20 - Acids, bases and pH Flashcards
A Bronsted lowry acid
proton donor
A Bronsted lowry base
proton acceptor
Conjugate acid-base pairs
- two species that can be interconverted by transfer of a proton
how can water act as a base
- accepts proton and becomes positive ion
monobasic, dibasic, tribasic
- Refers to the total number of hydrogen ions in the acid that can be replaced per molecule in an acid- base reaction
monobasic, dibasic, tribasic -HCL / CH3COOH / H2C02 / H3BO3
low value of [H+]…
high value of pH
high value of [H+]
low value of pH
how is pH measured
logarithmic scales of hydrogen ions
equation to convert H+ into pH
equation to convert pH into H+
dilution needed to convert a solution from pH 1 to pH 4
10 x 10 x 10 = 1000
dilution by factor of 1000
general equation for strong (monobasic) acid dissociation
- completely dissociates
for a strong acid dissociation what is special and needed for calculations
concentration of HA = concentration of H+
therefore - instead of using conc of H+ to find pH, can use conc of HA in -log equation
dilution -
strong and weak acids
fully vs partially dissociates
Ka
acid dissociation constant
Larger the value of Ka..
the further the equilibrium is to the right
greater dissociation and greater acid strength
Ka formula
why use pKa instead of Ka
Hard to compare Ka values because they have negative indices
convert ka into pKa
convert pKa into Ka
if Ka increases… (effect on pKa)
pKa decreases
how to calculate Ka from equilibrium concentrations
approx 1 - Ka
- HA dissociates to produce equilibrium concentration of H+ and A- that are equal
- Small concentration of H+ released from dissociation of water but this will be so small, it can be neglected compared to H+ of acid
The contribution of H+ from the water is negligible.
approx - 2 Ka
The dissociation of the HCOOH is small enough for the equilibrium concentration of HCOOH to be taken as the same as the initial concentration.
problems with approx 1
- If pH > 6 = then conc of H+ from dissociation of water will be significant compared to dissociation of weak acid
problems with approx 2
- Assumes that conc of acid is greater than the H+ conc at equilibrium
- Fine for weak acids with low Ka values
- When conc of H+ INCREASES = ‘stronger’ weak acids = it breaks down
- Difference between conc of acid at equilibrium and conc of acid at start – conc of H+ at equilibrium
doesnt work for stronger weak acids or very dilute solutions where the difference is significant
using approx - how can Ka be simplified for equilibrium
Calculate pH from Ka
- To calculate pH, need to find [H+] first.
- Rearrange formula for H+ conc
determination of Ka experimentally
- preparing a standard solution of weak acid of known concentration
- measuring pH of standard solution using a pH meter
- pH= acid concentration calculation or pH measurement
how can water be an acid and a base
Kw
ionic product of water - conc of H+ and OH- multiplied together
Kw - 1 x 10^-14
calculate the pH of water
For pH with whole numbers how to work out conc of H+ and OH-
indices add up to 14
if pH is not a whole number how to work out conc of H+ and OH-
how to find pH of a strong base
- as strong base - conc of base = conc of OH-
- use OH- and Kw to find conc of H+
- minus log it
state the two assumptions for ka / weak acid calculations
The contribution of H+ from the water is negligible.
The dissociation of the HCOOH is small enough for the equilibrium concentration of HCOOH to be taken as the same as the initial concentration.
