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Chemistry 100

Chemistry 100. Aqueous Reactions. Solutions. A solution is a homogenous mixture of two or more substances One substance (generally the one present in the greatest amount) is called the solvent The other substances - those that are dissolved - are called the solutes. The Solution Process.

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Chemistry 100

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  1. Chemistry 100 Aqueous Reactions

  2. Solutions A solution is a homogenous mixture of two or more substances One substance (generally the one present in the greatest amount) is called the solvent The other substances - those that are dissolved - are called the solutes

  3. The Solution Process Favourable interactions between the solute and the solvent drive the formation of a solution Example: NaCl (an ionic solid) dissolving in water Water is a polar fluid (i.e., possesses a permanent dipole)

  4. Electrolytes Salt is an ionic compound. NaCl is dissolved in water - the ions separate. The resulting solution conducts electricity . A solute with this property is called an electrolyte

  5. Strong Electrolytes • Strong electrolytes - completely dissociated • Some molecular compounds dissolve in water to form ions. • Dissolve HCl (g) in water. • All the molecules dissociate. So it is also a strong electrolyte.

  6. Weak and Nonelectrolytes • Weak electrolytes - only some of the molecules dissociate, i.e., acetic acid • Compounds that do not dissociate - nonelectrolytes • Sugars • Ureas • Alcohols

  7. Acids Acid - a substance that ionizes in water to form hydrogen ions H+. HCl (aq) H+ (aq) + Cl(aq) What is H+? A hydrogen atom without its electron - a bare proton.

  8. Monoprotic, diprotic, triprotic One molecule of HCl gives one H+ ion: HCl  H+ + Cl We say that HCl is monoprotic - one proton One molecule of sulphuric acid, H2SO4, has two hydrogens to give away. It is said to be diprotic. Phosphoric acid, H3PO4 is triprotic.

  9. Some Chemical Structures

  10. Acetic Acid CH3COOH (aq) ⇄ CH3COO- (aq) + H+ (aq) The double arrow - the system is in chemical equilibrium!!!! • Generally write as CH3COOH, not HC2H3O2. • Weak acid - doesn’t dissociate completely

  11. Bases Bases are substances that accept (react with) H+ ions. Hydroxide ions, OH, are basic. They react with H+ ions to form water: H+(aq) + OH (aq)  H2O (l) Ionic hydroxides like NaOH, KOH, Ca(OH)2 are basic. When dissolved in water they form hydroxide ions.

  12. Ammonia solution NOTE - only some NH3 molecules react with water. Ammonia is a weak electrolyte. When ammonia gas dissolves in water, some NH3 molecules react with water: NH3(aq) + H2O(l) ⇄ NH4+(aq) + OH–(aq)

  13. Strong and Weak Acids and Bases Acids and bases that are strong electrolytes are called strong acids and strong bases. Strong acids are more reactive than weak acids. Likewise for bases. Note exception - HF, a weak acid, is very reactive

  14. Acids you should know Chloric acid HClO3 Hydrobromic acid HBr Hydrochloric acid HCl Hydroiodic acid HI Nitric acid HNO3 Perchloric acid HClO4 Sulphuric acid H2SO4 Acetic acid CH3COOH (weak)

  15. Bases you should know Know the following bases: Strong bases a) Hydroxides of alkali metals: LiOH, NaOH, KOH b) Hydroxides of the heavy alkaline earth metals: Ca(OH)2, Sr(OH)2, Ba(OH)2 Weak base: ammonia solution NH3

  16. Metathesis reactions A metathesis reaction is an aqueous solution in which cations and anions appear to exchange partners. AX + BY  AY + BX AgNO3 (aq)+ NaCl (aq)  AgCl (s) + NaNO3 (aq)

  17. Metathesis reactions (cont.) Three driving forces Precipitate formation (insoluble compound) AgNO3(aq)+ NaCl(aq)  AgCl(s) + NaNO3(aq)

  18. Metathesis Reactions (Cont’d) Weak electrolyte or nonelectrolyte formation HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) Gas formation 2HCl(aq) + Na2S(aq) 2 NaCl(aq) + H2S(g)

  19. Neutralization Mix solutions of acids and bases - a neutralization reactions occurs. acid + base  salt + water Salt does not necessarily mean sodium chloride!!!! Salt - an ionic compound whose cation (positive ion) comes from a base and whose anion (negative ion) comes from an acid

  20. Precipitation Reactions Some ionic compounds are insoluble in water. If an insoluble compound is formed by mixing two electrolyte solutions, a precipitate results.

  21. Precipitation (Cont’d) Solubility - maximum amount of substance that will dissolve in a specified amount of solvent. Saturated solution of PbI2 contains 1 x 10-3 mol/L. A compound with a solubility of less than 0.01 mol/L - insoluble. More accurately - sparingly soluble.

  22. Solubility Fact 1 NaCl, K2CO3, (NH4)2S are all soluble All the common ionic compounds of the alkali metals are soluble in water. The same is true of the compounds containing the ammonium ion, NH4+.

  23. Solubility Fact 2 Anion exception, salts of NO3 nitrate none CH3COO  acetate none Cl  chloride Ag+, Hg22+,Pb2+ Br  bromide Ag+, Hg22+,Pb2+ I  iodide Ag+, Hg22+,Pb2+ SO42 sulphate Ca2+, Sr2+, Ba2+, Hg22+, Pb2+ Salts containing the following anions are soluble

  24. Solubility Fact 3 Salts containing the following anions are insoluble Anion exception, salts of S2 sulphide alkaline metal cations, NH4+, Ca2+, Sr2+, Ba2+, CO32 carbonate alkaline metal cations, NH4+ PO43 phosphate alkaline metal cations, NH4+ OH hydroxide alkaline metal cations, Ca2+, Sr2+, Ba2+,

  25. Reaction forming gases A metathesis reaction can occur due to the formation of a gas which is not very soluble in water. Examples involving hydrogen sulphide and carbon dioxide

  26. Reactions forming H2S A metathesis reaction occurs when hydrochloric acid is added to a sodium sulphide solution. 2HCl(aq) + Na2S(aq)  H2S(g) + 2NaCl(aq) Net ionic reaction: 2H+(aq) + S2(aq)  H2S (g)

  27. Reactions involving CO2 H2CO3(aq) CO2(g) + H2O(l) Carbonates and bicarbonates may be thought of as the salts of carbonic acid H2CO3 – unstable!!

  28. Ionic Equations Consider the reaction HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) The above is known as the molecular equation Note: the compounds are ionic (except water)!!

  29. Ionic Equations #2 Let’s show ionic compounds as ions H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) Na+(aq) + Cl–(aq) + H2O(l) Some ions appear on both sides of the equation.

  30. Out with the spectators! Remove ions that appear on both sides H+ (aq) + Cl– (aq) + Na+ (aq) + OH– (aq)  Na+ (aq) + Cl– (aq) + H2O (l) The unchanged ions are called spectators

  31. The Net Ionic Equation Note that the equation is balanced for both mass and charge!!! We are left with is the net ionic equation: H+(aq) + OH–(aq) H2O(l)

  32. Another ionic reaction Place zinc metal in a hydrochloric acid solution – hydrogen is evolved!! Zn (s) + 2HCl (aq)  ZnCl2 (aq) + H2 (g)

  33. Why use ionic reactions? • They summarize many reactions. • neutralization of any strong acid by a strong base is given by H+(aq) + OH–(aq) H2O(l) • The chemical behaviour of a strong electrolyte  behaviour of its constituent ions. • Ionic equations can be written only for strong electrolytes which are soluble.

  34. Concentrations • How do we express the concentration of a solution? • Percentage is one way. • 2% milk • 35% cream. (These are not true solutions)!!! • Some beer is 5% alcohol • Note: % measurements can be %w/w, %w/v, %v/v

  35. Molarity Must work in moles to do chemical arithmetic. Chemists - molarity as their unit of solution concentration

  36. Dilution The volumes can be either millilitres (mL) or litres (L). • Dilute a solution • more solvent is added but the amount (mass or moles) of solute is unchanged. M1V1 = M2V2

  37. Ionic Concentration • NaCl in water - totally ionized into Na+ and Cl ions. • A 2.0 M NaCl solution • Na+ concentration will be 2.0 M • Cl concentration also 2.0 M • A 2.0 M solution of K2CO3, • K+ concentration will be 4.0 M • The concentration of CO32 2.0 M.

  38. Oxidation and reduction A piece of calcium metal exposed to the air will react with the oxygen in the air 2Ca(s) + O2(g) 2 CaO(s) Ca has been converted to an ion Ca2+ by losing two 2 electrons. Dissolve Ca in acid Ca(s) + 2H+(aq) Ca2+(aq) + H2(g) Again the Ca has lost 2 electrons — oxidation

  39. Redox reactions In the last two reactions, the Ca atom lost two electrons. Where did they go? When one substance is oxidized, another is reduced. An oxidation-reduction reaction occurs. Or a redox reaction occurs. Oxidation: loss of electrons (more positive) Reduction: gain of electrons (less positive)

  40. Oxidation of Metals - by air • Many metals react with oxygen in the air. • Na and K do so explosively! • Fe rusts - at a cost of $billions each year! • Aluminum oxidizes • oxide layer forms a skin which prevent further oxidation. Al hides its reactivity. • Gold and platinum do not react with oxygen. • Silver tarnishes mainly because of H2S in the air. • What does copper do?

  41. Oxidation of Metals - by acids Many metals react with acids: metal + acid  salt + hydrogen gas Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) Metals may also be oxidized by the salts of other metals. Recall your lab experiment Fe(s) + CuSO4(aq)  Cu(s) + FeSO4(aq)

  42. Activity Series We has seen that some metals react with air, some also react with acids to give hydrogen. We have seen that some metals can be oxidized by ions of other metals. All this is summarized in the activity series.

  43. Activity Series A metal can be oxidized by any ionbelow it Metals above H, react with acids to give H2 The further up the series, the more readily the metal is oxidized See your textbook (p 136) for more elements Li  Li+ + e K  K+ + e Ba  Ba2+ + 2e Ca  Ca2+ + 2e Na  Na+ + e Mg  Mg2+ + 2e Zn  Zn2+ + 2e Fe  Fe2+ + 2e Pb  Pb2+ + 2e H  H+ + e Cu  Cu2+ + 2e Ag  Ag+ + e Au  Au3+ + 3e

  44. Some observations on the series Lead (Pb) is above H, so is Al. But these metals are not attacked by 6M HCl. They form very protective oxides. Cu reacts with nitric acid (HNO3) because that acid is a strong oxidizing agent in addition to being an acid. Gold (Au) and platinum (Pt) are valuable because they are (a) rare and (b) unreactive - they do not tarnish

  45. Oxidation Numbers Oxidation number - a fictitious charge assigned to atoms either by themselves or when combined in compounds as an electron bookkeeping device. There are a number of simple rules that chemists use to assign oxidation numbers.

  46. Assigning Oxidation Numbers • In any elemental form (atom or molecule), an atom is assigned a 0 oxidation number • e.g. He, Cu, N in N2, S in S8 • For a monatomic ion, the oxidation number equals the charge • e.g., -1 for Cl in Cl-, +2 for Ca+2, -2 for S-2

  47. Assigning Ox. Numbers (#2) • Fluorine’s oxidation number is -1 in any compound. • e.g. -1 for F in CF4, but 0 for F in F2 • Oxygen’s oxidation number is -2 except when combined with fluorine or in peroxides. • e.g. -2 for O in H2O and OH-, +2 for O in OF2, -1 for O in H2O2

  48. Assigning Ox. Numbers (#3) • For elements in Groups IA, IIA & most of IIIA, oxidation numbers are positive and equal to the group number. • e.g. +3 for Al in AlCl3, +1 for Na in NaCl, +2 for Mg in Mg SO4 • Hydrogen has a +1 oxidation number. Exceptions to this rule are the metallic hydrides, in which it is -1. • e.g., +1 for H in H2O and CH3OH, -1 for H in NaH

  49. Assigning Ox. Numbers (#4) • The sum of the oxidation numbers of the atoms in a neutral compound is zero; in a polyatomic ion, the sum equals the charge. • e.g. see OH- and H2O above, +6 for S in SO4-2

  50. Balancing Oxidation-Reduction (Redox) Equations (#1) • Assign oxidation numbers to all atoms in the equation. • Note - polyatomic ion that is unchanged in the reaction may be treated as a single unit with an oxidation number equal to its charge.

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