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Understanding Acids and Bases: Definitions and Equilibrium Calculations

Explore the definitions of acids and bases, equilibrium constants, and how to calculate pH and ionization constants. Compare acid/base strengths and learn about polyprotic acids.

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Understanding Acids and Bases: Definitions and Equilibrium Calculations

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  1. Chapter 15 Acids and Bases Suroviec Spring 2014

  2. I. Definitions of Acids/Bases • Arrhenius • Acid: something that releases H+ in water • Base: something that releases OH- in water • Brønsted-Lowry • Acid: substance that donates a proton • Base: substance that accepts a proton

  3. Monoprotic vs. Polyprotic • Monoprotic • acid – only has 1 proton to donate • base – only can accept 1 proton • Polyprotic • acid – can donate several protons • Base – can accept several protons

  4. Conjugate Acid-Base Pairs • A pair of compounds that differ by the presence of one H+ is called a conjugate acid-base pair • Every reaction between a Brønsted acid and Brønsted base involves H+ transfer and has 2 conjugate acid-base pairs

  5. II. Water Ionization • Water equilibrium Water ionization constant Kw = 1.00 X 10-14 at 25.0oC

  6. II. Water Ionization • pH scale • One way to describe the concentration of H+ in solution or the acidity of the solution

  7. D. Determining [H3O+] and [OH-] pH = 1.00 pH = 10.50

  8. IV. Equilibrium constants A. Acid • Acids and bases are either: • strong electrolytes: completely dissociates • weak electrolytes: ionizes very little in water • Ionization constants measure extent of ionization: • K>1: ionize extensively, strong acid/base • K<1: does not ionize extensively, weak acid/base

  9. B. Bases

  10. C. Compare Acid/Base strength • Look at the pKa value and compare to the acid strength

  11. D. Relationship between Ka, Kb and Kw • As acid strength increases Ka increases • As base strength increases Kb increases • Consider the relation between a conjugate acid-base pair • HB (aq) ⇌ H+ (aq) + B- (aq) KI= Ka of HB • B- (aq) + H2O ⇌ HB (aq) + OH- (aq) KII = Kb of B-

  12. III. Acid/Base reactions A. Strong Acid/Strong Base

  13. B. Strong – Weak

  14. V. Equilibrium Calculations • Determining K from pH 0.015 M HOCN has a pH of 2.67, what is the [H3O+]? What is the Ka of HOCN?

  15. B. Determining the Equilibrium Concentrations and pH from Ka • Weak acid ionization Given a weak acid (HA) concentration of 0.040M with a Ka of 4.0 x 10-9 what are the equilibrium concentrations of HA, A-, H3O+

  16. C. 5% rule • Codeine (Cod) is a powerful and addictive painkiller. It is also a weak base. • Write the reaction of weak base Cod with H2O • If the Ka of Hcod (the weak conjugate acid) is 1.2×10-8, what is the Kb of Cod? • Find the pH of a 2.0×10-3M solution of Cod in water.

  17. VI. Polyprotic Acids • Acids capable of donating more than 1 proton

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