What is the main cause of acid mine drainage? Use equations in your answer. What steps (successful or not?) have been taken to control acid mine drainage?

1. Before the Industrial Revolution, the concentration of CO₂ in Earth’s atmosphere was about 275 ppm. Considering the effect of CO₂ only, calculate the effect that the increase in CO₂ has had on the pH of precipitation.

2. What is the main cause of acid mine drainage? Use equations in your answer. What steps (successful or not?) have been taken to control acid mine drainage?

3. How would the pE-pH diagram called for in question 12 of the mid-term exam differ if the reaction:

1/2Fe(OH)₂•(Fe(OH)₃)₂(s) + 4H⁺(aq) +e^–(aq) = 3/2Fe²⁺(aq) + 4H₂O(l)        log K = 21.2

were included? The species on the left of the reaction is called ferrosoferric hydroxide.

Hints: Think about what new species need to be included and where they most likely would place on the diagram.

4. Organic complexing agent H₅X can be used to inactivate toxic metal (M) in water. H₅X needs to be completely dissociated to X^5- to complex metal M (MX is formed). H₅X forms the following species in water:

H₅X

H₄X^–                pK_a1 =1.5

H₃X^2-               pK_a2 =3.6

H₂X^3-               pK_a3 =7.8

HX^4-                pK_a4 =10.9

X^5-                   pK_a5 =11.5

1. In water what is the mass balance of all species X
2. Express the fraction (α) for species X^5- with respect to pH
3. Would metal M be complexed by our organic agent in natural water?
4. At the following pH’s what is the predominate ion of X present in natural water?
   • 67
   • 45
   • 45

HINT: you don’t really have to do any calculation in this question (you are of course always welcome to do so)

5. Explain in detail why chemical reactions in seawater need to be adapted from what we know in fresh water.

6. You and a friend are talking one evening, when your friend asks.  Have you ever heard of ocean acidification?  You are excited to tell your friend all about it since you are really into it. Write down how you would explain to your friend what ocean acidification is and what is causing it.  Also explain which areas are affected most in the ocean and why. Will our rivers and lakes be impacted the same way?

Answer two of the three questions below:

7. Water with an alkalinity of 2.5 mM has a pH of 7.0. Calculate [CO₂], [HCO₃^–], [CO₃^2-], and [OH^–]

(This is a brain teaser; just try to get as far as you can. You do need to make some assumptions)

8. Give reactions to show how rainfall weathers limestone.

Chemical weathering of limestone take place due to acid rain.

CaCO₃ (s) + H₃O⁺ (aq) = Ca²⁺ (aq) + HCO₃^– (aq)
HCO₃^– (aq) + H₃O⁺ (aq) = CO₂ (aq) + 2H₂O (l)
H₃O⁺ (aq) + CO₃^2- (aq) = HCO₃^– (aq) + H₂O (l)

What is the pH of the water in a limestone quarry? What is the [Ca²⁺]? Assume CO₂ partial pressure is 390 ppm. Hint: When you end up with a complex calculation assume a pH to eliminate terms of that equation. Don’t freak out, give it your best