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Chemistry A Level > Acid-Base Equilibria > Flashcards

Acid-Base Equilibria Flashcards

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1
Q

Strong acid dissociation in water eqn options

A

HA -> H+ + A-

HA + H2O -> H3O+ + A-

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2
Q

Oxonium ion

A

H3O+

Formed when hydrogen ions join with water molecules in solution

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3
Q

Strong acid definition

A

A proton donor that fully dissociates in water

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4
Q

Weak acid definition

A

A proton donor that partially dissociates in water

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5
Q

Base definition

A

Proton acceptor

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6
Q

Define pH

A

The negative log of the hydrogen ions concentration

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7
Q

Equation for water forming oxonium ions

A

H2O + H2O -> H3O+ + OH-

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8
Q

Assumptions when calculating pH

A
  • hydrogen ion concentration from water is insignificantly low (all hydrogen ions come from the acid)
  • acid concentration remains high (>1x10-5)
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9
Q

The greater the extent of dissociation…

A

The stronger the acid is

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10
Q

Equilibrium constant

A

[H3O+][A-] / [HA][H2O]

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11
Q

Ka =

A

[H+][A-] / [HA] moldm-3

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12
Q

Calculating hydrogen ion concentration of weak acids

A

Square root of: Ka[HA]

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13
Q

Assumption when calculating pH of a WEAK acid

A

concentration of HA does not change significantly on dissociation

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14
Q

Dilutions

A

Strong:
• 10x = +1pH
• 100x = +2pH
• 1000x = +3pH

Weak:
• 10x = +0.5pH
• 100x = +1pH
• 1000x = +1.5pH

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15
Q

For each 10x dilution

A

Strong acid pH increases by 1

Weak acid pH increases by 0.5

Weak acid is lower because it is diluted - some of the undissociated acid molecules split up

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16
Q

pKw

A

pH + pOH = 14

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17
Q

Kw - ionic product of water

A

1x10-14mol2dm-6

[H+][OH-]

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18
Q

Strong base

A

Particles dissociate completely to form hydroxide ions

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19
Q

Enthalpy change

A

Less exothermic with a weak acid, because some energy is used to dissociate

20
Q

The smaller the pKa…

A

The larger the Ka; the stronger the acid

21
Q

When the salt of a weak acid is placed in water…

A

It dissociates completely

MA -> M+ + A-

22
Q

Anion of weak acid + water

A

A- + H2O -> HA + OH-

Equilibrium
Therefore the salt of a weak acid forms a slightly alkali solution

23
Q

Buffer solution

A

Mixture of a weak acid (or base) and it’s salt

24
Q

Characteristic of a buffer

A

Resists dramatic changes in pH if small quantities of acid or alkali are added

25
Q

Adding acid to a buffer

A

MA -> M+ + A-
HA -> À- + H+

Acid equilibrium lies mainly to the left due to the high concentration of A- provided by the salt

If acid is added, equilibrium shifts left; taken up by A-

26
Q

Adding alkali to a buffer

A

OH- + H+ -> H2O
HA -> H+ + A-

Reduction in H+ is replaced by equilibrium shifting right

27
Q

Key factor of a buffer solution

A

Comparatively high concentration of weak acid ion (to remove acid added)

Comparatively high concentration of undissociated weak acid (to remove alkali added)

28
Q

Deriving the pH buffer equation

A
  • acid in a buffer is weak; hardly dissociated - HA does not change
  • A- taken as salt given
29
Q

pH buffer equation

A

H+ = Ka x ([ACID]/[SALT])

30
Q

pH curve for strong acid and strong base

A

Start at 1
Équivalence at 7
End at 13

31
Q

pH curve for weak acid and strong base

A

Start at 3
Equivalence at 8
End at 13

32
Q

pH curve for strong acid and weak base

A

Start at 1
Equivalence at 6
End at 11

33
Q

Indicator key choice

A

End point should be the same as equivalence

  • middle of the vertical section
34
Q

Define end point

A

The point at which an indicator changes colour

35
Q

What is an indicator?

A

A weak acid

HIn

36
Q

Indicator dissociation

A

HIn -> H+ + In-

HIn is one colour, In- is a different one

37
Q

Indicators in acidic solutions

A

[H+] shifts equilibrium left; colour 1

38
Q

Indicators in alkali solutions

A

H+ is removed
Equilibrium shifts right
Colour 2

39
Q

End points of indicators

A

Take place at a pH = pKa

40
Q

Finding pKa of a weak acid

A

Read off halfway to equivalence

41
Q

pH of blood

A

7.4

42
Q

Buffering systems in blood

A

Plasma, proteins, Hb, carbonate/hydrogencarbonate conjugate acid-base pairs

43
Q

Proteins act as buffers due to

A

Amine and carboxylic acid side chains

44
Q

Carbonate/hydrogencarbonate conjugate acid-base pairs

A

H2CO3 -> HCO3- + H+

45
Q

If the pH of blood drops

A

H+ is increasing
Equilibriums shifts left
Reducing H+

H2CO3 -> CO2 + H2O

As H2CO3 increases it produces CO2, which is removed by gas exchange

46
Q

If pH of blood increases

A

H+ decreases
Equilibrium shifts right
H+ created

Chemistry A Level (12 decks)

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