11.1 Describing Chemical Reactions Read the lesson title aloud to students.

Chemical Reactions Reactants form products in chemical reactions. Remind students that chemical reactions form new substances. Review with them the meanings of the terms reactant and product. Have students identify the reactants and products for all three images. Ask: What substance is common to all three reactions? Answer: oxygen Ask: Does oxygen function the same way in each reaction? Explain. Answer: No; in the rusting and burning, it is a reactant, and in the peroxide reaction, it is a product. Ask: What evidence do the photographs show of the presence of oxygen? Answer: Oxygen is available in the air, and oxygen formation can be inferred from the gas bubbles in the photograph for the peroxide reaction. Explain to students that many reactions occur without being able to see the substances react, but chemists use indirect evidence, such as color changes and gas formation, as evidence of a reaction.

Word Equations A word equation identifies the reactants and products of a chemical reaction. Methane and oxygen combine to make water and carbon dioxide. Have students identify the reactants and the product in the rusting reaction (car). Click to reveal the word equation for the reaction. Ask a student to write the word equation for the methane burning reaction (gas stove) on the board. Click to reveal the correct word equation. Ask a student to write the word equation for the hydrogen peroxide equation (injury) on the board. Hydrogen peroxide decomposes to form oxygen and water. Iron reacts with oxygen to yield iron(III) oxide.

CH4(g) + O2(g)  CO2(g) + H2O(g) Chemical Equations Chemical equations use chemical formulas to represent reactants and products. Letters in parenthesis tell us the state of matter the chemicals are in. CH4(g) + O2(g)  CO2(g) + H2O(g) Have students identify the chemical formulas for the reactants and products of each reaction. Click to reveal the chemical equation for the rusting reaction. Point out the use of the state-of-matter indicators. Ask a student to write the equation for the methane burning reaction on the board. Click to reveal the correct chemical equation for the methane burning reaction. Ask a student to write the equation for the hydrogen peroxide reaction on the board. Click to reveal the correct equation for the hydrogen peroxide reaction. H2O2(l)  H2O(l) + O2(g) Fe(s) + O2(g)  Fe2O3(s)

Catalysts & coefficients Catalysts are substances that speed up a reaction, but are not part of the reaction. It is placed over the arrow in the chemical reaction. Coefficients are numbers in front of the element or compound which are used to balance the reaction. The subscripts DO NOT change when balancing reactions.

Symbols Used in Chemical Equations Review the different types of symbols that may be used in chemical equations. Ensure students understand each of the different symbols.

Balancing Equations The number and type of all atoms must be the same in the reactants and products. A skeleton equation is one that is not yet balanced. Review with students the law of conservation of mass. Emphasize that mass, or matter, cannot be created or destroyed. Point out that in a chemical reaction, atoms are neither created nor destroyed; they are merely rearranged from their positions in the reactants to the products. Because of this, all of the atoms in the reactants must be accounted for in the products. A chemical equation is not complete until it is written as a balanced equation. Ask: Why does a skeleton equation describe an unbalanced reaction? Answer: The proper amounts of each substance have not been determined yet. Ask: What role does mass play in balancing a chemical equation? Answer: Atoms have mass. According to the law of the conservation of mass, any atom that is present in a reactant must also be present in a product for the masses to balance. Point out the number of carbon and oxygen atoms in the reactants and products for the carbon dioxide equation. Ask: Is this equation balanced? Answer: Yes. Click to reveal the skeleton equation for the formation of water. Ask: Is this equation balanced as written? Answer: No. If necessary, count the number of each type of atom with students. Ask a volunteer to identify what must be done to balance the equation. Click to reveal the balanced equation. Emphasize to students that equations can be balanced only by adding coefficients to the reactants and products. The chemical formulas themselves cannot be changed.

Balancing a Chemical Equation Balance the chemical equation for the reaction of aluminum (Al) with oxygen (O2) to form aluminum oxide (Al2O3): Review with students how to convert a problem statement into a chemical equation. Click to reveal the chemical equation for the reaction described. Ask: How many aluminum atoms are present in the reactants and products? Answer: 1 in reactants, 2 in products Click to show how to balance the aluminum atoms with the coefficient 2 on aluminum. Ask: Are the oxygen atoms balanced? Answer: No. Point out that there are 3 oxygen atoms in the products and only 2 in the reactants. Ask: What is the least common multiple of 2 and 3? Answer: 6 Explain that in this case, we must multiply the odd-numbered oxygen atoms by 2 to produce an even number. Click to reveal the equation showing the coefficient 2 on the aluminum oxide. Ask: What must be done to the equation now to balance the oxygen atoms? Answer: Multiply the oxygen gas in the reactants by 3. Click to reveal the equation with the oxygen multiplied by 3. Ask: Is the equation balanced? Answer: No, there are too many aluminum atoms in the products now. Explain that it is often necessary to keep adjusting the coefficients until they balance. Emphasize that as long as the ratios are maintained, there is never a danger of having coefficients that are too big. If all of the coefficients can be divided by a factor of 2, then they can all be reduced. Ask a volunteer to describe what must be done to balance the equation. Click to reveal the balanced equation.