acid-base equilibria p1 Flashcards
bronsted-lowry acid
proton donor
bronsted-lowry base
proton accepter
acid-base reaction involve the transfer of…
PROTONS
what ion causes a solution to be acidic?
H+ or more accurately H3O+ (oxonium ion)
what ion causes a solution to be alkaline?
OH-
ionisation of water equation
2H2O(l) ⇌ H3O+(aq) + OH-(aq)
OR
H2O(l) ⇌ H+(aq) + OH-(aq)
derive Kw using the equation for the ionisation of water
Keq = [H+][OH-]/[H2O]
[H2O]Keq = [H+][OH-]
[H2O] so large compared to [H+] and [OH-] that [H2O]Keq can be considered to be constant
therefore…
Kw = [H+][OH-]
what is the value of Kw at 298K?
1 x 10-14
what physical factors affect the value of Kw? how?
temperature - if increased the equilibrium moves to the right so Kw increases and the pH of pure water decreases
what is pKw?
sometimes pKw is used instead of Kw to make numbers more manageable
pKw = -logKw
Kw = 10-pKW
why is pure water still neutral even in pH does equal 7?
[H+]=[OH-]
expression for pH in terms [H+]
pH=-log[H+]
what is the relationship between pH and concentration of H+?
lower pH = higher concentration of H+
if two solutions have a pH difference of 1, what is the difference in [H+]?
a factor of 10
how do you find the [H+] from pH?
[H+] = 10-pH
how do you find [OH-] from pH (at 298K)?
find [H+], use Kw = [H+][OH-] to calculate [OH-]
what is different when finding [H+] from the concentration of diprotic and triprotic acids?
need to multiply the concentration of the acid by the number of protons to find [H+]
how do you calculate the pH of a strong alkaline solution?
use Kw to calculate [H+] from [OH-]
use pH = -log[H+] (pOH = -log[OH-])
strong acid
one which fully dissociates in water
HA –> H+ + A-
how do you calculate the pH of a strong acid?
pH = -log[H+]
strong base
one which fully dissociates in water
XOH –> X+ + OH-
what is the difference between concentrated and strong?
concentrated means many mol per dm3
strong refers to dissociation
what is a weak acid and a weak base?
do not fully dissociate in water
only partially dissociate into their ions
examples of strong acids
HCl, H2SO4, H3PO4
examples of strong bases
NaOH, CaCO3, Na2CO3
examples of weak acids
CH3COOH, any organic acid
example of a weak base
NH3
Ka expression
HA ⇌ H+ + A-
Ka = [H+][A-]/[HA]
how do you work out the pH of a weak acid?
use equation for Ka, subbing in values for [A-] and [HA]
use pH = -log[H+]
what is pKa?
sometimes used to make Ka values more manageable
pKa = -logKa
Ka = 10-pKa
what is a titration?
the addition of an acid/base of a known titrate to a base/acid of an unknown titrate to determine the concentration
an indicator is used to show the neutralization has occurred, as is a pH meter
TITRATION CURVE GRAPH!
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------- y - pH x - volume of acid/base addedequivalence point
the point at which the exact volume of base has been added to neutralise the acid or vice-versa
what happens to the pH of the solution around the equivalence point?
large and rapid change in pH, except in weak-weak titration
how do you calculate the concentration of a reactant if you know the volume and conc of the other reactant and the volume of the reactant added?
calculate moles of one reactant
use balanced equation to work out mols of the other
use conc = mol/vol to calculate concentration
what is the end point?
the volume of acid or alkali added when the indicator just changes colour
if the right indicator is chosen, equivalence point = end point
what 3 properties do a good indicator have?
sharp colour change (not gradual) - no more than one drop of acid/alkali needed for colour change
end point must be the same as the equivalence point, or titration gives wrong answer
distinct colour change so it is obvious when the end point has been reached
what indicator would you use for a strong acid-strong base titration?
phenolphthalein or methyl orange, but phenolphthalein is usually used as clearer colour change
what indicator would you use for a strong acid-weak base titration?
methyl orange
what indicator would you use for a strong base-weak acid titration?
phenolphthalein
what colour is methyl orange in acid? in alkali?
what pH does it change?
acid - red
alkali - yellow
changes at 4-5 approx same as pKa value
what colour is phenolphthalein in acid? in alkali?
what pH does it change?
acid - colourless
alkali - red/pink
changes at 9-10 approx same as pKa value
what is the half-neutralisation point?
when volume = half the volume that has been added at the equivalence point
buffer solution
a solution that resists change in pH when small amount of acid/alkali are added
what do acidic buffer solutions contain in general terms?
a weak acid and a soluble salt of that acid that fully dissociates
acidic buffer with added acid reaction
A- + H+ –> HA
acidic buffer with added alkali reaction
HA + OH- –> H2O + A-
how can you achieve an acidic buffer solution other than just mixing the constituents?
neutralise half of a weak acid (meaning the acid must be in excess) with an alkali - this forms a weak acid/soluble salt mixture
what do basic buffer solutions contain in general terms?
weak base and soluble salt of that weak base
how can you calculate the pH of buffer solutions?
use the Ka of the weak acid, sub in [A-] and [HA], calculate [H+] –> pH
how can you calculate the new pH of a buffer solution when acid or base is added?
calculate number of moles of H+ and A- and HA before acid or base is added
use equations to work out new moles of A- and HA
find [H+]
find pH
which buffer system maintains blood pH at 7.4? what happens when acid/alkali is added?
H+ + HCOs- CO2 + H2O
add OH- which reacts with H+ to form H2O, then shifts equilibrium left to restore H+ lost
add H+ equilibrium shifts to the right removing excess H+
what products are buffers found in?
shampoos, detergents -> important to keep pH right to avoid damage to skin, hair, fabrics
why is there a difference in enthalpy changes of neutralisation values for strong and weak acids?
enthalpy changes of neutralisation are always exothermic
this value is similar for strong acids and alkalis because the same reaction is occurring H+ + OH- –> H2O
weak acids have less exothermic enthalpy change of neutralisation because energy is absorbed to ionise the acid and break the bond to the hydrogen in the un-dissociated acid
