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Edexcel Chemistry A LEVEL - RM > TOPIC 12 - ACID + BASE EQIUILIBRIA > Flashcards

TOPIC 12 - ACID + BASE EQIUILIBRIA Flashcards

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1
Q

Bronsted-Lowry acid

eq

A

substance that is a proton donor (H+)

HA(aq) +H2O (l) –> H3O+ + A-

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2
Q

Bronsted-Lowry Base

eq

A

substance that can accept a proton - anything with a lone pair of electrons = dative covalent bond formed
B(aq) + H2O –> BH+ + OH-

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3
Q

strong acid vs weak acid

A

strong acid is able to completely dissociate.
weak acid partially dissociate (very slightly)
CH3COOH = CH3COO- + H+

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4
Q

weak bases

A

only slightly protonate in H20

NH3(aq) = NH4+ + OH-

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5
Q

examples of strong acids

A

HCl, H2SO4 (dibasic acid) , HNO3, HI, HBr, HClO, H3O+

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6
Q

examples of weak acids

A

HSO4- , H3PO4 (tribasic acid), HNO2, HF, HCN, NH4+, H2S, H2CO3

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7
Q

define pH

A
  • log[H+] = concen of H+ in sol

- always 2d.p

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8
Q

find [H+]

A

10^-pH

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9
Q

amphoteric substances

A

H2O & NH3 can act as an acid and base

H2O + NH3 = NH4+ + H20

equil favour direction of where the stronger acid/base produces weaker acid and weaker base

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10
Q

monoprotic and diprotic acid dissociation

A

H2SO4 –> H+ + HSO4-
HSO4- –> H+ + SO4-
calc pH the slightly lower pH of first ionisation as not many H+ from HSO4- dissociate

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11
Q

for every change in 1 unit of pH the times of dilution/ concentration

A

x10^-1 (concen H+) = pH 1
x10^-2 = pH 2
x 10^1 (concen H+) = pH -1

but for weak acids the pH will increase less than 1 unit

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12
Q

acid + base reaction involves and eq

A

involves the transfer of a proton
acid + base -> conjugate acid (paired with base) + conjugate base

A + B –> BH+ + A-

to identify need to by looking both directions of the reaction

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13
Q

conjugate acid (BH+)

A

base (B) accepts a proton and forms a species

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14
Q

conjugate base (A-)

A

acid (HA) donates the proton and species is formed

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15
Q

Stronger the acid

A

the weaker the conjugate base

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16
Q

weaker the acid

A

the stronger the conjugate acid

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17
Q

if asked to find pH when giving the concen of a base

A

Ph + Poh = 14
rearrange after finding
Poh= -log[OH-]
then find Ph = ANS

e.g. 0.15 NaOH mol/dm3. find pH?
[oh] = [0.15]
Poh = -log[0.15]
= 0.8239
= 14-0.8239
pH = 13.17
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18
Q

Ph + Poh = 14

only happens when

A

Kw = 1x10^-14

If Kw = 1x10^-15 then Ph + Poh = 15 etc

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19
Q

calc pH of weak acids

remember [H+]*mole ratio in equation

ALWAYS WRITE EQ

A 
- partial dissociation
HA = H+ + A-
Ka (expresses how easily the acid releases H+) 
Ka = [H+] [A-]/[HA]
due to 1:1 ratio 
Ka = [H+]^2/[HA]initial
rearrange to find [H=] then pH
can find pOH = 14 -pH
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20
Q

presentable/ manageable version of Ka

do not deal with standard form

A 
pKa = -logKa
Ka = 10^-pKa
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21
Q

the smaller the pKa

A 
the greater the Ka 
greater the dissociation
the stronger the acidity 
strength of acid increases
pH<7
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22
Q

pH of weak acids (Ka) assumptions

A
  1. [H+] = [A-] at equilibrium they have dissociated at 1:1 ratio
  2. the amount of dissociation is small so assume inital concen of undissociated acid is CONSTANT and the acid that has been dissociated is CONSTANT = Ka
23
Q

ionic product of water

A

Kc = [H+] [OH-] / [H20]
Kc*[H2O] = [H+][OH-]
constant as cocne on ions bigger than h2o =
Kw = [H+][OH-]
mol2/dm6
H20 dissociates slightly so endo so shit to right

24
Q

25’C Kw=

A

1x10^-14 mol2/dm6

25
Q

in pure water

A

[H+] = [OH-] are equal = 1x10^-14
[H+] = 1x10^-7 = [OH-]
or [H+]^2 = Kw
= pH of water = 7

26
Q

pKw

A

-logKw

27
Q

calc pH of strong base

A

either Kw or Poh+Ph = 14

depending on Kw

28
Q

find pH from concen of diprotic and triprotic acid

A

need to multiply concen of acid by the no. of protons to find [H+]

29
Q

Why is pure water still neutral even if pH does not equal 7?

Kw affected by temp similar to equil

A

does not mean pH = 7
but same
[H+] = [OH-]

30
Q

enthalpy change of neutralisation for strong acids and strong base

A

it will be similar = -57kJ/mol because the reaction btw them they are almost fully dissociated in aq sol

31
Q

enthalpy change of neutralisation for weak acids or weak base

A

is less than -57.6kJ/mol and there is a major difference as majority of acid molecules are undissociated so require less energy to dissociate them. So less heat energy given when CH3COOH & HCN are neutralised by NaOH

32
Q

HF is a weak acid but has a enthalpy change of neutralisation higher than -57kJ/mol why?

A

so more exothermic as the enthalpy of hydration releases more heat energy than the dissoiation molecules takes in

33
Q

pH of sol of strong acid and strong base?

A 
n(H+) = cxvx no.moles of H
N(OH-) = cxv 
which one is in excess 
xs (OH-) = n(OH-) - n(H+)
[OH-] = xs OH- / V  
Poh = -log[OH-]
pH calc
34
Q

What ion causes a solution to be acidic (2 marks)

A

H+ ions release in water combining with H2O to form H3O+ oxonium ions.

35
Q

What physical factors affect the value of Kw and how do they affect it

A

Temperature only - if temperature is increased, equilibrium moves to the right so Kw increases and the ph of the pure water decreases

36
Q

What is the relationship between pH and concentration of H+?

A

Lower pH = higher concentration of H+

37
Q

What is the difference between concentrated and strong

A

Concentrated means many mol per dm3, strong refers to amount of dissociation

38
Q

what is a buffer solution?

A

Buffer solution is a system that minimizes pH changes when addition of small amounts of an acid or base

39
Q

how buffers resist changes to pH when small amount of acid added?

A
  • write equil
  • influx of H+
  • pushes equil to left by reacting with reservoir of propanoate ions - remove H+
40
Q

how buffers resist changes to pH when small amount of base added?

A
  • added OH-
  • reacts with acid to form -oate ions (A-) and water
  • or reacts with H+ to form water
  • H+ concen drops - equil shifts right
  • dissociating more acid to upkeep he H+ concen therefore no change in pH
41
Q

composition of a buffer

A

make a sol of a weak acid + salt of the weak acid
- achieved by neutralise half of weak acid with an alkali - forms weak acid salt

opposite forbasic buffer sol = weak base + soluble salt

42
Q

facts about buffers

A
  • 2 equations
  • weak acid partially dissociates
  • salt of the acid completely dissociates
  • in Ka the total [A-] is the total [] present in the system from both equations
43
Q

2 assumptions abt buffers

A
  • considering [A-] is very small almost negligible due to partial dissociation. so total [A-] comes from salt of the weak acid
  • NEVER Ka = [H+]^2 /[HA]
44
Q

Buffer action in blood

pH = 7.4

A

H2CO3- carbonic acid (H+ + HCO3-) = CO2 + H20
addition of acid/base change equil
carbonic acid = buffer

45
Q

What is titration?

A

Addition of acid/base of known titration to a base/acid of unknown titration to determine the concen. An indicator is used to show that neutralization has occurred

  • clamp stand, burette in that is acid/alkali of known concen, conical flask, whit tile
  • calibrate and improve by maintaining constant temp
46
Q

calculate Ka for a weak acid from experimental data given the pH of a
solution containing a known mass of acid

A 
m (acid) = 0.49g
pH = 3
equil
Mr(acid) = 122g/mol
n(acid)
[acid] 
[H+] = 10^-3.00 = [A-]
calc Ka = [H+]^2/[HA]
compare with data book value
47
Q

obtain a pH titration curve

A
  • add smaller volumes and more accurate curve produced
48
Q

equivalence point

A

the point at which the exact vol of base has been added to just neutralise the acid. vertical point on curve and usually a large and rapid change in pH except in weak-weak titration

49
Q

end point

A

he vol of acid/lkali added when the indicator just changes colour.

50
Q

when does the equivalence point = end point

A

if the right indicator used

51
Q

what are the properties of a good indicator for a reaction? 3 marks

A
  • sharp colour change and not gradual - only 1 drop can cause change in colour
  • end point must be the same as the equivalence point or titration is wrong answer
  • distinct colour change so obvious when end point reached
52
Q

half neutralisation point

A

when half of volume added at equivalence point

- at half equivalence point pH = pKa = buffer

53
Q

titration curve and how to draw

y-axis = pH
x-axis = vol of _ added
A
  • strong acid + strong base
  • strong acid + weak base
  • weak acid + strong base
  • weak acid + weak base
    indicator look at data book
  • calc vol of half equivalence point

Edexcel Chemistry A LEVEL - RM (12 decks)

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