Download 1. What is a Chemical Reaction?

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What are Chemical Reactions?
1.
What is a Chemical Reaction?
YOU WILL LEARN:
o To identify and describe evidence (clues) that a chemical
reaction has occurred.
Lesson Summary
• A chemical reaction is the process by which atoms of one or more substances are
rearranged to form different substances(s) with new chemical and physical properties.
• A chemical reaction is another name for a chemical change.
• When substances chemically react, observations can be made that provide evidence or
clues that chemical changes are taking place.
• Evidence of chemical reactions include the formation of a gas, formation of a solid,
color change, or an energy change (energy can be taken in or given off).
Example of a chemical reaction:
2H2 + O2  2H2O
Read this as:
Two molecules of hydrogen plus one molecule of oxygen react to produce
molecules of
.
Fill in the missing items. Circle the symbols in the chemical reaction that represent the bold
words in the sentence above.
For each example below, write the evidence (or clue) that might indicate that a
chemical reaction has occurred.
Example
Evidence (clue)
Iron rusts
Alka seltzer is added to water
A hand warmer is taken out of its
package
Dry papers catch on fire
Two liquids are combined, a solid
forms and falls to the bottom of
the container
Vinegar is added to baking soda
Concepts and Challenges in Physical Science, Teacher’s Resources CD-ROM
(c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved.
Energy and Work: CHAPTER 14
Lesson Notes, page 1
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2.
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What is a Chemical Equation?
YOU WILL LEARN:
o To identify the reactants and products of a chemical equation.
o To determine whether a given chemical equation is balanced.
o To describe the Law of Conservation of Mass (matter cannot
be created or destroyed).
Lesson Summary
• A chemical equation is a statement that uses chemical formulas and coefficients to
describe a chemical reaction.
• Chemical equations are understood by chemists all over the world, regardless of their
native language.
• A chemical formula tells the composition of a compound using chemical symbols and
subscripts. Example: H2O
The letters represent the type of element, the #2
(subscript) indicates how many atoms of the element are in the substance. If no
subscript is present, then it means there is only 1 atom of that element in the substance.
• The reactants are the starting substances in the chemical reaction.
• The products are the substances formed in a chemical reaction.
Reactants (starting substances) are located to the left of the arrow in a chemical equation.
Products (substances produced) are located to the right of the arrow in a chemical equation.
(react to produce)
Reactants
Products
The Law of Conservation of Mass states that matter cannot be created or destroyed during a
chemical reaction. This means that the mass of the reactants must equal the mass of the
products.
Mass of Reactants = Mass of Products
Example: If you start with 80.2 g of reactants, you will produce
If you produced 200 g of product, that means you started with
g of products.
g of reactants.
What this also means is that the number atoms and type of element that you start with must
equal the number of atoms and type of elements that you produce. The chemical equation is
Concepts and Challenges in Physical Science, Teacher’s Resources CD-ROM
(c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved.
Energy and Work: CHAPTER 14
Lesson Notes, page 2
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then said to be balanced. Check the chemical equation below to see if it is a “balanced”
chemical equation.
2H2 + O2  2H2O
3.
What are exothermic and endothermic chemical reactions?
YOU WILL LEARN:
o To identify and describe an exothermic chemical reaction.
o To identify and describe an endothermic chemical reaction.
Lesson Summary
• A chemical reaction ALWAYS involves a change in energy.
• An EXOTHERMIC chemical reaction is when energy is given off or exits when the
reaction occurs. This can be in the form of heat or light.
• An ENDOTHERMIC chemical reaction is when energy is absorbed or enters when the
reaction occurs. This is when the reaction feels cooler in temperature.
Examples:
a. Athletic cold pack - exothermic or endothermic (circle one)? Why?
b. Hand Warmer – exothermic or endothermic (circle one)? Why?
c. Glowstick – exothermic or endothermic (circle one)? Why?
4.
What is the RATE OF REACTION?
What are some factors that may affect the speed of a chemical reaction?
Concepts and Challenges in Physical Science, Teacher’s Resources CD-ROM
(c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved.
Energy and Work: CHAPTER 14
Lesson Notes, page 3