CHem 111
Explain the "like dissolves like" principle and know some specific examples that document your explanation.
Polar substances are able to dissolve other polar substances. Water can dissolve salt because salt is an ion and ions break up completely in solution.
Write out chemical equations to explain why: aqueous sodium carbonate is basic, aqueous aluminum chloride is acidic, and aqueous potassium iodide is neutral.
1. Na₂CO₃(aq) + 2 H₂O(l)→2 NaOH(aq) + CO₂(aq) + H₂O(l) 2. AlCl₃(aq) + 3 H₂O(l)→Al(OH)₃(aq) + 3 HCl(aq) 3. KI(aq) + H₂O(l)→KOH(aq) + HI(aq)
You observed a reaction involving Crystal Drano which is a similar reaction to the one that took place in the first step of the alum synthesis. Write out this reaction equation.
2 Al(s) + 2 NaOH + 6H₂O → 3 H₂(g) + 2 NaAl(OH)₄
Be able to predict the mass and volume of CO2 that should be absorbed by a given mass of LiOH.
2LiOH(s)+ CO2(g) -> Li2CO3(s)+ H2O(g) PV=nRT P=Pressure V=Volume n= # moles R= constant (0.08206 L•atm/mol•K) T= temperature
When H2SO4 is added to one of the scratched up areas of an aluminum can bubbles are observed. Write a reaction equation to explain this observation. (If you do this correctly you will have an equation which explains why acids corrode most metals.)
3 H₂SO₄ + 2 Al → Al₂(SO₄)₃ + 3 H₂
Define calorimetry
A device that measures temperature changes from chemical reactions. This was used to determine heat produced from the burning of cheese balls and paraffin. It will measure the temperature change when the cheese ball is combusted
Define chromatography.
A method of separation in which the components to be separated are distributed between two phases, stationary and stationary phases.
Explain the complementary nature of absorbed/observed colors.
The observed color is the color transmitted, where the absorbed color is it's complementary color, which is not seen.
Describe the technique of colorimetry.
The process where a set of solutions of known concentrations of some light absorbing substances is placed in a series of containers. Unknown concentration solution can be determined by comparing with the calibration set.
What is meant by a small calorie "c" and a food calorie "C"?
A small calorie "c" is a scientific calorie and is 1000x smaller than a food calorie. A food calorie is 1000c and is how food is measured.
Given the volume of CO2 absorbed, be able to calculate the corresponding mass of CO2.
Use PV=nRT
Be able to define retention volume (Vr). What does a larger retention volume mean about a component's ability to interact with the liquid chromatography column?
VR= a measure of the amount of mobile phase required to elute (remove) each component of the mixture from the column) The larger the VR value, the more strongly the component interacts with the stationary phase (the higher the affinity for stationary phase, the more volume of mobile phase is required to "wash off" the component from the column)
Use the "like dissolves like" principle to explain the solubility experiments that you did with ethanol, pentanol, kerosene, and water in Section B.
A small puddle of each substance was placed on the reaction surface. Next, a sample of KI was placed in each puddle. KI is ionic and polar so it would dissolve in polar solvents. The polarity of the solvents would be ranked based on how quickly the KI dissolved.
What is an "alum"?
A variety of aluminum compounds that combine with aluminum sulfate and a group 1A metal sulfate
Explain what is meant by a "back titration." Why is this a better approach than a regular titration for the analysis of calcium carbonate in egg shells. Be able to calculate the calcium carbonate content of an eggshell (given the raw data and the pertinent molar masses) —i.e., the kind of calculation you had to do in Section F.
By adding excess acid, you will have gone pass the endpoint. You will then titrate the excess acid with base and come back to the endpoint you passed. This is a better approach because it is simple, rapid, and inexpensive.
Write an equation which explains why carbonated (not necessarily COLAs) beverages are acidic.
CO₂ + H₂O → H₂CO₃
Be able to use the dilution equation given in this experiment to solve dilution problems.
CiVi=CfVf C= concentration V= volume
SOLUBLITY
Compounds containing NO3-, SO42-, Cl-, Br-, or I- are soluble. Compounds containing 2- + + + OH-, PO43-, or CO3 are insoluble except when they are combined with Na , K , and NH4
What are the advantages and disadvantages of lithium hydroxide and activated carbon as CO2 scrubbers?
Cons: -CO2 is a product -they are not cost effective Pros: -carbon adsorbs gases on the high surface area -can regenerate by blowing a stream of air over the carbon
Name two sources of atmospheric carbon dioxide.
natural) aerobic respiration by humans and other animals, decomposition of organic matter, release from oceans, and volcanic eruptions, (anthropogenic) combustion of fossil fuels, such as coal and oil
Explain the difference between a line and continuous spectrum and give an example of a source of each type.
Continuous Spectrum- a spectrum in which the emitted radiation contains all frequencies within a region of the electromagnetic spectrum; rainbow and a light bulb Line Spectrum- a spectrum that is produced by excited atoms in the gas phase that only have certain frequencies; hydrogen gas
In Section C you did some titrations of HCl with NaOH, using a BTB indicator. What is observed at the endpoint, and what is the overall and net ionic equation? You should also know how to calculate the concentration of an unknown given titration results.
HCl(aq) + NaOH(aq) → NaCl (aq) + H₂O (l) H⁺(aq) + OH⁻(aq)→H₂O(l) Endpoint: yellow color → dark blue
You did a melting point test on the alum you made. What two things does this test tell you about your compound? Explain.
It confirms the identity of the compound and measures its purity. Impurities decrease the melting point and gives a bigger range of melting point.
Given the overall reaction equation for the synthesis of alum and the molar masses involved, you should be able to calculate the theoretical yield. If you are given the mass of alum actually obtained, you should be able to calculate the % yield.
% yield = (g alum actually obtained / g alum theoretically produced) * 100 %
When walking, an average person burns 0.453 Food Calories/mile-pounds. How many food calories does a 150-pound person burn after walking a mile? If a fully combusted cheese ball produces 5.6 Food Calories/g and an average cheese ball weighs about 1 gram, then how many cheese balls will a 150-pound person have to eat to walk a mile?
(150 lb)(.453C/mi-lb)=67.95 C/mi (67.95C/mi)(1g/ 5.6 C) = 12.13g = 13 cheese balls
If you are given the wavelengths of light observed by a particular student for the hydrogen emission spectrum you should be able to (a) know and be able to use the appropriate equations to determine the frequencies and the energies associated with these wavelengths, (b) be able to use the Bohr equation to calculate the wavelengths expected for the Balmer series of the hydrogen atom, (c) be able to quantitatively compare the results of (a) and (b)—i.e., know how to calculate the % error between a student's results and Bohr's. (You will be given the Bohr equation, the Rydberg constant and Planck's constant.)
(a) frequency= speed of light/wavelength energy= frequency x Planck's constant. (b) ΔE= -Rn x (1/nf2 - 1/ni2) (c) % difference = [(actual - theoretical)/ actual] x 100
Explain the effect on the spectrum of changing (a) the slit width and (b) the distance from the source of light to the spectroscope.
(a.) Slit with a wider width results in the visibility of the colors to be wider and the size of the spectrum stays the same. (b.) The closer you are to the light source the more intense the light/colors are
Given the mass of the scrubber and the volume of CO2 absorbed, be able to calculate the absorption capacity.
(mass of CO2 absorbed/mass of absorber)
Define the 4 major types of reactions that occur in aqueous solution and give an example of each type (i.e., be able to write out a balanced reaction equation for each type).
1. Acid/Base: acids react with bases to produce salt and H2O. 2. Precipitation: forms an insoluble product. 3. Redox: chemical species changing oxidation states. 4. Complexation: form a complex ion (metal ion with other substances covalently bonded to it.
Explain the difference between the corrosion of iron and the corrosion of aluminum.
Aluminum surface reacts rapidly with oxygen to form a thin film that stops further corrosion.
You used two different types of indicators in this experiment—red cabbage extract and bromothymol blue (BTB). Be able to cite one advantage of the BTB indicator over red cabbage extract.
BTB will keep longer and the color change is instantaneous. RCE needs to be kept cool to prevent degradation.
Know how to calculate amount of energy released using E = mCΔT.
E= energy m=mass c=specific heat ΔT= change in temperature
What kind of energy transitions were observed in this experiment? (spectroscopy)
Electronic energy transitions (valence electrons)
Outline in a few sentences how you calibrated your spectroscope
First, transparent graph paper was taken and the x-axis was labeled. Next, the outline of where it was taped on was traced and the slit was made to be .5mm wide. Then, a fluorescent light bulb with known wavelengths was put close to the slit, and the three visible wavelengths were documented. A graph with the wavelengths was made, and a calibration equation was then found using the wavelengths
To obtain good separation in chromatography, what factor must be maximized and what factor must be minimized?
Maximize: the component migration differences of all components so that each component has a clear, separate spot or band Minimize: the component spreading that occurs during the chromatographic process, so that the spots/bands are small/narrow
Be able to name the mobile phase and stationary phases in your paper chromatography experiment and be able to use the "like dissolves like" principle to rationalize relative Rf values for dye molecules.
Mobile phase: a fluid that flows through the porous stationary phase Stationary phase: a porous substance If the component has a stronger affinity for the mobile phase, it will spend more time in the mobile phase than in the stationary phase (LARGE Rf value) If the component has a stronger affinity for the stationary phase, it will spend more time in the stationary phase than in the mobile phase (SMALL Rf value)
Give two reasons why aluminum is expensive to produce, and two uses for the alum compound that you synthesized.
Most of the high-grade bauxite deposits occur outside of the United States, and the Hall-Heroult process is extremely energy intensive. Also, changes in the international situation and the depletion of these high-grade deposits are forcing the aluminum industry to use lower grade ores, with correspondingly higher prices. Uses are in think foil and soda cans.
Be able to calculate Rf values for developed chromatograms.
Rf= (distance moved by the component/distance moved by mobile phase front)
Explain the difference between a "serial" titration and a "single well" titration. Cite one advantage of each type (in comparison with the other).
Serial: add 1 drop to well 1, 2 drops to well 2, ... (progression of titration) Single: add 1 drop, observe, add another drop, observe. (higher precision)
Name two situations where removal of CO2 from air is important.
Submarines and spacecrafts where missions can last a long time without fresh air.
In Section B you tested for the presence of SO42−, K+, Al3+ and H2O. Be able to explain (a) how each test was performed, and (b) what should have been observed in each test.
Sulfate was tester for by reacting the alum wit BaCl2. A white precipitate of BaSO4 formed. K and H2O were tested for by the flame test. The flame turned purple which shows K in it and a popping noise was heard as the water evaporated. Al was tested for by reacting it with KOH and checking for the Al(OH)4 precipitate and by Al having an acidic pH level.
Draw the setup of a typical "discharge tube" and explain what the purpose of this setup is in this experiment.
The discharge tube contains H gas. The voltage source runs electrons trough the discharge tube, exciting the electrons in the gas. Its emission spectrum can then be seen but it is only excited to certain levels so you can only see certain frequencies.
One of the solutions you used is labeled "aqueous ammonia/ammonium hydroxide". Why is such a complicated name used? (Isn't ammonia a gas?)
The dissolved ammonia gas in the water comes out of solution very quickly and becomes NH3(g) again.
What is the fundamental reason why a substance absorbs visible light?
The energy of a particular photon of visible light matches the energy needed to promote an electron form a low energy state to a higher one.
What process removes the energy from excited food dye molecules in aqueous solution? (See the Background reading.)
The extra energy is removed continuously by the solvent molecules by bumping into the excited food dye molecules.
Define spectroscopy.
The study of the interaction of electromagnetic radiation with matter
Be able to describe the observations of OTO (from Chlorine test kit) with tap water and water passed through the Brita® filter. Explain.
When tap water was reacted, the solution turned yellow due to the high chlorine ion concentration. The Brita water turned a lighter shade of yellow due to the chlorine ions being removed. The chlorine was removed because it stuck to the activated carbons which have tiny pores so there is a high surface area to react.
Describe what is meant by the term "white light," and know the wavelength range (in nm) that white light covers. List the colors of the spectrum from the highest to the lowest frequency.
White light is also visible light in the spectrum of 400-750 nm
Be able to calculate a standard deviation by hand for a given set of data. You will not be given the equation for standard deviation.
s.d= xi= individual data point x ̅= mean or average value N= total number of data points