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AQA
Physical
3.1.12 Acids and Bases
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Cards (36)
Define 'Bronsted-Lowry Acid'
Proton donor
Define
'Bronsted-Lowry Base' Proton Acceptor
Formula for pH:
pH =
−log
([
H
+])
Acids are strong when they completely
dissociate
into
H+
ions in water
Acids are weak when they only slightly
dissociate
when dissolved in water, giving an
equilibrium
mixture
PKA scale
pKA = -
log
KA
so KA
10-pka
Using their Ka values,
how
can you tell which weak acid is stronger?:
Higher Ka value,
stronger
weak acid
Lower Ka value,
weaker
weak acid
What is the method of working out pH of a neutralised solution?
Work out
moles
of acid and base
Determine which is in
excess
If base in excess, use
Kw
for the excess
What is an indicator? A
weak acid
which has a different
colour
to its conjugate base
Buffer solution
: the pH does not change significantly if small amounts of acid or alkali are added to it
Acidic
buffer: made from a
weak acid
and the salt of that weak acid e.g ethanoic acid and sodium
ethanoate
Basic
buffer: made from a
weak
base and salt of that weak base e.g
ammonia
and ammonium chloride
Experimental procedure to form a titration curve:
Place a fixed amount of
alkali
solution into a beaker
Use a
burette
to add small portions of acid
Stir and measure
pH
after each addition of acid
If two solutions have a pH difference of 1, what is the difference in [H+ ]? A factor of
10
Define a Bronsted-Lowry base:
Proton acceptor
What ion causes a solution to be alkaline?
-OH
(
hydroxide
ion
)
What is the value of Kw at 298K?
1.0
x
10-1
What physical factors affect the value of Kw ? How do they affect it?
Temperature only
: if this is increased, the equilibrium moves to the
right
so Kw increases and the pH of pure water
decrease
Lower pH =
higher
concentration of
H+
Examples of
strong bases
: NaOH, CaCO3 , Na2CO3
Examples of
weak acids
: CH3COOH (ethanoic), any organic acid
Examples of
weak bases
:
NH3
Write a reaction for an acidic buffer with added alkali:
HA
+
OH-
→
H2O
+
A
What products are buffers found in?
Shampoos
, detergents → important to keep pH right to avoid damage to skin, hair, fabrics
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
+ ] →
pH
What do basic buffer solutions contain in general terms?
Weak base
and
soluble salt
of that weak base
What ion causes a solution to be alkaline?
-OH
(
hydroxide
ion
)
Write an equation for the ionisation of water
(2) 2H2O (l) ⇌
H3O
+ (aq) +
-OH
(
aq
)
OR
H2O (l) ⇌
H
+ (aq) + -OH (aq)
What is the value of K w at 298K?
1.0
x
10-14
Why is pure water still neutral, even if pH does not equal 7? [
H
+ ] = [
-OH
]
What is the relationship between pH and concentration of H+ ?
Lower
pH =
higher
concentration of H+
How do you find [H+ ] from pH? [
H
+ ] =
10-pH
How do you calculate the pH of a strong alkaline solution?
Use
Kw
to calculate [
H
+ ] from [
OH-
]
Use pH=
-log
[
H
+ ]
Strong acid:
One which fully
dissociates
in water (
HX
→
H
+ +
X
- )
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 amount of dissociation