Chapter 18, Problem 18.148P

(a)

Interpretation Introduction

Interpretation:

Carbon dioxide when dissolved in water undergoes multistep equilibrium process.  The reactions are,

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

Each step has to be classified as a Lewis or a Bronsted-Lowry reaction.

Concept Introduction:

Bronsted-Lowry reaction:

Any species that has the capability of accepting a proton, which requires a lone pair of electrons to bond to ${\text{H}}^{\text{+}}$ is said to be Bronsted-Lowry base.

Any species that has the capability of donating a proton (${\text{H}}^{\text{+}}$) is said to be Bronsted-Lowry acid.

According to Bronsted-Lowry theory, an acid-base reaction in which a proton is transferred from an acid to base is said to be Bronsted-Lowry reaction.

Example:

  ${\text{NH}}_{\text{3(g)}}{\text{+ HCl}}_{\text{(g)}}\to {\text{ NH}}_{\text{4}}{\text{Cl}}_{\text{(s)}}$

In the above reaction, $\text{HCl}$ acts as Bronsted-Lowry acid and donates proton which is accepted by ${\text{NH}}_{\text{3}}$ using its lone pair.  So, ${\text{NH}}_{\text{3}}$ is Bronsted-Lowry base.

Lewis reaction:

In an acid-base reaction, Lewis base donates electrons to the acid and Lewis acid accepts the electron pair to form a covalent bond between Lewis acid and Lewis base.

Example:

  $\text{A + B }\to \text{ A-B}$

In the above example, A is an acid and B is a base.  B donates electrons to A and forms an adduct.

(b)

Interpretation Introduction

Interpretation:

The $\text{pH}$ value of non-polluted rainwater in equilibrium with clean air has to be calculated.  (${\text{P}}_{{\text{CO}}_{\text{2}}}$ in clean air = ${\text{4×10}}^{\text{-4}}\text{ atm}$; Henry’s law constant for ${\text{CO}}_{\text{2}}$ at ${\text{25}}^{\text{°}}\text{C}$ is $\text{0}\text{.033 mol/L}\text{.atm}$ and ${\text{K}}_{\text{overall}}\text{ = 4}{\text{.5×10}}^{\text{-7}}$).

(c)

Interpretation Introduction

Interpretation:

The ${\text{[CO}}_{\text{3}}^{\text{2-}}\text{]}$ in rain water has to be calculated.  (${\text{K}}_{\text{a}}{\text{ of HCO}}_{\text{3}}^{\text{-}}\text{ = 4}{\text{.7 × 10}}^{\text{-11}}$).

(d)

Interpretation Introduction

Interpretation:

If the partial pressure of ${\text{CO}}_{\text{2}}$ in clean air doubles in the next few decades, the $\text{pH}$ of rainwater has to be calculated.