RELATIVE RATES OF REACTION OF STRONG AND WEAK ACIDS [PDF]

In this demonstration, magnesium metal is added to solutions of hydrochloric acid and acetic acid of equal concentration

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Idea Transcript


RELATIVE RATES OF REACTION OF STRONG AND WEAK ACIDS In this demonstration, magnesium metal is added to solutions of hydrochloric acid and acetic acid of equal concentration. Magnesium reacts with the acid to produce magnesium ions and hydrogen gas. 1.

Balance the equation for the reaction of Mg(s) with H+(aq). Mg (s) + 2 H+ (aq)

2.

Mg2+ (aq) + H2 (g)

Given the relative pKa values of acetic acid (4.8) and HCl (-8.0), which is the stronger acid? HCl

3.

4.

5.

Predict the outcome when the magnesium is added to the acid. (i)

HCl flask:

(ii)

CH3COOH flask:

Write down (or draw) what happened when the magnesium was added to each flask. (i)

HCl flask: gas produced rapidly expanding balloon

(ii)

CH3COOH flask: gas produced more slowly but balloon expands to the same size

Suggest why this is the case. CH3COOH is a weak acid so only a little H+ is present leading to slow reaction. However, it contains as much H+ as does HCl so as much H2 is ultimately produced.

Challenge Question 1.

Write out the equation for the reaction of acetic acid with magnesium. CH3COOH + Mg à CH3COOMg + H2

2.

The concentration of the hydrochloric acid solution is 1.0 M. Work out the pH of the solution. pH = -log10[H+(aq)] = -log10[1.0] = 0.0

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