Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter Presentation Bellringers Transparencies Standardized Test.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Nature of Chemical Reactions Chapter 6 Table of Contents Section 1 The Nature of Chemical Reactions Section 2 Reaction Types Section 3 Balancing Chemical Equations Section 4 Rates of Change

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 The Nature of Chemical Reactions Objectives Recognize some signs that a chemical reaction may be taking place. Explain chemical changes in terms of the structure and motion of atoms and molecules. Describe the differences between endothermic and exothermic reactions. Identify situations involving chemical energy. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 The Nature of Chemical Reactions Bellringer Methane, CH 4, is an organic compound that is the principal component of natural gas. Many people burn methane when cooking or heating homes. The chemical reaction of methane burning is shown in several ways below. Chapter 6 1. What else besides carbon dioxide and water is produced in this reaction that makes methane useful for cooking and heating?

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 The Nature of Chemical Reactions Bellringer 2. Complete the table below with the number of atoms of each element before and after the reaction. Chapter 6 3. How does the number of atoms of each element on the left side of the equation compare to the number on the right? What law does this demonstrate? 4. Use your answer to item 1 and the law of conservation of energy to guess whether there is more energy stored in the bonds among the atoms before the reaction or among the bonds of the atoms after the reaction.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 The Nature of Chemical Reactions Chemical Reactions Change Substances Chemical reactions occur when substances undergo chemical changes to form new substances. Production of gas and change of color are signs of chemical reactions. Chemical reactions rearrange atoms. A reactant is a substance or molecule that participates in a chemical reaction. A product is a substance that forms in a chemical reaction. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 The Nature of Chemical Reactions Energy and Reactions Energy must be added to break bonds. Many forms of energy can be used to break bonds: heat electricity sound light Forming bonds releases energy. Example: When gasoline burns, energy in the form of heat and light is released as the products of the isooctane-oxygen reaction and other gasoline reactions form. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 The Nature of Chemical Reactions Energy and Reactions, continued Energy is conserved in chemical reactions. Chemical energy is the energy released when a chemical compound reacts to produce new compounds. The total energy that exists before the reaction is equal to the total energy of the products and their surroundings. An exothermic reaction is a chemical reaction in which heat is released to the surroundings. An endothermic reaction is a chemical reaction that absorbs heat. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 The Nature of Chemical Reactions Energy and Reactions, continued The graphs below represent the changes in chemical energy for an exothermic reaction and an endothermic reaction. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Reaction Types Objectives Distinguish among five general types of chemical reactions. Predict the products of some reactions based on the reaction type. Describe reactions that transfer or share electrons between molecules, atoms, or ions. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Bellringer There are thousands of ways that more than one hundred elements can combine with each other to form different substances. Just as the elements can be sorted into families, the many reactions the elements undergo can be classified as a few basic types. The types of reactions are classified based on whether they involve combining atoms or smaller molecules to make larger molecules (synthesis), breaking down larger molecules into atoms or smaller molecules (decomposition), or having atoms of one element replace the atoms of another element within a compound (single- or double-displacement). Section 2 Reaction Types Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Bellringer, continued 1. In which reaction model do three elements combine to make a compound? 2. In which reaction model is a complex substance broken down into simpler parts? 3. Identify the reaction model in which one element reacts with a compound, leaving behind another element and a new compound containing the first element. 4. In which reaction model do two compounds react to form two different compounds? Section 2 Reaction Types Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Classifying Reactions A synthesis reaction is a reaction in which two or more substances combine to form a new compound. Synthesis reactions have the following general form: A + B → AB Example: In the following synthesis reaction, the metal sodium reacts with chlorine gas to form sodium chloride, or table salt. 2Na + Cl 2 → 2NaCl Section 2 Reaction Types Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Classifying Reactions, continued A decomposition reaction is a reaction in a single compound breaks down to form two or more simpler substances. Decomposition reactions have the following general form: AB → A + B Example: The following shows the decomposition of water. 2H 2 O → 2H 2 + O 2 Electrolysis is the process in which an electric current is used to produce a chemical reaction, such as the decomposition of water. Section 2 Reaction Types Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Classifying Reactions, continued A combustion reaction is the oxidation reaction of an organic compound, in which heat is released. Combustion reactions use oxygen as a reactant. Water is a common product of combustion reactions. In combustion the products depend on the amount of oxygen available for the reaction. Section 2 Reaction Types Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Classifying Reactions, continued A single-displacement reaction is a reaction in which one element or radical takes the place of another element or radical in the compound. Single-displacement reactions have the following general form: AX + B → BX + A Example: The single-displacement reaction between copper(II) chloride and aluminum is shown as follows. 3CuCl 2 + 2Al → 2AlCl 3 + 3Cu Section 2 Reaction Types Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Classifying Reactions, continued A double-displacement reaction is a reaction in which a gas, a solid precipitate, or a molecular compound forms from the apparent exchange of atoms or ions between two compounds. Double-displacement reactions have the following general form: AX + BY → AY + BX Example: The double-displacement reaction that forms lead chromate is as follows. Pb(NO 3 ) 2 + K 2 CrO 4 → PbCrO 4 + 2KNO 3 Section 2 Reaction Types Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Electrons and Chemical Reactions An oxidation-reduction reaction is any chemical change in which one species gains electrons and another species loses electrons. Oxidation-reduction reactions are often called redox reactions for short. Substances that accept electrons in a redox reaction are said to be reduced. Substances that give up electrons in a redox reaction are said to be oxidized. A radical is an organic group that has one or more electrons available for bonding. Polymerization reactions can occur when radicals are formed. Section 2 Reaction Types Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Balancing Chemical Equations Objectives Demonstrate how to balance chemical equations. Interpret chemical equations to determine the relative number of moles of reactants needed and moles of products formed. Explain how the law of definite proportions allows for predictions about reaction amounts. Identify mole ratios in a balanced chemical equation. Calculate the relative masses of reactants and products from a chemical equation. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Balancing Chemical Equations Bellringer You have already used scientific shorthand by writing symbols for elements and formulas for compounds. You can use these formulas to write chemical equations that summarize what happens during a chemical reaction and how much of each substance is involved. Examine the reaction model for the water synthesis reaction shown on the next slide, and answer the items that follow. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Balancing Chemical Equations Bellringer, continued 1. What is the difference between reaction models A and B? 2. Why is reaction model A not fully complete as written? (Hint: Consider how many atoms of each element exist before and after the reaction.)‏ 3. A friend tells you that an easier way to make sure the same number of atoms are on both sides of the equation is to change the subscript on the product so that it is H 2 O 2 instead of H 2 O. What’s wrong with this reasoning? (Hint: If you did this, would it still be a synthesis reaction for water?)‏ Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Balancing Chemical Equations Describing Reactions One way to record the products and reactants of a reaction is to write a word equation. Example: methane + oxygen → carbon dioxide + water A chemical equation is a representation of a chemical reaction that uses symbols to show the relationship between the reactants and the products. Chapter 6 In a chemical equation, such as the one above, the reactants, which are on the left-hand side of the arrow, form the products, which are on the right-hand side.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Balancing Chemical Equations Describing Reactions When the number of atoms of reactants matches the number of atoms of products, then the chemical equation is said to be balanced. Balancing equations follows the law of conservation of mass. You cannot balance chemical equations by changing chemical formulas themselves, because that would change the substances involved. To balance chemical equations, numbers called coefficients must be placed in front of the chemical formulas. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Balancing Chemical Equations Describing Reactions, continued When the numbers of atoms for each element are the same on each side, the equation is balanced, as shown below. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Balancing Chemical Equations Math Skills Balancing Chemical Equations Write the equation that describes the burning of magnesium in air to form magnesium oxide. 1. Identify the reactants and products. Magnesium and oxygen gas are the reactants that form the product, magnesium oxide. 2. Write a word equation for the reaction. magnesium + oxygen → magnesium oxide. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 3. Write the equation using formulas for the elements and compounds in the word equation. Remember that some gaseous elements, like oxygen, are molecules, not atoms. Oxygen in air is O 2, not O. Mg + O 2 → MgO 4. Balance the equation one element at a time. The same number of each kind of atom must appear on both sides. So far, there is one atom of magnesium on each side of the equation. Section 3 Balancing Chemical Equations Math Skills, continued Chapter 6 But there are two oxygen atoms on the left and only one on the right.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 4. Balance the equation one element at a time, continued To balance the number of oxygen atoms, you need to double the amount of magnesium oxide: Mg + O 2 → 2MgO Section 3 Balancing Chemical Equations Math Skills, continued Chapter 6 This equation gives you two magnesium atoms on the right and only one on the left. So you need to double the amount of magnesium on the left, as follows. 2Mg + O 2 → 2MgO

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 4. Balance the equation one element at a time, continued Section 3 Balancing Chemical Equations Math Skills, continued Chapter 6 Now the equation is balanced. It has an equal number of each type of atom on both sides. 2Mg + O 2 → 2MgO

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Balancing Chemical Equations Determining Mole Ratios The law of definite proportions states that a compound always contains the same elements in the same proportions, regardless of how the compound is made or how much of the compound is formed. Because the law of definite proportions holds true for all chemical substances in all reactions, mole ratios can be derived from balanced equations. Mole ratio is the relative number of moles of the substances required to produce a given amount of product in a chemical reaction. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Balancing Chemical Equations Determining Mole Ratios, continued The mole ratio for any reaction comes from the balanced chemical equation. Example: The equation for the electrolysis of water shows that the mole ratio for H 2 O:H 2 :O 2 is 2:2:1. 2H 2 O → 2H 2 + [1]O 2 If you know the mole ratios of the substances in a reaction, you can find the relative masses of the substances required to react completely. Relative masses can be found by multiplying the molecular mass of each substance by the mole ratio from the balanced equation. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 4 Rates of Change Objectives Describe the factors affecting reaction rates. Explain the effect a catalyst has on a chemical reaction. Explain chemical equilibrium in terms of equal forward and reverse reaction rates. Apply Le Châtelier’s principle to predict the effect of changes in concentration, temperature, and pressure in an equilibrium process. Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Bellringer Not all reactions happen at the same speed. Atoms, ions, and molecules can only interact when they are in close contact with each other. Below is a sample of zinc arranged in three different ways. Section 4 Rates of Change Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Bellringer, continued 1. In the reaction Zn + 2HCl → ZnCl 2 + H 2, which sample do you think would react the fastest? Why? 2. When you want to start a bonfire, why do you use many small sticks as kindling to start the larger logs? 3. Which do you think will react faster with hydrochloric acid, HCl–atoms of liquid zinc at its melting point or atoms of solid zinc at its melting point? (Hint: Which situation allows more contact among the particles?)‏ Section 4 Rates of Change Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Factors Affecting Reaction Rates For any reaction to occur, the particles of the reactants must collide with one another. Therefore, whatever will help particles collide with one another will speed up the reaction rate. Most reactions go faster at higher temperatures. Greater surface area speeds up reactions. Concentrated solutions react faster. Reactions are faster at higher pressure. Massive, bulky molecules react slower. Section 4 Rates of Change Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Factors Affecting Reaction Rates, continued A catalyst is a substance that changes the rate of a chemical reaction without being consumed or changed significantly. Catalysts are not reactants or products, because they are not used up in the reaction. Catalysts are often used in industry to make reactions go faster. Catalysts that slow reactions are called inhibitors. Section 4 Rates of Change Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Factors Affecting Reaction Rates, continued Enzymes are proteins that serve as biological catalysts. An enzyme is very specific, controlling one reaction or set of similar reactions. Most enzymes are fragile, and stop working above certain temperatures. The substrate is the reactant in reactions catalyzed by enzymes. Example: hydrogen peroxide is the substrate for catalase: Section 4 Rates of Change Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Equilibrium Systems Some changes are reversible. Example: the physical change represented below can go in either direction. Section 4 Rates of Change Chapter 6 Chemical equilibrium is a state of balance in which the rate of a forward reaction equals the rate of the reverse reaction. Systems in equilibrium respond to minimize change. Example: when the top is removed from a carbonated drink, the system is no longer at equilibrium, and CO 2 leaves as bubbles.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Equilibrium Systems, continued Le Châtelier’s principle predicts changes in equilibrium. Le Châtelier’s principle is a general rule that states that if a change is made to a system in chemical equilibrium, the equilibrium shifts to oppose the change until a new equilibrium is reached. Le Châtelier’s principle can be used to control reactions. Example: in a reaction that releases energy, if you raise the temperature, the equilibrium will shift to the left and make less products. Section 4 Rates of Change Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Understanding Concepts 1. Mg(s) + Cl 2 (g) → MgCl 2 (s) is an example of what type of chemical reaction? A.synthesis reaction B.decomposition reaction C.single-displacement reaction D.double-displacement reaction Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Understanding Concepts 2. Which of the following changes will not increase the rate of a chemical reaction? F.using an enzyme in a reaction G.adding an inhibitor to the reaction mixture H.increasing the concentration of the reactants I.grinding a solid reactant to make a fine powder Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Understanding Concepts 3. Which of the following is an endothermic chemical reaction? A.fireworks exploding in the sky B.photosynthesis in plant cells C.respiration in animal cells D.wood burning in a fireplace Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Understanding Concepts 4. Most chemical reactions proceed faster if the reactants are heated. How does the added heat affect reactant atoms or molecules? Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Understanding Concepts 4. Most chemical reactions proceed faster if the reactants are heated. How does the added heat affect reactant atoms or molecules? Answer: Addition of heat causes the particles to move faster and collide more often. The increase in collisions speeds up the reaction. Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Understanding Concepts 5. The reaction of glucose and oxygen to form carbon dioxide and water produces the same amount of energy inside living cells as it does by combustion. Analyze how this reaction can occur at body temperature in the cells, but not in the open air. Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Understanding Concepts 5. The reaction of glucose and oxygen to form carbon dioxide and water produces the same amount of energy inside living cells as it does by combustion. Analyze how this reaction can occur at body temperature in the cells, but not in the open air. Answer: Inside living cells, enzymes act as catalysts to reduce the amount of energy needed to start the reaction and to allow it to proceed at a lower temperature. Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Reading Skills Some metals react with water to form new compounds by displacing hydrogen from water molecules. Alkali metals are sufficiently reactive that this chemical reaction happens at room temperature. If a piece of cesium is placed in water, an explosion occurs as the hydrogen gas reacts with oxygen in the air. continued on next slide Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Reading Skills 6. Hydrogen and oxygen gases do not react spontaneously when they are mixed, unless energy is added to start the reaction. What is the source of energy that causes hydrogen to react explosively when cesium is added to water? Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Reading Skills 6. Hydrogen and oxygen gases do not react spontaneously when they are mixed, unless energy is added to start the reaction. What is the source of energy that causes hydrogen to react explosively when cesium is added to water? Answer: The reaction of cesium and water is extremely exothermic. This exothermic reaction provides the energy to initiate the reaction between hydrogen and oxygen. Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Interpreting Graphics 7.In each of these reactions, the chemical energy increases and then decreases, during the course of the reaction. What does the height of the “hill” on each graph represent? F.energy that must be added to start the reaction G.energy released as reactant molecules approach one another H.the potential energy of the chemical bonds in the molecules of the reactants I.The change in total chemical energy between the reactants and the products Standardized Test Prep Chapter 6

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Interpreting Graphics Standardized Test Prep Chapter 6 7.In each of these reactions, the chemical energy increases and then decreases, during the course of the reaction. What does the height of the “hill” on each graph represent? F.energy that must be added to start the reaction G.energy released as reactant molecules approach one another H.the potential energy of the chemical bonds in the molecules of the reactants I.The change in total chemical energy between the reactants and the products

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter Presentation Bellringers Transparencies Standardized Test.

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