Physical Unit 1.12: Acids & Bases (A2) Flashcards
(52 cards)
what is the Bronsted-Lowry definition for an acid, base & acid-base reaction/equilibria?
acid: proton donor
base: proton acceptor
acid-base reaction/equilibria: involves the transfer of protons
what is the pH scale?
logarithmic scale
a measure of hydrogen ion concentration
formula for pH (strong acids)
pH = -log[H+]
always to 2dp
define pH
-log[H+]
[H+] =
10^-pH
define monobasic & dibasic acid & give e.g.s
monobasic acid: acids where each molecule dissociates to form 1H+ ion e.g. HNO3, HCl
= monoprotic
dibasic = diprotic: each molecule dissociates to form 2 H+ ions e.g. H2SO4
define strong acid
(in solution) all molecules dissociate to form H+ ions
[H+] in dilution of a strong acid
[H+] = (volume of acid/total volume) x conc. acid
it’s the proportion of the volume x conc.
dilution of weak acid
[HA] = volume of acid/total volume x conc. acid
what is Kw, the ionic product of water?
water dissociates slightly:
H2O <–> H+ + OH- endothermic
conc. water is 55.6 moldm-3
Kw is derived from Kc of this dissociation
what is the formula for Kw? & derivation
Kc = [H+][OH-] / [H2O]
Kw = [H+][OH-]
define neutral
[H+] = [OH-]
how does temperature affect pH & neutrality of water & the value for Kw?
pure water is neutral
increasing temp. shifts equilibrium in endothermic direction, in forwards direction
= which increases [H+] & [OH-]
= pH decreases
but pure water stays neutral as [H+] = [OH-]
as temp. increases, Kw increases
what is Kw @ room temp.?
1x10^-14
@ stp in pure water, Kw =
pH =
Kw = [H+]^2 = 1x10^-14
[H+] = root 1x10^-14 = 10^-7
pH = 7.00
define monoprotic & diprotic base
monoprotic base: 1 mole of base accepts 1 mole of H+ ions
/
each molecule accepts 1 H+ ion
diprotic base: 1 mole of base accepts 2 moles of H+ ions
each molecule accepts 2 H+ ions
how do you calculate pH of a strong base?
use Kw = 1x10^-14 @stp
use [H+] = Kw/[OH-]
to calculate conc. of OH-, multiply conc. of substance by # moles of OH- in one molecule
to calculate conc. of base, divide conc. of substance by # moles of OH- in one molecule
how do you calculate the pH of a solution formed from the reaction b/w a strong acid & strong base?
- calculate moles H+
- calculate moles OH-
- calculate leftover moles at end, either of H+ or OH-
- calculate [H+] or [OH-] of leftover at end by mol/total vol.
- calculate pH
define weak acid & give e.g.
only a small fraction of the molecules dissociate to form H+ ions
e.g. carboxylic acids
weak acid equilibrium
HA <–> H+ + A-
HA = weak acid
A- = salt of weak acid
describe the salt of a weak acid
A- is slightly basic bc is accepts H+ to form HA
what is the formula for Ka, the acid dissociation constant for a weak acid?
what are the assumptions made?
Ka = [H+][A-] / [HA]
assumptions:
1. [HA] = original [HA], the conc. doesn’t change much due to equilibrium
2. [H+] only comes from the dissociation of HA & not from water/Kw
what is the formula for pKa?
pKa = -log(Ka)
Ka = 10^-pKa
important A- info
in a solution of pure weak acid dissolved in water, A- comes from the dissociation of weak acid (HA)
so [A-]=[H+]
if any ionic salt is dissolved in water, [A-] = [salt] & A- from the dissociation of weak acid is negligible