✨ Module 5: Acids, bases and pH

Question 1

Bronsted-Lowry acids are …
Bronsted-Lowry bases are …

Answer 1

Proton donors
Proton acceptors - H+ ions from water.

Question 2

Why do you never get H+ ions by themselves in water?

Answer 2

They’re always combined with H2O to form hydronium ions H3O+ (aq)

Question 3

What is a conjugate acid-base pair?

Answer 3

Contains two species that can convert between each other by transferring a proton.

Question 4

Write the two equations for neutralisation.

Answer 4

H+ (aq) + OH- (aq) = H2O (l)
H3O+ (aq) + OH- (aq) = 2H2O (l)
Either can be used.
The 2nd one is reversible and the second one can be simplifies to the top one.

Question 5

What are monobasic, dibasic and tribasic acids and some examples?

Answer 5

Refers to total no. hydrogen ions in an acid.
Mono - HNO3
Di - H2SO4
Tri - H3PO4

Question 6

Most insoluble bases are …

Answer 6

Metal oxides.

Question 7

More concentrated acid react more …

Answer 7

Vigorously with metals that dilute acids.

Question 8

Explain the 3 acid and base theories.

Answer 8

1. Lavoisier - acids have oxygen in them, but later shown that HCL and hydrogen sulfide (H2S) don’t have oxygen.
2. Arrhenius - acids release protons in aqueous solution, and bases release hydroxide ions. When they react together they form salt and water. Not true for ammonia.
3. Bronsted and Lowry - base is proton acceptor. Acids and bases react together to form conjugate pairs.

Question 9

How do you find the formula of a conjugate base?

Answer 9

The formula of the conjugate base is the formula of the acid with one less hydrogen.

Question 10

What is the pH scale?

Answer 10

Measures the conc of hydrogen ions in solution.

Question 11

pH of a solution with H+ conc of 2.45x10-3 moldm-3?

Answer 11

2.61

Question 12

Describe the relationship between pH1 and pH2.

Answer 12

pH1 is 10 times the conc H+ as a solution with pH2.

Question 13

How would I dilute a solution from pH1 to pH4?

Answer 13

Require dilution by 10x10x10 = 1000 times.

Question 14

Strong monobasic acid conc determination.

Answer 14

Conc of H+ ions is the same as the conc of the acid.

Question 15

What is Ka and when do we use it?

Answer 15

Acid dissociation constant - measures the position of equilibrium for the dissociation of an acid. We use it to find pH of a weak acid as their conc isn’t the same as H+ conc.

Question 16

How does Ka change with temperature?

Answer 16

For most acids, the dissociation is an endothermic process. According to Le Châtelier’s Principle, if you add heat to an endothermic process, the position of equilibrium moves to the right. Ka increases!
This is same for Kw!

Question 17

The larger the Ka …

Answer 17

The greater the dissociation and greater the acid strength.

Question 18

What is pKa value of a weak acid with Ka value of 1.48x10-4 moldm-3?

Answer 18

pKa = -log(1.48x10-4) = 3.83 (2dp)

Question 19

What is Ka value of a weak acid with pKa value 4.82?

Answer 19

10 to the power of -4.82 = 1.51x10-5 (2dp)

Question 20

The stronger the acid …
The weaker the acid …

Answer 20

The larger the Ka value and smaller the pKa value.
The smaller the Ka value and larger the pKa value.

Question 21

Compare the OH group and COOH group.

Answer 21

COOH is more acidic and readily donates a proton.

Question 22

All equilibrium constants vary with …

Answer 22

Temperature

Question 23

What effect does changing concentration have on Kw?

Answer 23

No effect - equilibrium will shift.

Question 24

In an acid solution of pH3

Answer 24

[H+] = 10-3
[OH-] = 10-11