ENVIRONMENTAL CHEMISTRY

Environmental Chemistry

1.

a) An exothermic redox reaction involves the release of energy in form of heat after an oxidation-reduction reaction.

b) The oxidation state of an element represents the charge of an atom after a redox reaction (Khan, n. d)

c) A redox reaction involves a reduction and oxidation reaction where an atom either gains or loses electrons. The process can also involve the gain or loss of oxygen atoms in an atom (Khan, n. d).

d) From the aspects above, it is important to note that energy is only released when electrons are transferred thereby losing their potential energy.

2.

a. During denitrification process which has the following balanced redox reaction: 5CH2O+ 4NO3-+4H+?2N2+ 7H2O+5CO2, CH2O is oxidized to CO2 while NO3- is reduced to N2. In addition, H+ is oxidized to H2O. In both CH2O and H+ there is addition of oxygen atom. In NO3 oxygen atom is reduced. Therefore, it would be prudent to note that NO3- acts as the oxidizing agent while CH2O and H+ are the reducing agents.

b.

i. The part that relates to air-water partitioning happens to be when water fills the pore spaces, the rate at which oxygen can diffuse through the soil drastically reduces (Reddy, 2008, pg. 173).

ii. Conditions quickly become anaerobic when soils are filled with water. This is more so the case owing to the fact that the diffusion of oxygen in such conditions tend to be low. In addition, after flooding begins, the rate at which oxygen is depleted increases leading to anaerobic conditions (Reddy, 2008). As a consequence, normal aerobic root respiration is not carried out owing to lack of oxygen that prevents the said process. In addition, availability of toxic materials and plant nutrients in the soil is largely affected. For this reason, plants in such soils must have certain adaptations to survive in the said environment.

iii. The thin oxidized layer of the soil consists of oxidized ions which are inclusive of, but they are not limited to; SO4, NO3-, Mn4+, Fe3+. On the other hand, compounds in the reduced layers of the soil include the sulfides, ammonia, manganous and ferrous salts.

iv. Figure 5.6 is an illustration of what happens during flooding whereby the soil is filled with water. In the inner layer of the soil, ions such as NO3, Mn2+, SO4-2, and Fe2+ are present. The upper layer consists of NH+, PO4-3, and H2S ions. The said figure happens to be corresponding with sequence of redox reactions whereby electrons are lost in one part and gained in the other part. In addition, hydrogen ions are either gained or lost. For instance, reduction occurs when SO4-2 gains an electron into the NO3-. It undergoes further reduction to O2. On the upper later of the soil, oxidation takes place where a hydrogen atom is added to ammonium ion.

3. a. In both benzene and organic matter degradation by bacteria, there happens to be growth of both cultures. Benzene is most likely degraded by bacteria when the reducing...

The free energy change at this condition happens to be -3070Kj/mol (Vogt et al., 2011). Given that the relationship between free energy change and redox potential is ?G0=-nFEo, then the redox potential for reduction of benzene would be 3070Kj/mol.

b. The stoichiometric equation for the reaction where Fe3+ is the oxidizing agent is

C6H6?+?18 H2O?+?30 Fe3+?? 6 HCO3-?+ 30 Fe2+?+?36 H+

Concentration of benzene is 64mg/L

1mg=10-3g thus 64mg=0.064g/L

Mass of benzene is given by mass of 6 carbons added to the mass of 6 hydrogen. Therefore,

Mass of C6H6=612.01+61.008=78.108g

Concentration=mass/volume

Therefore, volume of benzene=mass/concentration

V=78.108/0.064=1220.4375L

Moles=mass/volume

Moles of benzene= 78.108g/1220.4375L

=0.064g/L

The mole ratio for benzene: Fe3+=1:30

Therefore moles of Fe3+ would be 300.064=1.92g/L

Mass of Fe3+=55.84g

Volume of iron (III)=55.84/1.92=29.08L

Concentration...

…inclusive of, but they are not limited to; Rh, Zn, Cu (II), Ni, Co, Fe (II), and Mn. Type A metals are hard owing to their low polarizability and small size while type B metals are soft given that they are large in size and have high polarizability (Gadd, 1992). On the other hand, borderline metals are intermediate metals having both hard and soft metal ligands. Type A metals are highly stable owing to their weak electrostatic forces. Like type A metals, type B metals are also complex owing to strong covalent bonds. However, type B form more stable complexes compared to type A metals owing to the presence of both covalent bonds and electrostatic forces in their complexes. Type A metals are less toxic to the environment compared to type B metals which are highly toxic. The high toxicity of type B metals is often as a result of their strong affinity for sulfur owing to their high atomic numbers (Gadd, 1992). In addition, type A metals are present at high concentrations in the environment while both type B and borderline metals exist in the environment at low concentrations.

8. a) Concentration of n-propanol in water would be:

Mass of CH3CH2CH2OH=12.013+1.0088+16.00=60.094g=60094mg

1kg=1000g

500kg=500,000g=500mg

The number of moles present in 500mg will be: 500/60094=8.320310-3mg/L

Concentration=8.320310-3/1108=8.320310-11

C3H7OH + 4.5 O2 > 3 CO2 + 4 H2O

Mass of n-propanol=123+17+16+1=60g

ThOD=4.532=144/60=2.4gO2/g of n-propanol

ThOD=8.320310-112.4

=1.996910-10mg of O2/n-propanol

b) At 20Oc, the maximum dissolved oxygen is 1.996910-10 meaning that there would be minimum oxygen in the said water. According to Kutty (1987), the growth rate of fish will decrease leading to decrease in number of fish.

c) Apart from temperature, there are other factors that affect biological oxygen demand - for instance; gases such as carbon (IV) oxide and methane, decomposition of organic matter, and respiration of flora and fauna (Kutty, 1987). In addition, availability of iron-oxidation…