Chapter 18, Problem 76IAE

Interpretation Introduction

(a)

Interpretation:

It should be explained whether same observations will be made if Ca(OH)₂ is replaced by CaCl₂ in lime water test for CO₂.

Concept introduction:

Lime water test involved with following reaction series.

$\begin{array}{l}{\text{Ca(OH)}}_{\text{2}}\text{(aq) }\to {\text{ Ca}}^{\text{2+}}{\text{(aq) + 2OH}}^{\text{-}}\text{(aq)}\\ {\text{CO}}_2\text{(s)}+{\text{H}}_{\text{2}}\text{O (l)}\rightleftharpoons {\text{H}}_{\text{2}}{\text{CO}}_3\text{(aq)}\\ {\text{Ca(OH)}}_{\text{2}}\text{(aq)}+{\text{H}}_{\text{2}}{\text{CO}}_3\text{(aq)}\to {\text{CaCO}}_{\text{3}}(s)+H_{2}O(l)\end{array}$

When the solution become acidic:

${\text{CaCO}}_{\text{3}}(\text{s})+{\text{H}}_{\text{2}}{\text{CO}}_3\to \text{Ca}{\left({\text{HCO}}_{\text{3}}\right)}_2\left(\text{aq}\right)$

Interpretation Introduction

(b)

Interpretation:

It should be showed that the white precipitate will re-dissolve in 0.005 M Ca(OH)₂ solution but not in a saturated solution.

Concept introduction:

In both 0.005 M solution Ca(OH)₂ and saturated solution Ca(OH)₂ there is a complete ionization.

${\text{Ca(OH)}}_{\text{2}}\text{(aq) }\to {\text{ Ca}}^{\text{2+}}{\text{(aq) + 2OH}}^{\text{-}}\text{(aq)}$

The H₂CO₃ dissociate as follows:

$\begin{array}{l}{\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}(\text{aq})\underset{}{\rightleftharpoons }{\text{HCO}}_{\text{3}}{}^{-}(\text{aq})+{\text{H}}^{+}(\text{aq})\\ {\text{HCO}}_{\text{3}}{}^{-}(\text{aq})\underset{}{\rightleftharpoons }{\text{CO}}_{\text{3}}{}^{2-}(\text{aq})+{\text{H}}^{+}(\text{aq})\end{array}$

According to Le Chatelier’s Principle, if any change in temperature, pressure, volume or concentration in the system takes place at equilibrium, a new equilibrium will establish to counteract the change.