Solutions: Reactions of Ions & Molecules in Aqueous Solutions

1. SolutionsPart I: Reactions of Ions & Molecules in Aqueous Solns(from Chapter 4 Sec 1 thru 6of Jespersen 7TH ed) Dr. C. Yau Spring 2015 1

2. What is a Solution? A "solution" is a homogeneous mixture of one or more solute(s) and a solvent. solute + solvent solution The physical state of the solute and the solvent can be quite varied: salt solution NaCl (s) + H2O (l) air N2 (g) + O2 (g) bronze Cu (s) + Sn (s) booze CH3CH2OH(l) + H2O (l) 2

3. What is a Solution? The fact that it is a "homogeneous mixture" tells us that the components … • are present in a uniform mixture throughout the sample. • cannot be written with a formula. (Components are not in a fixed ratio as in a compound.) • cannot have a molar mass. • are not joined by "chemical bonds." 3

4. Concentration of a Solution The "concentration" of a solution refers to how much solute is present. A "concentrated solution" contains a large amount of solute. A "dilute solution" contains a small amount of solute. A "saturated solution" contains the maximum amount of dissolved solute. Any amount beyond the saturation point will remain undissolved. 4

5. Concentration of a Solution "Concentrated" and "dilute" are relativeterms: 1% 3% 5% A 3% solution is more concentrated than a 1% solution but more dilute than a 5% solution. A "saturatedsolution," however, is dependent on the solubility of the solute, under a particular condition, such as a specified temperature. e.g. Saturated soln of NaCl at 25oC is about 60%. 5

6. Solubility • A solute is said to be "soluble" if it dissolves appreciably in a given solvent. • It is said to be "insoluble" if it does not. • Solubility generally is affected by temperature. • Solids generally (but not always) dissolvebetter at a higher temperature. • Gases generally dissolve better at a lower temperature. • Sugar dissolves faster in hot water, but • you can “boil” off the gas from soda to make it flat.

7. What does a compound dowhen it dissolves? REMEMBER THIS!!!!! It depends… An ionic compounddissociates into ions which are then surrounded by solvent molecules. A molecular compoundstays intact as molecules as each molecule becomes surrounded by solvent molecules… EXCEPT for certain ACID molecules. How do you recognize acids from a given formula? Its formula generally has H in front: HCl, H2SO4

8. Ionic Compound in Solution Molecular Compound in Solution Na+ Cl C12H22O11 C12H22O11 C12H22O11 C12H22O11 C12H22O11 C12H22O11 C12H22O11 C12H22O11 C12H22O11 C12H22O11 Na+Cl Na+ClNa+Cl Cl Na+ClNa+ClNa+Na+Cl Na+ClNa+Cl C12H22O11(aq) NaCl (aq) Note: Sugar molecules are still intact. Note: Na+ and Cl- ions have separated in the water.

9. Electrolytes An "electrolyte" is a substance that conducts electricity when dissolved in water. What conducts electricity? …mobile, charged particles. Common charged particles involved in conducting electricity are… cations, anions, electrons

10. Common Electrolytes • Water-soluble ionic compounds.They separate into ions that become mobile in solution. • Strong acidsMolecules change into ions when dissolved in water and become mobile. REMEMBER THIS!!! Electrolytes consist of... • Water soluble ionic compounds, or • STRONG acids

11. What Else Conducts Electricity? • Ionic compounds are generally solids. As solids, the ions are NOT mobile, and therefore they do NOT conduct. • Ionic compounds in solution conduct because the ions become mobile. • Ionic compounds in their molten state (as a liquid) will conduct, because their ions become mobile.

12. What Else Conducts Electricity? • Metals conduct electricity because their electrons are mobile (move from atom to atom) • This is totally different from molecular and ionic compounds where electrons are confined within the molecule or ion.

13. Types of Electrolytes • Strong electrolytes are ones that conduct electricity well. (They fully dissociate into ions.) • Weak electrolytes are ones that conduct electricity weakly. (They dissociate only partially into ions.) • Nonelectrolytes are ones that do not conduct electricity. (They do not dissociate into ions.)

14. Ionic Compounds in Solution When table salt dissolves in water, we write... NaCl (s) NaCl (aq) or NaCl (s) Na+ (aq) + Cl- (aq) One f.u. of NaCl dissociates into two ions. One mole of NaCl dissociates into two moles of ions. Ex 4.1 (NH4)2SO4 (s) (NH4)2SO4 (aq) or (NH4)2SO4 (s) 2 NH4+ (aq) + SO42-(aq) In contrast, one mole of (NH4)2SO4 dissociates into threemoles of ions. Pract.Exercises 4.1 & 4.2 on p.160

15. Ionic Compounds in Solution How many moles of ions will 1 mole of ammonium phosphate form when dissolved in water? A. one mole B. two moles C. three moles D. four moles E. none of the above Write the equation showing the dissolution of ammonium phosphate. Remember to always include physical states.

16. Ionic Compounds in Solution How many moles of ions will 4 moles of zinc nitrate form when dissolved in water? A. 2 moles B. 3 moles C. 4 moles D. 12 moles E. none of the above Write the equation showing the dissolution of zinc nitrate. Remember to always include physical states.

17. How many moles of ions will form when one mole of barium sulfate is placed in water? Trick question! Answer is none!...hardly any to be exact. WHY? BaSO4 is not soluble in water! How do we know that?.... You need to memorize the solubility rules! (Back in your previous chemistry you were probably given these rules at exams...not any more!)

18. Know these solubility rules WELL. MEMORIZE THEM! Ionic compounds are solids to begin with. Soluble Compounds All cmpds of the alkali metals (Group IA) are soluble. Most common is Na+. All salts containing NH4+, NO3-, ClO4-, ClO3- and C2H3O2- are soluble. All cmpds of Cl-, Br-, I- are soluble except combined with Ag+, Pb2+ and Hg22+ (mercury(I) ion). All sulfates (salts containg SO42-) are soluble exept those of Ca2+, Sr2+, Ba2+, Pb2+ and Hg22+. Insoluble Compounds 5. All hydroxides and oxides are insoluble except those of NH4+, Group IA and Ca2+, Sr2+ and Ba2+. 6. Assume all other ionic compounds to be insoluble unless specified otherwise.

19. Using the Solubility Rules Which of the following are soluble in water? A. FeSO4 B. K2CO3 C. ammonium phosphate D. HgCl2 E. PbBr2 Which of the above are electrolytes?

20. Precipitation Reactions A "precipitate" is a solid formed when two aqueous solutions are mixed. It is NOT just any solid formed in a reaction. e.g. AgNO3 + NaCl AgCl + NaNO3 (aq) (aq) (s) (aq) AgCl is a precipitate (commonly abbreviated as ppt). CaCO3 (s) CaO (s) + CO2 (g) CaO is not considered a "precipitate." Why?

21. 3 Types of Equations for each reaction. When aqueous solutions of lead(II) nitrate and potassium iodide are mixed, a precipitate forms. First predict the products, then check solubility. Molecular Equation Pb(NO3)2 (aq) + 2KI (aq) PbI2 (s) + 2KNO3 (aq) Total Ionic Equation or Ionic Equation Pb2+ + 2NO3- + 2K+ + 2I- PbI2 + 2K+ + 2NO3- (aq) (aq) (aq) (aq) (s) (aq) (aq) Net Ionic Equation Pb2+ (aq)+ 2I- (aq) PbI2 (s) Spectator Ions: 2K+ and 2NO3- Check List: 1) Coefficients are at lowest ratio. 2) Net charge on left = net charge on right.

22. Fig. 5.7 p.161 NO3-(aq) K+(aq) PbI2 (s) Pb2+ NO3- K+ I-

23. Example 4.2 p.162 Write the molecular, ionic and net ionic equations for the rxn of aqueous solns of lead(II) chlorate and sodium iodide, which yields a precipitate. Remember to check 1) Coefficients are at lowest ratio. 2) What is the other check? Do Practice Exercises 4.3 & 4.4 on p.163.

24. Acids and Bases Acids • taste sour • release H+ ions (H3O+) when placed in water • turn blue litmus paper red. • react with bases to form a salt. Bases • taste bitter • feel soapy (slippery) • turn red litmus paper blue. • increase OH- ion concentration when placed in water • react with acids to form a salt.

25. Acids in Solution Simplified depiction of acid ionization: HCl (g) H+ (aq) + Cl- (aq) What exactly is H+? What is in a H atom? How does it become H+ ion? H+ is simply a proton. H+ is often referred to as a "proton." An acid is often called a "proton-donor."

26. H+ in Solution H+ ions do not exist as such in solution! The nuclear charge is totally exposed and is highly attractive to the water molecules surrounding it. Instead of H+ we will consider it as a "hydrated proton" and write it as H3O+ (aq) This is the hydronium ion. Learn this name and formula!

27. Ionization of Acids in Water HCl H2O Cl- H3O+ Collision of HCl molecule with water molecule Note that H+ ions are never formed. They immediately became part of the hydronium ion.

28. Ionization of Acids in Water Instead of… HCl (g) H+(aq) + Cl- (aq) we write… HCl (g) + H2O (l) H3O+(aq) + Cl- (aq) Write the equation for the ionization of nitric acid. Write the equation for the ionization of hydrobromic acid.

29. 3 Definitions for Acids and Bases Arrhenius Definition An acid is a substance that reacts with water to produce a hydronium ion, H3O+. A base is a substance that produces OH- ions in water. Brønsted-Lowry Definition An acid is a proton donor. A base is a proton acceptor. Lewis Definition...(must wait breathlessly for next semester)

30. Monoprotic and Polyprotic Acids A monoprotic acid can produce only one H+ per molecule. A polyprotic acid can produce more than one H+ per molecule. e.g. H2SO4 + H2O H3O+ + HSO4- HSO4- + H2O H3O+ + SO42- Overall Rxn: H2SO4 + 2H2O 2H3O+ + SO42-

31. Ionization of Phosphoric Acid Example 4.4 p.171 Phosphoric acid is a triprotic acid. Write the three ionization equations and name each ion formed. Watch out for the charges of these ions! Do Pract. Exer. 4.9 & 4.10 on p.171.

32. Acid rain!! Nonmetal Oxides in Water Nonmetal Oxides are considered "acidicanhydrides." "Anhydride"means without water. When water is added to an acidic anhydride it becomes an acid. SO3 + H2O H2SO4 NO2 + H2O HNO3 CO2 + H2O H2CO3

33. Metal Oxides in Water In contrast, metal oxides are considered bases. When placed in water, metal oxides become metal hydroxides: Na2O + H2O 2NaOH CaO + H2O Ca(OH)2 REMEMBER!! Nonmetal oxides in water become ACIDS. Metal oxides in water become BASES.

34. Strong & Weak Acids Strong acids are ones that dissociate 100% when placed in water. They are strong electrolytes. Weak acids are ones that dissociate only partially. They are weak electrolytes. Do not confuse "strong acid" with "concentrated acid." One has to do with... the concentration of acid in solution. The other reflects the nature of the acid itself.

35. Strong & Weak Acids HCl is a strong acid: HCl (aq) + H2O(l)H3O+ (aq)+ Cl-(aq) MEMORIZE THESE 7 COMMON STRONG ACIDS: HNO3 H2SO4 HClO4 HCl HBr HI HClO3 (HClO3 is borderline and we will considered it as being a strong acid.) All the other acids are weak acids.

36. Strong & Weak Acids Acetic acid is a weak acid. Write its ionization equation. HC2H3O2 + H2O H3O+ + C2H3O2- The two arrows show this is a reversible reaction, favoring the left side of the equation. Acetic acid is a weak electrolyte. Why? Write the ionization equation for another weak acid. Practice Exercise 4.5 & 4.6 on p.167

37. Bases • Metal oxides and compounds that contain OH-are bases because when placed in water they produce OH-. e.g. NaOH, Ba(OH)2, Na2O (Why?) Na2O + H2O 2NaOH 2. Molecular compounds that react with water to produce OH- are bases (even though they do not contain OH- themselves). NH3 (aq) + H2O (l) NH4OH (aq) Learn this!

38. Bases NH3 (aq) + H2O (l) NH4OH (aq) NH3 is ammonia. NH4+ is the ammonium ion. Note that ammonia has no OH- and yet it is a base. Why is it a base? Note the two arrows. This is a reversible reaction favoring the left side. NH3 is a weak electrolyte.

39. NH3 as a base NH3(aq) + H2O (l) NH4OH (aq) NH4OH (aq) is actually NH4+(aq) + OH- (aq) What exactly happened in this reaction? proton donor proton acceptor A proton (H+) is transferred from H2O to NH3. H2O is acting as a proton donor. NH3 is acting as a proton acceptor. Brønsted Definition!

40. N-Bases (nitrogen-bases) N-bases are the molecular compounds that behave as bases. They are similar to ammonia. CH3NH2 + H2O CH3NH3+ + OH- proton acceptorproton donor Note methylamine has no OH- but it is a base because it produces OH- when placed in water. Write the equation for the ionization of ethylamine: CH3CH2NH2

41. Ex 4.3 p.169 Dimethylamine, (CH3)2NH, is a base in water. It is used commercially as an attractant for boll weevils so they can be destroyed. This insect has caused more than 414 billion loss to the yield of cotton in the U.S. since it arrived from Mexico in 1892. Write an eqn for the ionization of (CH3)2NH in water. Practice Exer 4.7 & 4.8 on p. 169

42. brightly lit dimly lit dimly lit HCl (aq) HC2H3O2(aq) NH3 (aq) Fig 5.10 p.171

43. Neutralization Reaction The reaction of an acid with a base is called a neutralization reaction. (…but not all neutralizations end up with neutral solutions!) An acid-base reaction is often taught as producing water and a salt. This is not always true!

44. Neutralization Reaction It always produces a salt, but not always water. What is a salt? A salt is a compound formed by the reaction of an acid with a base. It is any ionic compound excluding ones that contain OH- or O2- (hydroxide or oxide ions). HCl + NaOH HOH + NaCl HCl + NH3 NH4Cl

45. Neutralization Based on Brønsted-Lowry Definition An acid is a proton donor A base is a proton acceptor. HCl + NaOHHOH + NaCl proton proton donor acceptor

46. Neutralization Forming Only a Salt (& No Water) HCl + NH3NH4Cl proton proton salt (no water) donor acceptor

47. Double Replacement Rxn AB + CD AD + CB “Switching partners” Why AD and not AC? Why CB and not BC? Generally, AB + CD AD + CB involves A+B- C+D- Rxn also known as “metathesis,” from Greek meaning “different order.”

48. Predicting Reactions For precipitation reactions, we can predict whether the reaction will “go” or “not go” by examining the solubility of the products. If one or more of the products is a solid (the precipitate) the reaction will go. Prediction of precipitation reactions hinges on the knowledge of the solubility rules.

49. Writing Precipitation Reactions Example 4.6 p. 175 Predict whether a rxn will occur when aqueous solns of Pb(NO3)2 and Fe2(SO4)3 are mixed. Write molecular, ionic and net ionic equations for it. Do Pract Exer. 4.13 & 4.14 p. 176

50. Writing Neutralization Reactions You can assume all acid-base reactions will go. We will not deal with the exceptions in this class. Exceptions will be covered in Gen Chem II. Writing ionic and net ionic equations is a little different from those of precipitation reactions. You MUST know which acids are strong and which are weak.