1 / 19

General Chemistry II 2302102

General Chemistry II 2302102. Acid and Base Equilibria. Lecture 2. i.fraser@rmit.edu.au Ian.Fraser@sci.monash.edu.au. Acids and Bases - 3 Lectures. Outline - 5 Subtopics. Autoionization of Water and pH (completed) Defining Acids and Bases (completed)

satya
Download Presentation

General Chemistry II 2302102

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. General Chemistry II2302102 Acid and Base Equilibria Lecture 2 i.fraser@rmit.edu.au Ian.Fraser@sci.monash.edu.au

  2. Acids and Bases- 3 Lectures Outline - 5 Subtopics • Autoionization of Water and pH (completed) • Defining Acids and Bases (completed) • Interaction of Acids and Bases with Water • (part-completed) • Buffer Solutions • Acid-Base Titrations

  3. Acids and Bases Objectives - Lecture 2 • By the end of this lecture AND completion of the set problems, you should : • Distinguish between strong & weak bases • Calculate equilibrium concentrations of acids & bases using basicity constants (Kb) & relate to Ka • Determine the direction of acid-base reactions • Understand the concept of buffer solutions, buffer action following dilution, following the addition of strong acid and following the addition of strong base.

  4. Acids & Bases - 2 Lectures Outline Introduction to acids and bases Strong & Weak Acids Conjugate acid-base pairs Common Ion Effect Lecture 1 • Bases • Buffers • Indicators • Titrations • Strong Acids • Weak Acids Lecture 2

  5. Acids & Bases - Lecture 2 Objectives By the end of this lecture AND completion of the set problems, you should be able to: • Distinguish between strong & weak bases • Calculate equilibrium concentrations of acids & bases using basicity constants (Kb) & relate to Ka • Determine the direction of acid-base reactions • Describe and calculate buffering of weak acids • Calculate concentrations from titration of a strong acid • Calculate concentrations and pKa from titration of a weak acid

  6. BASES

  7. B(aq) + H2O HB+(aq)+ OH-(aq) Equilibrium (‘Basicity’) constant, Kb. BASES Bases accept Protons and form acids.

  8. Bases accept Protons and form acids. B(aq) + H2O HB+(aq)+ OH-(aq) BASES pKb = -log10Kb Strong Bases Large Kb equilibrium lies to the right e.g. O2-(aq) Weak Bases Small Kb , large pKb equilibrium lies to left e.g.NH3(aq)

  9. NH4+(aq) + H2O NH3(aq) + H3O+(aq) Ka = 5.5 x 10-10 NH3(aq) + H2O NH4+(aq) + OH-(aq) Kb = 1.8 x 10-5 add equations: 2 H2O H3O+(aq) + OH-(aq) multiply equilibrium constants Kw = Ka x Kb BASES Relation between Kb and Kaof the conjugate acid.

  10. Assume NaNO2 (a salt) is completely ionised when dissolved, hence have 0.1 M NO2-. H2O(l) + NO2-(aq) HNO2(aq) + OH-(aq) initial 0.10 M 0 0 equilibrium (0.10 - x) M x M x M WEAK BASES Calculate [OH-], [H+] and pH when 0.1 mol of NaNO2 is dissolved in sufficient water to produce 1.0 L of solution. Ka for HNO2 = 4.5 x 10-4.

  11. H2O(l) + NO2-(aq) HNO2(aq) + OH-(aq) initial 0.10 M 0 0 equilibrium (0.10 - x) M x M x M WEAK BASES Assuming x << 0.10, then 2.2 x 10-12 = x2 thus, x = 1.5 x 10-6 M [OH-] = 1.5 x 10-6, [H+] = 1.0 x 10-14 / 1.5 x 10-6 so, [H+] = 6.7 x 10-9, pH = -log10(6.7 x 10-9) = 8.17

  12. H2O (l) + NH3 (aq) NH4+ (aq) + OH- (aq) Direction of Acid-Base Reactions • Acid-Base Reactions proceed spontaneously with the strongest acid and strongest base forming the weakest acid and the weakest base • Returning to: NH4+ is a stronger acid than H2O, and OH- is a stronger base than NH3 So reaction proceeds spontaneously to the left

  13. H2O(l) + NH4+(aq) H3O+(aq) + NH3(aq) For NH3, Kb = 1.8 x 10-5, pKb = 4.74 So pKa = 9.26 H2O (l) + H2O (l) H3O+(aq) + OH- (aq) For H2O, Ka = Kb = Kw = 1.0 x 10-14, So pKa = 14.00 Relative Strengths of Acids (Bases) • Pick the strongest acid (smallest pKa) NH4+ is a stronger acid than H2O, hence OH- is a stronger base than NH3

  14. Which direction is favoured here? HNO2 (aq) + CN- (aq) NO2-(aq) + HCN(aq) Relative Strengths of Acids (Bases) For HNO2, Ka = 4.5 x 10-4 so pKa = 3.35 For HCN, Ka = 7.2 x 10-10 so pKa = 9.14 HNO2 is a stronger acid than HCN, hence CN- is a stronger base than NO2-. Reaction strongly favours the right

  15. BUFFER SOLUTIONS (“BUFFERS”)

  16. HOCl(aq) + H2O(l) H3O+(aq) + OCl-(aq) initial 0.05 0 0.1 equilibrium (0.05 - z) z 0.1 + z BUFFER SOLUTIONS The presence of the conjugate base of an acid inhibits the ionization of the acid. e.g. a solution that contains 0.05M HOCl and 0.1M OCl- (Ka = 3.7 x 10-8) [HOCl] = 0.05 [OCl-] = 0.1 [H3O+] = 1.9 x 10-8 pH =7.72 [OH-] = 5.3 x 10-7

  17. BUFFER SOLUTIONS The presence of the conjugate base of an acid inhibits the ionisation of the acid. Compare the dissociation of 0.05 M HOCl in water with its dissociation in a solution containing 0.1 M OCl- ion. in water in 0.1 M OCl- [HOCl] 0.05 0.05 [OCl-] 4.3 x 10-5 0.1 [H3O+] 4.3 x 10-5 1.9 x 10-8 [OH-] 2.3 x 10-10 5.3 x 10-7 pH 4.37 7.73

  18. OCl-(aq) + H3O+(aq) HOCl(aq) + H2O(l) initial 0.1 M 1.85 x 10-8 0.05 M final 0.099 M 1.91 x10-8 0.051 M pH change 7.73 to 7.72 acid to water pH change 7.0 to 3.0 BUFFER SOLUTIONS BUFFER SOLUTION A mixture that contains a weak acid and its conjugate base. Addition of small amounts of acid or base result in only small changes of pH. Compare the effect of adding 10-3 M H3O+ to a solution containing of 0.05 M HOCl in water and 0.1M OCl- ion with the effect of adding 10-3 M H3O+ to pure water.

  19. Acids and Bases - End of Lecture 2 Objectives Covered in Lecture 2 • After studying this lecture and the set problems, • you should be able to: • Distinguish between strong & weak bases • Calculate equilibrium concentrations of acids & bases using basicity constants (Kb) & relate to Ka • Determine the direction of acid-base reactions • Understand the concept of buffer solutions, buffer action following dilution, following the addition of strong acid and following the addition of strong base.

More Related