Acids and Bases Flashcards
Equation for calculating pH
-log10[H+(aq)]
A small pH=
a high concentration of H+ ions (so a low concentration of OH- ions).
A large pH=
a low concentration of H+ ions (so a high concentration of OH- ions).
What does one pH unit correspond to?
A decrease in pH means an increase in H+ ions by a factor 10.
Equation for concentration of H+ ions
[H+] = 10^-pH
Kw equation
Kw = [H+] x [OH-] mol2dm-6
It is the ionic product of water (water ionises slightly).
Kw at 298K
1x10^-14 mol2dm-6.
What is required in a neutral solution in terms of ions?
[H+] = [OH-]
For every mole of water that dissociates, you would get an equal amount of H+ and OH- ions.
How does a low temperature affect pH?
A low temperature increases the pH because the concentration of H+ ions decrease. This is because the reaction is exothermic, so the backward reaction is favoured at equilibrium and more reactants are made (H2O).
How does a high temperature affect pH?
A high temperature decreases pH because the concentration of H+ ions increases. The reaction is endothermic so the forward reaction is favoured.
Kw equation (only for pure water)
Kw = [H+]^2
pKw equation
pKw = -log10Kw
To convert, use 10^-pKw
Strong monobasic alkalis
[OH-] = [alkali]
Releases 1 OH- ion.
Strong dibasic alkalis
[OH-] = 2[alkali]
Releases 2 OH- ions, have to multiply the [H+] concentration by 2 during calculations.
Weak acids and bases definition
They only partially dissociate into their ions when dissolved in water.
Strong acids and bases definition
They fully dissociate into their ions when dissolved in water.
Examples of weak acids
Carboxylic acids and carbonic acids.
Weak bases examples
Ammonia and amines.
A Bronsted-Lowry acid is …
A proton donor
A Bronsted-Lowry base is …
A proton acceptor
What is Ka?
The acid dissociation constant, same formula as Kc ([products]/[reactants]) moldm-3.
pKa =
-log10Ka
10^-pKa to convert
How does Ka affect the strength of acids?
The larger the value of Ka, the stronger the acid. In general, acids with Ka much smaller than 1 are classes as week and those with Ka much greater than 1 are strong.
General equation for a weak acid.
HA (reversible) H+ + A-
At equilibrium, there is a high concentration of reactants and a low concentration of products.
Weak acid assumption.
[HA] initially = [HA] equilibrium.
The position of equilibrium lies heavily to the reactant side - as very few acid molecules dissociate.
Weak acid key point
[H+] = [A-]
Ka =
[H+] x [A-]
[HA]
Ka if only weak acids are present
Ka = [H+]^2
[HA] initial
