Acids & Bases

Lesson 4Relative 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 [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:

HS- and HCO3- mixed together. Use Keq to find

which side the equilibrium favors?

Keq and Acid StrengthGiven:

HA + B- HB + A- Keq = 0.003

Which acid is stronger, HA or HB?

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.

Homework

Hebden Textbook

Page 133 Questions # 38-46  ( skip 44, we’ve already done it)