustion, cabon particles form soot and
carbon monoxide is one of the products.
Reaction rates depend on four things:
(1) Temperature - higher temp causes faster rates
(2) Concentration of the reactants - higher concentration
causes faster rates
(3) Surface area - larger surface area causes faster rates
(4) Catalyst - presence of a catalyst causes faster rates
A catalyst is a substance that increases the rate of a chemical
reaction but is not used up during the reaction.
Some examples of uses of catalysts are catalytic converters and
enzymes in your body.
Acids and Bases
There are several definitions for acids and bases. We will use
the classical Arrhenius definition of aqueous acids and bases.
Characteristics of an acid dissolved in water:
(1) Conducts electricity
(2) Changes the color of litmus from blue to red
(3) Tastes sour (never taste anything in the lab)
(4) Reacts with a base to neutralize it
(5) Reacts with active metals and releases hydrogen gas
Characteristics of a base dissolved in water
(1) Conducts electricity
(2) Changes the color of litmus from red to blue
(3) Reacts with an acid to neutralize its properties
(4) Tastes bitter (never taste anything in the lab)
(5) Reacts with organic matter to decompose it
The properties of an acid are due to the production of
hydronium ions (H3O+) when a hydrogen ion from the acid
combines with a water molecule.
A strong acid is one that ionizes almost completely in water
and produces a large number of hydronium ions.
A weak acid does not ionize as much and forms relatively few
hydronium ions.
Three well known strong acids are hydrochloric, nitric and
sulfuric.
Three weak acids are acetic, carbonic and citric. These are
found in foods such as vinegar, soft drinks, and citrus fruits.
The properties of a base are due to the formation of a
hydroxide ion (OH-) when dissolved in water. Some bases
contain the hydroxide ion before being dissolved in water but
ammonia combines with water molecules to form an
ammonium ion and a hydroxide ion. Therefore it is considered
to be a base.
There are strong bases and weak bases as with the acids.
NaOH, KOH and Ca(OH)2 are considered to be strong since
they ionize very well. NH4OH, Al(OH)3 and Mg(OH)2 are
considered to be weak bases.
In all aqueous solutions there exist some hydronium ions and
some hydroxide ions. The relative concentration of the
hydronium ion is what determines whether a solution is acidic
or basic.
The scale used to express the degree of acidity is called the pH
scale.
7 on the pH scale is considered neutral, neither acidic nor
basic. Blood, milk, and pure water have a pH very close to 7.
Substances that are acidic have a pH less than 7. Carbonated
water has a pH of 4, vinegar a pH of 3, stomach acid ranges
from 1 to 3.
Substances that are basic have a pH greater than 7. Baking
soda is about 8.5, milk of magnesia is about 10.5, and
household ammonia is about 12.
When an acid reacts with a base a neutralization reaction takes
place. During an acid-base reaction, an acid and a hydroxide
base react to give salt and water.
A salt is any compound formed from the positive ion of a base
and the negative ion of an acid. We call NaCl table salt and it
can be the result of a reaction between HCl and NaOH.
Another example is:
2HCl + Mg(OH)2 → 2HOH + MgCl2
This is one reaction that can occur when you take an antacid to
relieve your heartburn.
Acid can also be neutralized by the addition of a carbonate
such as calcium carbonate found in Tums or baking soda. In
both cases carbon dioxide, water and a neutral salt are
produced.
Single-Replacement Reactions
Oxidation is a type of reaction in which an element loses one or
more electrons.
Reduction is a type of reaction in which an element gains one
or more electrons.
A single replacement reaction that involves oxidation and
reduction occurs in the steel-making process.
2Fe2O3 + 3C → 4Fe + 3CO2
In this reaction, carbon is losing electrons so it is undergoing
oxidation. Iron is gaining back electrons it had lost to become a
free element so it is undergoing reduction.
This is called an oxidation-reduction reaction or redox for
short.
Although this type of reaction is named for oxygen, many other
elements undergo redox reactions with each other without
oxygen. Electrons must simply be transferred from the
substance that is oxidized to the substance that is reduced.
Metals are arranged on an activity series that ranks them
according to their tendency to undergo chemical reactions
during which they replace other metals in compounds.
Many single replacement reactions depend on the order of the
metals in this activity series.
Most metals are above hydrogen on the activity series so they
react rapidly or slowly with water to replace hydrogen.
Platinum and gold are below hydrogen so they do not react
with pure water.
Avogadro's Number
In chemistry quantities may be expressed in grams, kilograms,
liters or milliliters. These units are useful when making
measurements with a balance or graduated cylinder but are
cumbersome when working with equations and formulas.
The SI unit mole is more commonly used. A mole is defined as
6.02 x 1023 units of anything. Just like a dozen is 12 units of
anything a mole is a specific number of particles.
That number was chosen because 1 mole of a substance has a
mass in grams equal to its formula mass.
6.02 x 1023 is called Avogadro's number in honor of the Italian
physicist who first developed the concept of a diatomic
molecule of an element to help explain discrepancies in
Dalton's Laws.
Study
P 365 Terms
P 366 Matching Questions
P 366 Multiple Choice Questions
P 367 Fill in the Blank Questions
P 367 Questions 2, 3, 4, 6, 7, 8, 11, 12, 14, 16, 17, 19, 21, 22, 24,
25, 26, 27, 32, 35, 36, 37
P 369 Exercises 1, 3, 5, 7, 9