Chapter 3, Oxidation/Reduction in Aquatic Chemistry Video Solutions, Environmental Chemistry

Problem 1

The acid-base reaction for the dissociation of acetic acid is $\mathrm{HOAc}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{OAc}^{-}$, with $K_{\mathrm{a}}=$ $1.75 \times 10^{-5}$. Break this reaction down into two half-reactions involving $\mathrm{H}^{+}$. Break down the redox reaction, $\mathrm{Fe}^{2+}+\mathrm{H}^{+} \rightarrow \mathrm{Fe}^{3+}+1 / 2 \mathrm{H}_2$, into two half-reactions involving the electron. Discuss the analogies between the acid-base and redox processes.

Shazia Naz

Numerade Educator

03:09

Problem 2

Assuming a bicarbonate ion concentration $\left[\mathrm{HCO}_3^{-}\right]$of $1.00 \times 10^{-3} \mathrm{M}$ and a value of $3.5 \times 10^{-11}$ for the solubility product of $\mathrm{FeCO}_3$, what would you expect to be the stable iron species at $\mathrm{pH} 9.5$ and $\mathrm{p} E-8.0$, as shown in Figure 3.4?

Ayushi Sambyal

Numerade Educator

04:25

Problem 3

Assuming that the partial pressure of oxygen in water is that of atmospheric $\mathrm{O}_2, 0.21$ atm, rather than the 1.00 atm assumed in deriving Equation 3.58, derive an equation describing the oxidizing $\mathrm{p} E$ limit of water as a function of $\mathrm{pH}$.

Kenneth Jordan

Numerade Educator

01:55

Problem 4

Plot $\log \mathrm{PO}_2$ as a function of $\mathrm{p} E$ at $\mathrm{pH} 7.00$.

Susan Cooper

Numerade Educator

04:46

Problem 5

Calculate the pressure of oxygen for a system in equilibrium in which $\left[\mathrm{NH}_4^{+}\right]=\left[\mathrm{NO}_3^{-}\right]$at $\mathrm{pH} 7.00$.

Ronald Prasad

Numerade Educator

01:40

Problem 6

Calculate the values of $\left[\mathrm{Fe}^{3+}\right], \mathrm{p} E$, and $\mathrm{pH}$ at the point in Figure 3.4 where $\mathrm{Fe}^{2+}$ at a concentration of $1.00 \times 10^{-5} \mathrm{M}, \mathrm{Fe}(\mathrm{OH})_2$, and $\mathrm{Fe}(\mathrm{OH})_3$ are all in equilibrium.

Lottie Adams

Numerade Educator

01:16

Problem 7

What is the $\mathrm{p} E$ value in a solution in equilibrium with air ( $21 \% \mathrm{O}_2$ by volume) at $\mathrm{pH} 6.00$ ?

Ronald Prasad

Numerade Educator

01:43

Problem 8

What is the $\mathrm{p} E$ value at the point on the $\mathrm{Fe}^{2+}-\mathrm{Fe}(\mathrm{OH})_3$ boundary line (see Figure 3.4 ) in a solution with a soluble iron concentration of $1.00 \times 10^{-4} \mathrm{M}$ at $\mathrm{pH} 6.00$ ?

Anthony Han

Numerade Educator

02:11

Problem 9

What is the $\mathrm{p} E$ value in an acid mine water sample having $\left[\mathrm{Fe}^{3+}\right]=7.03 \times 10^{-3} \mathrm{M}$ and $\left[\mathrm{Fe}^{2+}\right]=3.71 \times 10^{-4} \mathrm{M}$ ?

Raj Aggarwal

Numerade Educator

01:40

Problem 10

At $\mathrm{pH} 6.00$ and $\mathrm{pE} 2.58$, what is the concentration of $\mathrm{Fe}^{2+}$ in equilibrium with $\mathrm{Fe}(\mathrm{OH})_3(s)$ ?

Lottie Adams

Numerade Educator

02:09

Problem 11

What is the calculated value of the partial pressure of $\mathrm{O}_2$ in acid mine water of $\mathrm{pH} 2.00$, in which $\left[\mathrm{Fe}^{3+}\right]=\left[\mathrm{Fe}^{2+}\right]$ ?

Joanna Josey

Numerade Educator

01:21

Problem 12

What is the major advantage of expressing redox reactions and half-reactions in terms of exactly one electron-mole?

Susan Hallstrom

Numerade Educator

01:33

Problem 13

Why are $\mathrm{p} E$ values that are determined by reading the potential of a platinum electrode versus a reference electrode generally not very meaningful?

Aadit Sharma

Numerade Educator

00:46

Problem 14

What determines the oxidizing and reducing limits, respectively, for the thermodynamic stability of water?

Ahmed Ali

Numerade Educator

01:13

Problem 15

How would you expect $\mathrm{p} E$ to vary with depth in a stratified lake?

Ben Nicholson

Numerade Educator

00:27

Problem 16

Upon what half-reaction is the rigorous definition of $\mathrm{p} E$ based?

David Collins

Numerade Educator

01:20

Problem 17

Analysis of water in a sediment sample at equilibrium at $\mathrm{pH} 7.00$ showed $\left[\mathrm{SO}_4^{2-}\right]=2.00 \times 10^{-5} \mathrm{M}$ and a partial pressure of $\mathrm{H}_2 \mathrm{~S}$ of 0.100 atm. Show with appropriate calculations if methane, $\mathrm{CH}_4$, would be expected in the sediment.

Adriano Chikande

Numerade Educator

03:16

Problem 18

Choose the correct answer from the following, explain why it is true, and explain why the other choices are untrue: High $\mathrm{p} E$ is associated with species such as $\mathrm{CH}_4, \mathrm{NH}_4^{+}$, and $\mathrm{Fe}^{2+}$
a. Low $\mathrm{p} E$ is associated with species such as $\mathrm{CO}_2, \mathrm{O}_2$, and $\mathrm{NO}_3^{-}$
b. Values of $\mathrm{p} E$ in bodies of water range from approximately $1 \times 10^{-7}$ to approximately $1 \times 10^7$
c. $\mathrm{p} E$ is a number, but cannot be related to anything real, such as is the case with $\mathrm{pH}$
d. $\mathrm{p} E$ uses convenient numbers to express electron activity over many orders of magnitude

Rajesh Singh

Numerade Educator

13:03

Problem 19

Match each of the following from the lettered list to the reaction that corresponds to it from the numbered list:
a. For one electron-mole
b. Reaction for standard electrode
1. $\mathrm{Fe}\left(\mathrm{H}_2 \mathrm{O}\right)_6^{2+} \leftrightarrow e^{-}+\mathrm{Fe}(\mathrm{OH})_3(s)+3 \mathrm{H}_2 \mathrm{O}+3 \mathrm{H}^{+}$
c. At upper $\mathrm{p} E$ limit of water
2. $\mathrm{H}_2 \leftrightarrow 2 \mathrm{H}^{+}+2 e^{-}$
d. Formation of a pollutant when anoxic
3. $1 / 8 \mathrm{NH}_4^{+}+1 / 4 \mathrm{O}_2 \leftrightarrow 1 / 8 \mathrm{NO}_3^{-}+1 / 4 \mathrm{H}^{+}+1 / 8 \mathrm{H}_2 \mathrm{O}$ water is brought to the surface
4. $2 \mathrm{H}_2 \mathrm{O} \leftrightarrow \mathrm{O}_2+4 \mathrm{H}^{+}+4 \mathrm{e}^{-}$

Shahina -

Numerade Educator

02:05

Problem 20

Of the following, the true statement regarding oxidation-reduction reactions and phenomena in natural water systems is:
a. At a $\mathrm{p} E$ higher than the oxidizing limit of stability, water decomposes to evolve $\mathrm{H}_2$
b. The production of $\mathrm{CH}_4$ at a very low $\mathrm{pE}$ is caused to occur by the action of bacteria
c. In the $\mathrm{p} E-\mathrm{pH}$ diagram for iron, the region of greatest area is occupied by solid $\mathrm{Fe}(\mathrm{OH})_2$
d. It is easy to accurately measure the $\mathrm{p} E$ of water with a platinum electrode
e. There are no $\mathrm{p} E-\mathrm{pH}$ limits for the regions of stability of $\mathrm{H}_2 \mathrm{O}$

Shahina -

Numerade Educator