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AQA CHEMISTRY A LEVEL > 1.12 Acids and bases > Flashcards

1.12 Acids and bases Flashcards

1
Q

What is an acid?

A

A proton donor

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2
Q

What is a base?

A

A proton acceptor

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3
Q

What is an alkali?

A

A soluble base

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4
Q

How do you find the pH of a solution? (equation)

A

-log₁₀[H⁺]

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5
Q

What is [H⁺]?

A

The concentration of hydrogen ions in the solution.

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6
Q

What is a strong acid?

A

When H⁺ fully dissociates from the acid.

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7
Q

What is a weak acid?

A

When H⁺ only partially dissociates from the acid.

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8
Q

What is a monoprotic acid?

A

Releases one H⁺ per molecule of acid.

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9
Q

What is a diprotic acid?

A

Releases two H⁺ per molecule of acid.

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10
Q

What is the equation used to find concentration or volume post-dilution?

A

C₁V₁ = C₂V₂

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11
Q

What is the ionic product of water, based on?

A

The fact that water only dissociates partly.

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12
Q

What is the dissociation of water equation?

A

H₂O ⇌ H⁺ + OH¯

H₂O + H₂O ⇌ H₃O⁺ + OH¯

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13
Q

What is the ionic product of water formula?

A

Kw = [H⁺][OH¯]

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14
Q

Why is the concentration of water not in the ionic product of water?

A

It is in so much excess, that the water concentration is pretty much constant.

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15
Q

What is Ka?

A

The weak acid dissociation constant.

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16
Q

What is the weak acid dissociation formula?

A

[HA]

17
Q

What is pKa?

A

The pH of a weak acid solution.

18
Q

How do you find pKa?

A

-log₁₀Ka

19
Q

When does pKa = pH?

A

When the weak acid is at the half neutralisation point where [A¯] = [HA]

20
Q

What is Ka when the concentration of [H⁺] = [A¯]?

A

[HA]

21
Q

Why does Kw vary with temperature?

A
  • The forward’s reaction is endothermic.
  • So increasing temperature shifts the equilibrium in favour of the forward’s reaction.
  • This, therefore, increases the Kw.
22
Q

How do you find the pH of a reaction between a weak acid & strong base? (when acid is in excess)

A
  1. Find acid (HA) moles and OH¯ (base) moles.
  2. Find out how much HA is in excess?
  3. Find A¯ moles and concentration.
  4. Find the excess HA concentration.
  5. Use Ka to find [H⁺] and then pH.
23
Q

How do you find the pH of a reaction between a weak acid & strong base? (when the base is in excess)

A
  1. Find acid (HA) moles and OH¯ (base) moles.
  2. Find out how much OH¯ is in excess.
  3. Use Kw to find [H⁺] and then find the pH.
24
Q

Outline what a titration reaction is?

A
  • Used to find the concentration of a solution by gradually adding a second solution to react with.
  • You need to know the equation of the reaction.
  • You need to know the concentration of the second solution.
  • It is all based on neutralisation between the two solutions.
25
Q

How do you know which indicator to use?

A
  • Must show the endpoint of the reaction with a distinct colour change.
  • The colour change must be rapid.
  • Must lie between the vertical part of the acid/base graph.
26
Q

What is the equivalence point between an acid and a base?

A

When [H⁺] = [OH¯]

27
Q

When is half neutralisation?

A

When the reaction is halfway between zero and the equivalence point.

28
Q

What does a buffer do?

A

Maintains an approximately constant pH, and combats the addition/dilution of small amounts of acid & base.

29
Q

What is an acidic buffer made of?

A
  • A weak acid

- A salt of that same weak acid

30
Q

How do you make an acidic buffer? (2 ways)

A
  1. Add a weak acid and a weak acid salt.

2. The half neutralisation point of a reaction between a weak acid & alkali.

31
Q

What is the pH of a buffer made from half neutralisation?

A

Because [HA] = [A¯]

pH = pKa

32
Q

How does the acid in an acidic buffer oppose the change and keep the pH constant?

A

The acid, HA, opposed changes when OH is added. The HA reacts with OH to form H₂O + A¯.

33
Q

How does the weak salt in an acidic buffer oppose the change and keep the pH constant?

A

The initial weak acid dissociation provides a small amount of A¯ so can’t oppose all the change from extra H⁺. The salt provides enough A¯, so when excess H⁺ is added it can react with the A¯ and use it up (by forming HA).

34
Q

How do you find the pH of an acidic buffer? (half neutralisation)

A
  1. Find HA & OH¯ moles
  2. Find HA & A¯ moles
  3. Find [HA] & [A¯]
  4. Use Ka to find [H⁺] and the pH.
35
Q

How do you find the pH of an acidic buffer? (acid + salt)

A
  1. Find HA & A¯ moles
  2. Find [HA] & [A¯]
  3. Use Ka to find [H⁺] and the pH.
36
Q

How do you find the pH after adding some acid/base to the buffer solution?

A
  1. Find original HA & A¯ moles
  2. Find the moles of what is added.
  • If you add acid (H⁺), add these moles to HA, and take them away from A¯.
  • If you add a base (OH¯), take these away from A¯, and take them away from HA.

AQA CHEMISTRY A LEVEL (28 decks)

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