Chapter 22: Q22.67CP (page 1020)

Question: Acid mine drainage (AMD) occurs when geologic deposits containing pyriteare 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:
${FeS}_{2} {(s) +O}_{2} (g) \to {Fe}^{2 +} (aq) + S {O_{4}}^{2 -} (aq)$
${Fe}^{2 +} {(aq) +O}_{2} (g) \to {Fe}^{3 +} {(aq) +H}_{2} O(l)$
${Fe}^{3 +} {(aq) +H}_{2} O(l) \to {Fe(OH)}_{3} {(s) + 12H}^{+} (aq)$
${FeS}_{2} {(s) + Fe}^{3 +} (aq) \to {Fe}^{2 +} (aq) + S {O_{4}}^{2 -} (aq)$

Short Answer

Expert verified

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

Step by step solution

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.

Identify the steps which increase the acidity

Let’s consider each steps:

$2 {FeS}_{2} (s) + 7 O_{2} (g) + 2 H_{2} O (l) \to 2 {Fe}^{2 +} (a q) + 4 {SO}_{4}^{2 -} (a q) + 4 H^{+} (a q)$ (1)

$4 {Fe}^{2 +} (a q) + O_{2} (g) + 4 H^{+} (a q) \to 4 {Fe}^{3 +} (a q) + 2 H_{2} O (l)$ (2)

$4 {Fe}^{3 +} (a q) + 12 H_{2} O (l) \to 4 Fe (OH)_{3} (s) + 12 H^{+} (a q)$ (3)

${FeS}_{2} (s) + 14 {Fe}^{3 +} (a q) + 8 H_{2} O (l) \to 15 {Fe}^{2 +} (a q) + 2 {SO}_{4}^{2 -} (a q) + 16 H^{+} (a q)$ (4)

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

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Identify the steps which increase the acidity

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