Strong and Weak Acids and Alkalis Flashcards

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In chemistry, what is meant by a strong acid?

A strong acid is one that fully ionises in water. Almost all its particles release hydrogen ions (H+).

In chemistry, what is meant by a weak acid?

A weak acid is one that only partly ionises in water. Only some of its particles release hydrogen ions (H+).

What is a strong alkali?

A strong alkali is one that fully ionises in water. Almost all its particles release hydroxide ions (OH-).

What is a weak alkali?

A weak alkali is one that only partly ionises in water. Only some of its particles release hydroxide ions (OH-).

Give one example of a strong acid and one example of a weak acid.

Hydrochloric acid (HCl) is a strong acid. Ethanoic acid (vinegar, CH3COOH) is a weak acid.

Give one example of a strong alkali and one example of a weak alkali.

Sodium hydroxide solution (NaOH) is a strong alkali. Aqueous ammonia (ammonia solution, NH3) is a weak alkali.

At the same concentration, which has the lower pH: a strong acid or a weak acid, and why?

A strong acid has the lower pH. It produces more hydrogen ions (H+) in the solution.

At the same concentration, which has the higher pH: a strong alkali or a weak alkali, and why?

A strong alkali has the higher pH. It produces more hydroxide ions (OH-) in the solution.

At the same concentration, which reacts faster with a metal: a strong acid or a weak acid, and why?

A strong acid reacts faster with a metal. This is because it has more hydrogen ions (H+) available to react.

What is the difference between a strong acid and a concentrated acid?

A strong acid fully ionises in water. A concentrated acid just has many acid particles in each dm3 of solution, whether it is strong or weak.

Does diluting a strong acid make it a weak acid? Explain your answer.

No. Diluting a strong acid does not change it into a weak acid. It is still a strong acid, but the solution has fewer hydrogen ions per dm3, so the pH is higher.

How is ionisation shown differently in equations for strong and weak acids?

For a strong acid you use a single arrow, for example: HCl(aq) → H+(aq) + Cl-(aq). For a weak acid you use a reversible arrow, for example: CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq).