Acid-Base equilibria Flashcards
What is an acid?
An acid is a substance that releases or provides H+ ions (proton donor).
What is a base?
A base is a substance that removes or accepts H+ ions (proton acceptor).
What is a strong acid?
One that almost totally dissociates into H+ ions and negative ions in solution in water (e.g HCl).
What is a weak acid?
One that only partially dissociates into H+ ions and negative ions in solution in water. The free ions are in equilibrium with the undissociated acid molecule (e.g CH3COOH).
What does concentrated mean?
There is a large amount of an acid or base dissolved in a set volume of water.
What does dilute mean?
There is a small amount of acid or base dissolved in a set volume of water.
What can be said about a substance which has a low pH?
It is a strong acid.
What can be said about a substance which has a high pH?
It is a strong alkali.
What can be said about a substance with pH 7?
It is neutral.
What is the general equation to find pH?
pH= -log10[H+]
What is the general equation to find [H+]?
[H+]= 10^-pH
What is Ka?
The acid dissociation constant.
How can you calculate Ka?
Ka = [H+][A-] / [HA]
What is the Ka value of a weak acid?
Ka is low.
What is the Ka value of a strong acid?
Ka is high.
How can you calculate pKa?
pKa= -log10(Ka)
How can we calculate the pH of water?
1.Kc = [H+][OH-] / [H2O] the amount of water that dissociates is so small we use Kw
2. Kw- the ionic product of water
Kw = [H+][OH-]
3. Kw= [H+]^2 as [H+]=[OH-] in pure water
4. Kw= [H+]^2 = 10^-14
5. [H+]= 10^-7 which is pH7
What is [H+] for strong acids?
In strong acids, all the H+ ions are released from the acid molecule, therefore [H+]=[acid].
What is [H+] for weak acids?
In weak acids, very little dissociation occurs, so the concentration of acid molecules is the same that we initially put in, therefore [H+]= √Ka x [acid].
What is [H+] for a strong base?
In strong bases, the concentration of the base gives the concentration of OH- ions, so we rearrange Kw, therefore [H+]= Kw/[OH-].
What is a buffer?
A buffer resists changes in pH as small amount of acid or alkali are added.
What is a buffer typically made from?
- A weak acid (e.g CH3COOH)
- A salt of the same acid with a strong base (e.g CH3COONa)
How does a buffer work?
In the buffer, there is a high concentration of the anion as the salt completely dissociates and the weak acid dissociates partly.
What happens when acid is added to a buffer?
The amount of H+ is increased, and this causes the equilibrium to shift to the left, removing the H+ ions by reaction with the dissociated weak acid.
What happens when alkali is added to a buffer?
The amount of OH- is increased, which removes some of the H+ ions present. This causes the equilibrium to shift to the right, producing H+ ions and the dissociated weak acid. The H+ ions replace those removed by reaction with the OH- ions.
What are some uses for buffers?
-Using or storing enzymes to ensure pH remains at the optimum value
-Storage of biological molecules, such as pharmaceuticals, which denature at the incorrect pH
How do we calculate the pH of a buffer?
We need to know the Ka of the weak acid and the ratio of the concentrations of the acid and the salt.
We use the expression Ka= [H+][A-] / [HA]
What is the Henderson-Hasselbalch equation?
pH= pKa + log([salt] / [acid])
What is an example of a buffer which maintains alkaline pH?
Ammonium chloride and ammonia solution.
What salt will be produced from a strong acid and a strong alkali?
A neutral salt, such as NaCl.
What salt will be produced from a strong acid and a weak alkali?
An acidic salt, such as NH4Cl.
What salt will be produced from a weak acid and a strong alkali?
A basic salt, such as CH3COONa.
Why is ammonium chloride an acidic salt?
It dissolves to produce free Cl- ions and NH4+ ions, which is a weak base. NH4+ dissolves further to NH3 and H+, releasing free H+ ions decreasing the pH to below 7.
Why is sodium ethanoate a basic salt?
It dissolves to produce free Na+ ions and CH3COO- ions, which is a weak acid. CH3COO- reacts with and removes H+ from the water to make CH3COOH, increasing the pH to above 7.n
