GRADE: Eighth

CONTENT STANDARD: Physical Science

CONTENT TOPIC: Matter

CONCEPT: In a chemical reaction two or more substances combine to produce one or more products with new properties.

CONTENT OBJECTIVE: 8A2.00 To understand chemical reactions

INSTRUCTIONAL OBJECTIVES: The learner will

2.01 Define a chemical reaction.
2.02 Describe the characteristics of a chemical reaction.
2.03 Explain that a chemical reaction involves a change in the properties of the substances and also a change in energy.
2.04 Classify chemical equations.
2.05 Balance chemical reaction as to type.
2.06 Explain how balancing equations relates to the law of conservation of mass.
2.07 Classify substances as metals or nonmetals based on their properties.
2.08 Classify reactions as endothermic or exothermic based on the energy change involved.

I. Definition of Chemical Reaction
II. Characteristics of Chemical Reaction
III. Types of Chemical Reaction
A. Synthesis
B. Decomposition
C. Single Replacement
D. Double Replacement
IV. Balancing Chemical Equations
A. Reactants
B. Products
C. Subscripts
D. Coefficients
V. Law of Conservation of Mass
VI. Properties of metals and nonmetals
VII. Types of Energy Changes
A. Endothermic
B. Exothermic

TN COMPONENT OF SCIENCE: Unifying Concepts of Science

TN GOAL:

To enable students to acquire scientific knowledge by applying concepts, theories, principles and laws from life/environmental, physical and earth/space sciences.

2.6 CONSERVATION - In any natural process the form may change but nothing is lost.

2.6a Although there can be transformations of matter and energy in changes, the sum of matter and energy is conserved.

BENCHMARK: Energy is neither created nor destroyed but only changes form.

BENCHMARK: As in all material systems, the total amount of matter remains constant, even though its form and location change.

BENCHMARK: No matter how atoms are rearranged, their total mass stays the same.

TIME REQUIRED:

5 days (2 lecture days and 3 days of activities)

paper, matches, salt, water, aluminum foil, vinegar, baking soda, cornstarch, nails, 2 jars with lids, sugar. Chemicals: iron filings, sulfur, silver nitrate solution, copper wire

This classroom connector addresses instructional objectives 2.01, 2.02, 2.03, and 2.04.

What do the souring of milk, the burning of paper, and the rusting of a nail have in common? (wait for answers) These are all examples of chemical reactions. A chemical reaction has occurred if two or more substances combine to produce one or more products which have properties that differ from the original substances. Can you give me some more examples of chemical reactions?

A chemical reaction consists of two main parts-reactants and products. The reactants are the starting substances in the reaction while the products are the substances that are formed as a result of the chemical reaction. Reactants are usually written on the left side of a chemical equation and products are written on the right side.

There are several ways to tell if a chemical reaction has occurred such as a change in color, gas given off, heat given off, heat required, or the formation of a new substance.

Chemical reactions also involve changes in energy. Energy is either absorbed as when pancake batter is cooked to produce pancakes or when energy is released as when a candle burns.

Chemical reactions may be classified as synthesis, decomposition, single replacement, or double replacement. A synthesis reaction has occurred when two substances combine to produce only one product. The general rule is element or compound + element or compound-->compound or A+B-->AB. A decomposition reaction has occurred when one compound decomposes to produce two or more substances. The general rule is AB-->A+B. In a single replacement reaction, one element replaces another element in a compound to produce a new compound. The general rule is element + compound or A+BC-->B+AC. In a double replacement reaction, two atoms switch places to produce two new compounds. The general rule is compound + compound or AB+CD-->AD+CB.

The following activities are suggested.

1. Determine whether the following activities are chemical or physical changes.

a. Tear a piece of paper (physical)

b. Strike a match (chemical)

c. Mix a small amount of salt with H₂O. (physical)

d. Make a small pan of aluminum foil and do the following in the pan:

1. Put a drop of H₂O in the pan. (physical)

2. Put a small amount of sugar in the pan and heat it. (chemical)

e. Add a small amount of vinegar to baking soda. (chemical)

f. Put a small amount of cornstarch in your hand and add a few drops of H₂O to form a dough. (chemical)

2. Preventing a chemical reaction activity:
a. Obtain 2 large nails. Paint one nail and leave the other one unpainted.

b. Put both nails in a jar with water. Cover the jar and let it stand several days.

c. Compare the appearance of the nails. (Painted nail does not form rust)

3. Activities for the types of reactions:
a. Synthesis-heat a small amount of Fe filings after mixing with a small amount of sulfur (Fe+S-->FeS)

b. Decomposition-Burn some sugar in a test tube (C₁₂H₂₂O₁₁+12O₂-->12CO₂+11H₂O)

c. Single replacement-Cover a piece of copper wire with silver nitrate solution. (Cu+AgNO₃-->CuNO₃+Ag)

d. Double replacement-Add several drops of silver nitrate solution to salt water.(A white precipitate, AgCl, forms) (NaCl+AgNO₃-->AgNO₃+NaNO₃)

This classroom connector addresses instructional objectives 2.05, 2.06, 2.07, and 2.08.

Ammonium chloride, sodium hydroxide pellets, thermometer, SOS pad (without soap)

Instruction: The law of conservation of mass states that atoms are neither created nor destroyed during a chemical reaction. The number of atoms of each element must be the same on both sides of the equation. That is, the number of atoms of reactant(s) must be equal the number of atoms of product(s).Balancing equation is an example of the law of conservation of mass. There are three steps to writing complete balanced equations. First, write a word equation for the chemical reaction. Substitute symbols and subscripts for each element or compound making sure the formulas are correct. Second, count the number of atoms of each element on both sides of the equation. If the numbers are the same, the equation is balanced. Third, if the numbers are not the same, the equation must be balanced by adding coefficients in front of the formulas until the number of atoms of each element is the same on both sides of the equation. Ask if the following equation is balanced: Mg+CuNO₃-->Mg(NO₃)₂+Cu answer:

No, there are not enough NO₃ groups on the left. A coefficient 2 should be placed in front of CuNO₃ on the left and in front of Cu on the right.

The equation is now balanced. (Practice balancing several equations such as the ones below. Then allow students to come up to the board or overhead and practice balancing equations. Finally, a worksheet of equations to balance should be given for independent practice)

Examples to balance:

1. Na+O₂-->Na₂O

Answer: (4Na+O₂-->2Na₂O)

2. Al+HCl-->AlCl₃+H₂

Answer: (2Al+6HCl-->2AlCl₃+3H₂

3. Cl₂+MgI₂-->MgCl₂+I₂

Answer: (already balanced)

4. Na+H₂O-->NaOH+H₂

Answer: (2Na+2H₂O-->2NaOH+H₂)

5. LiOH+H₃PO₄-->Li₃PO₄+H₂O

Answer: (3LiOH+H₃PO₄+3H₂O)

6. H₂O-->H₂+O₂

Answer: (2H₂O-->2H₂+O₂)

The elements which combine to form compounds can be broadly classified as metals or nonmetals. The differences between these two categories can be determined by discussing the chemical and physical properties of each type.

Metals have observable physical properties such as their shininess or luster, or their conductivity of heat and electricity. Most metals are ductile, that is, they can be drawn into wire. They are also malleable, which means they have the ability to be hammered into sheets. The chemical properties of metals are much harder to observe because they result from the way electrons are arranged in the atom. Metals have one, two, or three electrons in their outermost shell. These electrons are loosely held; metals have a tendency to lose these electrons.

Since metals lose electrons, they will react with water or other elements. A chemical reaction which results in the wearing away of the metal to change it into a metallic compound, is called corrosion. A rusting nail or tarnishing silver are examples of corrosion.

Nonmetals also have some observable physical properties. They are generally dull; that is, they have no luster. They are also soft; many can be cut with a knife. Nonmetals are not ductile, or malleable; instead, they break instead of bending so they are classified as brittle.

The chemical properties of nonmetals result from the fact that they have five, six, or seven electrons in their outermost shell. Nonmetals have a tendency to gain electrons to fill up the outermost level.

Generally, there is an energy change that occurs when a chemical reaction takes place. If heat must be added to the substance(s) in order for the reaction to occur, this change in energy is called endothermic. The baking of a cake or bread or heating the iron filing and sulfur to produce iron sulfide are examples of such a reaction. If heat is released it is a result of the chemical reaction, this energy change is called exothermic. Adding an acid to a metal, freezing of water, explosion of TNT are all examples of exothermic reactions.

1. To illustrate endothermic and exothermic reactions:
a. Endothermic-Dissolve ammonium chloride (NH₄Cl) in a small beaker containing about 15 ml of H₂O. Record starting temperature of H₂O before NH₄Cl is added and then temperature of solution after NH₄Cl is added.

b. Exothermic-Do same procedure as above except add sodium hydroxide pellets to the NaOH (Caution: NaOH is extremely caustic)

Disposal of NaOH may be done by adding vinegar (acetic acid) until the pH is about 7.

2. To illustrate corrosion:

Leave a wet SOS pad exposed to the air for about one week.

Worksheet of Equations to balance

1. HgO-->Hg+O₂

2. H₂O-->H₂+O₂

3. AI+Pb(NO₃)₂-->AI(NO₃)₃+Pb

4. K+H₂O--->KOH+H₂

5. Cl₂+LiI-->LiCl+I₂

6. AgNO₃+K₂SO₄-->Ag₂SO₄+KNO₃

7. NH₃+O₂-->NO+H₂O

8. SO₃-->SO₂+O₂

9. Mg(OH)₂+(NH₄)3PO₄-->Mg₃(PO₄)2+NH₃+H₂O

10. BaCO₃+HCl-->BaCl₂+H₂CO₃

11. Fe+AgC₂H₃O₂-->Fe(C₂H₃O₂)₂+Ag

12. KClO₃-->KCl+O₂

13. Mg+O₂-->MgO

14. Na₂O+H₂O-->NaOH

15. Zn+HCl-->ZnCl₂+H₂

The study of chemical reactions through the study of equations can be very exciting. You can observe how the law of conservation of mass works and how endothermic and exothermic reactions differ.

Reactants-starting substances in the reaction

Products-substances formed as a result of a chemical reaction

Synthesis reaction-the combination of two or more substances to produce one product

Decomposition-the breaking down of one compound into two or more components

Single replacement reaction-a reaction involving one element and one compound where the single element replaces an element in the compound to produce a new compound and that element

Double replacement reaction-a reaction in which the positive metallic ions in both compounds switch places to produce two new compounds

Law of conservation of mass-Atoms are neither created nor destroyed in a chemical reaction as a result of a chemical change

Subscript-a number written to the right and slightly below the symbol of an element to indicate the number of atoms of that element

Coefficient-a number written in front of the formula for an element or compound to indicate the number of atoms of that element or the number of molecules of that compound. This number multiplies the number of atoms of each element in the compound. For example 2H₂O = 4 Hydrogen atoms and 2 Oxygen atoms.

Luster-shininess

Ductile-ability of a substance to be drawn into a wire

Malleable-ability of a substance to be hammered into sheets

Corrosion-a chemical reaction which results in the wearing away of the metal to produce a metallic compound

Metal-element with 1, 2, or 3 electrons in the outermost shell; has a tendency to lose electrons when combining with other elements to form compounds

Nonmetal-element with 5, 6, or 7 electrons in the outermost shell, has a tendency to gain electrons when combining with other elements to form compounds

Endothermic-a reaction which requires the addition of heat in order to occur. Heat or some other form of energy must be absorbed

Exothermic-a reaction in which heat or some form of energy is released

This is the time this file has been accessed since 04/04/98.

The University of Tennessee at Martin is not responsible for the information or views expressed here.

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