test 2 Flashcards
(23 cards)
Q1
Write the balanced half-reaction involving
O2 that occurs in acidic waters when it oxidizes
organic matter.
The half reaction in acidic solution is
O2 + 4H+ + 4e- → 2H2O
Q2
How does temperature affect the solubility of
O2 in water? Explain what is meant by thermal
pollution.
solubility generally decreases with increasing temperature (thermal pollution)
Q3p1
Define BOD
BOD - The capacity of the organic and biological matter in a sample of natural water to consume oxygen, a process catalyzed by bacteria present.
Q3p2
Define COD
COD - the amount of oxygen used by biota in the process of assimilating the organic matter is determined chemically by oxidation. A strong oxidizing agent e.g., KMNO4 or Na2Cr2O7 in acid solution is used.
Q3p3
explain why BOD and COD values for the same water sample can differ slightly.
Values can be different because BOD measures the oxygen used by only biological processes and COD measure everything that uses up oxygen
Q3p4
Explain why natural waters can have a
high BOD.
???
I think its just because there is more biological activity but im not sure.
Q4
What do the acronyms TOC and DOC stand
for, and how do they differ in terms of what they
measure?
TOC - Total organic carbon (Measures both suspended organic carbon and dissolved)
DOC - Dissolved organic carbon (only measure dissolved organic carbon not suspended)
Q5
Write the half-reaction, used in the COD
titration, which converts dichromate ion to
Cr3# ion, and balance it.
Cr2O72- + 14H+ + 6e- -> 2Cr3+ + 7H2O
Q7
Draw a labeled diagram classifying the top and
bottom layers of a lake in summer as either
oxidizing or reducing in character, and showing
the stable forms of carbon, sulfur, nitrogen, and
iron in the two layers.
Top layer: Aerobic
CO2, H2CO3, HCO3^-,SO4^-,NO3^-,Fe(OH)3
Bottom: Anarobic
CH4, H2S, NH3, NH4^+, Fe^2+
Q9
Explain the phenomenon of acid mine drainage,
writing balanced chemical equations as
appropriate.
Not sure how to answer this yet
Q10
What is meant by the pE of an aqueous
solution? What does a low (negative) pE value
imply about the solution? What species determines
the pE value in aerated water?
pE is used by environmental scientists to illustrate redox equilibria in aquatic system.
Low pE = electrons are readily available i.e. water is reducing.
Q11
What is the acid and what is the base that
dominate the chemistry of most natural water
systems, and whose interaction produces
bicarbonate ion?
The acid-base chemistry of natural water dominated by the interaction of the carbonate ion, CO32-, and the weak acid carbonic acid, H2CO3 via the dissolution of carbon dioxide in air.
Q12
What is the source of most of the carbonate
ion in natural waters? What name is given to
waters that are exposed to this source?
Main source is Carbon dioxide in the air.
Other sources are: Microbial activity (respiration and degradation) and CO2 as it seeps through rocks also dissolves calcium carbonate from limestone.
They are called alkaline waters
Q13
Write the approximate net reaction between
carbonate ion and water in a system that is not
also exposed to atmospheric carbon dioxide. Is the
resulting water acidic, alkaline, or neutral?
H^+ + OH^- -> H2O(l)
H2CO3 -> H^+ + HCO3^-
The reduction of free protons results in a lower pH of the water. (Alkaline)
not 100% sure if this is correct***
Q16
Which are the most abundant ions in clean,
fresh, calcareous water?
Natural waters that are exposed to limestone are called calcareous waters. The dissolved carbonate ion acts as a base, producing its conjugate
weak acid, the bicarbonate ion, as well as hydroxide ion in the water:
Q17p1
What is the natural source of fluoride ion
in water?
The source of most F! is weathering of the mineral fluorapatite,- Ca5(PO4)3F.
Q17p2
How and why is the fluoride level in
drinking water artificially increased to about
1 ppm in many municipalities?
Increasing the public’s daily exposure to fluoride ion in order to reduce the incidence of dental caries. fluorosilicic acid, H2SiF6 is added to the water.
Q17p3
How can fluoride
be removed from natural water in which its
concentration is too high?
Excessive levels of fluoride are typically removed from water destined for drinking by passing it over a solid source of calcium ions.
Q18p1
Define the total alkalinity index
total alkalinity, is the number of moles of H” required to titrate one liter of a water sample to the (slightly acidic) end point.
Q18p2
Define phenolphthalein alkalinity index for water.
phenolphthalein alkalinity (also called carbonate alkalinity), which is a measure of the concentration of the carbonate ion and of other similarly basic anions.
Q19
Define the hardness index for water.
hardness index, which measures the total concentration of the ions Ca2+ and Mg2+,
Q20p1
Explain why aluminum ion concentrations in
acidified waters are much greater than those in
neutral water.
Although this value is very small, for every one-unit decrease of the pH, the concentration of aluminum ion increases by a factor of a thousand, so it reaches 10^-6 M at pH “ 5 and 10^-3 M at a pH of 4. Thus aluminum is much more soluble in highly acidified rivers and lakes than in those where pH values do not fall below 6 or 7.
Q20p2
How does the increased aluminum
ion level affect fish and trees?
It is thought that the principal deleterious effect of acid waters upon fish arises from the solubilization of aluminum from soil and its subsequent existence
as a free ion in the acidic water, as discussed in Chapter 4. Unfortunately,
the Al(OH)3 then precipitates as a gel on contact with the less acidic
gills of the fish, and the gel prevents the normal intake of oxygen from water,
thus suffocating the fish.
