Reactions in Solution

1. AP Chemistry Reactions in Solution

2. solution: a homogeneous mixture of two or more substances -The ______is present in greatest quantity. solvent -- Any other substance present is called a ______. solute aqueous solutions: solutions in which water is the dissolving medium (i.e., the solvent) electrolyte: any substance whose aqueous solution will conduct electricity e.g., HCl, NaCl, KOH -- as opposed to a nonelectrolyte, a substance in aqueous solution that will NOT conduct electricity e.g., any sugar (C6H12O6, C12H22O11) or any alcohol (CH3OH, CH3CH2OH)

3. + H H H H H H H H H H H H O O O O O O As a general rule, ionic solids dissociate into ions in aqueous solution. The partial (–) charge on the O and the partial (+) charge on the H atoms allow H2O to interact strongly with, and “pull out,” ions in the crystal lattice. Thus, ionic compounds are often strong electrolytes. d+ d+ d– – – – – – – + + + + + + – – – – – – + + + + + +

4. For molecular compounds, structural integrity of molecules is maintained. Substance may dissolve, but generally won’t split into ions. Thus, mol. comps. tend to be nonelectrolytes. -- major exceptions: acids and NH3 (ammonia) **When molecular compounds DO split into ions, it is called ionization, not dissociation.

5. lots of product lots of reactant Strong electrolytes exist almost completely as ions in aqueous solution. e.g., HCl(aq) H+(aq) + Cl–(aq) KCl(aq) K+(aq) + Cl–(aq) (note the one-sided arrow) (note the double arrow) Weak electrolytes produce only a small concentration of ions in reaching equilibrium. e.g., CH3COOH(aq) CH3COO–(aq) + H+(aq) HF(aq) H+(aq) + F–(aq)

6. hydrochloric, HCl hydrobromic, HBr hydroiodic, HI chloric, HClO3 perchloric, HClO4 nitric, HNO3 sulfuric, H2SO4 Some of the strong electrolytes are the strong acids and strong bases. STRONG ACIDS STRONG BASES the hydroxides of... Li, Na, K, Rb, Cs, Ca, Sr, Ba “strong base cations”

7. Be careful to distinguish between dissolution and dissociation/ionization in regard to strongs or weaks. For example, CH3COOH dissolves completely, but ionizes only slightly; it is therefore aweak electrolyte. On the other hand, Ba(OH)2dissolves very little, but the amount that does dissolve dissociates almost completely. Ba(OH)2 is astrong electrolyte. The question is: Of the amount that dissolves, what fraction dissociates/ionizes? If… “most” STRONG WEAK If… “not much”

8. Precipitation reactions are reactions in solution that form an insoluble product. The insoluble product is called a... precipitate. Solubility Guidelines for Selected Ions in Aqueous Solution CH3COO–, Alk+, no exceptions Soluble NO3–, NH4+ Br–, I–, Cl– except with Hg22+, Ag+, and Pb2+ SO42– except with Hg22+, Ba2+, Sr2+, and Pb2+ Insoluble except with Alk+ and NH4+ PO43–, CrO42–, CO32– except with NH4+ and “strong base cations” S2–, OH–

9. CH3COO–, Alk+, no exceptions Soluble NO3–, NH4+ Br–, I–, Cl– except with Hg22+, Ag+, and Pb2+ except with Hg22+, Ba2+, Sr2+, and Pb2+ SO42– Saul ‘Chuck’ Cooawlkay knows exceptions? Naaaah. sol CH3COO–Alk+ NO3– NH4+ Saul Brickell double-hugged Agatha… and Paul Bunyan, too. sol Br–, I–, Cl– Hg22+ Ag+ Pb2+ Saul Sulf ate two huge bars… and peanut butter, too. sol SO42– Hg22+ Ba2+ Sr2+ Pb2+

10. Insoluble except with Alk+ and NH4+ PO43–, CrO42–, CO32– except with NH4+ and “strong base cations” S2–, OH– “The poor crow was cold; he huddled with everyone, but Al K. said, “Naaaah.” PO43– CrO42– CO32– Alk+ NH4+ “Soooooo… You two are always combined.” “Naaaaht when we’re strongly basic.” NH4+ SBCs S2– OH–

11. Saul ‘Chuck’ Cooawlkay knows exceptions? Naaaah. sol CH3COO–Alk+ NO3– NH4+ the honorable, no-nonsense judge: Saul ‘Chuck’ Cooawlkay

12. Saul Brickell double-hugged Agatha… and Paul Bunyan, too. sol Br–,I–,Cl– Hg22+ Ag+ Pb2+ Saul Brickell double-hugging Agatha Saul Brickell double-hugging the Paul Bunyan trophy

13. Saul Sulf ate two huge bars… and peanut butter, too. sol SO42– Hg22+ Ba2+ Sr2+ Pb2+ the slightly sickened Saul Sulf two HUGE, “I’m-going- into-a-sugar-coma” bars peanut butter

14. The poor crow was cold; he huddled with everyone, but Al K. said, “Naaaah.” PO43– CrO42– CO32– Alk+ NH4+ the poor crow who was cold attempting to huddle with everyone the poor crow who was cold the somewhat aloof Al K. politely declining the crow’s overtures to huddle

15. ‘Soooooo… You two are always combined.’ “Naaaaht when we’re strongly basic.” NH4+ SBCs S2– OH– “OH!”

16. Writing Net Ionic Equations for Double Replacement Reactions ions • A “net ionic equation” only shows the _________ that were used to make the precipitate. • Some ions were always dissolved in water. These are called “________________ ions”. Ions that sat back and watched the reaction take place… but did not participate Example: CaCl2 (aq) + 2AgNO3(aq) Ca(NO3)2(aq) + 2AgCl (s) Overall IonicEquation Written as Ions Dissolved in Water: ___ (aq) + ___ (aq) + ___ (aq) + _____ (aq) ___ (aq) + _____ (aq) + _________(s) • Cancel out the spectator ions, and you are left with the Net Ionic Equation! ________ + _________  __________ spectator 2AgCl Ca+2 2Cl− 2Ag+ 2NO3 − Ca+2 2NO3 − 2Cl−(aq) 2Ag+(aq) 2AgCl(s) • Must be BALANCED

17. Writing Net Ionic Equations for Double Replacement Reactions KNO3 (aq) BaCO3 Practice Problem: Write the net ionic equation for the following reaction. K2CO3(aq) + Ba(NO3)2(aq) _________ + ________(s) Net Ionic Equation = _________________________________ (s) CO3 −2 (aq) + Ba+2(aq) BaCO3 • Must be BALANCED

18. Suppose you mix solutions of lead(II) nitrate and sodium iodide. Pb(NO3)2 NaI The ions present are... Pb2+, NO3–, Na+, I– Write the overall ionic equation… Pb2+ + 2 NO3– + 2 Na+ + 2 I– PbI2 + 2 NO3– + 2 Na+ (aq) (aq) (aq) (aq) (ppt) (aq) (aq) Cancel the spectator ionsto get the net ionic equation… Pb2+ + 2 I– PbI2 (aq) (aq) (ppt)

19. Acids and Bases For now, acids ionize in aqueous solutions to form a hydrogen ion (H+). -- “proton donors” -- monoprotic acids e.g., HCl, HNO3, CH3COOH -- diprotic acids e.g., H2SO4 H2SO4(aq) H+(aq) + HSO4–(aq) In soln… HSO4–(aq) H+(aq) + SO42–(aq) (only first ionization is complete) With weak acids, the first ionization isn’t even close to complete.

20. The pH Scale

21. For now, bases are substances that accept • hydrogen ions (i.e., protons). -- OH– is very basic H+(aq) + OH–(aq)  H2O(l) -- equation: H+ + OH– H2O The most common weak base is ammonia, NH3; it ionizes only about 1%. -- equation: NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)

22. Classify as a… strong electrolyte, weak electrolyte, or nonelectrolyte. LiOH strong HClO3 strong C6H12O6 non- HClO weak If 0.40 mol of each of the following are dissolved in 2.5 L of water, rank them from least to greatest electrical conductivity. HBrO2 Ca(CH3COO)2 CH3OH KNO3 CH3OH < HBrO2 < KNO3 < Ca(CH3COO)2

23. Neutralization Reactions • Neutral means the acid & base cancel out and are no longer present in the solution. • There are two ways to determine the moles of base that must be added to neutralize the acid (or vice-versa) • Molarity equation & Stoichiometry • Molarity equation & Titration (lab technique) Moles H+ = Moles OH-

24. Neutralization Reactions & Titrations • Lab technique designed to measure the volume needed to neutralize a solution. Substance of known concentration (M) Titrant is added to flask until the moles OH- = moles H+ Equivalence Point! Substance of unknown concentration (M)

25. A neutralization reaction has the form: ACID + BASE  SALT + WATER -- recall that “salt” means an ionic compound Write the balanced molecular equation when nitric acid reacts w/barium hydroxide. HNO3 Ba(NO3)2 2 + Ba(OH)2  + 2 H2O (aq) (aq) (aq) (l) NO3– NO3– H+ Ba2+ OH– Ba2+ H2O Write the net ionic equation for the above reaction. H+ + OH–  H2O

26. Write the balanced molecular eq. when perchloric acid reacts w/potassium hydroxide. HClO4 KClO4 + KOH  + H2O (aq) (aq) (aq) (l) ClO4– ClO4– H+ K+ OH– K+ H2O Write the net ionic equation for the above reaction. H+ + OH–  H2O For EVERY strong acid/strong base rxn, the net ionic equation is… H+ + OH– H2O.

27. Three other anions that act as bases (i.e., as p+ acceptors) are the sulfide ion (S2–), the carbonate ion (CO32–), and the bicarbonate ion (HCO3–). All react w/acids to form gases. -- sulfide ion: reacts w/acids to form H2S(g) e.g., HCl(aq) + K2S(aq) H2S(g) + KCl(aq) 2 2 Net ionic eq… 2 H+(aq) + S2–(aq)  H2S(g) Hydrogen sulfide is produced by anaerobic bacteria associated with water collection and treatment processes. It is corrosive, odorous, toxic, and contributes to the formation of acid rain.

28. -- carbonate and bicarbonate ions: react w/acids to form CO2(g) HNO3(aq)+ CaCO3(s) CO2(g) + H2O(l) + Ca(NO3)2(aq) 2 HCl(aq)+ NaHCO3(s or aq) CO2(g) + H2O(l) + NaCl(aq) The acidification of baking soda (i.e., sodium hydrogen carbonate or sodium bicarbonate) with vinegar (i.e., acetic acid) is a well-known reaction that has carbon dioxide as one of the products.

29. Molarity A solution’s concentration tells us the amount of solute per solvent. mol A common unit of concentration is molarity. M L Molarity=mole/Liter -- equation: What mass of magnesium nitrite is needed to make 3.25 L of a 0.35 M solution? Mg2+ NO2– Mg(NO2)2 mol = M(L) = 0.35 M (3.25 L ) = 1.1375 mol 1.1375 mol = 130 g Mg(NO2)2

30. Steps for Properly Mixing an Aqueous Solution 1. Fill an appropriate container (e.g., graduated cylinder or volumetric flask) mostly full of water (~80% full). This is an approximate technique and should take very little time. 2. Weigh out the proper amount of solute and mix it into the water from Step 1. 3. “Top off” the solution to the proper volume and mix. DONE.

31. strong electrolyte What is the conc. of sodium ions in a 0.025 M solution of sodium phosphate? Na+ PO43– Na3PO4 Na3PO4(aq) 3 Na+(aq) + PO43–(aq) 0.075 M 0.025 M

32. Dilutions Aqueous acids (and sometimes bases) can be purchased in concentrated form and diluted to any lower concentration. A purchased bottle of acid is called a concentrate or a stock solution. -- **Safety Tip: When diluting, add acid or base to water, not the other way around. C = conc. D = dilute Dilution Equation: MCVC = MDVD Conc. phosphoric acid is 14.8 M. What volume of concentrate is req’d to make 25.0 L of 0.500 M acid? (VC) = 14.8 0.500 (25.0) VC = 0.845 L

33. MCVC = MDVD

34. Serial Dilutions • Taking one concentrated solution and diluting it several times to get a series of solutions at various dilute concentrations!

35. Solution Stoichiometry NaOH H2SO4 V of gases at STP V of sol’ns What volume of 0.150 M sulfuric acid is needed to neutralize 26 g sodium hydroxide? H2SO4 + NaOH  Na2SO4 + H2O 2 2 26 g NaOH = 0.325 mol H2SO4 = 2.2 L of 0.150 M H2SO4 mass mass vol. vol. mol mol M L M L part. part.

36. What mass of lead(II) nitrate will consume 85.0 mL of 0.45 M sodium iodide? 2 2 Pb(NO3)2 + NaI  PbI2 + NaNO3 molNaI = M(L) = 0.45 M (0.085 L ) = 0.03825 molNaI Pb(NO3)2 NaI 0.03825 molNaI = 6.3 g Pb(NO3)2

37. Titrations If we don’t know a solution’s concentration, we react a second solution of known concentration – called a standard solution–with the first. Based on the stoichiometryof the reaction, we can determine the unknown solution’s concentration. -- This procedure is called a titration.

38. The equivalence pointof a titration occurs when stoichiometrically equivalent quantities are brought together. This point is identified by using indicators, which are chemicals whose color depends on the pH. A sudden color change indicates the end pointof the titration, which coincides closely with the equivalence point, and is usually considered to be “good enough.”

39. Titration Curve Graph • Sometimes the titration data is graphed and the equivalence point volume is read off the graph! ???? Spike in pH indicates the equivalence point

40. Titration Curve Graph ???? HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq)

41. Titration Curve Graph HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq)

42. If 56.0 mL of sodium hydroxide neutralize 19.0 mL of 0.235 M nitric acid, find the concentration of the base. mol H+ = mol OH– H+ + NO3– HNO3 0.235 M 0.235 M Na+ + OH– NaOH X M X M [ H+ ] VA = [ OH– ] VB 0.235 M (19.0 mL) [ OH– ] (56.0 mL) = [ OH–] = 4.465 M(ml) 56.0ml = MNaOH = 0.0797 M NaOH

43. Oxidation-Reduction (a.k.a., Redox) Reactions In redox reactions, electrons are transferred between species. Oxidationis the loss of electrons, so when a substance is oxidized, its charge… increases. Think about what O wants to do when it gets near stuff. Reduction is the gain of electrons, so when a substance is reduced, its charge… decreases.

44. Transfer of Electrons • The transfer of electrons is called an oxidation-reduction (REDOX) reaction. Leo Ger: lose e- oxidized; gain e- reduced Oil Rig: oxidation is loss; reduction is gain

45. Types of Redox Reactions • Many different types of reactions are also redox • Synthesis • Decomposition • Single Replacement • Combustion • Some reactions are NOT redox • Double replacement

46.  Common Oxidation States

47. (rare) Rules for Assigning Oxidation Numbers 1. Atoms in their elemental form have an oxidation number of zero. F2, Al, Mo 2. For a monatomic ion, the oxidation number is the charge on the ion. Ca2+, Pb4+, N3– 3. Nonmetals can have variable oxidation numbers. a. Oxygen is usually 2–, but in the peroxide ion (O22–) it is 1–. b. Hydrogen is 1+ when bonded to nonmetals, 1– when bonded to metals. **H– can happen only with the metals. c. Fluorine is 1–. Other halogens are usually 1–, but are + when combined w/oxygen. in BrO2–, Br is… 3+ 4. The sum of the oxidation numbers in a neutral compound is zero. H2O, Fe2O3

48. Practice Problems Br = 0 (element by itself) Assign Oxidation States to each element in the compounds below • Br2 • FeO • HF • K3PO4 • CN-1 Fe = +2 O = -2 (overall neutral) H = +1 F = -1 (overall neutral) K = +1 P = +5 O = -2 (overall neutral) C = +2 N = -3 (N=more electronegative; overall -1)

49. Determine the oxidation number of nitrogen in each of the following. NH3 N2O4 N= 3- H3= 3+ Rule Oxygen is 2–, • O4= 2- * 4 =8- • N2= 4+ * 2 = 8+ N= 3– NH3 is used in many types of cleaners. N= 4+ Why? monatomic ion, oxidation # is the charge on the ion. N2O4 is a key component of smog. NO3– Rule: Oxygen is 2–, N2 O3– = -2x 3 = -6 Therefore N= 5+ N= 0 Why? Atoms in their elemental form have an oxidation number of zero. N= 5+ Potassium nitrate (saltpeter) is used in the making of black powder. N2 makes up nearly 80% of Earth’s atmosphere.

50. Practice Problem Assign oxidation states to each element in the reaction below. Which element was oxidized? Which element was reduced? • 2Mg(s) + O2(g)  2MgO(s) Whenever one substance is oxidized, another is reduced. -2 0 0 +2 O gained e- = reduced Mg lost e- = oxidized (reduced) (oxidized) “oxidant” “reductant” “oxidizing agent” “reducing agent”