O) and carbon dioxide gas (CO 2

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1 Have you ever tried to balance a book on the top of your head while walking across a room? How do you compensate if you begin to feel the books sliding to one side? You might lean in the opposite direction or hold out your hands to steady the book. When a chemical reaction occurs, one or more substances is converted into another. A chemical reaction can be explained by writing a chemical equation. Chemical equations require balance of a different kind. What does it mean to balance a chemical equation? How does balancing a chemical equation reflect what happens during a chemical reaction? Writing All chemical reactions involve reactants and products. Reactants are the starting substances: the substances before the reaction happens. Reactants are normally written on the left side of a chemical equation. Products are the resulting substances: the substances after the reaction happens. Products are normally written on the right side of a chemical equation. In a chemical equation, the reactants are separated from the products with an arrow showing the direction of the chemical reaction. Therefore, in a typical chemical equation, the arrow points from left (the reactants) to right (the products). The chemical equation below shows what happens when you open a can of soda. Soda contains liquid water ( O) and carbon dioxide gas (CO 2 ). (This is why soda is also called carbonated water.) The pressure inside a sealed can causes the water and carbon dioxide to combine into a substance called carbonic acid ( CO 3 ). Opening a soda can releases this pressure. As a result, the carbonic acid undergoes a chemical reaction and is converted back into liquid water and carbon dioxide gas. (You see the CO 2 escaping into the air as fizz.) CO 3 (aq) O(l) + CO 2 (reactants) (products) The letters in parentheses next to the chemical formula for each molecule describe the physical state of each substance; they are called state symbols. For example, (aq) stands for aqueous and refers to a substance in solution; in addition, (s) refers to a solid, (l) refers to a liquid, and refers to a gas. Conservation of Mass in Chemical Reactions All chemical equations must follow the law of conservation of mass, which states that mass is neither created nor destroyed during chemical or physical changes. The law of conservation of mass describes all non-nuclear reactions. (As you learn in the companion Nuclear Chemistry, during a nuclear reaction the nucleus of an atom changes.) 1

2 Let s look at an example to demonstrate how mass is conserved during a typical chemical reaction. When calcium carbonate (CaCO 3 ) is heated, it decomposes into calcium oxide (CaO) and carbon dioxide (CO 2 ). The following equation describes this reaction: CaCO 3 (s) CaO(s) + CO 2 (reactants) (products) To determine the mass of each molecule, add the average atomic masses of each element in the chemical formula. You can find this information (measured in atomic mass units, or amu) in the periodic table. (For the sake of convenience, we shall round each value to the nearest whole number.) Calcium (Ca): 40 amu Carbon (C): 12 amu Oxygen (O): 16 amu Average atomic mass (measured in amu) is equivalent to molar mass (measured in grams per mole, or g/mol). For the remainder of this section, we shall write our calculations in g/mol. Now, find the sum of the masses of all the elements in one molecule of the reactant, calcium carbonate. (Remember that one molecule of CaCO 3 contains three atoms of oxygen.) 40 g/mol + 12 g/mol + (16 g/mol 3) = 100 g/mol (calcium) (carbon) (oxygen) Compare the mass of the reactant to the combined masses of the products, calcium oxide and carbon dioxide. (One molecule of CO 2 contains two atoms of oxygen.) (40 g/mol + 16 g/mol) + [12 g/mol + (16 g/mol 2)] = 56 g/mol + 44 g/mol (calcium oxide) (carbon dioxide) In other words, decomposing 100 g of calcium carbonate produces 56 g of calcium oxide and 44 g of carbon dioxide, and 56 g plus 44 g equals 100 g. The total mass of the reactants equals the total mass of the products. Mass is conserved. Balancing During a chemical reaction, the atoms in one group of molecules (the reactants) are rearranged to form new molecules (the products). According to the law of conservation of mass, the same atoms must be present in both reactants and products. Therefore, in a chemical equation, the same atomic symbols must appear to the left of the arrow and to the right of the arrow. Take another look at the chemical reaction from the previous example, in which calcium carbonate decomposes into calcium oxide and carbon dioxide: CaCO 3 (s) CaO(s) + CO 2 2

3 On the reactants side of the equation, there is one atom of calcium (Ca), one atom of carbon (C), and three atoms of oxygen (O). Likewise, on the products side of the equation, there is one calcium atom, one carbon atom, and three oxygen atoms. The equation is balanced. This is a relatively straightforward reaction. One molecule of reactant equals one molecule of one product and one molecule of another product. Most reactions are more complicated, however, involving different amounts of molecules on both sides of the equation. To balance the equation for such a reaction, you will need to use coefficients: numbers placed in front of a chemical formula to indicate the number of specific molecules present during a reaction. Let s look at an example involving the combustion (burning) of propane gas. In this reaction, propane ( ) reacts with oxygen (O 2 ) to produce water vapor ( O) and carbon dioxide (CO 2 ): + O 2 O + CO 2 The combustion of propane allows this steak to be cooked on a grill. How can we tell if this equation is balanced? Begin by counting the atoms of each element on both sides of the equation. You can do this by making a table, as shown on the following page. How many atoms in reactants? How many atoms in products? Are the atoms balanced? carbon (C) 3 1 no hydrogen (H) 8 2 no oxygen (O) 2 3 no We will need to use coefficients to balance this equation. Where should we begin? Balancing an equation often involves trial and error, but here are some guidelines: 1. First try to balance elements that appear in only one molecule on each side of the equation. 2. If several elements appear in only one molecule, begin with the element that has the fewest number of atoms. Let s take another look at the equation for the combustion of propane: + O 2 O + CO 2 3

4 Both carbon (C) and hydrogen (H) appear in one reactant molecule and one product molecule. Because the unbalanced equation contains fewer carbon atoms, let s begin with this element. The reactants side of the equation contains 3 carbon atoms, and the products side contains 1 carbon atom. To balance the carbon atoms, add a coefficient of 3 to the CO 2 molecule on the products side of the equation: + O 2 O + 3CO 2 This coefficient means the products side of the equation contains 3 molecules of CO 2. As a result, both sides of the equation now contain 3 carbon atoms. In addition, the products side of the equation now contains 7 oxygen atoms. ( O contains 1 oxygen atom, and 3CO 2 contains 6 oxygen atoms.) Because it appears in several products, however, let s wait to balance oxygen until we have balanced hydrogen. The reactants side of the equation contains 8 hydrogen atoms, and the products side contains 2 hydrogen atoms. To balance the hydrogen atoms, add a coefficient of 4 to the O molecule on the products side of the equation: + O 2 4 O + 3CO 2 This coefficient means the products side of the equation now contains 4 molecules of O. As a result, both sides of the equation now contain 8 hydrogen atoms. In addition, the products side of the equation now contains 10 oxygen atoms. (4 O contains 4 oxygen atoms and 3CO 2 contains 6 oxygen atoms.) We may now balance oxygen. The reactants side of the equation contains only 2 oxygen atoms. Fortunately, they appear together in a molecule with no other atoms (O 2 ). We can add a coefficient of 5 to O 2 without affecting the balance of other atoms in the equation. Here is the balanced equation for the combustion of propane: + 5O 2 4 O + 3CO 2. To confirm, count the atoms of each element on both sides of the equation. The products side contains 3 carbon atoms, 8 hydrogen atoms, and 10 oxygen atoms. The reactants side also contains 3 carbon atoms, 8 hydrogen atoms, and 10 oxygen atoms. The equation is balanced. If after working with each element, and the equation still is not balanced, you will need to change your coefficients. As before, begin with the element that appears in the fewest number of atoms. Continue to try new combinations of coefficients until you have balanced the equation. 4

5 Do not confuse coefficients with subscripts. When you change a subscript in a chemical formula, you are changing the type of molecule involved in the reaction. NEVER change subscripts to balance a reaction! When you change a coefficient in a chemical formula, you are changing the number of molecules involved in the reaction. For example, consider the reaction of oxygen gas (O 2 ) and hydrogen gas ( ) to produce water vapor ( O). + O 2 O The products side of the equation contains 2 hydrogen atoms and 2 oxygen atoms. The reactants side of the equation contains 2 hydrogen atoms and 1 oxygen atom. What happens if you add a subscript of 2 to the oxygen atom in the product? + O 2 O 2. By changing the subscript, you have changed the molecule. Instead of water ( O), the product is now hydrogen peroxide ( O 2 ). Instead, add a coefficient of 2 in front of the water molecule: + O 2 2 O By changing the subscript, you have changed the number of molecules. The product now contains two molecules of water ( O). Changing a subscript may not seem like a big deal, but there is a big difference between a molecule of water (left) and a molecule of hydrogen peroxide (right). Take another look at the equation we just considered: + O 2 2 O Is it balanced? If not, how do you balance it? You can check your answer on the next page. 5

6 Scientists in the Spotlight: Antoine Lavoisier In the late 1700s, Antoine Lavoisier was a French chemist who made many discoveries and was responsible for many changes in the way people viewed chemistry. He named the elements oxygen and hydrogen. He also discovered that sulfur is an element. However, one of his greatest achievements was to confirm the law of conservation of mass. To demonstrate that mass is conserved in a chemical reaction, Lavoisier studied the combustion of metals in a closed system. Prior studies of metal combustion showed that metals gained weight during the reaction. Lavoisier found that during metal combustion, the weight gained by the metal is from oxygen in the air reacting and combining with the metal. Using a closed system, Lavoisier was able to demonstrate that the mass of the products equals the mass of the reactants. In other words, mass is conserved. Lavoisier lived during the French Revolution, and though he supported many of its reforms, he made several enemies of high-ranking revolutionaries. In May of 1794, Lavoisier was arrested, convicted of treason, and sentenced to death. He was executed by the guillotine and buried in a common grave. Is this equation balanced? + O 2 2 O The answer is no. The products side contains 2 hydrogen atoms and 2 oxygen atoms, but the reactants side contains 4 hydrogen atoms and 2 oxygen atoms. To balance the equation, add a coefficient of 2 in front of the hydrogen gas molecule: 2 + O 2 2 O What do you know? Chemical equations are balanced when they have the same number and type of atoms on both the reactants side and products side of the equation. All of the equations in the first column, below, are unbalanced. Balance each equation by adding a coefficient in front of each molecule in the blanks labeled (a), (b), (c), and (d). (Not every equation has a (d).) If no coefficient is needed, write none in the appropriate cell. The first equation has been balanced for you. Chemical Equation (a) (b) (c) (d) (a)n 2 + (b) (c)nh 3 none (a)kclo 3 (b)kcl + (c)o 2 -- (a)ch 4 + (b)o 2 (c)co 2 + (d) O (a)s 8 + (b)o 2 (c)so 3 -- (a)ag 2 O (b)ag + (c)o 2 -- (a)na 2 SiF 6 + (b)na (c)si + (d)naf 6

7 Equations for Everyday Reactions There are several different categories of chemical reaction. Reviewing the chemical equations for common reactions in each category is an excellent way to practice balancing equations. Here are five important reaction types: combination, decomposition, combustion, single-replacement, double-replacement. Provide your child with the unbalanced form of each equation below (remove the coefficients), and challenge your child to balance the equation. Combination Reactions: Two or more reactants combine to form a larger product. This is also called a synthesis reaction. An important combination reaction is photosynthesis, the reaction that occurs in plants to convert water and carbon dioxide into glucose and oxygen gas. Here is the balanced equation: 6 O(l) + 6CO 2 C 6 H 12 O 6 (s) + 6O 2 Decomposition Reactions: A larger reactant breaks down into two or more products. A decomposition reaction happens inside an automobile s airbag: a substance called sodium azide breaks down into sodium and nitrogen gas, which inflates the airbag: 2NaN 3 (s) 2Na(s) + 3N 2 Combustion Reactions: All combustion reactions involve oxygen as a reactant and release heat. (Burning is a type of combustion the process of burning always involves oxygen and releases heat.) Water is a common product in many combustion reactions; for example, a burning candle produces water vapor. Rusting is another example of a combustion reaction. Here is the balanced equation for the formation of rust on iron, shown in the photograph at right: 4Fe(s) + 6 O(l) + 3O 2 4Fe(OH) 3 (s) Single-Replacement Reactions: In a typical single-replacement reaction, the reactants include a compound and a single atom. During the reaction, the single atom changes places with one of the ions in the compound. In other words, a single-replacement reaction looks like this: AX + B BX + A The reaction of aluminum and copper(ii) chloride to form copper and aluminum chloride is an example of a single-replacement reaction: 3CuCl 2 + 2Al 2AlCl 3 + 3Cu 7

8 Double-Replacement Reactions: In a double-replacement reaction, two reactants appear to exchange ions as they form two products. In other words, a double-replacement reaction looks like this: AX + BY AY + BX The reaction of lead nitrate and potassium iodide, shown in the photograph at right, is an example of a double-replacement reaction. Notice how the nitrate molecule (NO 3 ) and the iodide ion (I ) appear to change places in this equation: Pb(NO 3 ) 2 (aq) + 2KI(aq) PbI 2 (s) + 2KNO 3 (aq) Here are some questions to discuss with your child: Is this equation balanced or unbalanced? How do you know? Which element in this equation should you try to balance first? Why? What are some other examples of each type of chemical reaction? (You may need to research this question on the Internet or in a chemistry textbook.) 8