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Chemistry A Level > Acid-Base Equilibria > Flashcards

Acid-Base Equilibria Flashcards

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
Adding acid to a buffer
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
Adding alkali to a buffer
OH- + H+ -> H2O HA -> H+ + A- Reduction in H+ is replaced by equilibrium shifting right
27
Key factor of a buffer solution
Comparatively high concentration of weak acid ion (to remove acid added) Comparatively high concentration of undissociated weak acid (to remove alkali added)
28
Deriving the pH buffer equation
* acid in a buffer is weak; hardly dissociated - HA does not change * A- taken as salt given
29
pH buffer equation
H+ = Ka x ([ACID]/[SALT])
30
pH curve for strong acid and strong base
Start at 1 Équivalence at 7 End at 13
31
pH curve for weak acid and strong base
Start at 3 Equivalence at 8 End at 13
32
pH curve for strong acid and weak base
Start at 1 Equivalence at 6 End at 11
33
Indicator key choice
End point should be the same as equivalence - middle of the vertical section
34
Define end point
The point at which an indicator changes colour
35
What is an indicator?
A weak acid HIn
36
Indicator dissociation
HIn -> H+ + In- HIn is one colour, In- is a different one
37
Indicators in acidic solutions
[H+] shifts equilibrium left; colour 1
38
Indicators in alkali solutions
H+ is removed Equilibrium shifts right Colour 2
39
End points of indicators
Take place at a pH = pKa
40
Finding pKa of a weak acid
Read off halfway to equivalence
41
pH of blood
7.4
42
Buffering systems in blood
Plasma, proteins, Hb, carbonate/hydrogencarbonate conjugate acid-base pairs
43
Proteins act as buffers due to
Amine and carboxylic acid side chains
44
Carbonate/hydrogencarbonate conjugate acid-base pairs
H2CO3 -> HCO3- + H+
45
If the pH of blood drops
H+ is increasing Equilibriums shifts left Reducing H+ H2CO3 -> CO2 + H2O As H2CO3 increases it produces CO2, which is removed by gas exchange
46
If pH of blood increases
H+ decreases Equilibrium shifts right H+ created

Chemistry A Level (12 decks)

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