5.1.3 Acids, Bases and Buffers (incomplete)

Define ‘acid’

a proton donor

Define base

proton acceptor

What is a monobasic acid?

an acid which releases one proton per molecule when it dissociates in water

How many protons per molecule do monobasic acids release when in water?

1

Give an example of a monobasic acid

HCl

How many moles of OH- are required to neutralise a monobasic acid?

1

Define dibasic acid

an acid which releases 2 protons per molecule when it dissociates in water

How many protons per molecule do dibasic acids release when in water?

2

Give an example of a dibasic acid

H2SO4

Define tribasic acid

an acid which releases 2 protons per molecule when dissociated in water

How many protons per molecule do tribasic acids release when in water?

3

How many moles of OH- are required to neutralise a tribasic acid?

3

Give an example of a tribasic acid

H3PO4

Define salt

the compound formed when the H+ ion from an acid is replaced by a metal or other positive ion such as an ammonium ion

Ionic equation for a metal and acid

2H+ + Mg → Mg2+ + H2

Ionic equation for a carbonate and acid

CO32- + 2H+ → CO2 + H2O

Ionic equation for an alkali and acid

OH- + H+ → H2O

Ionic equation for an oxide and acid

O2- + 2H+ → H2O

Ionic equation for ammonia and acid

NH3 + H+ → NH4+

When does the release of a proton from an acid happen?

when an acid is added to water

Define strong acid

an acid which completely dissociates into its ions in solution

Define weak acid

an acid which partially dissociates into its ions in solution

Compare the symbols used for strong and weak acids

• strong = irreversible arrow

• weak = reversible arrow

What happens to the proton released from an acid in water?

it is accepted by water to form an oxonium ion

What happens when a proton from an acid is accepted by water

forma an oxonium ion (H3O + )

Draw an oxonium ion

Why does water act like a base?

it accepts a proton

Give another name for oxonium ion

hydrated hydrogen ion

Write the equation for the reaction between HCl and water

HCl = H2O → H3O+ + Cl-

Define conjugate acid-base pair

a pair of two species that transform into each other by the gain or loss of a proton

Write the reaction for ethanoic acid and water

CH3COOH + H2O → H3O+ + CH3COO-

Draw a diagram for the acid-base conjugate pairs in the dissociation of ethanoic acid

Why does CH3COOH act as an acid?

it donates a proton

Why does H2O act as a base?

it accepts a proton

Why does Ch3COO- act as a base?

it accepts a proton

Why does H3O+ act as an acid?

it donates a proton

What are the acid-base conjugate pairs in the dissociation of ethanoic acid?

• CH3COOH and CH3COO-

• H2O and H3O+

How do conjugate acids and bases differ?

only by a proton

Write 2 equations for the dissociation of H2SO4

• H2SO4 → H+ + HSO4-

• HSO4- (reversible) H+ + SO42-

Write 3 equations for the dissociation of H3PO4

• H3PO4 → H+ + H2PO4-

• H2PO4- (reversible) H+ + HPO42-

• HPO42- (reversible) H+ + PO43-

Describe the dissociation of weak and strong acids

• strong = dissociation is complete as only the forward rection takes place

• weak = dissociation is partial

Describe the concentration of protons in strong acids

high

Describe the concentration of protons in weak acids

low

Describe the equilibrium of a weak acid (homo/heterogenous)?

homogeneous

What is the equation for KA value?

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

What are the units for KA value?

moldm-3

What does KA value indicate?

the extent of acid dissociation

Is KA is a large value, where does equilibrium lie?

far to the right, indicating a strong acid

Describe dissociation is Ka is large?

a lot of the HA is dissociated into its ions

When Ka is small where does equilibrium lie?

far to the left

Describe dissociation is Ka is low

a lot of the HA is not dissociated into its ions

How does Ka change as an acid gets stronger?

it increases

Why are pKa values used instead of Ka values?

they are more manageable

How do you calculate pKa from Ka?

-log(Ka)

How do you calculate Ka from pKa?

10^(-pKa)

What does Ka change with?

temperature

Does Ka change with concentration of acid?

no

Describe the extent of the ionisation of water?

very small

How many water molecules dissociate into their ions?

1 in 500 million

What is the ionisation of water known as (2)

• self-dissociation

• self-ionisation

Is the ionisation of water equilibrium homogeneous or heterogeneous

heterogeneous

What is Kw?

ionic product of water

What is the equation for Kw?

Kw = [H+][OH-]

What are the units for Kw?

mol2dm-6

What is Kw at 25*C

1.00 × 10^-14

Is the self-dissociation of water endothermic or exothermic?

endothermic

Equation for pH?

pH = -log[H+]

What does a low pH indicate?

high [H+]

What does a high pH indicate?

low [H+]

Describe the pH scale

logarithmic

How to calculate H+ from pH

10^-pH

How do you find the pH of a strong monobasic acid?

• [H+] = [HA]

• find H+ using HA

• pH = -log[H+]

How do you find the concentration of a strong monobasic acid by pH?

[H+] = 10^-ph

[HA] = [H+]

How do you find the pH of a weak monobasic acid?

• Ka = [H+][A-] / [HA]

• Ka = [H+]²/[HA]

• [H+]² = Ka * [HA]

• square root

• pH = -log[H+]

What assumptions are made when calculating pH of a weak monobasic acid?

• [HA] dissociated = [HA] undissociated

• ionisation of water is minimal

What are the limitations of Ka in calculating the pH of a monobasic acid?

• stronger acids have a larger Ka value and greater degree of dissociation so the assumption that [HA] undissociated = [HA] dissociated may no longer be valid

• weaker acids have a low [H+] meaning that the dissociation of water will be significant and the assumption [H+] = [A-] may no longer be valid

How do you calculate the concentration of a weak monobasic acid from pH?

• [H+] = 10^-pH

• Ka = [H+]²/[HA]

• [HA] = [H+]²/Ka

How do you find the pH of a strong base?

• find [OH-] from [base]

• Kw = [OH-][H+]

• [H+] = Kw/[OH-]

• pH = -log[H+]

How do you find the concentration of a strong base from pH?

[H+] = 10^-pH

Kw = [OH-][H+]

[OH-] = Kw/[H+]

find concentration of base

What is an indicator used for in a titration?

determine the equivalence point

What is the equivalence point?

the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution. H+ concentration is equal to OH- concentration

What does a pH curve show?

how the pH of a solution changes during an acid-base titration

How can pH be measured during a titration?

using a pH probe

What is a pH meter?

an electrode that is dipped into a solution to read the pH

How many decimal places does a pH meter read to?

2

Give an example method of a pH titration?

1. using a pipette, add 25cm³ of 0.100 moldm^-3 HCl to a conical flask

2. place the electrode of the pH meter into the conical flask

3. fill a burette with 0.100 moldm^-3 NaOH

4. add the sodium hydroxide too the conical flask 1cm³ at a time

5. after each addition swirl the contemt

Draw a labelled diagram of a general pH curve

Draw a pH curve where the base is in the conical flask and the acid is added in the burette

Draw a labelled diagram of the pH curve with a strong acid and strong base

Draw a labelled diagram of a pH curve with a strong acid acid and a weak base

Draw a labelled diagram of a pH curve of a weak acid and strong base

Draw a labelled diagram of a pH curve of a weak acid and weak base

What are indicators?

chemical dyes that have a different colour in acid and alkali

Give 2 examples of indicators

• methyl orange

• phenolphthalein

What colour is methyl orange in acid?

red

What colour is methyl orange in alkali?

yellow

What are indicators usually?

weak acids where HA and A- are different colours

For methyl orange, what colour is the undissociated acid?

red

In methyl orange, what colour is the conjugate base?

yellow

What does the addition of an acid do to an indicator?

• increase in concentration of H+

• equilibrium shifts left to minimise increase

• H+ reacts with A- to form more HA