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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
HA + B ⇌ A- + BH+
E.g
CH3COOH (aq) + H2O (l) ⇌CH3COO- (aq) + H3O+ (aq)
ACID. BASE. Con. BASE Con. ACID

HA donates proton to form its conjugate base, A- (CH3COO-)
B accepts PROTON from HA to form its conjugate acid, BH+

(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 ionise them —> LESS EXOTHERMIC
- in this example:
energy needed to break O-H bond to release a H+
CH3COOH —> CH3COO- + H+ - shows partial ionisation

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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 of acid is small so we assume initial conc of undissociated acid has remained constant ( [HA]initial = [HA] equm)
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
What happens when H+ ions added to ethanol acid buffer?
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
What happens if OH- ions are added to ethanoaic acid buffer?
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
Ka expression for buffers and how to find [H+]?
Ka = [salt][H+]/[acid] Rearranged: [H+] = Ka x [acid]/[salt]
28
How blood pH controlled?
CO2 from aerobic respiration combines with water in blood to form carbonic acid (H2CO3) H2CO3 ⇌ 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 If CO2 present: carbonic acid CONC increases Equilibrium shifts right + produces more H+ and HCO3 - ions Excess HCO3- combine with H+ to control pH
29
What is a half equivalence point?
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
As concentration increases by a factor of 10 _________
PH decreases by 1 unit
31
Compare the pH of a strong acid and weak acid after dilution 10,100 and 1000 times?
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
32
What is the enthalpy change of neutralisation for string acids /bases?
-57/-58 Kj/mol Bc acid/alkali are fully ionised and undergo neutralisation by this reaction: OH- (aq) + H+ (aq) —> H2O (l)
33
What are diprotic acids?
2 mol H+ per mole (Multiply conc of acid by 2 to get [H+] or multiply moles by 2 in calcs)
34
Ways to make acidic buffers?
***Weak acid + its salt of its conjugate base*** (ethanoic acid + sodium ethanoate (CH3COONa)) - weak acid partially dissociates/conj base salt FULLY DISSOCIATES CH3COONa(aq) ➔ CH3COO−(aq) + Na+(aq) CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq) ***EXCESS weak acid + STRONG BASE*** (excess ethanoic acid + sodium hydroxide) Excess weak acid remains in solution after base fully consumed - remaining weak acid PARTIALLY DISSOCIATES CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq) In both cases: equm solution contains large amounts of CH3COOH (HA) and CH3COO- (A-)
35
Ways to make basic buffers?
[base] and [salt] > [OH-] Weak base + salt of its conj acid (ammonia + ammonium chloride) SALT fully dissociates , WEAK BASE partially dissociates NH4Cl(aq) ➔ NH4+(aq) + Cl-(aq) NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq) **As a result, solution contains large amount of NH4 + and NH3** EXCESS weak base + STRONG ACID (excess ammonia+ HCl)
36
Example of BASIC BUFFER and how it works?
NH3 + H20 ⇌ NH4 + + OH- Add H+ : H+ conc increases. Some H+ reacts with OH- ions forming H2O Equilibrium shifts right to replace consumed OH- ions Some H+ ions react with NH3 to form NH4 + ***These reactions removes most of added H+ ions - minimise change in pH*** Add OH- : OH- conc increases - react with NH4+ to form NH3 and H2O —> equm shifted left, removes OH- ions from solution - ***minimise change in pH*** In both cases: [B] (NH3) and [BH+] (NH4 +)in reserve so stay constant
37
Devise an experiment to determine acid dissociation constant , Ka, for a solution of ethanoic acid, CH3COOH of unknown conc?
**Titrate** ethanoic acid with strong base Measure **pH at reg intervals** Plot **pH against vol ** of strong base (titration curve) Use graph to find **pH at half equivalence point** At half neutralisation: **PH = pKa so Ka = 10^-pH**
38
CH3COOH + H2O —> CH3COO- + H3O + Give a reason why only the proton from COOH group, and not from the methyl group, is donated to a water molecule?
Loss of hydrogen from methyl group would produce carboanion with no stabilisation
39
Predict with reason, the sign of enthalpy change for the ionisation of water?
+ because Kw increases with temperature

Chemistry A- Level (26 decks)

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