2. 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

3. 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

4. Chemical Reaction Section 1 The Nature of Chemical Reactions Chapter 6

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

6. 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

7. Reaction Model Section 1 The Nature of Chemical Reactions Chapter 6

8. 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

9. 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

10. 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

11. 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

12. Synthesis Reaction Section 2 Reaction Types Chapter 6

13. 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

14. Decomposition Reaction Section 2 Reaction Types Chapter 6

15. Electrolysis Section 2 Reaction Types Chapter 6

16. Classifying Reactions 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

17. Combustion Reaction Section 2 Reaction Types Chapter 6

18. Classifying Reactions 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

19. Single Displacement Section 2 Reaction Types Chapter 6

20. Classifying Reactions 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

21. Double Displacement Reaction Section 2 Reaction Types Chapter 6

22. 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

23. Redox Reactions Section 2 Reaction Types Chapter 6

24. 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

25. 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.

26. 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

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

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

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

30. Chemical Equation Section 3 Balancing Chemical Equations Chapter 6

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

32. 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. 2. Write a word equation for the reaction. magnesium + oxygen → magnesium oxide. Chapter 6

33. 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 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 Chapter 6 But there are two oxygen atoms on the left and only one on the right.

34. 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 + O 2 → 2MgO Section 3 Balancing Chemical Equations 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

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

36. 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. reaction. Chapter 6

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

38. 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

39. 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

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

41. Factors Affecting Reaction Rates 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

42. Catalyst Section 4 Rates of Change Chapter 6

43. Factors Affecting Reaction Rates 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

44. Inhibitors Section 4 Rates of Change Chapter 6

45. Enzyme Section 4 Rates of Change Chapter 6

46. 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.

47. Equilibrium Section 4 Rates of Change Chapter 6

48. Equilibrium Section 4 Rates of Change Chapter 6

49. Equilibrium Systems 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

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

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