What’s a Brønsted-Lowry acid?
A proton donor (H+)
What’s a Brønsted-Lowry base?
A proton acceptor (H+)
How can weak and strong acids be shown within an equation?
By the arrows used.
Equilibrium / reversible reaction signs suggest a weak acid.
A single arrow suggests it is strong.
What’s a conjugate acid-base pair?
Two species that can be interconverted by transfer of proton e.g. HCl and Cl-.
In the forward reaction HCl releases a proton to form its conjugate base, Cl-.
In the reverse direction, Cl- accepts a proton to form its conjugate acid, HCl.
What are the conjugate acid-base pairs in the reaction:
HCl + OH- H2O + Cl-
HCl + OH- H2O + Cl-
Acid 1 Base 2 Acid 2 Base 1
HCl donates a proton which is accepted by OH-.
In reverse, H2O donates a proton which is accepted by Cl-.
What are the conjugate acid-base pairs in the reaction:
HCl + H2O H3O+ + Cl-
HCl + H2O H3O+ + Cl-
Acid 1 Base 2 Acid 2 Base 1
HCl donates a proton which is accepted by H2O.
H3O+ donates a proton in the reverse which is accepted by Cl-.
What’s a monobasic acid?
An acid with one hydrogen atom, able to donate 1 proton.
What’s a dibasic acid?
An acid with two hydrogen atoms, able to donate 2 protons.
What’s a tribasic acid?
An acid with three hydrogen atoms, able to donate 3 protons.
What do acids react with? (4)
What do they form?
Metals (salt and hydrogen)
Carbonates (salt, water and CO2)
Metal oxides (salt and water)
Hydroxides / alkalis (salt and water)
What is the full and ionic equation for the reaction between HCl and Mg?
Full:
2HCl (aq) + Mg (s) -> MgCl2 (aq) + H2 (g)
Ionic:
2H+ (aq) + Mg (s) -> Mg 2+ (aq) + H2 (g)
What is the full and ionic equation for the reaction between H2SO4 and Mg?
Full:
H2SO4 (aq) + Mg (s) -> MgSO4 (aq) + H2 (g)
Ionic:
2H+ (aq) + Mg (s) -> Mg 2+ (aq) + H2 (g)
What is the mathematical relationship between pH and [H+ (aq)] ?
pH = -log[H+ (aq)]
It’s inverse is:
[H+] = 10^(-pH)
Being logarithmic, each pH has a scale difference of 10; so a pH of 1 has 10 times the concentration of a pH 2.
How is pH calculated?
pH = -log[H+ (aq)]
How is the concentration of protons / H+ ions within a substance calculated (using pH)?
[H+ aq] = 10 ^(-pH)
What is the concentration of protons in a substance of pH 2 and pH 8?
pH 2: 10^-2 = 0.01 moldm-3
pH 8: 10^-8 = 0.00000001 moldm-3
What is the ‘strength’ of an acid?
The extent of dissociation of an acid.
The stronger the acid, the greater the dissociation of H+ ions.
Weak acids only partially dissociate.
What is Ka?
The acid dissociation constant.
It shows how acidic a solution is - the greater the value, the further the equilibrium is to the right and the stronger the acid.
How is Ka calculated?
Ka = [H+][A-] / [HA]
How is the acid dissociation constant calculated?
Ka = [H+][A-] / [HA]
What is pKa?
A negative log value of Ka to compare pH using simpler numbers than the large numbers for Ka.
How is pKa calculated?
pKa = -logKa
How can Ka be calculated using pKa?
Ka = 10^-pKa
What are the values of Ka and pKa like for strong and weak acids?
For strong acids, Ka is large and pKa is a small value.
For weak acids, Ka is small and pKa is large.
What happens to weak acids (HA) in equilibrium?
HA H+ + A-
[H+] depends on the the concentration of the acid of acid [HA] and acid dissociation Ka.
What assumptions are made abut the Ka equation:
Ka = [H+][A-] / [HA]
What are the limitations?
Assumptions:
1) [H+] = [A-]
H20 H+ + OH-
Water also releases H+ ions but this is ignored.
2) [HA] at the start = [HA] at equilibrium
We assume that the dissociation of H+ ions is so small that the concentration of acid does not change.
Limitations:
1) Only works with strong-weak acids. With weak-weak acids, the dissociation is significant.
2) Only works with weak-weak acids. With strong-weak acids, more has been dissociated so difference in concentration will be greater between the start and equilibrium.
What is Kw?
The ionic product of water.
How is Kw calculated?
Kw = [H+][OH-]
At 25’C, Kw = 1*10^-14 and the pH of water is 7.
How is the ionic product of water calculated?
Kw = [H+][OH-]
How can the pH of a strong base be calculated?
Using the concentration of the base and ionic product of water (Kw).
How would the concentrations of H+ and OH- ions in a solution of oH 3.25 at 25°C be calculated?
1) Use calc to find [H+].
10^-3.25 = [H+] = 5.62*10^-4
2) Use [H+] and Kw to calculate [OH-].
[OH-] = Kw/[H+] = 110^-14 / 5.6210^-4
= 1.78*10^-11 moldm-3
What is the pH of a solution with [OH-] of 2*10^-2 moldm-3 at 25°C?
1) Calculate [H+] from Kw and [OH-].
[H+] = 110^-14 / 210^-2
= 5*10^-13
2) Calculate pH
pH = -log[H+]
= 12.30
What is the pH of a solution of 0.075 moldm-3 NaOH at 25°C?
1) Convert NaOH into OH-
NaOH is a strong monoacidic alkali which completely dissociates.
[OH-] = [NaOH] = 0.075 moldm-3
2) Use Kw and [OH-] to find [H+]
[H+] = 1.33*10^-13 moldm-3
3)Calculate pH
pH = -log[H+]
=12.88