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N Chemistry A Level > Topic 12 Acids > Flashcards

Topic 12 Acids Flashcards

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1
Q

Define pH

A

-log [H+]

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2
Q

How to find [H+] ?

A

10 ^-pH

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3
Q

What is a strong acid?

A

Dissociates completely

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4
Q

What is a weak acid?

A

Hardly dissociates in water

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5
Q

What is the Bronsted-Lowry definition of acid and alkali?

A

Acid = proton donors
Alkali = proton acceptors

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6
Q

How to find Kw?

A

[H+][OH-]

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7
Q

What is the Kw of water at room temperature? Always the same.

A

1 x 10^-14 mol2 dm-6

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8
Q

What is the pH of 0.1, 0.01 and 0.001 moldm-3 of an HCl solution?

A

pH 1, 2, 3 respectively

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9
Q

Calculate the pH of a 0.05moldm-3 H2SO4 solution

A

H2SO4 -> 2H+ + SO4 2-
1:2 ratio
0.05x2 = 0.1
= pH 1

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10
Q

Why is H (a bare proton) more unlikely to exist in an aqueous solution but instead react w water to form H3O+?

A

They are highly reactive, very unstable on their own, so will react with water

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11
Q

Why is the Ka calculation an approximation?

A
  • ignores H+ made by water
  • ignores dissociation in acids
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12
Q

What’s a conjugate pair?

A

A species before and after losing/gaining an H+ ion

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13
Q

What does p mean?

A

-log

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14
Q

Why is pure water not alkaline in certain conditions?

A

[H+] =[OH-] , not exceeded [H+]

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15
Q

What is the pH of magnesium hydroxide compared to calcium hydroxide?

A

Calcium hydroxide is more alkali
Because G2 hydroxide solubility increases down
More solubility = more OH- dissolved and released
Hence detected by pH probe
So more alkali

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16
Q

How to calculate Ka?

A

[H+]^2 / [acid]

17
Q

How to compare Ka values and determine which ones more acidic?

A

The greater the Ka value for an acid is relative to the strength of the acid
Due to Bigger numerator in equation

18
Q

How to find pH of buffer?

A

[H+] = Ka x ([acid]/[salt])
Then
-log[H+]

19
Q

What are 3 things that affect pH? (In buffers)

A
  • ratio of [acid]:[salt]
  • type of acid (diff Ka)
  • temp (changes Ka)
20
Q

How to find pH of weak acid?

A

Ka = [H+]^2 / [acid]
rearrange and -log(ans)

21
Q

What is the role of buffers in blood?

A
  • to keep pH of blood plasma constant at 7.35-7.45
  • used in respiration and metabolism
22
Q

What is the equilibrium equation of buffers in blood?

A

H2CO3 ⇌ HCO3- + H+
all (aq)

23
Q

What is the buffer named in blood?

A

carbonic acid-hydrogencarbonate buffer mixture

24
Q

Why is chloroethanoic acid a stronger acid than ethanoic acid?

A
  • more stable anions
  • Cl- draws e-s to itself (more chloros=more stable)
  • reduces negative charge on O
  • more chloros = more stable (less attractive) = stronger acid (as happier to dissociate H+)
25
Q

Write the reaction of ammonia and water.

A

NH3 + H2O ⇌ NH4+ OH-

26
Q

Explain why the pH of solution 1.0 moldm-3 ammonia is less than pH 14

A

It is a weak base
So equilibrium lies to the left (NH3 + H2O ⇌NH4+ + OH-)
OH- produced is nowhere close to 1moldm-3 (as it hardly dissociates already)

27
Q

What is a buffer solution meant?

A

Solution that keeps pH of mixture relatively constant when small volumes of acid/alkali are added

28
Q

Suggest a buffer solution for ammonia

A

Ammonium chloride
- salt of base

29
Q

Hydrogen carbonate (HCO3-) act as a weak acid in aqueous solution. Write an equation for this equilibrium

A

HCO3- ⇌ H+ + CO3-

30
Q

How does buffer work when small amounts of acids and alkali are added?

A

the equilibrium of the buffer solution will shift to oppose the change

31
Q

When to use buffer/Kw/weak acid equations to find pH in titration questions?

A

When stated buffer, use buffer
When acid in excess (alkali reacts to make salt) use buffer eq
When alkali in excess, subtract excess moles and use Kw
Know what acids are strong and weak. If obscure, assume weak

32
Q

Why is the salt of weak acid alkaline?

A

Because it accepts protons from water and forms OH- and acid

33
Q

What must be assumed when calculating Ka acid (2 marks)

A
  • that [H+] and [HCOO-] are equal, no further dissociation took place
  • [HCOOH] at equilibrium is equal to original [HCOOH]

N Chemistry A Level (21 decks)

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