To React or Not to React! Answer Key
Instructions: Read the article. Answer all the questions on a separate sheet of paper. You may write on this article.
Chemical reactions cause chemical changes. They involve the breaking and making of chemical bonds. All chemical reactions involve
a change in substances and a change in energy. However, neither matter nor energy is created or destroyed in a chemical reaction.
The fact that matter is not created or destroyed in a chemical reaction is called the law of conservation of mass. In order for
chemical reaction equations to show that no matter is created or destroyed, chemical reaction equations must be balanced. In other
words, chemical reaction equations must show all the same atoms in the reactants and the products. They will just be arranged in
different ways to show that a chemical reaction has occurred. There are many ways atoms can be rearranged in compounds. Thus,
there are many types of chemical reactions. This article reviews five common chemical reactions.
Question 1: What is the law of conservation of mass?
Matter or mass is not created or destroyed as a result of a chemical reaction.
Question 2: Why do we balance chemical reaction equations?
Balancing a chemical reaction equation shows that no matter is created or destroyed. When the equation is balanced there
are the same number of atoms of each element on both sides of the arrow.
Question 3: What happens to chemical compounds in a chemical reaction?
The chemical bonds that hold compounds together are broken and rearranged in a chemical reaction.
SYNTHESIS REACTION: In a synthesis reaction two or more simple substances combine to form a more complex substance. In other
words, two or more reactants yield one product. For example, hydrogen gas combined with oxygen gas can produce a more complex
substance…water! The chemical equation for this synthesis reaction looks like:
reactant + reactant  product
In order to visualize a synthesis reaction look at the cartoon to the right. In the
cartoon, the skinny bird (reactant) and the worm (reactant) combine to make one
product, a fat bird.
Question 4: How could you identify a synthesis reaction?
In a synthesis reaction there are two or more reactants on the left side of the arrow. There is just one product on the right side
of the arrow. The reactants were combined to make the product.
Question 5: Find an example of a synthesis reaction in your worksheets or textbook. Explain why you picked the reaction that you did.
Answers will vary.
DECOMPOSITION REACTION: In a decomposition reaction a more complex substance breaks down into simpler parts. One reactant
yields 2 or more products. For example, water can be broken down into hydrogen gas and oxygen gas. The chemical equation for this
decomposition reaction looks like:
reactant  product + product
In order to visualize a decomposition reaction look at the cartoon to the right. In
this cartoon the egg (the reactant), which contained the turtle at one time, now
has opened and the turtle (product) and eggshell (product) are now two
separate substances.
Question 6: Compare synthesis and decomposition reactions. How are they similar? How are they different?
Both synthesis and decomposition reactions are types of chemical reactions. They both have one compound on one side of the arrow
and more than one compound on the other side. But, they are different because decomposition starts with one compound in the
reactants and breaks it apart. Synthesis is the opposite, starting with more than one reactant and putting them together.
Question 7: Find an example of a decomposition reaction in your worksheets or textbook. Explain why you picked the reaction that
you did.
Answers will vary.
SINGLE REPLACEMENT REACTION: In a single replacement reaction a single
uncombined element replaces another in a compound. Two reactants yield two products. For
example when zinc combines with hydrochloric acid, the zinc replaces hydrogen. The
chemical equation for this single replacement reaction looks like:
reactant + reactant  product + product
In order to visualize a single replacement reaction look at the cartoon to the right. Notice, the guy in the dark shirt steals the dance
partner of the guy in the white shirt. So, the lone reactant traded places with one of the atoms that were part of a compound.
Question 8: How could you identify a single replacement reaction?
In a single replacement reaction there are two reactants and two products. One of the reactants is a single element and it trades
places with one element in a compound.
Question 9: Find an example of a single replacement reaction in your worksheets or textbook. Explain why you picked the reaction
that you did.
Answers will vary.
DOUBLE REPLACEMENT REACTION: In a double replacement reaction parts of two compounds switch places to form two new
compounds. Two reactants yield two products. For example when silver nitrate combines with sodium chloride, two new compounds-silver chloride and sodium nitrate are formed because the sodium and silver switched
places. The chemical equation for this double replacement reaction looks like:
reactant + reactant  product + product
In order to visualize a double replacement reaction look at the cartoon to the right. In
the cartoon there are two different men with different hats on the reactants side. The
men switch hats.
Question 10: Compare single and double replacement reactions. How are they similar? How are they different?
Both single replacement and double replacement reactions involve two reactants and two products. They are both types of chemical
reactions involving a substance switching places with another. They are different because a single replacement starts with a
compound and a single element and ends with a compound and a single element, but a double-replacement has two compounds on
both sides of the arrow and a part of both compounds switch places.
Question 11: Find an example of a double replacement reaction in your worksheets or textbook. Explain why you picked the reaction
that you did.
Answers will vary.
COMBUSTION REACTION: Combustion means burning. In a combustion reaction a hydrocarbon and oxygen gas react to form
carbon dioxide and water. A hydrocarbon is a compound made primarily of carbon and hydrogen. For example, when propane reacts
with oxygen, carbon dioxide and water are formed. The chemical equation for this combustion reaction look like:
Question 12: How could you identify a combustion reaction?
In a combustion reaction the reactants are always some hydrocarbon (carbon and hydrogen compound) and oxygen. The products
are always carbon dioxide and water.
Question 13: Find an example of a combustion reaction in your worksheets or textbook. Explain why you picked the reaction you did.
Answers will vary.
ENERGY OF CHEMICAL REACTIONS: Chemical reactions always involve a change in energy because energy is required to break
and make chemical bonds. However, energy is neither created nor destroyed. Energy is absorbed or released in chemical reactions.
Chemical reactions in which energy is absorbed are called endothermic. This means that energy is required for the reaction to occur.
The energy absorbed is often heat energy or electrical energy. Chemical reactions in which energy is released are called exothermic.
The energy that is released was originally stored in the chemical bonds of the reactants. Often the heat given off causes the product(s)
to feel hot. Any reaction that involves combustion (burning) is an exothermic chemical reaction.