5C - Acids & pH definitions

Question 1

Acid

Answer 1

Proton donor in aqueous solution

Question 2

Base (2)

Answer 2

• Proton acceptor in aqueous solution
• A compound tht neutralises an acid to form a salt

Question 3

Alkali

Answer 3

A type of base that dissolves in water forming OH^- ions

Question 4

Salt

Answer 4

The product of reaction in which the H⁺ ions of an acid is replaced by metal or ammonium ions

Question 5

Conjugate acid

Answer 5

A species that releases a proton to form a conjugate base

Question 6

Conjugate base

Answer 6

A species that accepts a proton to form a conjugate acid

Question 7

Monobasic acids

Dibasic acids

Tribasic acids

Answer 7

Monobasic acids = Reacts with one base as it has only 1 proton

Dibasic acids = Reacts with 2 bases as it has 2 protons

Tribasic acids = Reacts with 3 bases as it has 3 protons

Question 8

Brønsted - Lowry acid

Brønsted - Lowry base

Answer 8

Brønsted - Lowry acid = Proton donor

Brønsted - Lowry base = Proton acceptor

Question 9

Strong acid

Weak Acid

Answer 9

Strong acid = Acid which completely dissociates in solution

Weak acid = Acid that partially dissociates in solution

Question 10

pH = ?

∴ [H⁺] = ?

Answer 10

Question 11

pH and strong acids

Answer 11

HA → H⁺ + A^-

[HA] = Conc of Acid

[H⁺] = [HA]

Assumptions:

• All of HA dissociates

Question 12

Acid dissociation constant (Ka) = ?

Assumptions?

Answer 12

Ka = [H⁺] [A^-] / [HA]

Assumptions:

• Contribution of [H⁺] from H₂O is negligable
• [H⁺] = [A^-]
• No acid dissociate ∴ [HA]_equil = [HA]_initial

∴ Ka = [H⁺]² / [HA]

[H⁺] = √(Ka x [HA])

Question 13

PKa = ?

Answer 13

PKa = -log(Ka)

Question 14

Strong alkalis

Answer 14

[OH^-] = [Alkali]

Assumption:

• Fully dissociates

Question 15

Ionic product of water = ?

Formula and alternative method

Answer 15

K_w = [OH^-] [H⁺]

At 25°C, K_w = 1 x 10^-14

Units = mol²dm^-6

OR

pOH = 14 - pH because pH + pOH = 14

Question 16

Buffer solution

Answer 16

A solution that minimises the change in the pH by small additions of H⁺ or OH^-

Question 17

Acid buffers

Assumptions

Answer 17

WEAK ACID + SALT OF WEAK ACID → ACID BUFFER

HA ⇌ H⁺ + A^-

NaA → Na⁺ + A^- (Na is just used as an example)

Ka = [HA] [A^-] / [HA] ∴ [H⁺] = Ka x [HA] / [A^-]

Assumptions:

• [H⁺] from H₂O is negligable
• [A^-] from acid is negligable, all [A^-] comes from salt
• [HA]_equil = [HA]_initial i.e. no acid dissociates

Question 18

Alkaline buffers

Through example

Answer 18

WEAK BASE + SALT OF WEAK BASE → ALKALINE BUFFER

Through example:

NH₃ + H₂O ⇌ NH₄⁺ + OH^-

NH₄Cl → NH₄⁺ + Cl^-

Add H⁺:

• H⁺ + OH^- → H₂O
• [H⁺] increases will reduce [OH^-] ratio ∴ equil shifts to right
• NH₃ will react with more H₂O to produce more OH^-
• Buffer action maintained until BASE used up

Add OH^-:

• Increase [OH^-], equil shifts to left
• More NH₄+ will react wiht excess OH^-
• Buffer action maintained until SALT used up