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Year 13 Chemistry > Acids, Bases, Ka, Kw > Flashcards

Acids, Bases, Ka, Kw Flashcards

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1
Q

What is a Brønsted acid?

A

A Brønsted acid is a species that can donate a proton.
For example, hydrogen chloride (HCl) is a Brønsted acid as it can lose a proton to form a hydrogen (H+) and chloride (Cl-) ion

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2
Q

What is a Brønsted base?

A

A Brønsted base is a species that can accept a proton.
For example, a hydroxide (OH-) ion is a Brønsted base as it can accept a proton to form water

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3
Q

How can a hydroxide ion act as a base?

A

OH- (aq) + H+ (aq) → H2O (l)

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4
Q

What happens in an equilibrium reaction?

A

In an equilibrium reaction, the products are formed at the same rate as the reactants are used.

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5
Q

At equilibrium, what is present in the solution?

A

At equilibrium, both reactants and products are present in the solution.

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6
Q

What species are present when ethanoic acid (CH3COOH) is at equilibrium after partially dissociating in solution?

A

CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H3O+ (aq)

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7
Q

What is a conjugate acid-base pair?

A

A conjugate acid-base pair is two species that are different from each other by an H+ ion

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8
Q

What is a monobasic acid?

A

Acids, such as HCl, that contain one ionisable hydrogen atom in each molecule are called monobasic acids.
When HCl reacts with NaOH we can see that one hydrogen is replaced by a sodium atom
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (aq)

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9
Q

What is a dibasic acid?

A

Dibasic acids contain two ionisable hydrogen atoms per molecule, for example H2SO4.

Ionisation of such acids occurs in two steps.

When H2SO4 reacts with NaOH we can see that two hydrogens are replaced by two sodium atoms
H2SO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + 2H2O (aq)

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10
Q

What are tribasic acids?

A

Tribasic acids contain three ionisable hydrogen atoms per molecule, for example H3PO4.

Ionisation of such acids occurs in three steps.

When H3PO4 reacts with NaOH we can see that three hydrogens are replaced by three sodium atoms
H3PO4 (aq) + 3NaOH (aq) → Na3PO4 (aq) + 3H2O (aq)

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11
Q

Ionic equation for acid + alkali

A

H+ (aq) + OH- (aq) → H2O (l)

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12
Q

What is a weak acid?

A

A weak acid is an acid that partially dissociates in aqueous solutions

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13
Q

Why is position of equilibrium more over to the left in weak acid dissociation?

A

The weak acid only slightly dissociates, therefore there are more molecules of weak acid than H+ and A- ions.

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14
Q

What is Ka?

A

Ka is an equilibrium constant called the acid dissociation constant.
The higher the Ka, the more the acid dissociates.

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15
Q

What does a large Ka value show?

A

The acid is strong

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16
Q

What does a small Ka value show?

A

The acid is weak

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17
Q

Define what is meant by ‘strength’ of an acid

A

Strength is the extent of dissociation of an acid.
It is measured as Ka.

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18
Q

What is pKa and why is it used?

A

The range of values of Ka is very large and for weak acids, the values themselves are very small numbers
For this reason it is easier to work with another term called pKa.

pKa = -logKa

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19
Q

What does the acidity of a solution depend on?

A

The acidity of an aqueous solution depends on the number of H+ (H3O+) ions in solution.

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20
Q

What is pH defined as?

A

pH = -log[H+]

where [H+] is the concentration of hydrogen ions in mol dm–3.

21
Q

How can the concentration of H+ of a solution be calculated if the pH is known?

A

[H+] = 10-pH

22
Q

How many times more acidic is a solution at pH 5 than a solution at pH6?

A

pH 5 is 10 times more acidic than pH 6.

23
Q

How do you calculate the pH of a strong acid?

A

Strong acids are completely ionised in solution

HA (aq) → H+ (aq) + A- (aq)

Therefore, the concentration of hydrogen ions, H+, is equal to the concentration of acid, HA.

The number of hydrogen ions formed from the ionisation of water is very small relative to the [H+] due to ionisation of the strong acid and can therefore be neglected.

The total [H+] is therefore the same as the [HA]

24
Q

How can the pH of strong bases be found?

A

Strong bases are completely ionised in solution

BOH (aq) → B+ (aq) + OH- (aq)

Therefore, the concentration of hydroxide ions [OH-] is equal to the concentration of base [BOH].

The concentration of OH- in solution can be used to calculate the pH using the ionic product of water.

Once the [H+] has been determined, the pH of the strong alkali can be found using pH = -log[H+].

25
Q

How can we calculate the pH of weak acids?

A

The pH of weak acids can be calculated when the following is known:
- The concentration of the acid
- The Ka value of the acid

From the Ka expression we can see that there are three variables.

However, the equilibrium concentration of [H+] and [A-] will be the same since one molecule of HA dissociates into one of each ion.

So we can simplify the equation to:
[H+]2 = Ka x [HA]

26
Q

What are the limitations of Ka?

A
  • [H+] at equilibrium is equal to the [A-] at equilibrium because they have dissociated according to a 1:1 ratio.
    This is because the amount of H+ from the dissociation of water is insignificant.

The amount of dissociation is so small that we assume that the initial concentration of the undissociated acid has remained constant.
So initial [HA] is equal to the [HA] at equilibrium.
If the acid is stronger, the dissociation will be greater, therefore the difference in the values for initial [HA] and [HA] at equilibrium will be greater.

27
Q

How do you work out the pH of a new solution?

A

To work out the pH for a new solution, you must know which reactant is in excess.

e.g. if mols of OH- is in excess, then subtract mols of H+ from mols of OH-. Then work out concentration of OH-, and substitute into Kw expression.

28
Q

What are the concentrations of [H+] and [OH-] in water?

A
  • In water the concentrations of [H+] and [OH-] are the same, which is why we consider water to be neutral
  • This means we just have to take the square root for the value of Kw to calculate [H+]
29
Q

If we react equal amounts of weak acid and strong base…

A

the resulting salt will slightly react with water to form an alkaline solution.

30
Q

Define ‘enthalpy change of neutralisation’

A

The enthalpy change when an acid and a base react to form one mole of water

31
Q

Why is the enthalpy change of neutralisation the same for all strong acids?

A

Since strong acids and bases completely dissociate in water, every strong acid releases one mole of H+ and every strong base releases one mole OH- for every mole of the compound you start with. Therefore, the energy released when one mole of H+ and one mole of OH- combine (the enthalpy of neutralization) is about 57kj/mole for every strong acid and base.

32
Q

How would you find the pH of a strong acid partially neutralised by a base?

A
  • Moles of strong acid left over
  • Moles of H+ left over
  • / volume of partially neutralised solution
  • [H+]
  • Plug into formula for pH
33
Q

How would you rearrange Kw to find [H+]?

A

[H+] = Kw / [OH-]

34
Q

Outline how to find the pH of a solution of a strong base

A

The first step is to find the concentration of OH- ions.
Use this to find out the concentration of the base.
[H+] = Kw / [OH-]
pH = -log[H+]

35
Q

As we go down Group 2, the solubility of the hydroxides…

A

increases

36
Q

Why does Ba(OH)2 have a higher pH than Mg(OH)2?

A
  • As you go down group 2, the solubility of the hydroxides increases.
  • More Ba(OH)2 dissolves in water than Mg(OH)2, meaning more OH- ions are released.
37
Q

What are the factors that affect the pH of a solution?

A
  • Amount of dissociation
  • Solubility
  • Concentration of H+ ions
  • Temperature
38
Q

What is Ka?

A

Kc but for a weak acid.
Units = moldm-3.

39
Q

What do Ka values allow us to do?

A

Compare the strength of acids.
The higher the Ka value, the stronger the acid.

40
Q

Ka expression

A

Ka = [H+][A-] / [HA]

41
Q

What assumptions are made in the Ka expression?

A

1) The equilibrium concentrations of H+ and A- are equal.
2) The initial concentration and equilibrium concentration of HA are equal.

42
Q

How to find H+ from Ka expression

A

[H+] = square root of Ka x [HA]
Because [H+][A-] = H squared

43
Q

How to find [HA] if given concentration of [H+]

A

[HA] = [H+]2 / Ka

44
Q

What is pKa?

A

-log(Ka)

45
Q

The stronger an acid, the ____ its pKa

A

lower

46
Q

What is the vertical region of a pH curve?

A

Shows equivalence point - shows point at which reaction is complete

47
Q

When is an indicator e.g. phenolphthalein, methyl orange suitable to be used in an acid base titration?

A

When its colour change occurs in the vertical region.

48
Q

Why does a weak acid-weak base titration have no suitable pH indicators?

A

Because it does not have a vertical region.

Year 13 Chemistry (59 decks)

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