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A2 Chemistry > Acids & Bases > Flashcards

Acids & Bases Flashcards

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1
Q

What is an acid and a base

A

Acid - Proton donor
Base - proton acceptor

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2
Q

What’s the formula of Kw

A

[H+] [OH-]

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3
Q

What’s the value of Kw at 25C

A

10^-14

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4
Q

How do you find [H+] from a pH

A

10^-pH

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5
Q

How does concentration of H+ ions [H+] effect pH?

A

Increasing [H+] = decreased pH
Decreasing [H+] = increased pH

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6
Q

How do you find pH of pure water

A

when neutral Kw = [H+]^2
so [H+] = square root Kw

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7
Q

How does increasing the temperature of water effect pH

A

The dissociation of water is endothermic
- increasing the temp would push the equilibrium to the right
- so [H+] increases
- so pH decreases

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8
Q

how to get Ka from pKa

A

Ka = 10^-pKa

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9
Q

What’s the equation for weak acids dissociation

A

HA —> H+ + A-

HA is the weak acid
A- is the salt

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10
Q

How do you find pH of a weak acid on its own

A

Ka = [H+]^2 / [HA]

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11
Q

How do you work out pH of a DILUTED strong acid

A
  1. [H+] old x old volume/new volume
  2. pH -log
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12
Q

How do you work out pH DILUTED base

A
  1. [OH-] old x old volume/new volume
  2. [H+] = Kw/[OH-]
  3. pH -log
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13
Q

Why is H20 not shown in the Kw expression

A

H20 is almost constant

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14
Q

Why does Kw increase as temperature increases

A

Equilibrium is endothermic in the forward reaction
The equilibrium shifts right to oppose the increase in temperature

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15
Q

Why is the pH probe washed in between readings

A

So the different solutions don’t contaminate eachother

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16
Q

How does a buffer resist changes in pH when a small amount of acid is added

A

1 - write down reversible reaction for the buffer eg CH3COOH -> H+ + CH3COO-
2 - when acid is added [H+] increases
3- so the equilibrium shifts to the left to reduce the conc of H+ ions to what it was originally
4- overall [H+] remains roughly constant

17
Q

How does a buffer resist changes in pH when a small amount of base is added

A

1 - write down reversible reaction for the buffer eg CH3COOH -> H+ + CH3COO-
2 - when OH- is added it reacts with H+ ions so [H+] decreases
3- so the equilibrium shifts to the right to increase the conc of H+ ions to what it was originally
4- overall [H+] remains roughly constant

18
Q

What is the equation that represents when a base is added to a buffer solution

A

HA + OH- —> A- + H20

19
Q

What is the equation that represents when a acid is added to a buffer solution

A

H+ + A- —> HA

20
Q

Explain why diluting with water does not change the pH of a buffer solution

A

[HA] / [A-] is constant upon addition of water therefore [H+] is constant

21
Q

Describe a method to construct a pH curve 7m

A
  1. Transfer 25cm3 of acid to a conical flask using a pipette
  2. Measure initial pH of the acid with a pH meter
  3. Add alkali in small amounts 2cm3 noting the volume added
  4. Stir the mixture to equalise the pH
  5. Measure and record the pH to 1.d.p
  6. Repeat 3-5 but when approaching endpoint add smaller volumes of alkali
  7. Add until the alkali is in excess
22
Q

Why do you need to calibrate the pH meter

A

Because they can lose accuracy when they’re stored for periods of time

23
Q

How can you improve the accuracy of pH meter

A

By maintaining constant temperature

24
Q

What are examples of acids strong and weak

A

Strong - HCl, H2SO4, HNO3
Weak - CH3COOH

25
Q

What are examples of bases strong and weak

A

Strong - NaOH, Ca(OH)2
Weak - NH3

26
Q

What is the pH at equivalence point of a strong acid + strong base

A

pH 7

27
Q

What is the pH at equivalence point of a strong acid + weak base

A

Less than pH 7

28
Q

What is the pH at equivalence point of a weak acid + strong base

A

More than pH 7

29
Q

What is the pH at equivalence point of a weak acid + weak base

A

Normally 7 but hard to determine

30
Q

In a buffer when you add ACID how do you work out moles of HA and A-

A

Mol of HA = initial moles + moles of acid

Mol of A- = initial moles - moles of acid

31
Q

In a buffer when you add a BASE how do you work out moles of HA and A-

A

Mol of HA = initial moles - moles of base

Mol of A- = initial moles + moles of base

32
Q

What’s the equation to calculate [H+] in a buffer solution

A

Ka x [HA] / [A-]

33
Q

How do you work out Ka from pH

A

10^-pH

34
Q

How does a COOH group in a acid effect it’s strength

A
  • it contains two very electronegative oxygen atoms
  • therefore has a negative inductive effect
  • making the acid stronger
35
Q

How does a CH3 group in a acid effect it’s strength

A
  • has a positive inductive effect
  • making the acid weaker
36
Q

How does polarity of OH effect acid strength

A
  • the O-H bond is more polarised
  • so there is more dissociation info H+ ions
  • making the acid stronger
37
Q

How do you do a buffer calc when it’s a neutralisation

A
  1. Work out initial moles of acid and base
  2. Take excess away from the other
  3. Do excess/total vol to get [HA]
  4. Do initial moles of thing not in excess/total vol to get [A-]
  5. Find [H_] by rearranging Ka expression

A2 Chemistry (16 decks)

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