Strength of Acids and Alkalis

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Strength of Acids and Alkalis

• By SK
  Chan

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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
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What happens when weak acid is dissolved in water?
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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)

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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)

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Question 1

• Suggest 3 different properties to distinguish
• 0.1 M H2SO4
• and
• 0.1M CH3COOH

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Three different properties

• pH
• Rate of reaction with metals, carbonates, etc.
• Conductivity

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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.

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Difference between Strength and Concentration

• Strength
• degree of ionization
• Concentration
• quantity of acids in
• unit volume

How are they differ from each other?
10M Ethanoic acid
10M Hydrochloric acid
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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
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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
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Reading Assignment

• Find the names and formula of two weak alkalis.
• Describe and explain three different methods to
  distinguish them from strong alkalis.