Q22.67CP - Chemistry: Molecular Nature Of Matter And Change

Question

Question: Acid mine drainage (AMD) occurs when geologic deposits containing pyrite  are exposed to oxygen and moisture. AMD is generated in a multistep process catalysed by acidophilic (acid-loving) bacteria. Balance each step and identify those that increase acidity:

1.  ${\mathbf{\text{FeS}}}_{\mathbf{\text{2}}}{\mathbf{\text{(s) + O}}}_{\mathbf{\text{2}}}\mathbf{\text{(g)}}\mathbf{\to }{\mathbf{\text{Fe}}}^{\mathbf{\text{2 + }}}\mathbf{\text{(aq) + S}}{{\mathbf{\text{O}}}_{\mathbf{\text{4}}}}^{\mathbf{\text{2 - }}}\mathbf{\text{(aq)}}$
2.  ${\mathbf{\text{Fe}}}^{\mathbf{\text{2 + }}}{\mathbf{\text{(aq) + O}}}_{\mathbf{\text{2}}}\mathbf{\text{(g)}}\mathbf{\to }{\mathbf{\text{Fe}}}^{\mathbf{\text{3 + }}}{\mathbf{\text{(aq) + H}}}_{\mathbf{\text{2}}}\mathbf{\text{O(l)}}$
3.  ${\mathbf{\text{Fe}}}^{\mathbf{\text{3 + }}}{\mathbf{\text{(aq) + H}}}_{\mathbf{\text{2}}}\mathbf{\text{O(l)}}\mathbf{\to }{\mathbf{\text{Fe(OH)}}}_{\mathbf{\text{3}}}{\mathbf{\text{(s) + 12H}}}^{\mathbf{\text{ + }}}\mathbf{\text{(aq)}}$
4.   ${\mathbf{\text{FeS}}}_{\mathbf{\text{2}}}{\mathbf{\text{(s) + Fe}}}^{\mathbf{\text{3 + }}}\mathbf{\text{(aq)}}\mathbf{\to }{\mathbf{\text{Fe}}}^{\mathbf{\text{2 + }}}\mathbf{\text{(aq) + S}}{{\mathbf{\text{O}}}_{\mathbf{\text{4}}}}^{\mathbf{\text{2 - }}}\mathbf{\text{(aq)}}$

Step-by-Step Solution

Answer

Steps (1), (3), and (4) enhance acidity, because ${\mathrm{H}}^{+}$ is generated

Step 1: Concept Introduction

Acid mine drainage (AMD) is the discharge that occurs when water interacts with accessible rocks containing sulphur-bearing minerals, causing sulphuric acid and dissolved iron to develop. Heavy metals such as copper, lead, and mercury are dissolved in the acidic run-off, polluting ground and surface water.

Step 2: Identify the steps which increase the acidity

Let’s consider each steps:

     $2{\text{FeS}}_2( \text{s})+7{\text{O}}_2\left(g\right)+2{\text{H}}_2\text{O}\left(\text{l}\right)\to 2{\text{Fe}}^{2+}\left(aq\right)+4{\text{SO}}_4^{2-}\left(aq\right)+4{\text{H}}^{+}\left(aq\right)$          (1)

           $4{\text{Fe}}^{2+}\left(aq\right)+{\text{O}}_2\left(g\right)+4{\text{H}}^{+}\left(aq\right)\to 4{\text{Fe}}^{3+}\left(aq\right)+2{\text{H}}_2\text{O}\left(\text{l}\right)$                         (2)

                $4{\text{Fe}}^{3+}\left(aq\right)+12{\text{H}}_2\text{O}\left(\text{l}\right)\to 4\text{Fe}(\text{OH}{)}_3\left(s\right)+12{\text{H}}^{+}\left(aq\right)$                         (3)

 ${\text{FeS}}_2\left(s\right)+14{\text{Fe}}^{3+}\left(aq\right)+8{\text{H}}_2\text{O}\left(\text{l}\right)\to 15{\text{Fe}}^{2+}\left(aq\right)+2{\text{SO}}_4^{2-}\left(aq\right)+16{\text{H}}^{+}\left(aq\right)$        (4)

Therefore, the steps which increase the acidity are 1, 3 and 4