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TOPIC V: Ions in Aqueous Solutions

TOPIC V: Ions in Aqueous Solutions. LECTURE SLIDES: Precipitation Reactions Solubility : Acids, Bases, Salts Net Ionic Equations Acid/Base Reactions Gas Forming Reactions. Kotz & Treichel, Chapter 5, Sections 5.1- 5.6.

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TOPIC V: Ions in Aqueous Solutions

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  1. TOPIC V: Ions in Aqueous Solutions • LECTURE SLIDES: • Precipitation Reactions • Solubility : Acids, Bases, Salts • Net Ionic Equations • Acid/Base Reactions • Gas Forming Reactions Kotz & Treichel, Chapter 5, Sections 5.1- 5.6

  2. Chapter FiveDouble Replacement Reactions in Aqueous Solutions:AB(aq) + CD(aq) AD + CB A) Precipitation reactions B) Acid/ Base reactions C) Gas Formation reactions NOTE: All these reactions are often represented by a balanced “net ionic equation” which we will meet as we look at these types!

  3. “The Game Players” Generally, two aqueous solutions, each containing a 100% ionized solute, a “strong electrolyte,” are required to commence action in any of these three types of reactions. AB(aq) A+(aq) + B-(aq) CD(aq) C+(aq) + D-(aq) In order to appreciate what is happening, we must examine all terms ...

  4. “AQUEOUS SOLUTIONS” “AQUEOUS SOLUTION” = homogeneous mixture of some solute in a specific solvent, water HOMOGENEOUS:uniform compositionthroughout, one phase. Liquid solution: transparent, no boundaries, layers, bubbles or solid particles visible.

  5. “Strong Electrolytes” Double Replacement reactions in aqueous solutions occur when both reactants are “100% ionized” in aqueous solutions. H2O AB ----------> A+(aq) + B-(aq) H2O CD ----------> C+(aq) + D-(aq) The solutes described above are called “strong electrolytes”. To better understand this concept, let’s consider the following demonstration and video clip.

  6. DEMONSTRATION!!! Compounds which form ions in water solution are considered “electrolytes” because their presence allows the solution to conduct electric current. SOLUTES TO BE BE TESTED AS ELECTROLYTES: salt (NaCl), sugar (C12H22O12), hydrochloric acid (HCl), acetic acid (vinegar, HC2H3O2), ammonia (NH3), alcohol (CH3CH2OH), sodium hydroxide (NaOH) We are going to see what happens to the demo light bulb when the electrodes are immersed in first pure water, and then into water containing these solutes.

  7. If there are NO ions present in solution, the liquid or solution will NOT conduct a current and the light bulb will not “light up” If the solute present in the solution is completely ionized, the solution will readily conduct a current and the light bulb will “light up brightly” If the solute is ionized to a small extent (“mostly molecular”), then the light bulb may “glow faintly”

  8. Summary, Results:

  9. We can sort out our results into three categories, based on our observations: Strong electrolytes:allow current to flow through solution: NaCl, HCl, NaOH, all 100% ionized in solution Weak Electrolytes:allow a small amount of current to flow through solution: HC2H3O2 and NH3 (aq) as “NH4OH”, small amount of ionic presence, mostly “molecular” in nature Non Electrolytes: No ions, no current: molecular in nature: water, alcohol (CH3CH2OH), sugar (C12H22O12).

  10. STRONG ELECTROLYTES: REACTANTS, DOUBLE REPLACEMENT REACTIONS Salts: metal or ammonium cation , monatomic or polyatomic anion: NaCl K2SO4 AgNO3 NH4BrO3 CuI2 Strong Bases: Soluble Metal hydroxides: NaOH, KOH Strong Acids: H written first in the formula, Strong:HCl, HBr, HI, H2SO4, HNO3

  11. Salts in water: Cation, not H+, Anion, not OH-

  12. BASES IN WATER: Cation not H+; Anion: OH-

  13. ACIDS IN WATER: H+Cation; Anion, not OH-

  14. Group Work 5.1: Electrolytes in Solution

  15. Double Replacement Reactions “go to completion” because collisions between some of the mixed anions and cations causes precipitates, molecules or gases to form, removing ions from solution. Our first type of reaction between aqueous solutions containing electrolytes involves forming a precipitate when the solutions are mixed. This type of reaction goes to completion because ions are removed from solution as an insoluble precipitate.

  16. Reactions in Aqueous Solutions #1: Precipitation reactions AB(aq) + CD(aq) AD(s) + CB(aq) [A+(aq) +B-(aq)]+ [C+(aq)+ D-(aq)] AD(s) + [C+(aq)+B-(aq)] “salt A(aq) + salt B (aq) salt C (s) + salt D (aq)”

  17. NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3 (aq)

  18. Net Ionic Equations When we finish writing and balancing a double replacement reaction, we go several steps further: • we prepare a “total ionic equation” in which all • soluble electrolytes are representing as separate ions • we cancel out any ion present on both sides of the • equation • the final result is called the “net ionic equation”. • We will practice this procedure as we go through the • various types of double replacement reactions.

  19. Total balanced equation: AB(aq) + CD(aq) AD(s) + CB(aq) [A+(aq) +B-(aq)]+ [C+(aq)+ D-(aq)] AD(s) + [C+(aq)+B-(aq)] A+(aq) + D-(aq)  AD(s) Total Ionic Equation: Net Ionic Equation:

  20. Total equation: NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3 (aq) Total Ionic Equation: Na+ (aq) + Cl- (aq) + Ag+(aq) + NO3- (aq) AgCl(s) + Na+ (aq) + NO3- (aq) Net Ionic Equation: Ag+(aq) + Cl- (aq) AgCl(s)

  21. SOLUBILITY OF “Strong Electrolytes” IN WATER To predict when a precipitate will form, we need to know some solubility guidelines: Acids: Mostly water soluble, commercially available in water solution Salts and Bases: if both cation and anion are large in size and small in charge, ( +1,- 1), it is probably soluble in H2O. Checkout following tables ...

  22. THE CATION IS: Na+ K+ NH4+ Always! ORTHE ANION IS*: Cl-, Br-, I- ClO4-, ClO3- NO3- SO42- CH3CO2- *Mostly..... The Electrolyte is Usually Water Soluble if:

  23. There are a few notable exceptions to the solubility guide on the last slide, principally the ones noted below, which you should be aware of: Insoluble in Water: AgCl, PbI2 BaSO4

  24. Group Work 5.2: SOLUBILITY WORKSHEET #1

  25. GROUP WORK 5.3: SOLUBILITY WORKSHEET #2 Name H2O Soluble?

  26. Reactions in Aqueous Solutions #1 Precipitation Reactions This type of reaction goes to completion if and only if any recombination of the reactant ions produces an insoluble precipitate. Let’s Consider two possible reaction sequences: K2SO4 (aq) + (NH4)3PO4 (aq)---> ? CuSO4 (aq) + (NH4)3PO4 (aq)---> ?

  27. K2SO4 (aq) + (NH4)3PO4 (aq)---> ? Step One: Write out the four reactant ions, decide if any combination is insoluble: Cations: Anions: K+ (no ppt) SO42- (usually soluble) NH4+ (no ppt) PO43- (ppt???) Decision: No cation to precipitate with the phosphate ion, therefore, no reaction! K2SO4 (aq) + (NH4)3PO4 (aq) ---> NR, no reaction

  28. CuSO4 (aq) + (NH4)3PO4 (aq)---> ? Step One. Write out each reactant ion. Decide if any combination of ions will produce a precipitate: Cations: Anions: Cu2+(ppt???) SO42- (usually soluble) NH4+ (no ppt) PO43- (ppt???) Decision: The copper(II) ion will form an insoluble precipitate with the phosphate ion and therefore reaction will occur.

  29. Step Two: since reaction will occur to form insoluble copper(II) phosphate, determine the correct formula for the products. Cu2+ (PO4)3- ---> Cu3(PO4)2 (NH4)+ + (SO4)2- ---> (NH4)2SO4 SMART ACTION: DOUBLE CHECK YOUR FORMULAS! Charge per ion: 2+ 3- 1+ 2- (NH4)2SO4 Cu3(PO4)2 Total Charge: 6+ 6- 2+ 2-

  30. Step Three: Do a total balanced equation: 3CuSO4 (aq) + 2(NH4)3PO4 (aq) ---> Cu3(PO4)2 (s) + 3 (NH4)2SO4 (aq) Step Four: Do a total ionic equation, showing the actual species involved in solution: [3 Cu2+ (aq)+ 3 SO42- (aq)] + [6 NH4+ (aq) + 2 PO43- (aq) ] ---> Cu3(PO4)2 (s) + [6 NH4+ (aq) + 3 SO42- (aq) ]

  31. Step Five: Prepare a “net ionic equation” for this reaction, by eliminating any ion which appears on both sides of the equation, A “SPECTATOR ION” : 3Cu2+ (aq)+ 3SO42- (aq) + 6NH4+ (aq) + 2 PO43- (aq) ---> Cu3(PO4)2 (s) + 6NH4+ (aq)+ 3SO42- (aq) NET IONIC EQUATION 3Cu2+ (aq)+ 2 PO43- (aq) ---> Cu3(PO4)2 (s)

  32. GROUP WORK 5.4: NET IONIC EQUATION a) Na2CO3 (aq) + Al(NO3)3 ( aq) ---> ? b) K2CO3(aq) + NH4NO3 (aq) ---> ? c) CrCl3 (aq)+ K3PO4(aq) --->? 1. Decide which of above is NR (“no reaction”) by writing down all four ions involved For reaction(s) which “go to completion”: 2. Complete Product Formulas 3. Balance Equation 4. Do Total Ionic Equation 5. Do Net Ionic Equation

  33. Reactions in Aqueous Solutions: #2 Acid/Base Reactions Acid + Base ---> Salt + H2O These reactions are double replacement, like the precipitation reactions we just studied. However, in this case, the reaction will ALWAYS go to completion because an un-ionized molecule, water, is formed. In both cases (precipitation; acid/base), the removal of ions from solution causes the reaction to go to completion...

  34. ACIDS are defined in the most common (“Arrhenius”) system as substances which increase the H+ ion concentration when dissolved in water. ACIDS may be recognized by the convention of writing H first in the formula of the compound; in general the formula contains H plus some cation except OH-. Strong acids are 100% ionized in aqueous solutions and therefore “strong electrolytes”; Weak Acids are generally <5% ionized in aqueous solutions and therefore “weak electrolytes”.

  35. Let’s consider names and formulas of common acids and bases which we meet in these reactions: COMMON STRONG ACIDS: HCl Hydrochloric acid HBr Hydrobromic acid HI Hydroiodic Acid HNO3 Nitric Acid HClO4 Perchloric Acid H2SO4 Sulfuric Acid Note names: learn!

  36. COMMON WEAK ACIDS:* H3PO4 Phosphoric Acid H2CO3 Carbonic Acid H2SO3 Sulfurous Acid H2S Hydrosulfuric Acid CH3CO2H Acetic Acid * and many, many more..... Learn Names, Recognize!

  37. BASES are defined in the Arrhenius system as substances which increase the OH- ion concentration when dissolved in water. BASES may be recognized as the combination of some metal or ammonium cation plus the OH- or O2- anion. The list of strong bases in water is severely limited by the lack of solubility of most metal hydroxides and oxides in water; all of these compounds do however react with acids to yield salts plus water.

  38. COMMON STRONG BASES (LiOH Lithium Hydroxide) NaOH Sodium Hydroxide KOH Potassium Hydroxide These are the only bases classified as strong electrolytes because they are the only ones soluble in water. Other metal hydroxides and oxides are basic but insoluble.

  39. AMMONIA AS A WEAK BASE As we have seen earlier: NH3(g) + H2O ---> NH4+(aq) + OH-(aq) <------------- 99% 1% This reaction with water produces a small number of hydroxide ions in water solution, so aqueous ammonia is considered a weak base.

  40. SALTS IN AQUEOUS SOLUTION All common salts which are water soluble are strong electrolytes, 100% ionized in the aqueous solution. Salts can be recognized by their formulas, in which a metal or ammonium cation is written first and some anion second.

  41. NET IONIC EQUATIONS FOR ACID BASE REACTIONS NOTE: If any acid and base are mixed together and at least one of them is in aqueous solution , a reaction will always occur and go quickly to completion: the formation of the water molecule from the H+ of the acid and the OH- or O2- of the base is very energy releasing and exothermic. Acid + Base -----> Salt + H2O Both strong and weak types react in this fashion!

  42. Net Ionic Equations for Acid /Base Reactions 1. Recognize that all reactions between acids and bases go to completion if at least one of them is in aqueous solution. 2. Decide on formula of salt product to accompany the H2O formed. 3. Balance the Equation 4. Do a Total Ionic Equation 5. Do a Net Ionic Equation

  43. Reaction #1 HCl(aq) + KOH(aq) ---> ? ---> 1. H2O + salt 2. Formulas of products: Combine the H and OH to form water, and the leftover ions to make the salt: Cl- + K+ ----> KCl 3.Balance the equation (ok as written): HCl(aq) + KOH(aq) ---> HOH(l)+ KCl (aq) 4. Total Ionic Equation: H+(aq) + Cl-(aq) + K+ (aq) + OH-(aq) -----> H2O(l) + K+ (aq) + Cl- (aq)

  44. 5. IDENTIFY THE SPECTATORS: H+ (aq) + Cl- (aq) + K+ (aq) + OH- (aq) -----> H2O(l) + K+ (aq) + Cl- (aq) NET IONIC EQUATION: H+ (aq) + OH- (aq) -----> H2O(l)

  45. Now, another: 1. H3PO4 (aq) + NaOH(aq) ---> ?-----> H2O + salt 2. Predict the salt formula and complete equation: ( PO4)3- + Na+ ---> Na3PO4 H3PO4 (aq) + NaOH(aq) ---> H2O(l) + Na3PO4(aq) 3. balance the equation: H3PO4 (aq) + 3 NaOH(aq) --->3H2O(l) + Na3PO4(aq)

  46. H3PO4 (aq) + 3 NaOH(aq) --->3H2O(l) + Na3PO4(aq) 4. Convert into total ionic equation, noting that the phosphoric acid is a weak electrolyte, about 99% molecular, so we do not show its ions in water: H3PO4 (aq) + 3 Na+(aq) + 3 OH- (aq) ---> 3H2O(l) + 3 Na+(aq) + PO43-(aq) 5. NET IONIC EQUATION: H3PO4 (aq) + 3 OH- (aq) ---> 3H2O(l) + PO43-(aq)

  47. Group Work 5.5 Do Net Ionic Equations for the following: 1. Fe2O3 (s) + HCl(aq) ---> ? 2. H2CO3 (aq) + KOH (aq) --->? 3. H2SO4 (aq) + Ba(OH)2 (s) -----> ?

  48. 5.5 Gas-Forming Reactions Finish up on Net Ionic Equations: third type of double replacement reactions: 1. Precipitation Reactions 2. Acid/Base Reactions 3. Gas Formation Reactions Two types considered: Sulfide salt + acid -----> salt + H2S(g) Carbonate salt + acid -----> salt + CO2(g) + H2O Formation of gaseous product causes reaction completion

  49. Sulfide Salt + Acid Sulfide salt + acid ----> new salt + H2S(g) K2S(aq) + H2SO4 (aq) -----> K2SO4 (aq) + H2S(g) 2 K+(aq) + S2- (aq)+ 2 H+2 (aq)+ SO42- (aq) -----> 2 K+(aq) + SO42- (aq) + H2S(g) 2 H+2 (aq) + S2- (aq)-----> H2S(g)

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