Presentation on theme: "Strong and Weak Acids and Bases"— Presentation transcript:
1 Strong and Weak Acids and Bases UNIT IVStrong and Weak Acids and Bases
2 What is a Strong Acid?Strong Acid- An acid which is 100% ionized in a water solution. Ex.) HCl(g) + H2O(l) H3O+(aq)+ Cl-(aq)Single arrow (goes to completion) = Strong acidEx.) What is the [HCl(g)] in 1 M HCl?Ex.) What is [H3O+] in 0.20 M HCl
3 What is a Strong Acid? In a Strong Acid: [H3O+] = [Acid] Ex.) What is [H3O+] in 0.60 M HNO3
4 What is a Weak Acid?Weak Acid: An acid which is less than 100% ionized in solution. (In Chem 12 WA’s are usually significantly less than 100% ionized…usually < 5% ionized) In a solution of a weak acid, most of the molecules don’t ionize.Ex.) HF (g) H2O(l) H3O+ (aq) F- (aq)(Molecules) (Double arrow!) (Ions)id13.swf
5 Strength Versus Concentration Any acid (weak or strong) could have high or low concentration.Weak & Strong refers to % ionizationConcentration the moles of acid dissolved per litreExamples:10.0 M HCl conc. and strong [H3O+] = 10.0 M0.001 M HCl dilute and strong [H3O+] = M10.0 M HF conc. and weak [H3O+] = low0.001 M HF dilute and weak [H3O+] = very low*Concentration of IONS determines CONDUCTIVITY!
6 The Strong AcidsThe strong acids are the top six acids listed on the “Relative Strengths of Bronsted-Lowry Acids and Bases”, namely:Name FormulaPerchloric acid HClO4Hydriodic acid HIHydrobromic acid HBrHydrochloric acid HClNitric acid HNO3Sulphuric acid H2SO4(Note: H2SO4 is only strong for the first dissociation:H2SO4 → H+ + HSO4-)
7 The Weak AcidsAll the acids listed on the LEFT side of the table in the white section are “weak” acids.H3O+ H+ + H2O net result of adding any strong acid to waterHIO3 H+ + IO3-THE WEAK ACIDSH2O H+ + OH-Note: The bottom two species on the left NEVER act as acids....SINGLE ARROW GOING BACKWARDS!
8 What is a Strong Base?Strong Base- A base which is 100% ionized in a water solution. Ex. Ba(OH)2(s) Ba2+(aq) + 2OH-(aq) Ex.) What is the [OH-] in 0.10 M NaOH? Ex.) What is the [OH-] in 0.10 M Ba(OH)2 ?
9 [OH-] = [Base] x # of OH’s in Formula What is a Strong Base?For a strong base:[OH-] = [Base] x # of OH’s in Formula
10 What is a Weak Base?Weak Base: A base which is less than 100% ionized in solution.Ex.) NH3(aq) + H2O(l) NH4+ (aq) + OH- (aq)
11 The Strong BasesAny substance which dissociates completely to produce OH-, O2- or NH2- is a Strong Base:Alkali Metal Hydroxides (Group 1)Ex. LiOH, NaOH, KOH, RbOH, CsOHAlkaline Earth Metal Hydroxides (Group 2) Ex. Ca(OH)2, Ba(OH)2 , Sr(OH)2The hydroxide ion, OH-, found on the lower right side of the table, is the net result of adding any strong base to water.
12 The Weak BasesAll the bases listed on the RIGHT side of the table in the white section are “weak” bases.H2OIOTHE WEAK BASESPO43-H2O H+ + OH- net result of adding any strong base to water
13 SpectatorsThe CONJUGATE of a STRONG ACID or a STRONG BASE is a SPECTATOR.These are the top 5 (not 6) “bases” on the right.CIO4-I-Br They are so weak that they cannot react with H2O to form OH-Cl-NO3-
14 H----Cl + H2O H3O+ (aq) + Cl-(aq) SpectatorsH----Cl H2O H3O+ (aq) Cl-(aq)weak So weak , it cannot takean H+ from H2O or even H3O+Also, the alkali metal ions and alkaline earth metal ions are considered spectators since they are the conjugates of strong bases.
15 Amphiprotic Species are found on both sides of the table e.g.) HSO4- can act as acids (donate H+’s) or as bases (accept H+’s)to look at an amphiprotic species as an acid, you must find it on the left side:ExampleHCO3- is a ________er acid than C6H5OHHCO3- is a ________er acid than H2O2
16 Amphiprotic Speciesto look at an amphiprotic species as a base, you must find it on the right side:ExampleHCO3- is a _________er base than C6H5O73-HCO3- is a _________er base than Al(H2O)5(OH)2+HSO4- in shaded region on top right will not act as a base in water….but it is not a spectator (like NO3- is). Why not? (HSO4‑ is also found on the left side quite a way up, it is a relatively “strong” weak acid.)
17 The Levelling Effect What is [H3O+] in 1.0 M H3O+ ? ______ What is [H3O+] in 1.0 M HNO3? ______ What is [H3O+] in 1.0 M HCl ? ______Acids from HClO4 to H2SO4 are 100% ionized in waterSo, even though HClO4 is above HCl on the chart, it is no more acidic in a water solution.
18 H3O+ is the strongest acid that can exist in The Levelling EffectH3O+ is the strongest acid that can exist inundissociated form in water solution.All stronger acids ionize to form H3O+.The only way you can tell which strong acid is “stronger” is to react them in a non-aqueous (not H2O) solvent.Eg) HClO4 + H2SO4 H3SO4+ + ClO4- It is found that HClO4 donates a proton to H2SO4, not the other way around, so HClO4 is a stronger acid than H2SO4. This is not important in Chemistry 12. This would not happen in a water solution…in H2O, they would both form H3O+.
19 The Levelling EffectThe strongest base which can exist in high concentrations in water solution is OH-The two stronger bases below it will react with water completely to form OH-.Eg) O2- + H2O OH- + OH-OR…O2- + H2O 2OH-Ex. What is the final [O2-] in 1.0 M Na2O ?
20 The Levelling Effect Write the equation for NH2- reacting with H2O. Hebden Textbook Pages Questions #21-27
21 Acid-Base Equilibria & Relative Strengths of Acids & Bases Example Mix some H2PO4- and some CO32- H2PO4- + CO32- HCO3- + HPO42- (A) (B) (A) (B) At equilibrium, which will be favoured, reactants or products?
22 Acid-Base Equilibria & Relative Strengths of Acids & Bases Equilibrium favours the sidewith the weaker acid.ExampleWill HSO3- + HCO3- H2CO3 + SO32- favour reactants or products?
23 Acid-Base Equilibria & Relative Strengths of Acids & Bases Mixing Two Amphiprotic Ions (products not given)ExampleComplete the reaction and determine which is favoured, reactants or products? (Also, compare the relative strengths of the bases.)HSO4- + H2PO4-
24 Acid-Base Equilibria & Relative Strengths of Acids & Bases NOTICE:The STRONGER ACID is on the same side as the STRONGER BASE. (The stronger acid has the weaker conjugate base.)The WEAKER ACID is on the same side as the WEAKER BASE.Equilibrium favours the side with the weaker conjugate acid and the weaker conjugate base.
25 Acid-Base Equilibria & Relative Strengths of Acids & Bases Starting with “Salts” The amphiprotic ions are often products of the dissociation of salts.Spectator ions must be discarded.ExampleComplete the net ionic reaction and state whether equilibrium favours reactants or productsNaHSO3 + K2HPO4 →
26 Keq and Acid Strength If products are favored Keq is large (>1) If reactants are favored Keq is small (<1)Ex) Given:HA + B- HB + A- Keq = 0.003Which acid is stronger, HA or HB?Hebden Textbook Page 133 Questions # 38-46