George Mason University General Chemistry 211 Chapter 4 Three Major Classes of Chemical Reactions

1. George Mason University General Chemistry 211 Chapter 4 Three Major Classes of Chemical Reactions Acknowledgements Course Text: Chemistry: the Molecular Nature of Matter and Change, 7th edition, 2011, McGraw-Hill Martin S. Silberberg & Patricia Amateis The Chemistry 211/212 General Chemistry courses taught at George Mason are intended for those students enrolled in a science /engineering oriented curricula, with particular emphasis on chemistry, biochemistry, and biology The material on these slides is taken primarily from the course text but the instructor has modified, condensed, or otherwise reorganized selected material.Additional material from other sources may also be included. Interpretation of course material to clarify concepts and solutions to problems is the sole responsibility of this instructor.

2. Chapter 4Classes of Chemical Reactions • The Role of Water as a Solvent • Polar Nature of Water • Ionic Compounds • Covalent Compounds • Writing Equations for Aqueous Ionic Reactions • Precipitation Reactions • Formation of a Solid • Predicting a Precipitate • Acid-Base Reactions • Formation of Water • Acid-Base Titrations • Proton Transfer in Acid-Base Reactions

3. Chapter 4Classes of Chemical Reactions • Oxidation-Reduction (REDOX) Reactions • Movement of Electrons • Redox Terminology • Oxidation Number • Balancing Redox Equations • Redox Titrations • Elements in Redox Reactions • Reaction Reversibility and the Equilibrium State

4. Water as a Solvent • Nearly all reactions in the environment and, especially in organisms, take place in water • Water is a covalent molecule where the shared pair of electrons is attracted more strongly toward the oxygen nucleus forming a partially negative charged “pole” near the Oxygen nucleus and a partially positively charged pole near the hydrogen proton nucleus • The H-O-H arrangement forms an angle (104.5o) • The bent shape and polar bonds produce a polar molecule

5. Solubility • Substances (solutes) soluble in water • In some cases, the force of the attraction between the ions’ solid substance form mixed with water is so strong that it cannot be overcome by the interaction of the ions with the polarized water molecules. These materials will be insoluble in water • The interaction with water depends on the structure of the molecule. • If the interaction between ion and water is strong, the substance will be soluble • If the interaction is weak. the substance will not be very soluble

6. Solubility • Substances (solutes) soluble in water • If a covalent molecule contains polar groups, they will interact well with the polar solvent water, i.e., they will dissolve! • A few covalent molecules, such as HCl, dissociate completely into ions • In general, covalent compounds which produce ions in an aqueous solution interact with the water molecules to form either: • H+(aq) or H3O+ (aq) ions (acidic solutions) or • OH–(aq) ions (basic or alkaline solutions)

7. Ions in Aqueous Solution • Ionic Compounds as Electrolytes • In an Ionicsolid, the oppositely charged ions (Cations & Anions) are held together by electrostatic attraction • In aqueous solution, the electrostatic attraction is replaced with Water molecules • Many ionic compounds dissociate into independent ions when dissolved in water • Compounds that “freely” dissociate into independent ions in aqueous solution are called electrolytesbecause the solutions are good conductors of electricity 7

8. Ions in Aqueous Solution • Molecular Compounds as Electrolytes • Not all electrolytes are ionic compounds • Molecular compounds consist of individual molecules whose physical state depends on intermolecular forces (to be discussed later), not ionic attractions • Some molecular compoundsdissociate into ions • The resulting solution is electrically conducting, and is called an electrolyte

9. Ions in Aqueous Solution • Molecular Compounds as Nonelectrolytes • Some molecular compounds dissolve but do not dissociate into ions • These compounds contain their own polar bonds, which interact with the polar bonds of water • They do not conduct an electric current, i.e., they produce a nonconducting solution • The compounds are referred to as nonelectrolytes

10. Ions in Aqueous Solution • Ionic Theory of Solutions • Strong and Weak Electrolytes • A strong electrolyteis an electrolyte that exists in solution almost entirely as ions • Most ionic solids that dissolve in water do so almost completely as ions, so they are strong electrolytes

11. Ions in Aqueous Solution • Ionic Theory of Solutions • Strong and Weak Electrolytes • A weakelectrolyteis an electrolyte that dissolves in water to give a relatively small percentage of ions • Most soluble molecular compounds are either: nonelectrolytes or weakelectrolytes

12. Ions in Aqueous Solution • Molecular and Ionic Equations • A molecular equationis one in which the reactants and products are written as if they were molecules, even though they may actually exist in solution as ions Note that Ca(OH)2, Na2CO3, and NaOH are all soluble compounds, but CaCO3 is a solid, i.e., a precipitate

13. Ions in Aqueous Solution • Molecular and Ionic Equations • An ionic equation, however, represents strong electrolytes as separate independent ions. This is a more accurate representation of the way electrolytes behave in solution

14. Ions in Aqueous Solution • Molecular and Ionic Equations • Complete and net ionic equations • A complete ionic equationis a chemical equation in which strong electrolytes (such as soluble ionic compounds) are written as separate ions in solution

15. Ions in Aqueous Solution • Molecular and Ionic Equations • Complete and net ionic equations (con’t) • A net ionic equationis a chemical equation from which the spectator ions have been removed • A spectator ionis an ion in an ionic equation that does not take part in the reaction • Ex. Reaction between Calcium Nitrate [Ca(NO3)2] & Potassium Carbonate [K2CO3] Potassium (K+) and Nitrate (NO3-) are spectator ions that do not participate in the reaction

16. Practice Problem Limestone (Calcium Carbonate – CaCO3) is insoluble in water but dissolves when Hydrochloric Acid (HCl) is added. Write balanced total and net ionic equations Ans: Calcium Carbonate dissolves in HCl(aq) because the Carbonate ion, a base, reacts with the acid to form H2CO3 which decomposes into CO2(g) and H2O(l). CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2CO3(aq) Total ionic equation: CaCO3(s) + 2H+(aq) + 2 Cl–(aq) → Ca2+(aq) + 2 Cl–(aq) + H2O(l) + CO2(g) Net ionic equation: CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)

17. Types of Chemical Reactions • Most of the reactions we will study fall into one of the following categories • Precipitation Reactions • Acid-Base Reactions • Oxidation-Reduction Reactions

18. Types of Chemical Reactions • Precipitation Reactions • A precipitation reaction occurs in aqueous solution because one product is insoluble • A precipitate is an insoluble solid compound formed during a chemical reaction in solution • For example, the reaction of Sodium Chloride with Silver Nitrate forms AgCl(s), an insoluble precipitate

19. Types of Chemical Reactions • Precipitation Reactions • Predicting Precipitation Reactions • To predict whether a precipitate will form, we need to look at potential insoluble products. • The following table lists eight solubility rules for ionic compounds. These rules apply to the most common ionic compounds ClO4-

20. Solubility Rules

21. Solubility Rules (con’t) General Solubility Rules • All Compounds of the Ammonium Ion (NH4+) and the Alkali Metal (Group IA) cations are “Soluble” • All Nitrates and Acetates (Ethanoates) are “Soluble” • All Chlorides, Bromides and Iodides are “Soluble” except those of Ag, Pb, Hg • All Sulfates are “Soluble” except those of Ag, Pb, Hg(I), Ba, Sr, Ca • All Carbonates, Sulfites and Phosphates are “Insoluble” except those ofAmmonium (NH4+), and Alkali metal (Group IA) cations • All Hydroxides are “Insoluble” except those of NH4+, Alkali Metal (Group IA) cations • All Sulfides are “Insoluble” except those of NH4+, Alkali Metal (Group Ia) cations and Alkali Earth metal (Group II) cations • All Oxides are “Insoluble” except those of Calcium, Barium, and Alkali Metal(group IA) cations, which actually react with water to form hydroxides

22. Types of Chemical Reactions • Precipitation Reactions • Predicting Precipitation Reactions • Suppose you mix together solutions of Nickel(II) Chloride, NiCl2, and Sodium Phosphate, Na3PO4 • How can you tell if a reaction will occur, and if it does, what products to expect? Con’t

23. Types of Chemical Reactions • Precipitation Reactions (Con’t) • Predicting Precipitation Reactions (Con’t) • Precipitation reactions have the form of an “exchange reaction” • An exchange (or metathesis) reaction is a reaction between compounds that, when written as a molecular equation, appears to involve an exchange of cations and anions

24. Types of Chemical Reactions • Precipitation Reactions (Con’t) • Predicting Precipitation Reactions (Con’t) • Now that we have predicted potential products, we must balance the equation • We must verify that NiCl2 and Na3PO4 are soluble and then check the solubilities of the products

25. Types of Chemical Reactions • Precipitation Reactions (Con’t) • Predicting Precipitation Reactions (Con’t) • The Table of Solubility (slides 19-21) indicates that the reactants, Nickel(II) Chloride and Sodium Phosphate are both soluble • Looking at the potential products we find that Nickel(II) Phosphate is not soluble • Sodium Chloride is soluble

26. Types of Chemical Reactions • Precipitation Reactions (Con’t) • Predicting Precipitation Reactions (Con’t) • To see the reaction that occurs on the ionic level, we must rewrite the molecular equation as an ionic equation • We predict that a reaction occurs because Nickel(II) Phosphate is insoluble and precipitates from the reaction mixture

27. Types of Chemical Reactions • Precipitation Reactions • Predicting Precipitation Reactions (con’t) • First, write strong electrolytes (the soluble ionic compounds) in the form of ions to obtain the complete ionic equation

28. Types of Chemical Reactions • Precipitation Reactions • Predicting Precipitation Reactions (con’t) • After canceling the spectator ions, you obtain the net ionic equation – The “Essential Equation”

29. Practice Problem When solutions of Barium Chloride and Sodium Sulfate are mixed, the spectator ions in the resulting reaction are a. only Ba2+ b. only SO42– c. only Na+ d. only Cl– e. both Na+ and Cl– Ans: e BaCl2 + Na2SO4  BaSO4 (s) + 2Na+ + 2Cl- BaSO4 (s) Precipitate does participate in reaction Na+ & Cl-Spectator ions do not participate in reaction

30. Practice Problem Predict whether a chemical reaction occurs when Potassium Fluoride (KF) is mixed with Strontium Nitrate (Sr(NO3)2) Molecular Equation: 2KF(aq) + Sr(NO3)2 SrF2(s) + 2KNO3(aq) Total Ionic Equation: 2K+(aq) + 2F- (aq) + Sr2+(aq) +2NO3-(aq)  SrF2(s) + 2K+(aq) + 2NO3-(aq) Net Ionic Equation: Sr2+(aq) + 2F-(aq)  SrF2(s) Strontium Fluoride is a solid, therefore a reaction has occurred K+ & NO3- are spectator ions and do not participate in the reaction

31. Types of Chemical Reactions • Acid-Base Reactions • The Arrhenius Concept • The Arrhenius concept defines acids as: substances that produce Hydrogen ions, H+, when dissolved in water • An example is Nitric Acid, HNO3, a molecular substance that dissolves in water to give H+ and NO3-

32. Types of Chemical Reactions • Acid-Base Reactions • The Arrhenius Concept • The Arrhenius concept defines bases as substances that produce Hydroxide ions, OH-, when dissolved in water • An example is Sodium Hydroxide, NaOH, an ionic substance that dissolves in water to give sodium ions and hydroxide ions

33. Types of Chemical Reactions • Acid-Base Reactions • The Arrhenius Concept • The molecular substance Ammonia, NH3, is a base in the Arrhenius view • It yields Hydroxide ions when it reacts with water

34. Types of Chemical Reactions • Acid-Base Reactions • The Brønsted-Lowry Concept • The Brønsted-Lowry concept of acids and bases involves the transfer of a proton (H+) from the acid to the base • In this view, acid-base reactions are: proton-transfer reactions

35. Types of Chemical Reactions • Acid-Base Reactions • The Brønsted-Lowry Concept • In the reaction of Ammonia with Water • The H2O molecule is the acid because it donates a proton • The NH3 molecule is a base, because it accepts a proton H+

36. Types of Chemical Reactions • Acid-Base Reactions • The Brønsted-Lowry Concept • The H+(aq) ion associates itself with Water to form H3O+(aq) • this “mode of transportation” for the H+ ion is called the “Hydronium” ion

37. Types of Chemical Reactions • Acid-Base Reactions • The Brønsted-Lowry Concept • The dissolution of Nitric Acid, HNO3, in water is therefore a proton-transfer reaction • Where HNO3 is an acid (proton donor) and H2O is a base (proton acceptor) H+

38. Types of Chemical Reactions • Acid-Base Reactions • Strong and Weak Acids and Bases • A strong acidis an acid that ionizes completelyin water; it is a strong electrolyte Common Strong Acids: HCl, HBr, HI, HNO3, H2SO4, HClO4

39. Types of Chemical Reactions • Acid-Base Reactions • Strong and Weak Acids and Bases • A weak acidis an acid that only partiallyionizes in water; it is a weak electrolyte • The Hydrogen Cyanide molecule, HCN, reacts with water to produce a small percentage of ions in solution

40. Types of Chemical Reactions • Acid-Base Reactions • Strong and Weak Acids and Bases • A strong baseis a base that is present entirelyas ions, one of which is OH-; it is a strongelectrolyte • The hydroxides of Group IA and IIA elements, except for Beryllium Hydroxide, are strong bases

41. Types of Chemical Reactions • Acid-Base Reactions • Strong and Weak Acids and Bases • A weak baseis a base that is only partially ionized in water; it is a weakelectrolyte • Ammonia, NH3, is an example

42. Types of Chemical Reactions • Acid-Base Reactions • Strong and Weak Acids and Bases • It is important to be able to identify an acid or base as strong or weak • In an ionic equation, strong acids and bases are represented as Separate Ions • Weak acids and bases are represented as: Undissociated “molecules”in ionic equations

43. Types of Chemical Reactions • Acid-Base Reactions • Reactions of Weak Acids • Acetic Acid is a weak acid (dissociates very little) • Molecular: • Total Ionic: • Net Ionic: Note: The Na+ ions are called “Spectator ions” and cancel out

44. Types of Chemical Reactions acid base salt • Acid-Base Reactions • Neutralization (Acid-Base) Reactions • One of the chemical properties of acids and bases is that they neutralize one another • A neutralization reaction is a reaction of an acid and a base that results in an ionic compound and water • The ionic compound that is the product of a neutralization reaction is called a salt

45. Types of Chemical Reactions • Acid-Base Reactions • Neutralization (Acid-Base) Reactions • The net ionic equation for each acid-base neutralization reaction involves a transfer of a proton • Consider the reaction of the strong acid, HCl(aq) and a strong base, LiOH(aq)

46. Types of Chemical Reactions • Acid-Base Reactions • Neutralization Reactions • Writing the strongelectrolytes in the form of ions gives the complete ionic equation

47. Types of Chemical Reactions • Acid-Base Reactions • Neutralization Reactions • Canceling the spectator ions results in the net ionic equation. Note the proton transfer H+

48. Types of Chemical Reactions • Acid-Base Reactions • Neutralization Reactions • In a reaction involving HCN(aq), a weak acid, and KOH(aq), a strong base, the product is KCN, a strong electrolyte • The net ionic equation for this reaction is: H+ Note the proton transfer

49. Practice Problem Complete and balance the following neutralization equation (in aqueous solution); include phase labels Then write the net ionic equation Ba(OH)2 + HC2H3O2 Ans: Molecular: Ba(OH)2 + 2HC2H3O2  Ba(C2H3O2)2 (aq) + 2H2O(l) Total Ionic: Ba+2(aq) + 2OH-(aq) + 2H+(aq) + 2C2H3O2-(aq)  Ba+2(aq) + 2C2H3O2-(aq) + 2H2O(l) Net Ionic: 2OH-(aq) + 2H+(aq)  2H2O(l) (Ba+2 & C2H3O2- are spectator ions)

50. Types of Chemical Reactions • Acid-Base Reactions • Acid-Base Reactions with Gas Formation (Displacement Reactions) • Carbonates react with acids to form CO2, Carbon Dioxide gas and water • Sulfites react with acids to form SO2, Sulfur Dioxide gas and water