Acids, Bases and Buffers Flashcards
What is the Bronsted-Lowry definition of an acid?
Proton donor
What is the Bronsted-Lowry definition of a base?
Proton acceptor
What does acid-base equilibria involve?
The transfer of protons
How do you find the pH of a strong acid?
pH = –log₁₀[H⁺]
How do you find the H⁺ concentration from pH?
10⁻ᵖᴴ
How dissociated is water?
Slightly dissociated
What is the expression for the ionic product of water?
Kw = [OH⁻][H⁺]
How do you calculate the pH of a strong base?
1) Kw = [OH⁻][H⁺]
2) Rearrange for [H⁺]
3) Substitute into pH = –log₁₀[H⁺]
Define strong acids/weak acid
Fully dissociated in solution
Define weak acid/weak base
Partially dissociated
What is the dissociation constant for a weak acid?
Ka
How is Ka related to Kc?
Kc [H₂O] = Ka
What is the Ka expression for the equation:
CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
Ka = [CH₃COO⁻][H⁺] / [CH₃COOH]
How do you calculate the pH of a weak acid?
Ka = [H⁺]² / [CH₃COOH]
- solve for [H⁺]
- use equation pH = –log₁₀[H⁺]
How does temperature affect Ka?
When you increase temperature, you increase Ka
What is the ‘end point’?
When the indicator changes colour
What is the ‘equivalence point’?
When [H⁺] = [OH⁻]
What indicator should be used for a strong acid/ weak base titration?
methyl orange
What indicator should be used for a strong base/weak acid titration?
phenolphthalein
How would you measure the pH change for a weak acid/weak base titration and why?
A pH probe because there is no sharp increase in pH.
How to calculate the pH during a titration?
1) Calculate moles
2) Find whichever is in excess (x)
3) Find the concentration from the moles of x left
4) Find pH
What is monoprotic?
Donates only one proton
What is a buffer?
A buffer is able to resist changes to pH when moderate amounts of acid or base are added
What makes up an acidic buffer?
Weak acid + Salt from the weak acid and strong base
What equations are involved in acidic buffers?
1) HA ⇌ H⁺ + A⁻
2) NaA → Na⁺ + A⁻
In acidic buffers, what happens when you add more acid (H⁺)?
The system opposes the addition of H⁺ by shifting the position of equilibrium to the left. It is able to do this because of the large reservoir of A⁻ (conjugate of base). Hence this ensures that the conc. of H⁺ remains almost constant.
H⁺ + OH⁻ → H₂O
In acidic buffers, what happens when you add alkali (OH⁻)?
The system opposes the addition of OH⁻ by shifting the p.o.e to the right, it is able to do this because of the large reservoir of HA. Hence this ensures that the conc. of H⁺ remains almost constant.
What makes a basic buffer?
Weak base + salt from weak base and strong acid
What equations are involved in basic buffers?
e.g. NH₃ + NH₄Cl
1) NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
2) NH₄Cl → NH₄⁺ + Cl⁻
In basic buffers, what happens when you add more acid (H⁺)?
The system opposes this by shifting the p.o.e to the right, it is able to do this because of the large reservoir of NH₃. Hence this ensures that the conc. of H⁺ remains almost constant.
In basic buffers, what happens when you add alkali (OH⁻)?
The system opposes this by shifting the p.o.e to the left. It is able to do this due to large reservoir of NH₄⁺ (conjugate acid). Hence this ensures that the conc. of H⁺ remains almost constant.
How do you calculate the pH of a buffer solution?
1) Calculate moles of HA (acid) and A⁻(salt) and find which is in excess
2) Find the conc. of both by using moles of excess left and original moles of the other.
3) Find the pH
At half equvalence [HA]=?
[HA] = [A⁻] pH = pKa
