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KEY CONCEPT The masses of reactants and products are equal.

BEFORE, you learned NOW, you will learn • Chemical reactions turn • About the law of conservation reactants into products by of mass rearranging atoms • How a chemical equation • Chemical reactions can be represents a chemical reaction observed and identified • How to balance a simple • The rate of chemical reactions chemical equation can be changed

VOCABULARY THINK ABOUT law of conservation What happens to burning matter? of mass p. 79 coefficient p. 82 You have probably watched a fire burn in a fireplace, a campfire, or a candle flame. It looks as if the wood or candle disappears over time, leaving a small pile of ashes or wax when the fire has finished burning. But does matter really disappear? Combustion is a chemical reaction, and chemical reactions involve rearrangements of atoms. The atoms do not disappear, so where do they go?

Careful observations led to the discovery of the conservation of mass. The ashes left over from a wood fire contain less mass than the wood. COMBINATION NOTES In many other chemical reactions, mass also appears to decrease. Take notes on the That is, the mass of the products appears to be less than the mass conservation of mass using combination notes. of the reactants. In other reactions, the products appear to gain mass. For example, plants grow through a complex series of reactions, but where does their extra mass come from? At one time, scientists thought that chemical reactions could create or destroy matter. During the 1780s the French chemist Antoine Lavoisier (luh-VWAH- zee-ay) showed that matter can never be created or destroyed in a chemical reaction. Lavoisier emphasized the importance of making very careful measurements in his experiments. Because of his methods, he was able to show that reactions that seem to gain mass or lose mass actually involve reactions with gases in the air. These gases could not be seen, but their masses could be measured.

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An example of Lavoisier’s work is his study of the reaction of the metal mercury when heated in air. In this reaction, the reddish- orange product formed has more mass than the original metal. Lavoisier placed some mercury in a jar, sealed the jar, and recorded the total mass of the setup. After the mercury had been heated in the jar, the total mass of the jar and its contents had not changed. Lavoisier showed that the air left in the jar would no longer support burning—a candle flame was snuffed out by this air. He concluded that a gas in the air, which he called oxygen, had combined with the mercury to form the new product. Lavoisier carefully meas- ured both the reactants Lavoisier conducted many experiments of this type and found in all and the products of cases that the mass of the reactants is equal to the mass of the products. chemical reactions. This conclusion, called the law of conservation of mass, states that in a chemical reaction atoms are neither created nor destroyed. All atoms present in the reactants are also present in the products.

Check Your Reading How did Lavoisier investigate the conservation of mass?

Conservation of Mass SKILL FOCUS Why is it important to measure the masses of Measuring reactants and products?

PROCEDURE

1 Measure 2 tsp of baking soda. Use a funnel to put the baking soda in a balloon. MATERIALS • teaspoon 2 Pour 2 tsp of vinegar into the plastic bottle. • baking soda • funnel 3 Secure the balloon over the mouth of the bottle with the balloon • balloon hanging to the side of the bottle. Find and record the mass of the • vinegar experimental setup. • plastic bottle • balance 4 Lift the balloon so that the baking soda drops into the bottle. Observe for five minutes, and then find and record the mass TIME of the setup again. 35 minutes

WHAT DO YOU THINK? • Did the mass of the experimental setup change? • How do your observations demonstrate the conservation of mass?

CHALLENGE What do you think you would have observed if you had not used the balloon? Explain.

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Chemical reactions can be described by chemical equations. The law of conservation of mass states that in a chemical reaction, the total mass of reactants is equal to the total mass of products. For example, the mass of sodium plus the mass of chlorine that reacts with the sodium equals the mass of the product sodium chloride. Because atoms are rearranged in a chemical reaction, there must be the same number of sodium atoms and chlorine atoms in both the reactants and products. Chemical equations represent how atoms are rearranged in a chemical reaction. The atoms in the reactants are shown on the left side of the equation. The atoms in the products are shown on the right side of the equation. Because atoms are rearranged and not created or destroyed, the number of atoms of each different element must be the same on each side of the equation.

Check Your Reading How does a chemical equation show the conservation of mass?

In order to write a chemical equation, the information that you need to know is • the reactants and products in the reaction • the atomic symbols and chemical formulas of the reactants and products in the reaction Carbon dioxide is a gas that animals exhale. • the direction of the reaction The following equation describes the formation of carbon dioxide from carbon and oxygen. In words, this equation says “Carbon reacts with oxygen to yield carbon dioxide.” Notice that instead of an equal sign, an arrow appears between the reactants and the products. The arrow shows which way the reaction proceeds—from reactants on the left to the product or the products on the right. reactants direction of reaction product C + O 2 CO 2 Remember, the numbers below the chemical formulas for oxygen and carbon dioxide are called subscripts. A subscript indicates the number of atoms of an element in a molecule. You can see in the equation above that the oxygen molecule has two oxygen atoms, and the carbon dioxide molecule also has two oxygen atoms. If the chemical formula of a reactant or product does not have a subscript, it means that only one atom of each element is present in the molecule.

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Chemical equations must be balanced. Remember, chemical reactions follow the law of conservation of mass. Chemical equations show this conservation, or equality, in terms of atoms. The same number of atoms of each element must appear on both sides of a chemical equation. However, simply writing down the chemical formulas of reactants and products does not always result in equal numbers of atoms. You have to balance the equation to make the number of atoms equal on each side of an equation.

Balancing Chemical Equations To learn how to balance an equation, look at the example of the combustion of natural gas, which is mostly methane (CH 4). The reactants are methane and oxygen. The products are carbon dioxide reminder and water. You can write this reaction as the following equation. Oxygen is always a reactant in a combustion reaction. Unbalanced Equation

CH 4 +O2 CO 2 + H2 O C H O C O H O

H O O H

H

H

This equation is not balanced. There is one C on each side of the equation, so C is balanced. However, on the left side, H has a subscript

of 4, which means there are four hydrogen atoms. On the right side, H reading tip has a subscript of 2, which means there are two hydrogen atoms. Also, As you read how to balance there are two oxygen atoms on the left and three oxygen atoms on the the equation, look at the illustrations and count the right. Because of the conservation of mass, you know that hydrogen atoms. The number of each atoms do not disappear and oxygen atoms do not suddenly appear. type of atom is shown below the formula. You can balance a chemical equation by changing the amounts of reactants or products represented.

• To balance H first, add another H 2O molecule on the right. Now, both C and H are balanced. • There are now two oxygen atoms on the left side and four oxygen atoms on the right side. To balance O, add another O 2 molecule on the left.

Balanced Equation

CH 4 ++O2 O2 CO 2 ++H2 O H 2 O

C H O O C OH O H O

H O O O H H

H

H

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Using Coefficients to Balance Equations The balanced equation for the combustion of methane shows that one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water. The equa- tion can be simplified by writing 2O 2 instead of O 2 + O 2, and 2H 2O instead of H 2O + H 2O. The numbers in front of the chemical formulas are called coeffi- cients.Coefficients indicate how many molecules take part in the reaction. If there is no coefficient, then only one molecule of that type takes part in the reaction. The balanced equation, with coefficients, for the combustion of methane is shown below.

reminder Balanced Equation with Coefficients A subscript shows the + + number of atoms in a CH 4 2O 2 CO 2 2H 2O molecule. If a subscript is coefficient subscript changed, the molecule represented by the formula Chemical formulas can have both coefficients and subscripts. is changed. In these cases, multiply the two numbers together to find the number of atoms involved in the reaction. For example, two water molecules (2H 2O) contain 2 • 2 = 4 hydrogen atoms and 2 • 1 = 2 oxygen atoms. Remember, coefficients in a chemical equation indicate how many molecules of each type take part in the reaction. Only coefficients can be changed in order to balance a chemical equation. Subscripts are part of the chemical formula for reactants or products and cannot be changed to balance an equation. Changing a subscript changes the substance represented by the formula. For example, the equation for the combustion of methane can- not be balanced by changing the formula CO 2 to CO. The formula CO 2 represents carbon dioxide gas, which animals exhale when they breathe. The formula CO repre- sents carbon monoxide gas, which is a very different compound from CO 2. Carbon monoxide gas is poisonous, and breathing too much of it can be fatal. The combustion of Check Your Reading Why are coefficients used to balance equations? methane (CH 4) is used to melt glass.

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Balancing Equations with Coefficients

The steps below show how to balance the equation for the synthesis reaction between nitrogen (N 2) and hydrogen (H 2), which produces ammonia (NH 3).

1 Count the atoms. Neither N nor H is balanced. The reactants contain two atoms Tip: Listing the number of each of N and H, but the atoms of each element product contains one N makes it easy to see which atom and three H atoms. elements must be balanced.

2 Use a coefficient to add Tip: When adding coeffi- atoms to one side of the cients, start with the equation. A coefficient of 2 reactant or product that on NH balances the number 3 contains the greatest num- of N atoms. ber of different elements.

3 Add a coefficient to another reactant or Tip: Make sure that the product. Adding a coefficients in your bal- coefficient of 3 to H 2 anced equation are the on the left side of the smallest whole numbers equation balances the possible—that is, they number of H atoms on have no common factor both sides. Now the other than 1. equation is balanced.

APPLY Balance the following equations. 1. Hg + O 2 HgO 2. Zn + HCl ZnCl 2 + H 2

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Using the Conservation of Mass A balanced chemical equation shows that no matter how atoms are rearranged during a chemical reaction, the same number of atoms must be present before and after the reaction. The following example demonstrates the usefulness of chemical equations and the conserva- tion of mass.

The decomposition of sodium azide (NaN 3) is used to inflate automobile air bags. Sodium azide is a solid, and the amount of sodium azide needed in an air bag fills only a small amount of space. In fact, the amount of sodium azide used in air bags is only about

The decomposition of 130 grams—an amount that would fit in a large spoon. An inflated sodium azide is used air bag, though, takes up much more space even though it contains to inflate air bags in the same number of atoms that entered the reaction. The reason is automobiles. illustrated by the chemical equation for this reaction.

Balanced Equation

2NaN3 2Na + 3N 2 According to the balanced equation shown above, three molecules of nitrogen gas are formed for every two molecules of sodium azide that decompose. Because the nitrogen is a gas, it fills a much greater volume than the original sodium azide. In fact, 67 liters of nitrogen gas are produced by the 130 grams of sodium azide in the reaction. This amount of nitrogen is enough to quickly inflate the air bag during a collision—the decomposition of sodium azide to sodium and nitrogen takes 0.03 seconds.

Check Your Reading Why must chemical equations be balanced?

KEY CONCEPTS CRITICAL THINKING CHALLENGE 1. State the law of conservation 4. Communicate Describe 6. Synthesize Suppose a of mass. Lavoisier’s experiment with mer- container holds 1000 hydrogen 2. Write the chemical equation cury. How does this experiment molecules (H 2) and 1000 that shows sodium (Na) and show the law of conservation oxygen molecules (O 2) that of mass? react to form water. How many chlorine (Cl 2) combining to form table salt (NaCl). 5. Synthesize Suppose a log’s water molecules will be in the container? Will anything else 3. Is the following equation mass is 5 kg. After burning, the be in the container? balanced? Why or why not? mass of the ash is 1 kg. Explain what may have happened to If so, what? CO C + O 2 the other 4 kg of mass.

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