O - Acids, Bases, pH and buffers Flashcards
Give the Bronsted-Lowry definition of an acid
Proton donors
Give the Bronsted-Lowry definition of a base
Proton acceptors
Give the equation for the reaction between an acid and water
HA (aq) + H₂O (l) → H₃O⁺ (aq) + A⁻ (aq)
Give the equation for the reaction between a base and water
B (aq) + H₂O (l) → BH⁺ (aq) + OH⁻ (aq)
Define a strong acid and give an example
Strong acids ionise almost completely in water (nearly all H⁺ released)
Example: Hydrochloric acid
HCl (g) + H₂O (l) → H⁺ (aq) + Cl⁻ (aq)
(this reaction is reversible but the equilibrium is well to the right)
Define a strong base and give an example
Strong bases ionise almost completely in water
Example: Sodium Hydroxide
NaOH (s) + H₂O (l) → Na⁺ (aq) + OH⁻ (aq)
(this reaction is reversible but the equilibrium is well to the right)
Define a weak acid and give an example
Weak acids only ionise very slightly in water, so small numbers of H⁺ ions are formed. An equilibrium is set up which is well over to the left
Example: Ethanoic acid
CH₃OOH (aq) + H₂O (l) ⇌ CH₃OO⁻ (aq) + H⁺ (aq)
Define a weak base and give an example
Weak bases only ionise slightly in water. An equilibrium is set up which is well over to the left
Example: Ammonia
NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
Describe how acids and bases react
Acids donate their protons to the base
HA (aq) + B (aq) ⇌ BH⁺ (aq) + A⁻ (aq)
Describe what happens when you add an acid to water
The water acts as a base and accepts the protons:
HA (aq) + H₂O (l) → H₃O⁺ (aq) + A⁻ (aq)
Define conjugate pairs
A pair made between an acid and a base either side of the equilibrium which transfer an Hydrogen ion between them
State the conjugate pairs in the following reaction:
HCl + H₂O ⇌ H₃O⁺ + Cl⁻
- HCl and Cl⁻ form a conjugate pair where HCl is the acid
- H₂O and H₃O⁺ form a conjugate pair where H₃O⁺ is the acid
State the conjugate pairs in the following reaction:
B + H₂O ⇌ BH⁺ + OH⁻
where B is a base
- B and BH⁺ form a conjugate pair where BH⁺ is the acid
- H₂O and OH⁻ form conjugate pair where H₂O is the acid
Describe how water can behave as both an acid and a base
It can act as an acid by donating a proton or as a base by accepting a proton
Both hydroxonium ions and hydroxide ions will always be present at the same time due to the equilibrium:
H₂O ⇌ H⁺ + OH⁻
Describe why the equilibrium lies to the left in the following reaction:
H₂O ⇌ H⁺ + OH⁻
Give an expression for the equilibrium constant Kc
Because water only dissociates a small amount
Kc = [H⁺][OH⁻] / [H₂O]
Define Ionic product of water
The Ionic Product of Water, Kw, is the equilibrium constant for the reaction in which water undergoes an acid-base reaction with itself.
Kw = Kc x [H₂O] = [H⁺][OH⁻]
What makes a solution neutral?
An equal concentration of H⁺ and OH⁻ ions
[H⁺] = [OH⁻]
Why is the pH logarithmic?
Because the concentration of hydrogen ions can vary enormously
Give the equation to find the pH of a solution
pH = -log₁₀[H⁺]
Name 2 strong monoprotic acids
Hydrochloric Acid (HCl) Nitric Acid (HNO₃)
Describe the relationship between the concentration of hydrogen ions and the acid concentration for a strong monoprotic acid
Each mole of acid produces 1 mole of hydrogen ions
[HA] = [H⁺]
Describe how do you find the pH of a strong base
[H⁺] is linked to [OH⁻] by the ionic product of water Since Kw = [H⁺][OH⁻] [H⁺] = Kw / [OH⁻] And thus the pH of a base is: pH = -log₁₀[H⁺] = -log₁₀(Kw / [OH⁻])
Give the equation for the equilibrium constant Ka for the reaction:
HA ⇌ H⁺ + A⁻
Ka = [H⁺]² / [HA]
Define pKa
pKa is the negative base-10 logarithm of the acid dissociation constant of a solution
pKa = -log₁₀Ka
Ka = 10⁻ᵖᴷᵃ
The lower the pKa, the stronger the acid
