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Advanced Higher Chemistry > Physical Chemistry 1.2 - Chemical Equilibrium - Acids and Bases, Salts, Buffers and Indicators > Flashcards

Physical Chemistry 1.2 - Chemical Equilibrium - Acids and Bases, Salts, Buffers and Indicators Flashcards

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1
Q

What is an acid?
What is a base?

A

a proton donor
a proton acceptor

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2
Q

Water is amphetoric. What does this mean?

A

it can act as an acid and a base

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3
Q

The equation for the ionisation of water demonstrates that water is ___.
H2O + H2O ⇌ H3O+ + OH-

A

amphetoric

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4
Q

Another word for a hydrogen ion is a ___.

A

proton

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5
Q

For every acid there is a ___ base, formed by the ___ of a proton.
For every base there is a ___ acid, formed by the ___ of a proton.

A

conjugate, loss.
conjugate, gain

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6
Q

A strong acid or base is one that…
A weak acid or base is one that…

A

completely dissociates into ions in solution (→).
only partially dissociates into ions in solution, and is therefore in equilibrium (⇌)

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7
Q

Since acids donate protons, they react to form _ve ions.
Since bases accept protons, they react to form _ve ions.

A

-ve, +ve

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8
Q

At the end of a reaction involving a strong acid or base, there will be no ___ left.

A

reactant (acid/base)

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9
Q

Name 3 strong acids and 3 weak acids.

A

Strong: Hydrochloric (HCl), Nitric (HNO3), Sulfuric (H2SO4)
Weak: Ethanoic (CH3COOH), Carbonic (H2CO3), Sulfurous (H2SO3)

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10
Q

Name some strong bases and weak bases.

A

Strong: any solution of metal hydroxides: Solidum hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH)
Weak: Ammonia (NH3), Amines (NH2R, NHR2)

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11
Q

What is a conjugate acid-base pair?

A

a pair of species - one an acid and the other a base - they create each other by transferring a proton when the reaction takes place

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12
Q

Since weak acids and bases only partially dissocate, they are a type of ___ reaction.
This means they can be expressed by __.

A

equilibrium.
K

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13
Q

What is the equation for the dissocate of an acid?
What does K look like for this equation?

A

HA + H2O ⇌ A- + H3O+
Ka = [H3O+] [A-] / [HA]

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14
Q

Why is H2O not included in the expression for Ka?
What is A-?

A

it’s a liquid (and therefore has a value of 1).
the conjugate base of the acid

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15
Q

The relative dissocation of an acid can also be represented by…

A

pKa.
(pKa = - log (Ka)

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16
Q

Since pKa is the - log of Ka, it provides an inverse value, meaning as Ka decreases, pKa ___.

A

increases.

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17
Q

The pH of a weak acid can be calculated by pH = ?
*in db

A

pH = 1/2 pKa - 1/2 log (c)
*where c is concentration of acid in mol L-1

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18
Q

What is pH a measure of?
What is a neutral solution?

A

the concentration of hydrogen ions in solution.
a solution in which the concentration of H+ ions is equal to the concentration of OH- ions.

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19
Q

Are neutral solutions always pH 7?
Why is this?

A

Nope.
Because neutrality is defined by an equal concentration of H+ and OH- ions, and does not care how many there are, as long as they’re the same.
pH on the other hand is purely a measure of H+ ion concentration (in mol/L), and does not care how many OH- ions there are. pH only cares about the number of moles of H+ ions in a certain volume.

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20
Q

What are salts made of?

A

an acid and a base

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21
Q

What will the relative strength of the parent acid and base determine in a salt?

A

it’s pH (when it dissocates into ions in solution)

22
Q

A strong acid and strong base will produce…
A strong acid and weak base will produce…
A weak acid and strong base will produce…
A weak acid and weak base will produce…

A

a neutral salt.
an acidic salt.
an alkaline (basic) salt
nothing

23
Q

All salts will fully ___ in water.

A

dissociate

24
Q

Why will a salt made of a strong acid and a strong base create a pH neutral solution?

A

because the conjugate partners of the acid and base are present in equal amounts, so they react with H+ and OH- ions the same amount

25
Q

The equilibrium of ___ is always present in solutions.

A

water

26
Q

A strong acid plus a weak base will produce an ____ salt because…

A

acidic, because OH- ions are being reacted with, and therefore removed from the water equilibrium.

27
Q

The equilibrium of water then…

A

shifts to the right to replenish the OH- ions, creating more H+ in the process

28
Q

When explaining why salt solutions are acidic or alkali, you must refer to the water equilibrium, explaining that the ions produced by ___ of the salt are reacting with either __ or __.

A

dissocation, H+ or OH-

29
Q

In an alkaline salt, the concentration of _ ions is greater than that of _ ions.
This is because the ___ ions of the salt are ___ with the _ ions, removing them from solution.
This shifts water’s ___ to the product side, producing more ___ and ___ (in a __ ratio).

A

OH-, H+.
conjugate, reacting, H+.
equilibrium, H+ and OH-, 1:1

30
Q

Salts are named after…

A

their parent base, and acid

31
Q

What is the name of the acid and base which reacted to produce sodium ethanoate?
The name of the ___ comes first.

A

sodium hydroxide, Ethanoic acid.
base

32
Q

What is a buffer solution?

A

a solution in which the pH remains approximately constant when a small amount of acid or alkali is added

33
Q

What are the two types of buffer and how are they made?

A

acidic and alkali - made by reacting a weak acid or alkali with one of its salts

34
Q

When explaining how the pH of a buffer remains constant, which equations should you write out and make reference to?

A
  1. The salt (which completely dissocates into +ve and -ve ions, one of which is reactive, the other unreactive)
  2. The weak acid/base (this is an equilibrium reaction - which will shift to the right (products) should the H+/OH- on the products side react with added OH-/H+)
  3. Water (the equilibrium of water is far to the left, but will shift right if H+/OH- is reacted with, but will produce H+ and OH- in equal amounts, not affecting pH
35
Q

You should then suggest which ions would react with the added __ or __ ions, to keep the __ constant.

A

H+, OH-, pH

36
Q

Take the buffer of ammonia and ammonium chloride:
NH3 + H2O ⇌ OH- + NH4+
NH4+Cl- -> NH4+ + Cl-
1. Is this buffer acidic or alkali?
2. If a small volume of acid is added, what will happen?
3. What about an alkali?

A
  1. alkali.
  2. the H+ ions produced by the acid will react with the OH- ions produced by the weak base dissocating, which will cause its equilibrium to shift to the right to replenish them
  3. The OH- ions produced by the alkali will react with the +ve ions of the salt (NH4+)
37
Q

How does an acidic buffer solution cope with added acid?
How about a base?

A
  1. the H+ ions from the acid react with the -ve ions of the salt
  2. the OH- ions from the base will react with the H+ ions from the weak acid’s equilibrium, which will be replaced by the shifting of the equilibrium to the right
38
Q

Which equation tells you the pH of a buffer?
*in db

A

pH = pKa - log(acid/salt).
The pKa of an acid is the - log of Ka, and has values provided in the db

39
Q

What are indicators?

A

weak acids with the equation:
HIn + H2O ⇌ In- + H3O+

40
Q

The colour of an indicator molecule is ___ from the colour of its ion.

A

different

41
Q

The colour of an indicator solution is determined by the ratio of ___ to ___.

A

reactants, products

42
Q

What is the acid dissocation constant? (indicator edition)

A

KIn = [In-] [H3O+] / HIn

43
Q

What is the theoretical point at which colour change occurs?
At what point does the colour change become visible?

A

when the concentration of HIn = In- or when KIn = H3O+, (given that: KIn = [H3O+][In-] / [HIn] )
Orrr when pKIn = pH.

when the concentration of HIn and In- differ by a factor of 10

44
Q

The pH range over which a colour change occurs can be estimated by which equation? *in db

A

pH = pKIn ±1

45
Q

How can you tell if an indicator is suitable for use in a solution?

A

the pH range of the indicator fits within the vertical section of the tritration curve

46
Q

Why do indicators change colour?

A

because the addition of H+ or OH- shifts their equilibrium, changing the concentrations of the coloured HIn and In- molecules.
(i think the other reactants and products make up the difference so that the ratio of reactants to products (K) remains the same?)

47
Q

If KIn = H3O+, what does pKIn =?

A

pH

48
Q

Different indicators have different ___ ___ (there is a table on pg 14 of the data booklet)

A

pH ranges

49
Q

What is the pH range of an indicator?

A

the range of pHs over which it changes colour

50
Q

Over, or beneath an indicator’s pH range, it will appear a ___ colour, because the equilibrium is very far to one ___.

A

uniform, side

51
Q

Explain fully why a solution of the salt sodium 4-hydroxybenzoate has a pH greater than 7.
(2 marks)

A

Conjugate base of a weak acid,
removes/reacts with H+ from the water

Causing the water equilibrium to shift to the right hand side

Advanced Higher Chemistry (22 decks)

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