Chapter 20 - Acids, bases, and PH Flashcards
What is the important chemical component in an acid?
Hydorgen
Briefly describe the arrhenius model of acids and bases
- Acids dissociate and release H+ ions in aqeous solution (e.g. water)
- Alkalis (soluble base) dissociate and release OH- ions in aqueous solution
- H+ ions are neutralised by OH- ions to form water. H+(aq) + OH-(aq) -> H2O(l)
What is a Bronsted-lowry acid?
Species that donates a proton
What is a Bronsted-Lowry base?
species that accepts a proton
What is a conjugate acid-base pair?
contains two species that can be interconverted by transfer of a proton
In the dissociation of HCl to H+ and Cl- , identify the conjugate acid and base
HCl releases a proton, so therefore is a conjugate acid
Cl- accepts a proton, so therefore is the conjugate base
Give the acid base pairs in this equilibria:
HCl(aq) + OH-(aq) H2O(l) +Cl-(aq)
HCl - donates H+ - Acid 1
OH - accepts H+ - base 2
H20 - donates H+ - acid 2
Cl - accepts H+ - base 1
When water is the base, what’s formed?
H3O+ - hydronium ion
What do the terms monobasic, dibasic and tribasic refer to?
total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction
How can ionic equations be simplified?
Cancel the spectator ions (don’t react)
What’s the word equation for reaction of an acid and a metal?
Acid + metal -> salt +hydrogen
What’s the word equation for reaction of acids and a carbonate?
acid + carbonate -> salt + water + CO2
What’s the word equation for reaction of acids with a base?
acid + base -> salt + water
What’s the word equation for reaction of an acids with an alkali?
acid + alkali -> salt + water
What is the relationship between PH and concentration of H+?
Low value of [H+(aq)] = high PH
high value of [H+(aq)] = low PH
What;s the equation for working out PH from [H+]?
PH = -log[H+(aq)]
How do you work out [H+(aq)] using PH?
[H+(aq)] = 10-PH
What change in [H+] does 1 PH number increase give
x10 difference
How is the PH calculated for a strong acid?
Assume it fully dissociates
How is the new PH calculated for a strong acid on dilution?
Work out change in concentration of HA and therefore [H+]
[H+] x original vol/ new total vol
or
C1V1 = C2V2
then put back into PH = -log[H+]
What is Ka?
- Acid dissociation constant
- standardised at 25ºC
How do you work out Ka?
using HA(aq) H+(aq) +A-(aq)
Ka = [H+(aq)] [A-(aq)]/ [HA(aq)]
What is pKa value formula? (used to make Ka data more comparible)
pKa = -logKa
How do you get Ka from pKa?
Ka = 10-pKa
Why are pKa values used and not Ka?
Much more manageable than Ka. Easier to compare relative acidic strengths using pKa strengths using pKa values than Ka values
What does a lower pKa and larger Ka value tell us?
Stronger acid
What does a lower Ka value and larger pKa value tell us?
Weaker acid
What information is provided by Ka values?
Strength of acid/ extent of dissociation
What is Kw?
Ionic product of water
Ions in water (H+ and OH-) multiplied together
What’s the equation for Kw?
Kw = [H+(aq)] [OH-(aq)]
What is the value of Kw?
1 x 10-14 mol2dm-6
What are monobasic and dibasic bases?
- Monobasic bases are bases that release one OH- ion e.g. NaOH, KOH, NH3
- Dibasic bases are bases that release two OH- ions e.g. Ba(OH)2, Ca(OH)2
How do you find PH of a strong base?
[OH-] = [Base]
[H+] = Kw/[OH-]
PH = -log[H+]
How do you calculate PH of reaction between strong acid and a strong base?
- ) Calculate H+ moles
- ) Calculate OH- moles
- ) Calculate XS H+ or OH- (larger mol - small mol)
- ) Calculate XS [H+] or [OH]
- ) Calculate PH
What are the 2 assumptions made about weak acids?
- ) [H+]eqm = [A-]eqm
- ) [HA] ≈ [HA]initial
[HA] at equilibrium virtually same as it was before any of it dissociated as so little dissociates
How do you calculate [H+] of weak acids using Ka?
[H+]2 = Ka x [HA]
[H+] = (Ka x [HA])1/2
How do you calculate PH of weak acids?
Ka = [H+]2/[HA]
[H+] = (Ka x [HA])1/2
PH = -log[H+]
How do you calculate PH for the solution formed from reaction between weak acid and a strong base?
For every mole of OH- added, one mole of HA is used up and one mole of A- is formed.
- ) calculate moles HA (if it is still HA and not H+ - weak acids)
- ) Calculate OH- moles
- ) Calculate XS HA or OH-
IF XS is HA:
- ) Calculate HA mol left + A- formed
- ) Calculate [HA] left over + [A] formed
- ) Use Ka to find [H+]
- ) Find PH
If XS OH-:
- ) Calculate [OH-]
- ) Use Kw to find [H+]
- ) Find PH
How is the PH of a diluted solution of a weak acid?
Find concentration of weak acid when diluted:
using C1V1 = C2V2
Calculate [H+] :
[H+]2 = Ka x [diluted weak acid]
PH = -log[H+]
What’s the relation between [H+] and [OH-] in pure water
They’re the same
