Download Science24-UnitA-Section3.4

Science 24 - Section 3.4 - Chemical Equations
Name: ______________________________________
In Chapter 1 you studied chemical formulas. When chemical formulas are used to represent substances in a reaction,
you get a chemical equation. Chemical equations are used to represent all reactions. In mathematics you work with
equations. What you do to one side of the equation must be done to the other side to keep the equation balanced.
Similarly, in chemical equations you need have the same number of atoms on each side to balance the equation.
1. Previously, you studied the neutralization reaction that occurs when you combine vinegar and baking soda. If
you were able to measure the mass of the reactants and then the mass of products in this reaction, how do
you think these masses would compare?
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Law of Conservation of Mass
Experiments like the one involving vinegar and baking soda are similar to the experiments Antoine Lavoisier
performed. Lavoisier’s experiments led to the development of the Law of Conservation of Mass. In a chemical reaction
the mass of the reactants is always equal to the mass of the products.
Turn to page 52 of the textbook. Read introductory paragraphs of “Chemical Equations” and the information in “The
Law of Conservation of Mass.” In “Try This!” the statement “Allow the gas to escape” should read, “​Do NOT​ allow gas
to escape.”
2. What chemical did Antoine Lavoisier start with for his experiment?
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3. What were the products of Antoine Lavoisier’s experiment?
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4. How is the experiment Antoine Lavoisier performed similar to the baking soda and vinegar experiment
described in the reading?
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a. Name the law that describes the results of Antoine Lavoisier’s experiment. What does this law state?
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Staying Balanced
Chemists use chemical equations to represent chemical reactions. These equations must be balanced to show the
proper proportion of reactants and products. Balancing an equation means making sure an equation has the same
elements and the same number of atoms of each element on both sides. Read “Staying Balanced” on page 53 of the
textbook for further information on balanced chemical equations.
5. Why do chemists use a chemical equation rather than a word equation to represent a reaction?
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Let's first look at simple chemical equations. Read the information in “Math Connect: Checking the Balance of Simple
Equations” on page 53 of the textbook. Pay special attention to the steps for checking the balance.
6. Use the sample problem on page 54 of the textbook, 2 H​2​O​(l)​ → 2H​2​(g)​ + O​2​(g)​ to answer the following.
a. How many molecules of water are in the chemical equation? _________________
b. How many molecules of oxygen are on the right side of the equation? _________________
c. How many molecules of hydrogen are on the right side of the equation? _________________
d. How many atoms of each substance are on the left side of the equation? _________________
e. How many atoms of each substance are on the right side of the equation? _________________
f.
Is the equation balanced? Why? ___________________________________________________
Chemical Equation Balancing Worksheet
Answer questions 1 and 2 of “What Did You Find Out?”
7. Which reactions are not balanced? How do you know?
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8. Explain in your own words how this activity illustrates the Law of Conservation of Mass.
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Check Your Understanding
9. Look carefully at the equations below. Which ones are balanced? Show your thinking and any calculations.
a. Cl​2(g)​ + 2KI​(g)​ → 2KCl​(g)​ + I​2(s)
b. 3Li​(s)​ + 2H​2​O​(l)​ → 2LiOH​(l)​ + H​2(g)
c. Zn​(s)​ + H​2​SO​4(l)​ → ZnSO​4(s)​ + H​2(g)
10. In your own words, explain why equations must be balanced. Compare a balanced chemical equation to what
actually happens during the reaction.
11. Look back at the four reactions in the Find Out Activity above. Classify each reaction as a composition
reaction, decomposition reaction, combustion reaction, or neutralization reaction.