Describing Chemical Reactions

Transcription

1 Name Date Class Chemical Reactions Guided Reading and Study Describing Chemical Reactions This section explains how to show chemical reactions with symbols. It also states the principle of conservation of mass, and identifies three categories of chemical reactions. Use Target Reading Skills After you read the section, reread the paragraphs that contain definitions of Key Terms. Use all of the information you have learned to write a meaningful sentence using each Key Term. a. chemical equation: b. reactant: c. product: d. conservation of mass: e. open system: f. closed system: g. coefficient: _ h. synthesis: i. decomposition: _ j. replacement:

2 Chemical Reactions Name Date Class Chemical Reactions Guided Reading and Study What Are Chemical Equations? 1. What is a chemical equation? 2. Is the following sentence true or false? Chemical equations use symbols instead of words to summarize chemical reactions. 3. If a molecule of carbon dioxide is involved in a chemical reaction, how is it represented in the chemical equation for the reaction? 4. The substances you have at the beginning of a chemical reaction are called the. 5. The substances you have when a chemical reaction is complete are called the. 6. What do you read the arrow in a chemical equation as meaning? 7. Label each formula in the chemical equation below as either a reactant or a product. Fe + S FeS a. Fe b. S c. FeS 8. Circle the letter of each statement that is true about chemical equations. a. Chemical equations have no real structure. b. A chemical equation summarizes a reaction. c. The formulas for the reactants are written on the right. d. Symbols in the equation show the reactants and the products. Conservation of Mass 9. Is the following sentence true or false? All the atoms present at the start of a reaction are present at the end. 10. At the end of a chemical reaction, what is the total mass of the reactants compared to the total mass of the products?

3 Name Date Class Chemical Reactions Guided Reading and Study Describing Chemical Reactions (continued) 11. What is the principle called the conservation of mass? 12. Describe an open system. 13. What is an example of a closed system? Balancing Chemical Equations 14. When is a chemical equation balanced? 15. How many atoms of oxygen are there on each side of the following chemical equation: 2 Mg + O 2 2 MgO? 16. Circle the letter of each chemical equation that is balanced. a. H 2 + O 2 H 2 O b. Mg + O 2 MgO c. Na + O 2 Na 2 O d. 2 H 2 O 2 2 H 2 O + O A number placed in front of a chemical formula in a chemical equation is called a(n). 18. What does a coefficient tell you?

4 Chemical Reactions Name Date Class Chemical Reactions Guided Reading and Study 19. Tell why this chemical equation is not balanced: H 2 + O 2 H 2 O. 20. Write a balanced equation for this reaction: Oxygen reacts with hydrogen to yield water. Classifying Chemical Reactions 21. In what three categories can chemical reactions be classified? 22. Which category of chemical reactions comes from a term that means to put things together? 23. Complete the table about the three categories of chemical reactions. Categories of Chemical Reactions Category Description Example Chemical Equation a. Two or more substances combine to make a more complex compound. 2 SO 2 + O H 2 O H 2 SO 4 Decomposition b. 2 H 2 O 2 2 H 2 O + O 2 c. One element replaces another in a compound, or two elements in different compounds trade places. 2 CuO + C 2 Cu + CO 2

5 Name Date Class Chemical Reactions Guided Reading and Study Describing Chemical Reactions (continued) Classify each of the following equations as synthesis, decomposition, or replacement. 24. a. CaCO 3 CaO + CO 2 b. 2 Na + Cl 2 2 NaCl c. Mg + CuSO 4 MgSO 4 + Cu

6 Chemical Reactions Name Date Class Chemical Reactions Section Summary Describing Chemical Reactions Key Concepts What information does a chemical equation contain? What does the principle of conservation of mass state? What must a balanced chemical equation show? What are three categories of chemical reactions? A chemical equation is a short, easy way to show a chemical reaction. Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction. All chemical equations have a common structure. A chemical equation tells you the substances you start with in a reaction and the substances you get at the end. The substances you have at the beginning are called the reactants. When the reaction is complete, you have new substances called the products. The formulas for the reactants are written on the left side of the equation, followed by an arrow ( ). You read the arrow as yields. The formulas for the products are written on the right side of the equation. When there are two or more reactants or products, they are separated by plus signs. The principle called conservation of mass was first demonstrated in the late 1700s. The principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products. In an open system, matter can enter from or escape to the surroundings. A match burning in the air is an example of an open system. You cannot measure the mass of all the reactants and products in an open system. A closed system is a system in which matter cannot enter from or escape to the surroundings. A sealed plastic bag is an example of a closed system. A closed system allows you to measure the mass of all reactants and products in a reaction. To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation. An equation is balanced when it accurately represents conservation of mass. To balance a chemical equation, you may have to use coefficients. A coefficient is a number placed in front of a chemical formula in an equation. It tells you how many atoms or molecules of a reactant or a product take part in the reaction. Many chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement. When two or more elements or compounds combine to make a more complex substance, the reaction is called a synthesis reaction. The reaction of hydrogen and oxygen to make water is a synthesis reaction. A reaction called a decomposition reaction breaks down compounds into simpler products. For example, hydrogen peroxide decomposes into water and oxygen gas. When one element replaces another in a compound, or when two elements in different compounds trade places, the reaction is called a replacement reaction.

7 Chemical Reactions Name Date Class Chemical Reactions Review and Reinforce Describing Chemical Reactions Understanding Main Ideas Balance the equations on the lines below. State whether the reaction is a synthesis, decomposition, or replacement reaction. Given Equation Balanced Equation Type of Reaction 1. FeS + HCl FeCl 2 + H 2 S a. b. 2. Na + F 2 NaF a. b. 3. HgO Hg + O 2 a. b. Answer questions 4 and 5 on a separate sheet of paper. 4. Describe in words the chemical composition of the molecules involved and the reaction represented by the equation: 2H 2 + O 2 2 H 2 O 5. Use the principle of conservation of mass to explain why the equation in question 4 is balanced. Building Vocabulary Match each term with its definition by writing the letter of the correct definition in the right column on the line beside the term in the left column. 6. chemical equation 7. decomposition reaction 8. coefficient 9. product 10. reactant 11. conservation of mass 12. synthesis reaction 13. replacement reaction a. substance present after a reaction b. reaction in which substances combine to form a more complex compound c. uses symbols and formulas to show chemical reactions d. reaction in which one element replaces another in a compound e. substance present before a reaction f. number telling how many molecules of a substance are involved in a chemical reaction g. reaction in which compounds are broken down into simpler products h. principle that states that matter is not created or destroyed during a chemical reaction

8 Name Date Class Chemical Reactions Enrich The Decomposition of Water You learned in Section 2 that hydrogen gas and oxygen gas can react to produce water. The reverse of this reaction can also occur. In other words, water can be broken down to make hydrogen gas and oxygen gas. The breakdown of water is a decomposition reaction. The unbalanced equation for this reaction is shown below. H 2 O H 2 + O 2 Water Hydrogen gas Oxygen gas For this reaction to occur, there must be an electric current through the water as shown in the figure below. Two wires are connected to a battery, and the free ends of the wires are put into a beaker of water that contains a small amount of sulfuric acid. The sulfuric acid helps to increase the flow of current through the water. Wires Battery Beaker Hydrogen gas Oxygen gas Water with a small amount of sulfuric acid Answer the following questions on a separate sheet of paper. 1. Write a balanced equation for the decomposition of water. 2. How many atoms of hydrogen are on the left side of the balanced equation? How many oxygen atoms? How many hydrogen atoms are on the right side of the balanced equation? How many oxygen atoms? 3. The water in a beaker has a mass of 18 g. An electric current is turned on in the water for two hours. Afterward the water has a mass of only 16 g. What happened to the missing mass? 4. Water decomposes to make 4 g of hydrogen gas and 32 g of oxygen gas. What mass of water decomposed? How do you know? 5. Look at the figure above. How can you tell that a reaction is occurring?