Acid-base equilibria

Question 1

What is the Bronsted-Lowry acid-base theory?

Answer 1

Bronsted-Lowry acid-base theory is the theory in which:
Acids are defined as proton donors.
Bases are defined as proton acceptors.

Question 2

What is a monoprotic acid/monobasic

Answer 2

an acid that can donate one proton per molecule (eg. HCl)

Question 3

What is a diprotic acid?

Answer 3

A diprotic, or dibasic, acid can donate two protons per molecule (eg. H2SO4)

Question 4

How is pH defined?

Answer 4

-log[H+], usually just through the equation pH = -log[H+]

Question 5

What do strong acids do when in solution?

Answer 5

Strong acids fully ionise (dissociate) to produce H+ ions when in solution in water.

Question 6

What do weak acids do in solution?

Answer 6

Weak acids partially ionise (dissociate) to produce H+ ions when in solution in water

Question 7

What is a conjugate base

Answer 7

it is formed when a base donates a proton

Question 8

What is a conjugate acid

Answer 8

it is formed when a base accepts a proton

Question 9

What is the Kw equation

Answer 9

Kw = [H+][OH-] / [H2O]

Question 10

is H2O included in the Kw equation? why?

Answer 10

No, because it is a liquid in its natural state and so doesn’t match the products that are aqueous, so we don’t include it so the Kw equation is homogeneous

Question 11

How is pKa defined

Answer 11

mathematically, pKa = -log(Ka)

Question 12

what is the acid dissociation constant?

Answer 12

Ka = [H+][A-] / [HA], (where the weak acid is denoted as HA, and its salt is denoted as A-)

Question 13

What is a buffer solution?

Answer 13

a mixture of a weak acid and its salt, that will resist changes in pH when small amounts of acid or alkali are added

Question 14

What is the enthalpy of neutralisation

Answer 14

enthalpy change when one mole of water is formed by neutralisation between an acid and a base, under standard conditions (298K, 100kP), with all reactants and products in their standard states.

Question 15

what is a conjugate acid base pair

Answer 15

a pair of species on opposite sides of an equation which are related, and differ only by their number of protons.

Question 16

What dp is pH given to

Answer 16

2 dp

Question 17

You have 1.5 M solution of a weak acid, HA. Its pKa is 2.88. Find its pH.

Answer 17

• Use the Ka equation -> Ka = [H+][A-] / [HA]
• 10^-pKa = Ka = 1.32x10^-3
• 1.32 x 10^-3 = [H+]^2 / 1.5
• rearrange to find [H+]^2 = 1.5 x 1.3x10^-3
• Find [H]+
• calculate pH
  Answer = 1.35

Question 18

What is the Ka equation

Answer 18

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

Question 19

What is the anatomy of a pH curve graph

Answer 19

• Half equivalence point
• equivalence point
• Buffer region (in weak acids)
• At the half equivalence point, pH = pKa

Question 20

What is the equivalence point on a pH curve?

Answer 20

The straight portion of a graph, appears when the same amount of moles (assuming they act in a 1:1 ratio) of acid and alkali have been added.
It should always pass through pH 7.00

Question 21

How is half equivalence point determined?

Answer 21

• it is half the quantity of acid/alkali added compared to the equivalence point

Question 22

What does a STRONG ACID STRONG ALKALI pH curve graph look like

Answer 22

• long straight portion in the middle
• very high pH for alkali and very low pH for acid

Question 23

What does a STRONG ACID WEAK ALKALI pH curve graph look like?

Answer 23

• Low pH for acid
• Weaker pH for alkali than a strong one
• middle length of straight portion in the middle

Question 24

What does a WEAK ACID STRONG ALKALI pH curve graph look like?

Answer 24

• higher pH for acid than a strong one
• High pH for alkali
• middle length of straight portion in the middle

Question 25

What does a WEAK ACID WEAK ALKALI pH curve graph look like?

Answer 25

• High pH for acid, low pH for alkali
• very short straight portion in the middle

Question 26

What are indicators?

Answer 26

They are usually weak acids that have two colours; one in protonated species ( H+ still attached) and the other in deprotonated species (H+ species removed)

Question 27

Why do we only add a few drops of indicator?

Answer 27

• Indicators are weak acids themselves, and so they will start to shift the equilibrium depending on what environment they are in (acid or alkali)