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Standardized Test Prep Resources Chapter Presentation Bellringers Transparencies Standardized Test Prep Math Skills Visual Concepts

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

Section 1 The Nature of Chemical Reactions Chapter 6 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.

Section 1 The Nature of Chemical Reactions Chapter 6 Bellringer Methane, CH4, 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. 1. What else besides carbon dioxide and water is produced in this reaction that makes methane useful for cooking and heating?

Section 1 The Nature of Chemical Reactions Chapter 6 Bellringer 2. Complete the table below with the number of atoms of each element before and after the reaction. 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.

Chemical Reactions Change Substances Section 1 The Nature of Chemical Reactions Chapter 6 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.

Section 1 The Nature of Chemical Reactions Chapter 6 Chemical Reaction

Signs of a Chemical Reaction Section 1 The Nature of Chemical Reactions Chapter 6 Signs of a Chemical Reaction

Chapter 6 Energy and Reactions Energy must be added to break bonds. Section 1 The Nature of Chemical Reactions Chapter 6 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.

Section 1 The Nature of Chemical Reactions Chapter 6 Reaction Model

Energy and Reactions, continued Section 1 The Nature of Chemical Reactions Chapter 6 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.

Energy and Reactions, continued Section 1 The Nature of Chemical Reactions Chapter 6 Energy and Reactions, continued The graphs below represent the changes in chemical energy for an exothermic reaction and an endothermic reaction.

Chapter 6 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 Section 2 Reaction Types 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).

Chapter 6 Bellringer, continued Section 2 Reaction Types 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?

Classifying Reactions Chapter 6 Section 2 Reaction Types 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 + Cl2 → 2NaCl

Chapter 6 Section 2 Reaction Types Synthesis Reaction

Classifying Reactions, continued Chapter 6 Section 2 Reaction Types 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. 2H2O → 2H2 + O2 Electrolysis is the process in which an electric current is used to produce a chemical reaction, such as the decomposition of water.

Decomposition Reaction Chapter 6 Section 2 Reaction Types Decomposition Reaction

Chapter 6 Section 2 Reaction Types Electrolysis

Classifying Reactions, continued Chapter 6 Section 2 Reaction Types 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.

Chapter 6 Section 2 Reaction Types Combustion Reaction

Classifying Reactions, continued Chapter 6 Section 2 Reaction Types 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. 3CuCl2 + 2Al → 2AlCl3 + 3Cu

Chapter 6 Section 2 Reaction Types Single Displacement

Classifying Reactions, continued Chapter 6 Section 2 Reaction Types 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(NO3)2 + K2CrO4 → PbCrO4 + 2KNO3

Double Displacement Reaction Chapter 6 Section 2 Reaction Types Double Displacement Reaction

Electrons and Chemical Reactions Chapter 6 Section 2 Reaction Types 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.

Chapter 6 Section 2 Reaction Types Redox Reactions

Chapter 6 Objectives Demonstrate how to balance chemical equations. Section 3 Balancing Chemical Equations Chapter 6 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.

Section 3 Balancing Chemical Equations Chapter 6 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 Bellringer, continued Section 3 Balancing Chemical Equations 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 H2O2 instead of H2O. What’s wrong with this reasoning? (Hint: If you did this, would it still be a synthesis reaction for water?)

Chapter 6 Describing Reactions Section 3 Balancing Chemical Equations 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. 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.

Chapter 6 Describing Reactions Section 3 Balancing Chemical Equations Chapter 6 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.

Law of Conservation of Mass Section 3 Balancing Chemical Equations Chapter 6 Law of Conservation of Mass

Describing Reactions, continued Section 3 Balancing Chemical Equations Chapter 6 Describing Reactions, continued When the numbers of atoms for each element are the same on each side, the equation is balanced, as shown below.

Reading a Chemical Equation Section 3 Balancing Chemical Equations Chapter 6 Reading a Chemical Equation

Section 3 Balancing Chemical Equations Chapter 6 Chemical Equation

Balancing a Chemical Equation by Inspection Section 3 Balancing Chemical Equations Chapter 6 Balancing a Chemical Equation by Inspection

Section 3 Balancing Chemical Equations Chapter 6 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 Math Skills, continued Section 3 Balancing Chemical Equations Chapter 6 Math Skills, continued 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 O2, not O. Mg + O2 → 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. But there are two oxygen atoms on the left and only one on the right.

Chapter 6 Math Skills, continued Section 3 Balancing Chemical Equations Chapter 6 Math Skills, continued 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 + O2 → 2MgO 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 + O2 → 2MgO

Chapter 6 Math Skills, continued 2Mg + O2 → 2MgO Section 3 Balancing Chemical Equations Chapter 6 Math Skills, continued 4. Balance the equation one element at a time, continued 2Mg + O2 → 2MgO Now the equation is balanced. It has an equal number of each type of atom on both sides.

Determining Mole Ratios Section 3 Balancing Chemical Equations Chapter 6 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.

Law of Definite Proportions Section 3 Balancing Chemical Equations Chapter 6 Law of Definite Proportions

Determining Mole Ratios, continued Section 3 Balancing Chemical Equations Chapter 6 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 H2O:H2:O2 is 2:2:1. 2H2O → 2H2 + [1]O2 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 Objectives Describe the factors affecting reaction rates. 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 Section 4 Rates of Change 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.

Chapter 6 Bellringer, continued Section 4 Rates of Change 1. In the reaction Zn + 2HCl → ZnCl2 + H2, 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?)

Factors Affecting Reaction Rates Chapter 6 Section 4 Rates of Change 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.

Factors Affecting Reaction Rate Chapter 6 Section 4 Rates of Change Factors Affecting Reaction Rate

Factors Affecting Reaction Rates, continued Chapter 6 Section 4 Rates of Change 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.

Chapter 6 Section 4 Rates of Change Catalyst

Factors Affecting Reaction Rates, continued Chapter 6 Section 4 Rates of Change 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:

Chapter 6 Section 4 Rates of Change Inhibitors

Chapter 6 Section 4 Rates of Change Enzyme

Chapter 6 Equilibrium Systems Some changes are reversible. Section 4 Rates of Change Equilibrium Systems Some changes are reversible. Example: the physical change represented below can go in either direction. 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 CO2 leaves as bubbles.

Chapter 6 Section 4 Rates of Change Equilibrium

Chapter 6 Section 4 Rates of Change Equilibrium

Equilibrium Systems, continued Chapter 6 Section 4 Rates of Change 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.

Factors Affecting Equilibrium Chapter 6 Section 4 Rates of Change Factors Affecting Equilibrium

Le Châtelier’s Principle Chapter 6 Section 4 Rates of Change Le Châtelier’s Principle

Chapter 6 Section 4 Rates of Change Concept Mapping

Understanding Concepts Chapter 6 Standardized Test Prep Understanding Concepts 1. Mg(s) + Cl2(g) → MgCl2(s) is an example of what type of chemical reaction? A. synthesis reaction B. decomposition reaction C. single-displacement reaction D. double-displacement reaction

Understanding Concepts Chapter 6 Standardized Test Prep Understanding Concepts 1. Mg(s) + Cl2(g) → MgCl2(s) is an example of what type of chemical reaction? A. synthesis reaction B. decomposition reaction C. single-displacement reaction D. double-displacement reaction

Understanding Concepts Chapter 6 Standardized Test Prep 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

Understanding Concepts Chapter 6 Standardized Test Prep 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

Understanding Concepts Chapter 6 Standardized Test Prep 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

Understanding Concepts Chapter 6 Standardized Test Prep 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

Understanding Concepts Chapter 6 Standardized Test Prep Understanding Concepts 4. Most chemical reactions proceed faster if the reactants are heated. How does the added heat affect reactant atoms or molecules?

Understanding Concepts Chapter 6 Standardized Test Prep 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.

Understanding Concepts Chapter 6 Standardized Test Prep 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.

Understanding Concepts Chapter 6 Standardized Test Prep 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.

Chapter 6 Reading Skills Standardized Test Prep 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

Chapter 6 Reading Skills Standardized Test Prep 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?

Chapter 6 Reading Skills Standardized Test Prep 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.

Interpreting Graphics Chapter 6 Standardized Test Prep 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

Interpreting Graphics Chapter 6 Standardized Test Prep 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