Acids & Bases Lesson 4 Relative Strengths of Acids and Bases

Review of “Strong & Weak” Acids and Bases

Strong acids and bases They 100% ionize in solution. The ionization is NOT at equilibrium  - one-way reactions. -In other words, reverse rxns do not occur at all

Weak acids and bases They DON’T 100% ionize in solution. The ionization is at equilibrium -two-way reactions. -In other words, reverse rxns do occur.

Recall: The larger the Ka, the stronger the acid. The greater the [H 3 O + ] The larger the Kb, the stronger the base. The greater the [OH - ] Also, on your table; acids get stronger moving up. bases get stronger going down.

Relative Strengths When a bronsted-Lowry acid and base react, the position of the equilibrium results from the relative strengths of the acids and bases involved.

Relative Strengths When you compare two acids, the stronger acids are more able to donate H + ions so they will always drive the reaction to the position where they are donating their H + protons.

Relative Strengths of Acids & Bases H 2 PO CO 3 2-  HCO HPO 4 2- (A) (B) (A) (B) At equilibrium, which will be favored, reactants or products? Steps to take: Identify acids and bases Identify the STRONGER acid; H 2 PO 4 - KNOW that where you find your stronger acid, the stronger base will be on that same side of the equation. ALSO, it means your other side of the equation will have the weaker acid and the weaker base. Equilibrium will always favor/shift to the side containing the weaker acid.

Relative Strengths of Acids & Bases Equilibrium favors the side with the weaker acid. TRY: Will HSO HCO 3 -  H 2 CO 3 + SO 3 2- favor reactants or products?

Relative Strengths of Acids & Bases Example: Complete the reaction and determine which is favored; reactants or products? HSO H 2 PO 4 -

Relative Strengths of Acids & Bases TRY: Complete the reaction and determine which is favored; reactants or products? HSO 4 - +NO 2 -

Relative Strengths of Acids & Bases “Salts” All salts dissociate 100% in water. Ex: will the equilibrium favor reactants or products? (NH 4 ) 2 CO 3

Another way to figure out which side is favored Keq= K a (reactant acid) K a( product acid) If Keq is large (>1) products are favored If Keq is small (<1) reactants are favored

TRY: HS - and HCO 3 - mixed together. Use K eq to find which side the equilibrium favors?

Keq and Acid Strength Given: HA + B -  HB + A - Keq = Which acid is stronger, HA or HB?

TRY: The following equilibrium favors products Se -2 + HSO 4 -  HSe - + SO 4 -2 a)Which is the stronger of the two acids? b)If NaHSe was added to the solution, how would the equilibrium be affected?

Relative Strengths of Acids & Bases SUMMARY: The STRONGER ACID is on the same side as the STRONGER BASE. (The stronger acid will form the weaker conjugate base on the other side) 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. Keq= K a (reactant acid) K a( product acid) If Keq is large (>1) products are favored If Keq is small (<1) reactants are favored

Relative Strengths of Acids & Bases SUMMARY: Keq= K a (reactant acid) K a( product acid) If Keq is large (>1) products are favored If Keq is small (<1) reactants are favored Remember, Keq represents [products]/[reactants] meaning… The bigger Keq is, the more products are favored.

Relative Strengths of Acids & Bases SUMMARY: -When asked to write the equilibria which occur when pairs of substances are put together, you need to: Look at the table to see which one is a stronger acid and therefore will donate their proton. Use this strong acid to give its proton and let the ‘base’ accept that proton When you add an H to a substance, you add a +1 charge. Make sure your total charges are balanced(same) on each side of the equation.

Homework Hebden Textbook Page 133 Questions # ( skip 44, we’ve already done it)