Understanding Chemical Reactions: A Comprehensive Guide

1. Chapter 5: Chemical Reactions and Equations

2. Questions for Consideration • What happens in a chemical reaction? • How do we know whether a chemical reaction takes place? • How do we represent a chemical reaction with a chemical equation? • How are chemical reactions classified? How can the products of different classes of chemical reactions be predicted? • How do we represent chemical reactions in aqueous solution?

3. Chapter 5 Topics: • What is a Chemical Reaction? • How Do We Know a Chemical Reaction Occurs? • Writing Chemical Equations • Predicting Chemical Reactions • Representing Reactions in Aqueous Solution

4. Introduction • Chemical reactions occur all around us. • How do we make sense of these changes? What patterns can we find? Figure 5.1 Figure 5.29 Figure 5.8F

5. 5.1 What is a Chemical Reaction? • A chemical reaction is a chemical change. • A chemical reaction occurs when one or more substances is converted into one or more new substances. • Reactant – a substance that we start with that undergoes a change • Product – a new substance that forms during the reaction • Products differ from reactants only in the arrangement of their component atoms.

6. 5.2 How do We Know a Chemical Reaction Occurs? • What observations might indicate that a chemical reaction has taken place? Figure 5.7

7. What clues tell you a reaction is likely occurring? Figure 5.8

8. Physical Clues of a Chemical Reaction • Common observations that may accompany a chemical reaction are: • change in color • production of light • formation of a solid (such as a precipitate in solution, or smoke in air, or a metal coating) • formation of a gas (bubbles in solution or fumes in the gaseous state) • absorption or release of heat (sometimes appearing as a flame)

9. 5.3 Writing Chemical Equations • A chemical equation is a symbolic representation of a chemical reaction. • A chemical equation shows: • the formulas for reactants and products • the physical states of each substance (s, l, g, aq) • relative numbers of reactants that combine and products that form • special conditions required for the reaction, such as constant heating.

10. Chemical Reactions • When hydrogen gas is ignited in the presence of oxygen, an explosive reaction occurs producing gaseous water molecules. • Is mass conserved? Replace with updated Figure 5.6 Figure 5.5 Figure 5.6

11. Writing Chemical Equations • Balanced equation • The number of atoms of each element is the same in the products as in the reactants. • Conservation of mass is always maintained.

12. Writing Chemical Equations • When a powdered mixture of aluminum metal and iron(III) oxide is heated, it reacts to form liquid iron metal and aluminum(III) oxide. Al(s) + Fe2O3(s)  Al2O3(s) + Fe(l) Figure 5.9

13. Chemical Equations Must be Balanced • Al(s) + Fe2O3(s)  Al2O3(s) + Fe(l) • Is this equation balanced? • What has to change? Figure from p. 177

14. Balance Equations with Coefficients • Al(s) + Fe2O3(s)  Al2O3(s) + Fe(l) • We balance the atoms in equations with coefficients. • The aluminum atoms are not balanced so we place a coefficient of 2 in front of the Al reactant: 2Al(s) + Fe2O3(s)  Al2O3(s) + Fe(l) • The iron atoms are not balanced so we place a coefficient of 2 in front of the Fe product: 2Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(l) Figure from p. 177

15. Writing Chemical Equations Methane + oxygen  carbon dioxide + water CH4(g) + O2(g)  CO2(g) + H2O(g) The number of atoms of each element in the unbalanced equation is shown below. These numbers were obtained by multiplying the subscript to the right of the element’s symbol by the stoichiometric coefficient.

16. Writing Chemical Equations CH4(g) + O2(g)  CO2(g) + H2O(g) First, we look at the carbon atoms. Since the number of carbon atoms on the reactant side is already equal to the number of carbon atoms on the product side, we don’t need to add coefficients. Next, we look at the hydrogen atoms. Currently, there are four hydrogen atoms on the reactant side and 2 hydrogen atoms on the product side. Thus, we need to add a coefficient of 2 in front of water to make the hydrogen atoms equal. CH4(g) + O2(g)  CO2(g) + 2H2O(g)

17. Writing Chemical Equations CH4(g) + O2(g)  CO2(g) + 2H2O(g) Finally, we look at the oxygen atoms. Currently, there are 2 oxygen atoms on the reactant side and 4 oxygen atoms (combined from carbon dioxide and water) on the product side. Thus, we add a coefficient of 2 in front of the oxygen gas. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) Now the equation is balanced!

18. A General Approach to Balancing Equations • Identify the reactants and products and write their correct formulas. • Write a skeletal equation including physical states. • Change coefficients one at a time until the atoms of each element are balanced. (Start with the elements that occur least often in the equation.) • Make a final check by counting the atoms of each element on both sides of the equation.

19. Activity: Balancing Equations • Balance the following equations: • Potassium chlorate  potassium chloride + oxygen • Aluminum acetate reacts with potassium sulfate to form potassium acetate and aluminum sulfate • Hexane (C6H14) reacts with oxygen gas to form carbon dioxide and water • Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas

20. Activity Solutions: Balancing Equations • Potassium chlorate  potassium chloride + oxygen First, translate the chemical names into chemical formulas: Potassium chlorate  potassium chloride + oxygen K+ ClO3− K+ Cl− O2(diatomic) KClO3(s)  KCl(aq) + O2(g) Next, balance the chemical equation: 2KClO3(s)  2KCl(aq) + 3O2(g)

21. Activity Solutions: Balancing Equations • Aluminum acetate reacts with potassium sulfate to form potassium acetate and aluminum sulfate Aluminum acetate + potassium sulfate  Al(C2H3O2)3 + K2SO4 potassium acetate + aluminum sulfate KC2H3O2 + Al2(SO4)3 Al(C2H3O2)3(aq) + K2SO4(aq)  KC2H3O2(aq) + Al2(SO4)3(s) Balanced: 2Al(C2H3O2)3(aq) + 3K2SO4(aq)  6KC2H3O2(aq) + Al2(SO4)3(s)

22. Activity Solutions: Balancing Equations • Hexane (C6H14) reacts with oxygen gas to form carbon dioxide and water Hexane was given. Oxygen gas is diatomic. Carbon dioxide has 1 carbon atom and 2 oxygen atoms (using the Greek prefixes) and water has 2 hydrogen atoms and 1 oxygen atom. C6H14(l) + O2(g)  CO2(g) + H2O(g) Balanced: C6H14(l) + 9.5O2(g)  6CO2(g) + 7H2O(g) Fractional coefficients are not acceptable, so we multiply all coefficients by 2: 2C6H14(l) + 19O2(g)  12CO2(g) + 14H2O(g)

23. Activity Solutions: Balancing Equations • Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas Zinc + Hydrochloric acid  Zinc chloride + hydrogen Zn(s) + HCl(aq)  ZnCl2(aq) + H2(g) Balanced: Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)

24. 5.4 Predicting Chemical Reactions • Since chemical reactions are rearrangements of atoms, let’s look at the possible rearrangements for the following two elements and two compounds. • A and B are elements, either monatomic or diatomic. • C, D, E, and F might be atoms, monatomic ions, or polyatomic ions • When making a list of the possible reactions, do not react more than two elements or compounds. Figure from p. 183

25. Each compound could react to form its elements: Figure from p. 183 Compound AB could react similarly.

26. The elements could combine to form a compound: Figure from p. 184 Similarly, an element could combine with a compound to form a more complex compound: Figure from p. 184

27. An element could react with a compound to form a new element and a new compound: Figure from p. 184 In the example, why did zinc displace copper instead of chlorine?

28. Two compounds can combine to form one new compound: Figure from p. 184

29. Two compounds can react to form two new compounds: Figure from p. 179 In the example, why did copper ion combine with sulfide ion instead of sodium ion?

30. Summary of Reactions • All of the combinations we looked at fall into one of these four categories: Table from p. 184

31. Summary of Reactions • These four categories have names:

32. Activity: Classes of Reactions • Classify each of the following as a decomposition, combination, single-displacement, or double-displacement reaction. • NH3(g) + HCl(g)  NH4Cl(s) • CuCl2(aq) + Na2S(aq)  CuS(s) + 2 NaCl(aq) • NiSO3(s)  NiO(s) + SO2(g) • Ca(s) + PbCl2(aq)  CaCl2(aq) + Pb(s)

33. Activity Solutions: Classes of Reactions • Classify each of the following as a decomposition, combination, single-displacement, or double-displacement reaction. • NH3(g) + HCl(g)  NH4Cl(s) Combination • CuCl2(aq) + Na2S(aq)  CuS(s) + 2 NaCl(aq) Double-displacement • NiSO3(s)  NiO(s) + SO2(g) Decomposition • Ca(s) + PbCl2(aq)  CaCl2(aq) + Pb(s) Single-displacement

34. Activity: Classify each of the following reactions: Decomposition Combination Single-displacement Double-displacement Figures from p. 183-184

35. Activity: Can you classify any of these reactions? Figure 5.8

36. Activity: Classify this reaction Figure 5.2 Single Displacement

37. Activity: Classify these reactions: Combination Decomposition

38. Activity: Classify these reactions: Single Displacement Double Displacement

39. Activity: Classify this reaction A Figure from p. 204 Combination

40. Activity: Classify this reaction B Figure from p. 206 Decomposition

41. Activity: Classify this reaction b. Figure from p. 207 Single Displacement

42. Activity: Classify this reaction A. Figure from p. 208 Decomposition

43. Activity: Classify the following reactions, based on the changes happening at an atomic/molecular level. double displacement double displacement decomposition single displacement combination single displacement single displacement single displacement decomposition decomposition double displacement combination double displacement combination decomposition combination 1. AlF3(aq) + 3H2O(l)  Al(OH)3(s) + 3HF(aq) 2. BaCl2(aq) + Na2SO4(aq)  BaSO4(s) + 2NaCl(aq) 3. Ca(OH)2(s)  CaO(s) + H2O(g) 4. Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g) 5. CaO(s) + CO2(g)  CaCO3(s) 6. Cl2(aq) + 2NaI(aq)  2NaCl(aq) + I2(aq) 7. Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s) 8. Fe(s) + 2HCl(aq)  FeCl2(aq) + H2(g) 9. H2SO3(aq)  H2O(l) + SO2(g) 10. 2HgO(s)  2Hg(l) + O2(g) 11. KOH(aq) + HNO3(aq)  KNO3(aq) + H2O(l) 12. 4Li(s) + O2(g)  2Li2O(s) 13. Na2S(aq) + 2HCl(aq)  2NaCl(aq) + H2S(g) 14. NH3(g) + HCl(g)  NH4Cl(s) 15. NiCO3(s)  NiO(s) + CO2(g) 16. P4(s) + 10F2(g)  4PF5(g)

44. Decomposition Reactions (CD  C + D) • In a decomposition reaction, a single compound breaks down into elements and/or simpler compounds. 2HgO(s)  2Hg(l) + O2(g) Figure 5.13 Before Figure 5.13 After Figure 5.14

46. Activity: Predicting Products heat PtCl4(s) → Pt(s) + 2Cl2(g) • Predict the products and balance the equation for the following decomposition reaction. Platinum(IV) chloride decomposes to its elements: PtCl4(s) → heat

47. Combination Reactions (A + B  AB) • During a combination reaction, two substances combine to form a single compound. • Most metals react with most nonmetals to form ionic compounds. • A nonmetal may react with a more reactive nonmetal to form a molecular compound. • A compound and an element may combine to form another compound if one exists with a higher atom: atom ratio. • 2CO(g) + O2(g)  2CO2(g) Figure 5.18 Figure 5.19

48. Combination Reactions

49. Activity: Predicting Products • We can often predict the products of combination reactions involving metal and nonmetal element reactants. • Predict the product and balance the following equation: • Al(s) + Br2(l)  Figure 5.16 2Al(s) + 3Br2(l)  2AlBr3(s)

50. Single-Displacement Reactions (A + CD  AD + C) • In a single-displacement, a free element displaces another element from a compound to produce a different compound and another free element. Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s) Replace with updated Figure 5.23 Figure 5.23 Figure 5.22