Strength of Acids and Alkalis

By SK Chan

Why do acids/alkalis show different properties?

Acid/Alkali pH value

Electrical conductivity

0.1 M Ethanoic acid 4 Low

0.1 M Hydrochloric acid

1 High

0.1 M aqueous ammonia

9 Low

0.1 M sodium hydroxide

13 high

What happens when weak acid is dissolved in water?

Weak acid

Only slightly ionized in water to give aqueous hydrogen ions, H+

(aq).

It is found that about 1% of 0.1 M ethanoic acid molecule is ionized in water.

CH3COOH(aq) CH3COO-(aq) + H+(aq)

How about Strong acid?

An acid that is completely ionized in water to give hydrogen ions, H+(aq).

HCl(aq) H+(aq) + Cl-(aq)

0.1 M 0 M 0 M (before ionization)

0 M 0.1M 0.1M (after ionization)

Question 1

Suggest 3 different properties to distinguish:

0.1 M H2SO4

and

0.1M CH3COOH

Three different properties

pH Rate of reaction with

metals, carbonates, etc. Conductivity

Check point

What is the term used to describe the formation of hydrogen ions from acid molecule?

Explain in your word the definition of weak acid. Name one weak and one strong acid. Suggest methods to distinguish weak acid from

strong acids.

Difference between Strength and Concentration

Strength

degree of ionization

Concentration

quantity of acids in

unit volume 10M Ethanoic acid

10MHydrochloric acid

How are they differfrom each other?

Question 2

Which of the following has a lower pH?

A 0.1 M hydrochloric acid

B 0.1 M ethanoic acid

C 0.1 M sulphuric acid

D 0.1 M nitric acid

Ans. C

Question 3

Which of the following solutions has the HIGHEST concentration of hydrogen ions?

A 20 cm3 of 2M sulphuric acid

B 100 cm3 of 0.2 M hydrochloric acid

C 100 cm3 of 4M ethanoic acid

D 100 cm3 of 2M sodium hydroxide

Ans. A

Reading Assignment

Find the names and formula of two weak alkalis.

Describe and explain three different methods to distinguish them from strong alkalis.