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Review

Review. 1. What is the Bronsted -Lowry definition of an acid? Of a base? 2. Identify five properties of an acid and five properties of a base.

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Review

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  1. Review • 1. What is the Bronsted-Lowry definition of an acid? Of a base? • 2. Identify five properties of an acid and five properties of a base. • 3. State the Arrhenius definition of acids and bases. Explain why ammonia is classified as a base under the Bronsted-Lowry definition but night not be under the Arrhenius definition. • 4. On p. 603 in the reaction between ammonia and water the ammonia is a B-L base. Why is water the B-L acid in this equation?

  2. Section 2: Determining the Strength of Acids and Bases Chapter 18: Acids, Bases, and Salts

  3. Strong and Weak Acids • Strong Acid- like HCl readily transfers H+ ions to water to form H3O+ ions • Almost all HCl molecules dissociate into ions • Strong electrolytes • Ex: HCl + H2O H3O+ + Cl- • If the reaction is complete we use a single arrow in the equation • Weak acid- like acetic acid (HC2H3O2) does not readily transfer H+ ions to water • Very few molecules dissociate • Poor electrolytes • Ex: HC2H3O2 + H2O H3O+ + C2H3O2- • If in equilibrium use a double headed arrow

  4. Strong and Weak Bases • Strong bases have the greatest affinity for H+ ions • The OH- ion is a strong base. • Are strong bases good electrolytes? • Calcium oxide aka lime-most widely used base because O2- strongly attracts H+ ions • Ex: O2- + H2O 2OH- • Weak bases-only partially reacts with water to form OH- ions • The most common weak bases are ammonia and polyatomic ions with negative charges like carbonate and phosphate • In equilibrium • Ex: CO32- + H2O HCO3- + OH-

  5. Strength of Conjugate Acid-base Pairs • HCO3 + H2O H3O+ + CO32- • AcidConjugate Base • Looking at table 18-15 CO32- is a strong base relative to the weak acid HCO3-, this means that the reaction will proceed toward the acid side, or right to left. The reverse reaction will also occur. Because HCO3- is a relatively weak acid, CO32- is a relatively strong base.

  6. Strength of Conjugate Acid-base Pair • CN- + H2O HCN + OH- • Look at the table below. This means that the cyanide ion is relatively a stronger base than its conjugate acid HCN • Which way will this reaction tend to proceed? Is HCN a strong/weaker conjugate acid compared to HF?

  7. The Acid Dissociation Constant • HA + H2O H3O+ + A- • Where H is a hydrogen atom and A is the rest of the acid molecule • HA is a weak acid so it partially dissociates in water to a state of equilibrium • Keq=[H3O+][A-]/[HA][H2O] • This is the equation for the equilibrium constant from chapter 16 which states that when products and reactants are in a state of equilibrium a certain ratio of their concentrations has the same value, Keq

  8. The Acid Dissociation Constant • Keq=[H3O+][A-]/[HA][H2O] • Since the concentration of water [H2O] is a constant in dilute solutions we can move it to the left side getting • Keq[H2O]=[H3O+][A-]/[HA] • Because the product of any two constants is another constant it can be rewritten as Ka=[H3O+][A-]/[HA] where Ka is equivalent to Keq[H2O] • Ka is called the acid dissociation constant which is a measure of the strength of an acid • The higher the Ka, the stronger the acid • All weak acids have Ka<1 • See p. 610 table 18-17

  9. The Base Dissociation Constant • Derived from equilibrium rxn with water • B + H2O HB+ + OH- • Kb=[HB+][OH-]/[B] • The base dissociation constant is a measure of the strength of a base • See table p. 611 • All weak bases have a Kb<1 • The higher the Kb, the stronger the base

  10. Calculating Dissociation Constants • Read through sample problem 1 on p. 612 • Practice Problem 1 • You try Practice Problem 2

  11. More Practice • Ammonia is a weak base. If the initial concentration of ammonia is 0.150M and the equilibrium concentration of OH- is 1.6 x 10-3 calculate Kb.

  12. Acid Base Properties of Salts • Salts are strong electrolytes, they dissociate in water to form cations and anions • Salt Hydrolysis Reactions- the reactions of ions from salts to form H3O+ or OH- • Predict whether a salt hydrolysis reaction produces an acid or basic solution by looking at the acid and base from which the salt was formed. • There are 4 possibilities…

  13. Acid Base Properties of Salts • 1. Salts of strong acids and strong bases • These salt solutions are neutral • Ex. NaOH + HClNaCl + H2O • 2. Salts of strong acids and weak bases • These salt solutions are acidic • Ex. NH3 + HCl NH4Cl • 3. Salts of weak acids and strong bases • These salt solutions are basic • Ex. 2NaOH + H2CO3 Na2CO3 + 2H2O • 4. Salts of weak acids and weak bases • These solutions can be acidic, basic, or neutral depending on the relative strengths of the acids and bases from which the salt is formed. • We won’t discuss these in this chapter

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