chapter 20 Flashcards
What is a Brønsted Lowry acid?
a species that can donate a proton
hydrogen chloride (HCl) is a Brønsted acid as it can lose a proton to form a hydrogen (H+) and chloride (Cl-) ion
What is a Brønsted Lowry base?
a species that can accept a proton
A hydroxide (OH-) ion is a Brønsted base as it can accept a proton to form water
What is a conjugate acid-base pair?
A conjugate acid-base pair is two species that are different from each other by an H+ ion.
CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H3O+ (aq)
What does it mean for an acid to be classified as mono basic/dibasic or tribasic?
It means the number of bases that they can donate protons to in a reaction (this depends on how many H+ per molecule that they can give up in a reaction)
Example of mono basic acid + what this means
HCL, It contains one ionisable hydrogen atom in each molecule.
Example of dibasic acid + what this means
H2SO4, It contains two ionisable hydrogen atoms per molecule. Ionisation occurs in two steps.
Example of tribasic acid + what this means
H3PO4, It contains 3 ionisable hydrogen atoms per molecule. Ionisation occurs in 3 steps.
Acid + metal =
acid + metal → salt + hydrogen
2HCl (aq) + Zn (s) → ZnCl2 (aq) + H2 (g)
acid + metal oxide =
acid + metal oxide → salt + water
2HCl (aq) + CaO (s) → CaCl2 (aq) + H2O (l)
acid + metal carbonate =
acid + metal carbonate → salt + water + carbon dioxide
2HNO3 (aq) + CuCO3 (s) → Cu(NO3)2 (aq) + H2O (l) + CO2 (g)
acid + alkali =
acid + alkali → salt + water
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
What is a weak acid?
A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions.
eg. ethanoic acid/ carbonic acid
(equilibrium is more to the left- there are lots more weak acid molecules than H+ and A- Ions.)
What is the equilibrium constant for a weak acid?
Ka
What is the equilibrium constant expression for Ka?
Ka = [H+] [A-] / [HA]
this can be simplified to [H]^2 / [HA]
What are the units of Ka?
mol dm-3
A high Ka value means…?
The higher the value of Ka the more dissociated the acid and the stronger it is
A low Ka value means….?
The lower the value of Ka the weaker the acid
Why do we use pka instead of Ka?
Its difficult to compare numbers with negative indices, so we use pka as it is whole numbers.
How do you go from Ka to pKa?
pKa = -log(Ka)
How do you go from pKa to Ka?
Ka = 10^-(pKa)
A high pKa value means…?
The acid is weak.
A low pKa value means…?
The acid is strong and more dissociated.
How do you work out pH?
pH= -log[H+]
H+ = conc of hydrogen atoms in moldm-3
How do you calculate [H+] with the pH?
[H+] = 10^-pH
How much does each pH value differ by?
The pH scale is a logarithmic scale with base 10
This means that each value is 10 times the value below it.
pH 1 = 10^-1
PH 2 = 10^-2
What is the ionic product of water?
Kw
What’s the value of Kw?
1 x 10^-14 mol^2dm^-6 @ 298k
What’s the equation for Kw?
Kw = [H+][OH-] / [H20]
Kw x [H20] = [H+][OH-]
What approximations are made with Ka?
HA dissociates to create concentrations of H+ and A-
this is what allows us to write [H+]^2. We also ignore the very small conc of H+ from water dissociating.
As the dissociation of weak acids is small we assume that the concentration of the weak acid is the same at the start and the end.
How do you work out pH with Ka?
square root of Ka x [HA] = H+
so you then do -log[H+]
How do you calculate the pH of a strong acid?
Strong acids completely ionise within solution.
So [H+] = [HA]
How do you calculate the pH of a strong base?
Strong bases completely ionise within solution.
So H+ = Kw (1x10^-14) / [OH-]
You then use the H+ to work out pH.
Kw = [OH-] x [H+]
