20) Acids, bases and pH Flashcards
Define BrØnsted-Lowry acid
a species that is a proton donor
Define BrØnsted-Lowry base
a species that is a proton acceptor
Define conjugate acid-base pair
two species that can be interconverted by the transfer of a proton
Define conjugate acid
a species that releases a proton to form a conjugate base
Define conjugate base
a species that accepts a proton to form a conjugate acid
How can acid-base pair by clearly linked together?
labelling acid 1 base 1, acid 2 base 2
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Dissociation does not take place unless…?
water is present
Define hydronium ion
H3O+ (aq); active ingredient in any aqueous acid and conjugate acid for water
What is the simplified form of a hydronium ion?
H+
What do the terms monobasic, dibasic and tribasic refer to?
the total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction
Define spectator ions
ions that do not change during a reaction and therefore, can be cancelled out from ionic equations
What do ionic equations help to do?
emphasise the role of H+
How should you write a solid in an ionic equation?
with the full formula
What is the ionic equation for neutralisation of acids with alkalis?
H+(aq) + OH-(aq) -> H2O(l)
For any solution at 25 degrees C, pH < 7 indicates?
decreasing acidity
For any solution at 25 degrees C, pH = 7 indicates?
neutral
For any solution at 25 degrees C, pH > 7 indicates?
increasing alkalinity
How can pH be measured?
indication paper
pH meter
pH = ?
-log [H+(aq)]
[H+(aq)] = ?
10^-pH
A low value of [H+] matches a _ value of pH and vice versa.
high
pH is a _ scale, this means that a pH of 1 has _ times the [H+] as a solution with a pH of 2?
logarithmic
10
Give the equation for the pH of strong acids
[H+(aq)] = [HA(aq)]
because a strong acid completely dissociates its H+
Define acid dissociation constant Ka
equilibrium constant that shows the extent of dissociation of a weak acid
Ka = ?
( [H+(aq)] [A-aq)] ) / [HA(aq)]
Does temperature change Ka?
yes
The larger the numerical value of Ka, the further the equilibrium is to _ and the _ dissociation and acid strength?
right
greater
pKa = ?
-log Ka
Ka from pKa?
Ka = 10^-pKa
The stronger the acid, the _ the Ka val. and the _ the pKa value?
larger
smaller
Give the 2 key equations for the pH of weak acids
- ka = [H+(aq)]^2 / [HA(aq)]
2. [H+(aq)] = √(Ka x [Ha])
What 2 approximations is the key equation for the pH of weak acids based upon?
- [H+(aq)]eqm = [A-(aq)]eqm
the small amount of H+ from the dissociation of water is neglected - [HA(aq)]start = [HA(aq)]eqm
dissociation of weak acids is small so any decrease in [HA] from dissociation is neglected
Give the simplified equation for the pH of weak acids
Ka = ( [H+(aq)]eqm x [H+(aq)]eqm ) / ( [HA(aq)]start - [HA(aq)]eqm )
How can Ka be determined experimentally?
preparing a standard solution of the weak acid of known conc. and measuring the pH of the standard solution using a pH meter
When does the approximation that [H+(aq)]eqm = [A-(aq)]eqm for weak acids break down?
for very weak acids or very dilute solutions as if pH > 6 the dissociation of water becomes significant
When does the approximation that [HA(aq)]start = [HA(aq)]eqm for weak acids break down?
for stronger weak acids w/ Ka > 10^-2 moldm^-3 or very dilute solutions as [H+] becomes significant and there is a real difference between [HA(aq)]eqm and [HA(aq)]start - [HA(aq)]eqm
Define ionic product for water Kw
the product of the ions formed by partial dissociation of water, given by Kw = [H+(aq)] [OH-(aq)]
At 25 degrees C, Kw = _ therefore, neutral water has a pH of _ as [H+] = _?
1.00 x 10^-14 mol2dm-6
7
[OH-(aq)]
If [H+] > [OH-] in a solution it is _?
acidic
If [H+] < [OH-] in a solution it is _?
alkaline
Define strong base
an alkali that completely dissociates in solution e.g. NaOH
Can bases be monobasic, dibasic and tribasic?
yes
Give the key equation for the pH of solutions of strong bases
[H+(aq)] = Kw / [OH-(aq)]
uses concentration of the base and the ionic product of water
HSO3- can act as either a BrØnsted-Lowry acid or a BrØnsted-Lowry base. Explain this statement, including equations for the reactions of HSO3-(aq) with H+(aq) and OH-(aq)
can accept protons acting as a base
HSO3-(aq) + H+(aq) -> H2SO3
can donate protons acting as an acid
HSO3-(aq) + OH-(aq) -> SO3 + H2O (neutralisation)
