Chapter 7, Section 2 Key Concept: The masses of reactants and products are equal. BEFORE, you learned Chemical reactions turn reactants into products by rearranging atoms Chemical reactions can be observed and identified The rate of chemical reactions can be changed NOW, you will learn About the law of conservation of mass How a chemical equation represents a chemical reaction How to balance a simple chemical equation THINK ABOUT What happens to burning matter? 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. In many other chemical reactions, mass also appears to decrease. That is, the mass of the products appears to be less than the mass 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-vwahzeeay) 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. An example of Lavoisier s work is his study of the reaction of the metal mercury when heated in air. In this reaction, the reddishorange 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 conducted many experiments of this type and found in all cases that the mass of the reactants is equal to the mass of the products. 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? 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 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 + O2 CO2 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. 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 and water. You can write this reaction as the following equation. Unbalanced Equation CH 4 + O 2 CO 2 + H 2 O
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 has a subscript of 2, which means there are two hydrogen atoms. Also, there are two oxygen atoms on the left and three oxygen atoms on the right. Because of the conservation of mass, you know that hydrogen atoms do not disappear and oxygen atoms do not suddenly appear. You can balance a chemical equation by changing the amounts of reactants or products represented. To balance H first, add another H2O 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 O2 molecule on the left. Balanced Equation CH 4 + O 2 CO 2 + H 2 O C 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 equation 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 coefficients. 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. Balanced Equation with Coefficients CH 4 + 2O 2 CO 2 + 2H 2 O Chemical formulas can have both coefficients and subscripts. 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 cannot 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 represents 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. Check Your Reading Why are coefficients used to balance equations? Questions for Chapter 7, Section 2 KEY CONCEPTS 1. State the law of conservation of mass. 2. Write the chemical equation that shows sodium (Na) and chlorine (Cl2) combining to form table salt (NaCl). 3. Is the following equation balanced? Why or why not? CO C + O2 CRITICAL THINKING 4. Communicate Describe Lavoisier s experiment with mercury. How does this experiment show the law of conservation of mass? 5. Synthesize Suppose a log s mass is 5 kg. After burning, the mass of the ash is 1 kg. Explain what may have happened to the other 4 kg of mass. CHALLENGE 6. Synthesize Suppose a container holds 1000 hydrogen molecules (H2) and 1000 oxygen molecules (O2) that
react to form water. How many water molecules will be in the container? Will anything else be in the container? If so, what?