Gen 6 - Acids and Bases

Question 1

What is a Bronsted Acid ?

Answer 1

An acid is a proton (ie. H⁺) donor

Question 2

HA + H₂O ⇔ ? + ?

Answer 2

H₃O⁺ + A^-

Question 3

What is the equation for the equilibrium constant Ka ?

Answer 3

Question 4

Name the two catergories of Acids in general chemistry ?

Answer 4

• Binary Acids
• Oxy Acids

Question 5

Describe binary acids ?

Answer 5

Acids composed of hydrogen and a non-metal e.g. HCl

Question 6

Describe the trend displayed by halogen containing binary acids ?

Answer 6

As halogen size increases, acid strength also increases.

Bond length also increases, which means bond strength decreases, leading to increased acidity.

Question 7

What are Oxyacids ?

Answer 7

Anions containing a non-metal oxygen such as hydroxide or nitrate ions. They contain:
H - O - X

Question 8

Describe the acidity of Oxyacids ?

Answer 8

The more oxygen atoms bound to the central atom, the more acidic the oxyacid.

Question 9

What are diprotic acids ?

Answer 9

Acids with two protons e.g. H₂SO₄.

Question 10

Describe the equilibrium constant Ka for diprotic or polyprotic acids ?

Answer 10

The acids would have Ka values for each of its protons, e.g. Ka1 for the first, Ka2 for the second.

The acids ionise in successive steps with their own Ka’s.

Ka1 is typically > Ka2

Question 11

Give 8 examples of strong acids

Answer 11

1. Perchloric HClO₄
2. Chloric HClO₃
3. Nitric HNO₃
4. Hydrochloric HCl
5. Sulfuric H₂SO₄
6. Hydrobromic HBr
7. Hydroiodic HI
8. Hydronium ion H₃O⁺

Question 12

What is a base ?

Answer 12

A proton accepter

Question 13

B + H₂O ⇔ ? + ?

Answer 13

HB⁺ + OH^-

Question 14

What is the equilibrium constant for a base ?

Answer 14

Question 15

Give examples of strong and weak acids ?

Answer 15

Strong bases include any hydroxide of the Group 1A metals e.g. Na, K, Li etc…

The most common weak bases are ammonia and any organic amine.

Question 16

Why is water amphoteric ?

Answer 16

It can act as an acid and a base

Question 17

How does Ka relate to relative acid strength

Answer 17

The greater the magnitude of Ka, the stronger the acid (the more the reaction proceeds to the right).

Question 18

What is a conjugate acid-base pair ?

Answer 18

The acid [HA] and the base produced when it ionises [A-].

E.g. HCN and CN^-

Question 19

Describe relative strength in conjugate acid-base pairs ?

Answer 19

If the main acid or base is strong, the conjugate acid or base will be weak, and vice versa.

Question 20

Explain why an amino acid is an example of a conjugate acid-base ?

Answer 20

It has two groups, the acidic carbonyl group, and the base amine group.

Question 21

Give a biological example of a Zwitterion ?

Answer 21

In blood plasma (pH 7.4) the predominant form of amino acids is COO^-, NH₃⁺

Question 22

What is the ion product constant for water at STP ?

Answer 22

Kw = [H+] [OH-] = 1.0 x 10 ^-14

This is the equilibrium constant for the reaction in which water undergoes an acid-base reaction with itself.

Question 23

What is the pH of a solution ?

Answer 23

An expression of the concentration of hydrogen ions in a solution.

pH = -log₁₀[H⁺]

Question 24

If the [H+] of solution of pure water is [H+] = 10^-7, what is the pH ?

Answer 24

7.0

Question 25

If pH is > 7, how is the solution described ?

Answer 25

Alkaline (basic)

Question 26

If the pH is < 7, how is the solution described ?

Answer 26

Acidic

Question 27

How can pH be measured quantitatively and qualitatively ?

Answer 27

Quantitative = pH meter

Qualitative - indicator

Question 28

What is the most commonly encountered weak acid ?

Answer 28

Acetic acid HOAc

Question 29

If the Ka of an acid is 1.75 x 10 ^-15, what is the relative strength of the acid ?

Answer 29

Weak

N.B. Log 10=1, Log 1=0)

Question 30

What is a salt ?

Answer 30

An ionic compound in which the anion is not OH^-, or O^2-, and the cation is not H⁺.

Question 31

What is a neutralisation reaction ?

Answer 31

When acids and bases react with each other, forming a salt and water.

Question 32

What is a Bronsted base ?

Answer 32

The salt of a weak acid

Question 33

What is K_h of a salt ?

Answer 33

The hydrolysis constant of the salt.

Question 34

What is the equation for a strong acid + a strong base ?

E.g. HCl + NaOH

Answer 34

HCl + NaOH ⇔ NaCl + H₂O

N.B. Salts in which the cation and anion are both conjugates of a strong base and a strong acid form neutral solutions.

Question 35

What is the equation for strong acid and a weak base ?

E.g. HCl + NH₃

Answer 35

HCl + NH₃ ⇔ NH₄Cl

N.B. Salts that are formed based on a strong acid and a weak base for acidic solutions.

Question 36

What is the equation for a weak acid and a strong base ?

E.g. HOAc + NaOH

Answer 36

HOAc + NaOH ⇔ NaOAc + H₂O

Question 37

What is the equation for a weak acid and a weak base ?

E.g. HOAc + NH₃

Answer 37

HOAc + NH₃ ⇔ NH₄OAc

The pH is dependent on the relative strengths of the acid and the base

Question 38

What is a buffer ?

Answer 38

A solution that resists change in pH when a solution is diluted , or a small amount of acid or base is added.

Question 39

What does a buffer solution consist of ?

Answer 39

A mixture of a weak acid and its salt, or a weak base and its salt.

Question 40

Why are buffers made up of weak acids/bases, and not strong ?

Answer 40

Strong acids/bases do not have any buffering capacity as the dissociation would be irreversible.

Question 41

When is the buffering capacity of a solution at its maximum ?

Answer 41

When pH = pKa

Question 42

What is an amphoteric species ?

Answer 42

A substance, e.g. water, that can act as either an acid or a base. Many metal oxides and hydroxides also act amphoterically.

Question 43

What is the general purpose of a titration reaction ?

Answer 43

To determine the concentration of a given sample of acid or base (analyte).

Question 44

What is the end/equivalent point of a titration ?

Answer 44

They are not always the same thing.

The end point is where a physical change in solution (e.g. colour) occurs.

The equivalence point is where a reaction is theoretically complete.

In practice, this difference can usually be seen as negligable.