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Chemistry A- Level > Acid-Base > Flashcards

Acid-Base Flashcards

1
Q

What is a Bronsted-Lowry acid?

A

Substance that can donate a proton

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2
Q

What is Bronsted-Lowry Base?

A

Substance that accepts a proton

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3
Q

What is a conjugate acid-base pair?

A

Pair of reactants/products that are linked to each other by the transfer of a proton
E.g
CH3COOH (aq) + H2O (l) ⇌CH3COO- (aq) + H3O+ (aq)
ACIDBASE Conjugate BASE Con. ACID

(Substance with bigger Ka will act as acid)

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4
Q

Calculate pH and [H+]?

A

PH = -log[H+]
[H+] = 10^-pH

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5
Q

What is meant by ‘the pH scale is logarithmic scale with base 10’?

A

Each value is 10x the value below it
E.g pH 5 is 10x more acidic than pH 6

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6
Q

What is a strong acid and examples?

A

Strong acid is an acid that dissociates completely in aqueous solutions
E.g HCl - hydrochloric acid
HNO3 - nitric acid
H2SO4 - sulfuric acid

HA —> H+ + A-
HA= Strong acid
H+ ions = formed from dissociation
IRREVERSIBLE REACTION (equilibrium shifted right)

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7
Q

In a monoprotic strong acid ————————-
Also what assumption is made when strong acids ionise?

A

The conc of H+ ions = conc of strong acid

No. Hydrogen ions formed from ionisation of water is very small relative to [H+] due to ionisation of strong acid so NEGLECTED

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8
Q

What are weak acids? E.g?

A

Partially dissociate when dissolved in water, giving an equilibrium mixture
E.g most organic acids (ethanoic acid)
HCN - hydrocyanic acid
H2S - hydrogen sulfide
H2CO3 - carbonic acid

HA —> H+ + A-
Equilibrium is LEFT / equilibrium is ESTABLISHED
Due to PARTIAL DISSOCIATION , more molecules of HA (weak acid) than H+ and A- ions

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9
Q

Why is the enthalpy of neurtralsation of strong acids and strong bases very similar?

A

Acid/bases are fully ionised and the reaction is:
H+ + OH- —> H2O
- in each strong acid-strong bases reaction

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10
Q

Why is the enthalpy of neutralisation less exothermic for weak acids and weak bases?

A

Only partially ionise so energy needed to fully ionised them —> LESS EXOTHERMIC

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11
Q

What is Ka and example of an expression?

A

Ka= acid dissociation constant (mol dm-3)
- indicate extent of dissociation
HIGHER Ka = more dissociated = stronger acid

Ka = [H+] [A-] / [HA]
For weak acids as an equilibrium is established

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12
Q

When writing Ka expression for weak acids, what assumption is made?

A

Conc of H+ ions due to ionisation of water is NEGLIGIBLE
[H+]eqm = [A-]eqm as they have dissociated in a 1:1 ratio
Amount of dissociation is small so we assume initial conc of undissociated acid has remained constant
So can simplify to Ka = [H+]^2/[HA]initial

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13
Q

What is ionic product of water , Kw and how is it derived?

A

Kc = [H+][OH-] / [H2O]
Rearranged to:
Kc x [H2O] = [H+][OH-]
Bc [H2O] is music bigger than conc of ions, we asssume value is constant and make it Kw

Kw = [H+(aq)][OH-(aq)]
Kw =1x10^-14 mol2dm-6

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14
Q

Relationship between Kw and pKw?

A

pKw = -logKw

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15
Q

Relationship between pKa and Ka?

A

pKa = -logKa
- used bc for weak acids, Ka are very small numbers so pKa easier to work with
(pKa values lie with 3 and 7, for weak acids)

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16
Q

For strong bases , the [OH-] is = to_________________

A

Conc of the base

17
Q

Why is pure water/neutral solutions neutral?

A

[H+] = [OH-]
Using Kw = [H+][OH-] then when neutral :

Kw=[H+]^2 and [H+] = √Kw
At 25C , [H+] = √1×10^-14 = 1x10^-7
= pH 7

18
Q

What is an acid base indicator?

A

Weak acid which dissociates to give an anion of a different colour
Weak acid HIn:
HIn ⇌ In- + H+
Colour1. Colour 2

Equilibrium will shift left/right due to Chatelier’s principle if acidity changes
- colour changes over a pH range

19
Q

When is endpoint of a reaction reached?

A

When:
[HIn] = [In-]

And at this point if Ka = [H+][In-]/[HIn] , then:
Ka = [H+] OR pKa = pH

20
Q

Best indicators for strong acid-strong base reactions?

A

PH changes from 4-10 ,so indicator must change colour in this range :
METHYL RED/PHENOLPHTHALEIN

21
Q

Best indicator for weak acid-strong base reaction?

A

PH changes from 7 to 10 so:
PHENOLPHTHALEIN

22
Q

Best indicator for STRONG ACID-WEAK BASE reactions?

A

Ph changes from 4 to 7 so:
METHYL RED
Can use METHYL ORANGE TOO

23
Q

Best indicator for WEAK ACID -WEAK BASE reactions?

A

No sudden pH change so no suitable indicators for these titrations
- end point not easily determined

24
Q

What is a buffer solution?

A

Solution where pH doesn’t change significantly if small amount of acid or alkali added to it

Buffer can consist of:
Weak acid - conjugate base
Weak base - conjugate acid

25
Q

What happens when H+ ions added to ethanol acid buffer?

A

CH3COOH ⇌ H+ + CH3COO-
Ethanoic acid. Ethanoate ion
- reserve supplies of HA (CH3COOH) and A- ions (CH3COO-)
Equilibrium shifts left - H+ ions react with CH3COO- ions to form more CH3COOH until equilibrium re-established
due to reserve supply of CH3COO-/ CH3COOH, conc of these doesn’t change much
PH remains reasonably constant

26
Q

What happens if OH- ions are added to ethanoaic acid buffer?

A

The OH- react with H+ to form water so H+ conc DECREASES
Equilibrium shifts right - more CH3COOH molecules ionise to form more H+and CH3COO- until equilibrium is re-established

large reserve supply of CH3COOH, conc of CH3COOH doesn’t change much when CH3COOH dissociates to form more H+ ions
Reserves of CH3COO- the conc of CH3COO- doesn’t change much
PH remains reasonably constant

27
Q

Ka expression for buffers and how to find [H+]?

A

Ka = [salt][H+]/[acid]
Rearranged: [H+] = Ka x [acid]/[salt]

28
Q

How blood pH controlled?

A

CO2 from aerobic respiration combines with water in blood to form solution containing H+

CO2 (g) + H2O (l) ⇌ H+ (aq) + HCO3- (aq)
HCO3- is buffer
If conc of H+ not controlled —> ACIDOSIS (too much acid in body fluids) —> body malfucntion —> coma

Equilibrium shifts left/right to keep pH constant

29
Q

What is a half equivalence point?

A

Stage of titration at which exactly half the amount of acid/base has been neutralised
E.g REACTION BETWEEN NaOH and CH3COOH

NaOH (aq) + CH3COOH (aq) → CH3COONa (aq) + H2O (l)
- can assume [HA]=[A-] so [CH3COOH (aq)] = [CH3COO- (aq)]
PKa = pH at equivalence point

30
Q

As concentration increases by a factor of 10 _________

A

PH decreases by 1 unit

31
Q

Compare the pH of a strong acid and weak acid after dilution 10,100 and 1000 times?

A

STRONG ACID: diluting strong acid by 10x , increase pH by 1 unit , diluting by 100x, increase pH by 2 units

WEAK ACID: diluting weak acid by factor of 10, increase pH by 0.5

Chemistry A- Level (23 decks)

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