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:
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.

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.
4
2
2
2
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 H
2
O in the products.
4
2
2
2
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.
4
2
2
2

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
2
4
2
4
2
4
2
4
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
SO
4
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
2H
2
+ O
2
→ 2H
2
O

2. Decomposition - One reactant is broken down into two or more substances as products.
Mercury (II) Oxide → Mercury + Oxygen
2HgO → 2Hg + O
2
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 → ZnCl
2
+ H
2

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
H
2
SO
4
+ 2NaOH → Na
2
SO
4
+ 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 (H
3
O
+
) 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. NH
4
OH, 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:
2
2

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.
2
3
2
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 10 23 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 10
23
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