Fundamentals on Chemistry: Acids and Bases

Question 1

Definition of an Arrhenius acid and base

Answer 1

An acid ionises in water to give H+ and anions

A base ionises in water to give OH- and cations

Question 2

Spectator ions

Answer 2

Ions which do not partake in the reaction

Question 3

Oxonium ion

Answer 3

H3O+ (sometimes called hydronium ion)

Question 4

Definition of a Bronsted-Lowry acid and base

Answer 4

An acid donates H+ in a chemical reaction

A base accepts H+ a chemical reaction

Question 5

Why is the Bronsted-Lowry definition an improvement on the Arrhenius definition?

Answer 5

Not all acid-base reactions happen in water

Question 6

Definition of a Lewis acid and base

Answer 6

An acid accepts a pair of electrons

A base donates a pair of electrons

Question 7

Why are all Bronsted-Lowry acids Lewis acids but not all Lewis acids and Bronsted-Lowry acids?

Answer 7

A Bronsted-Lowry acid donates a proton and in the process of this, accepts a pair of electrons therefore all Bronsted-Lowry acids are Lewis acids. However, not all acid-base reactions involve an H+ ion so not all Lewis acids are Bronsted-Lowry acids

Question 8

What is a strong acid and a strong base

Answer 8

A strong acid is a powerful proton donor

A strong base is a powerful proton acceptor

The strength of an acid and a base is their most fundamental property

Question 9

What is Ka?

Answer 9

Ka is the acid dissociation constant. The greater the degree of dissociation, the larger the value of Ka

Ka values can range from very large and positive to very small and negative so pKa is used

Question 10

What is pKa?

Answer 10

pKa = -log10(Ka)

When Ka is large, pKa is small/negative
When Ka is small, pKa is large/positive

Question 11

Why do strong acids appear to have the same pKa in water and how can this be solved?

Answer 11

Strong acids fully dissociate in water meaning they will all show the same pKa. To solve this issue, a process called solvent levelling is used. This is where a solvent which is a poorer H+ acceptor so the difference in dissociation is noticeable

Question 12

Amphoteric

Answer 12

When a molecule can act as an acid or a base. Sometimes called amphiprotic

Question 13

How to calculate pH

Answer 13

pH = -log10[H3O+]

Question 14

The pH of water

Answer 14

At 298K, the pH of water is 7.0, pH can vary with temperature. The higher the temperature, the lower the pH

Question 15

Assumptions of weak acids

Answer 15

1) [H3O+] = [A-]
- expect in extremely dilute solution
- in pure water [H3O+] = 10^-7 mol dm-3

2) There is very little dissociation of the weak acid

Question 16

What is the degree of dissociation?

Answer 16

The ratio of dissociated ions to the starting compound

Question 17

Acid strength of the hydrogen halides?

Answer 17

HI > HBr > HCl&raquo_space; F

The acid strength depends on the position of equilibrium when reacted with water

Question 18

Bond dissociation energy order D(H-X)

Answer 18

HF > HCl > HBr > HI

Question 19

How can the bond hydrogen halides equilibrium with water be pulled to the right

Answer 19

X- can hydrogen bond with the oxonium ion which contributes positively and pulls the equilibrium position to the right

Question 20

What happens when X- = F-

Answer 20

The hydrogen bonding is very strong that it ‘masks’ the acidity of the oxonium ions

While HI, HBr, and HCl all have low pKa values (meaning they are acidic), HF has a pKa value of 3.2 meaning it is a weak acid

There is an effect from the strength of the H-X bond but a larger effect from the hydrogen bonding

Question 21

What is an oxo acid?

Answer 21

An oxo acid is one which contains an O-H group and will have a double bond between oxygen and another
element

Many of these oxo acids have more than one ionisable hydrogen

The general form for an oxo acid is HmEOn

Question 22

How many equilibriums does each basic molecule have?

Answer 22

Monobasic - one equilibrium
Dibasic - two equilibriums
Tribasic - three equilibriums

It is harder to remove H+ from an anion due to the electrostatic attraction

Question 23

What happens to the value of pKa as n-m increases

Answer 23

The value of pKa will decrease as the value of n-m increases

Question 24

How many resonance forms would NOOH have?

Answer 24

• There are 2 resonance forms
• n-m = 1
• Negative charge delocalised over 2 oxygen atoms

Question 25

Which is more stable HClO3 or HClO4?

Answer 25

HClO4 is more stable as it has more resonance forms meaning the negative charge is spread over more electronegative atoms than in HClO3

Question 26

Bell's Rule

Answer 26

pKa = 8-5(n-m) This is an empirical equation

Question 27

Superacids and superbases

Answer 27

Superacids are acids with very low pKa values Superbases are bases with very high pKa values, and are almost all derived from reactions with alkali metals

Question 28

Buffer solutions

Answer 28

A buffer solutions resists change in pH on addition of small amounts of acid and base Consists of either: - Weak acid and salt of its conjugate base - Weak base and salt of its conjugate acid

Question 29

Buffer capacity

Answer 29

The amount of acid or base which can be added to a buffer solution before it no longer resists change in pH

Question 30

Henderson-Hasselback equation

Answer 30

pKa = pH - log10[A-]/[HA] Derived from: -log10(Ka) = -log10[H3O+] - log10[A-]/[HA]

Question 31

Why should the pKa of a solution be close to the pH

Answer 31

Using the Henderson-Hasselback equation, when [A-] = [HA] then [A-]/[HA] = 1 so log10[A-]/[HA] = 0. Therefore pH = pKa + 0 so pKa will be relatively close to the pH of a solution

Question 32

Amino acids

Answer 32

- Amino acids are the building blocks of proteins - Proteins are chains of many amino acids arranged in 3D - In the human body, there are 20 known amino acids all with the same structure - Never found in a neutral form --> always at least one charge on the molecule but there may be no overall charge as it may be zwitterionic

Question 33

What is a zwitterion?

Answer 33

A zwitterion is a molecule which is neutral and contains an equal number of positive and negative charges within it

Question 34

Isoelectric point

Answer 34

The pH at which an amino acid will form a zwitterion. The symbol for isoelectric point is pI