acids & bases lesson 4 relative strengths of acids and bases
TRANSCRIPT
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 [H3O+]
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 & BasesH2PO4
- + CO32- HCO3
- + HPO42-
(A) (B) (A) (B)
At equilibrium, which will be favored, reactants or products?
Steps to take:
Identify acids and bases
Identify the STRONGER acid; H2PO4-
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 HSO3- + HCO3
- H2CO3 + SO32- favor reactants or
products?
Relative Strengths of Acids & Bases
Example: Complete the reaction and determine which is favored; reactants or products?
HSO4- + H2PO4
-
Relative Strengths of Acids & Bases
TRY: Complete the reaction and determine which is favored; reactants or products?
HSO4- +NO2
-
Relative Strengths of Acids & Bases
“Salts”
All salts dissociate 100% in water.
Ex: will the equilibrium favor reactants or products?
(NH4)2CO3
Another way to figure out which side is favored
Keq= Ka(reactant acid)
Ka(product acid)
•If Keq is large (>1) products are favored
•If Keq is small (<1) reactants are favored
TRY: The following equilibrium favors products
Se-2 + HSO4- HSe- + SO4
-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 & BasesSUMMARY: • 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= Ka(reactant acid)
Ka(product acid)
• If Keq is large (>1) products are favored • If Keq is small (<1) reactants are favored
Relative Strengths of Acids & BasesSUMMARY: Keq= Ka(reactant acid)
Ka(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 & BasesSUMMARY:
- 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.