Chemistry Unit 2
Wear your safety goggles!
Chemistry Unit 1
Structure of
an Atom
Protons and
an Atoms
identity
Valence
Electrons and
Reactivity
The Periodic
Table
Chemistry Unit 2
Chemical
Formulas
Chemical
Reactions
Chemical
Equations
Law of
Conservation
of Mass
Teks for Unit 1 and 2
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(5) Matter and energy. The student knows that matter is composed of atoms and has
chemical and physical properties. The student is expected to:
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(A) describe the structure of atoms, including the masses, electrical charges, and
locations, of protons and neutrons in the nucleus and electrons in the electron cloud;
(B) identify that protons determine an element's identity and valence electrons
determine its chemical properties, including reactivity;
(C) interpret the arrangement of the Periodic Table, including groups and periods, to
explain how properties are used to classify elements;
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(D) recognize that chemical formulas are used to identify substances and determine
the number of atoms of each element in chemical formulas containing subscripts;
(E) investigate how evidence of chemical reactions indicate that new substances with
different properties are formed; and
(F) recognize whether a chemical equation containing coefficients is balanced or not
and how that relates to the law of conservation of mass.
8.5 D Questions
• The chemical formula for water is H2O. What do the H
and O represent? What does the subscript 2 mean?
• The chemical formula for hydrogen peroxide is H2O2.
What does the chemical formula tell you about the
hydrogen peroxide molecule?
• Is CO the same as CO2? Explain.
• One calcium (Ca) atom combines with two chlorine (Cl)
atoms to make calcium chloride. What is the chemical
formula of calcium chloride?
• Suppose that all you know about a molecule is its
chemical formula. What can the chemical formula tell
you?
8.5 D Key Concepts
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An element is made up of only one type of atom. Hydrogen, helium, and oxygen are examples of elements.
Elements are pure substances. A pure substance is matter that has the same chemical composition throughout
and cannot be separated into its parts by physical means.
Elements form compounds. A compound is a pure substance that forms when two or more elements join
chemically in a fixed proportion.
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Just as an element is made up of one kind of atom, a compound is made up of one kind of molecule. A molecule is
a group of two or more atoms held together by very strong chemical bonds. These bonds form between atoms
that share or transfer electrons. A molecule is the smallest unit of a compound that has all the properties of that
compound.
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A molecule can be made up of more than one atom of the same element. For example, two atoms of oxygen join
to form a molecule of oxygen gas.
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A molecule can also be made up of two or more different elements. A water molecule is made up of two hydrogen
atoms and one oxygen atom. The illustration below is a model of a water molecule.
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Molecules are identified by chemical formulas. A chemical formula is a group of chemical symbols and numbers
that shows the kinds and numbers of atoms in a molecule.
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The formula for a water molecule is H2 O. H is the chemical symbol for hydrogen. O is the chemical symbol for
oxygen. The number 2 in the formula is called a subscript. It shows that the molecule contains two atoms of
hydrogen. The O has no subscript. That means that the molecule contains only one atom of oxygen.
Chemistry Vocabulary, Unit 2, part 1
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pure substance: matter that has the same chemical composition throughout
compound: a substance that forms when two or more elements join chemically
molecule: a group of two or more atoms held together by chemical bonds
chemical formula: a group of chemical symbols and numbers that shows the
kinds and numbers of atoms in a molecule
How many chemical formulas can
you name?
Matter
Compound
Pure
Substance
Smallest unit
is a molecule
Smallest unit
is an atom
H2O
H
Chemical Formulas
Subscripts
And Lettering
carbon dioxide
CO2
Chemical
Formulas of
Household
Items
Common Name
Chemical Name
Chemical Formula
aluminum foil
vitamin C
aluminum
ascorbic acid
Al
C6 H8 O6
vinegar
acetic acid (diluted) CH3 COOH
table salt
sodium chloride
NaCl
road salt
calcium chloride
CaCl2
table sugar
sucrose
C12 H22 O11
baking soda
sodium bicarbonate NaHCO3
wood alcohol
(solvent)
methanol
hydrogen peroxide
hydrogen peroxide
H2 O2
(diluted)
dry ice
carbon dioxide
CH3 OH
CO2
nail polish remover acetone
CH3 COCH3
chalk, some antacidscalcium carbonate
CaCO3
lighter fluid
butane
C4 H10
tincture of iodine
iodine
I2
soda water
carbonic acid
(diluted)
H2 CO3
Chemical
Formula
CaC03
Common
Name
calcite
Atoms
Present
1 Calcium
1 Carbon
3 Oxygen
Chemical
Formula
CaC03
Common
Name
calcite
Atoms
Present
1 Calcium
1 Carbon
3 Oxygen
Chemical
Formula
CaC03
Common
Name
calcite
Atoms
Present
1 Calcium
1 Carbon
3 Oxygen
NaCl
C8H10N4O2
Mg3Al2Si3O12
O3
C6 H12 O6
NaCl
C8H10N4O2
Mg3Al2Si3O12
O3
C6 H12 O6
NaCl
C8H10N4O2
Mg3Al2Si3O12
O3
C6 H12 O6
Chemistry Vocabulary, Unit 2
pure substance: matter that has the same chemical composition throughout
compound: a substance that forms when two or more elements join chemically
molecule: a group of two or more atoms held together by chemical bonds
chemical formula: a group of chemical symbols and numbers that shows the
kinds and numbers of atoms in a molecule
physical change: a change in the form or appearance of a substance without a
change in the identity of the substance
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chemical reaction: a process in which chemicals react, or change, to form new
types of matter
precipitate: a solid that forms during a chemical reaction in a solution
reactant: a substance that enters into a chemical reaction
product: a substance that forms during a chemical reaction
combustion reaction: a chemical reaction that occurs when oxygen combines
with certain other substances to release heat
rusting: a slow chemical reaction between oxygen and a metal
chemical reaction: a process in which chemicals react, or change, to form new
types of matter
reactants: the chemicals that enter into a chemical reaction; also called
reagents
products: the chemicals that form during a chemical reaction
chemical equation: a statement that shows the reactants and products of a
8.5 E Questions
• What is a chemical reaction?
• What is the difference between a physical
change and a chemical reaction?
• How can you tell that a chemical reaction has
taken place?
• Are the products of a chemical reaction the
same as the reactants? Explain why or why
not.
• What types of chemical reactions are there?
8.5 E Key Concepts
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A physical change alters the form or appearance of a substance without changing the identity of the
substance. During a physical change, no changes occur in the elements or compounds that make up
the matter. For example, when water boils, water vapor is released. The water changes from a liquid
to a gas, but it is still water.
A chemical reaction is a process in which elements and compounds combine in new ways to form
new substances
Usually a physical reaction can be easily reversed. For example, to change water vapor back into a
liquid, you simply let it cool down. A chemical reaction, on the other hand, is not easily reversed.
Some signs that indicate a chemical reaction has occurred include a change in color and the release
of heat or light. Sometimes a precipitate forms. A precipitate is a solid that forms during a chemical
reaction that takes place in a solution.
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Some chemical reactions cause release of a gas. If you drop an antacid tablet into water, a chemical
reaction occurs. The bubbles that you see in the water are carbon dioxide given off by a reaction
between a compound in the tablet and the water. But release of gas does not always mean a
chemical reaction has taken place. For example, when water boils, this change from liquid water to
water vapor is a physical change, not a chemical change.
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The reactants are the chemicals that enter into a chemical reaction. The products are the chemicals
that form during a chemical reaction. For example, when acetic acid (vinegar) combines with sodium
bicarbonate (baking soda), acetic acid and sodium bicarbonate are the reactants. The products are a
salt, water, and a gas—carbon dioxide.
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A combustion reaction is a chemical reaction that occurs when oxygen combines with certain other
substances to release heat. A combustion reaction takes place when something burns. When wood
burns, water vapor and carbon dioxide are given off. The ash that remains is very different from the
wood that burned. It is a new substance with different properties.
Rusting is another example of a chemical reaction. Rusting is a slow reaction between oxygen and a
metal. A nail contains iron. Over time, the iron combines with oxygen in the air to form rust. The
surface of the nail changes from smooth and shiny to rough and reddish brown. The rust is a
different substance from iron.
Reaction Types
• Physical Reaction
• Chemical Reaction
Types of Reactions
Types of Reactions
Types of Reactions
Types of Reactions
Types of Reactions
Types of Reactions
Combustion Reaction
Combustion Reaction
C25H52 + 38 O2 → 25 CO2 + 26 H2O
1. What are the reactants in the chemical reaction that occurs when a
candle burns?
2. The products of the reaction are carbon dioxide and water vapor. What
two types of energy are also released?
3. What happened when your teacher placed a glass jar over the burning
candle? Why do you think this happened?
C25H52 + 38 O2 → 25 CO2 + 26 H2O
1. What are the reactants in the chemical reaction that occurs when a
candle burns?
2. The products of the reaction are carbon dioxide and water vapor. What
two types of energy are also released?
3. What happened when your teacher placed a glass jar over the burning
candle? Why do you think this happened?
4. Explain How do you know this is a combustion reaction?
4. Explain How do you know this is a combustion reaction?
5. Apply How do you know this is a chemical reaction and not just a
physical change?
5. Apply How do you know this is a chemical reaction and not just a
physical change?
Evidence of a Chemical Reaction
Precipitation
Gas Bubbles
Change in
color or
temperature
N2 + H2 → NH3
Types of Chemical Reactions
Oxidizing
Combustion
8.5 F Questions
• Why are coefficients included in a chemical equation?
• What is the law of conservation of mass? How does it
apply to a chemical reaction?
• If 12 oxygen atoms enter a chemical reaction, must the
same 12 oxygen atoms be included in the products of
the reaction? How do you know?
• How can you tell if a chemical equation is balanced or
unbalanced?
• When balancing a chemical equation, which element
should you balance first?
8.5 F Key Concepts
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A chemical equation shows the reactants and products of a chemical reaction. The reactants are shown on the left side of
the equation, and the products are shown on the right side.
Here is an example of a chemical equation: CH4 + O2 CO2 + H2 O
This equation shows that methane (CH4) and oxygen (O2) react to form carbon dioxide (CO2) and water (H2 O).
The equation above is an unbalanced equation, which means that the numbers of atoms on the left and right sides of the
arrow do not match. The equation shows two oxygen atoms in the reactants (on the left side), and three oxygen atoms in
the products (on the right side). Also, there are four hydrogen atoms in the reactants and only two hydrogen atoms in the
products.
The balanced form of the chemical equation shown above is as follows:
CH4 + 2 O2 → CO2 + 2 H2 O
Placing a coefficient of 2 in front of H2 O makes four hydrogen atoms on both sides of the equation. Then, placing a
coefficient of 2 in front of O2 makes four oxygen atoms on both sides of the equation.
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A balanced chemical equation shows how a reaction agrees with the law of conservation of mass. Every atom has a specific
mass, and that mass cannot be increased or decreased by a chemical reaction. So, if four oxygen atoms enter a chemical
reaction, then four oxygen atoms must leave the reaction as well.
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In balanced chemical equations, the coefficients show the ratios in which chemicals combine and form. In the reaction
between methane and oxygen described above, the ratio of the reactants is 1:2—one methane molecule to two oxygen
molecules. This shows that 10 methane molecules will combine with 20 oxygen molecules, or that 1,000 methane
molecules will combine with 2,000 oxygen molecules.
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When you balance an equation, remember the following:
– If no coefficient is visible in front of a reactant or product, the coefficient of that reactant or product is 1.
– You might not need to change every coefficient when balancing a chemical equation.
– A coefficient in front of a reactant or product affects each element inside that reactant or product. For example,
putting a coefficient of 2 in front of H2 O means that there are four hydrogen atoms and two oxygen atoms.
– Begin by balancing the element that appears in the fewest places in the equation.
When you balance an equation,
remember the following:
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If no coefficient is visible in front of a reactant or product, the coefficient of
that reactant or product is 1.
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You might not need to change every coefficient when balancing a chemical
equation.
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A coefficient in front of a reactant or product affects each element inside that
reactant or product. For example, putting a coefficient of 2 in front of H 2 O
means that there are four hydrogen atoms and two oxygen atoms.
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Begin by balancing the element that appears in the fewest places in the
equation.
Balancing Chemical Reactions
• Unbalanced Reaction
– CH4 + O2 CO2 + H2 O
• Balanced Reaction
– CH4 + 2 O2 CO2 + 2 H2 O
Evidence of a Chemical Reaction
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Color Change
Temperature Change
Release of Gas Bubbles
Precipatate
Light
Odor