Acid/Base pH
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Acid Rain (BOTH: do Part A. On B part follow instructions)
Prepared by: James A. Skipper, East Mississippi Community College
Purpose: You are to learn about causes of acid rain and how we can detect and identify the gaseous
molecules responsible.
Discussion:
In this lab, you will investigate the effect of acid anhydrides on the waters of the earth. Acid anhydrides are
nonmetal oxides such as NO2, CO2, and SO3 that react with water to form acids. You will model clouds,
rain, natural lakes and other water sources.
One of the major sources of acid rain and smog (acid fog) is the automobile. Internal combustion engines
produce nitrogen oxides. By compressing air (nitrogen and oxygen) in combustion chambers (cylinders) at
high temperatures, the nitrogen and oxygen react to form NOx's nitrogen oxides. Since several nitrogen
oxides are possible, we refer to them collectively as NOx's. Nitric oxide, NO, is the most common to be
formed initially but it rapidly reacts to form nitrogen dioxide, NO2 which dissolves in water to form nitric
acid. Nitric acid dissociates to form nitrate ions, NO3-, which are used by plants as fertilizer for making
proteins.
Another source of nitric oxide and thus nitric acid in the water cycle is lightning. Lightning breaks oxygen
and nitrogen molecules. The fragments may reassemble to form ozone and nitric oxide.
Another major cause of acid rain and smog is the burning of sulfur – containing fuels such as wood, oil and
coal. Sulfur oxidizes to form sulfur dioxide gas. Sulfur dioxide undergoes further oxidation to form sulfur
trioxide. Sulfur trioxide dissolves and reacts with water to form a dilute solution of sulfuric acid, a very
strong acid. Strong acids dissociate in water almost completely. If sulfuric acid forms, we should be able to
identify the sulfate ion in solution.
Acid rain is natural and is a part of the nitrogen cycle, and the sulfur cycle. Without it, the land would
become depleted of nutrients that plants need and life would cease. The problem is that in some parts of the
world man's activities have increased the amount of acid forming gases to the point that environmental
damage is being done by acid rain.
In this lab we will generate NOx's and SO2 to simulate the formation of acid rain and smog. We will create
model lakes to see the effects of these gases on the Earth's waters. We will use bromcresol green (BCG) as
an indicator to observe pH changes, a mixture of starch and iodide ions to observe oxidation and use
barium ions to detect the sulfate ion.
Materials:
2 M H2SO4 (sulfuric acid)
0.5 M Na2SO3 (sodium sulfite)
0.5 M KNO2 (potassium nitrite)
0.0l MNaHCO3 (sodium bicarbonate)
KI/ starch indicator
BaCl2 (barium chloride)
BCG (bromcresol green) indicator
NaOCl (sodium hypochlorite or " bleach")
2M NH3 (ammonia)
6M HCl (hydrochloric acid) 2 plastic Petri dishes
clear plastic cup
Safety: You will be using very small quantities of ammonia (an irritant) and acids (corrosive). You should
wear safety glasses. If you get any on yourself, wash it off with water. Keep the lid closed on the Petri
dishes after you start generating gases. Because some of these gases are toxic, minimize your exposure to
the gases produced even though the quantities are small.
Acid/Base pH
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Disposal of Waste: Neutralize acids with ammonia or sodium hydroxide then discard in waste container.
Iodine should be reduced to its colorless from with sodium thiosulfate.
Reference: Chemtrek Small Scale Experiments for General Chemistry by Dr. Stephen Thompson,
Chapter 16, Prentice Hall.
Procedure
You are to record your observations and your interpretation of the reactions you observe in you laboratory
notebook. Keep good notes. (Do data tables)
Part A: A Probe System to Detect Acidity and Alkalinity We cannot see molecules directly but we can
detect them and interpret what they are doing by using indicators or "probes."
Use clean dry Petri dishes as your containers for these tests. Colors can be best seen against a white
background. White precipitates and gas bubbles can be best seen against a black background. Use both
types of backgrounds. Do a Data table with observations.
1. Place a drop of BCG indicator on the dish
2. Add a drop of H2SO4 close by? Observation? Interpretation? No effect, right?
3. What is the effect of H2SO4 on BCG? Try it and see. Add a drop of BCG to a drop of sulfuric acid.
4. What is the effect of H2SO4 on BaCl2? Add a drop of one to the other. When you are looking for a white
precipitate latter in this experiment, it will not form that fast nor will it be that white.
5. Add a drop of ammonia close to a drop of BCG and cover. Explain what you think is happening. Do not
allow ammonia to mix with chlorine gas or chlorine bleach. They react to form chloroamine, a poisonous
gas.
6. In a clean dry Petri dish, add a drop of bleach then add a drop of starch/KI solution close by and cover.
Observe for two or three minutes. Chlorine gas, a powerful oxidizing agent, is released from chlorinated
water and from chlorine bleach (sodium hypochlorite). After observing the starch indicator for a minute
or more, add a drop of sulfuric acid to the bleach and cover quickly. What effect did the acid have on the
decomposition of the bleach? Add a drop of sodium sulfite to starch/KI mixture. Do you think sodium
sulfite is an oxidizing or a reducing agent?
Questions:
What do you think happens to the chlorine which is added to our water supplies (swimming pools,
drinking water) to disinfect the water?
How do you detect an acid? A base? An oxidizing agent? A reducing agent? The sulfate ion?
Write a paragraph about observations and conclusions.
Acid/Base pH
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\Part B: The Effect of ______ on pH Changes in Surface Water. You are to apply what you learned
and design experiments to test the effects of some of these variables. Site visual evidence to each. Use
the pollutant source from A. Write a procedure, data table of measured results (time effects, Measure
distances, etc.) and observations, and conclusion for each of the two experiments you are responsible
for. Partner 1 Do 1 & 3. Partner 2 do 2 & 4.
1. What is the effect of distance from the pollution source on the rate at which the model lakes become
acidic?
2. What is the effect of the surface area? Make a big lake with 3 drops of BCG, and then make 3 onedrop "lakes." Compare the rate of color change in the two areas.
3. What is the effect of the size of the lake? Compare the rate at which small lakes become acidic to
large lakes.
4. What is the effect of buffers (use sodium bicarbonate) on the rate the lake becomes acidic.
5. Write a paragraph about observations and conclusions for each.