CHEM120L Study Guide - Phenolphthalein, Potassium Hydrogen Phthalate, Incandescent Light Bulb

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Experiment 1
Sunday, November 27, 2005
3:59 PM
Need to Know
Know the reaction equations, how the reactions occur (ex. spontaneously, by heating, etc.) and
the physical descriptions and color of both the reactants and products
oEx. CuO (s) (black solid) + H2SO4 (aq) + H2O --> Cu(SO)4 (aq) (blue solution) + H2O
Know how to calculate percent yield (i.e. CuSO4 was obtained from the reaction of CuO with
sulfuric acid. If 2.5 g of CuSO4 was obtained from 5.0 g of CuO, what is the percent yield?)
Purpose
To synthesize different copper compounds
Theory
Many organic and inorganic compounds are synthesized by the chemical industry even though
they can be found in nature, because a limited natural supply or expensive extraction process
may make synthesis more economical
Things we have to take into consideration when synthesizing:
oAvailability of equipment
oPercentage yield
oValue of by-products
This experiment illustrates the synthesis of several copper compounds from metallic copper:
oCu -> Cu(NO3)2 -> Cu(OH)2 -> CuO -> CuSO4--5H2O -> Cu
We expect to get the same mass of copper at the end than what we started with…in order to do
so, we must prevent loss by:
oAvoiding spattering while boiling
oNot leaving product on the sides of beakers
oNot spilling the product
oPurifying precipitates by washing efficiently then drying completely before weighing
Procedure
Part 1: Synthesis of Copper(II) Nitrate and Copper(II) Hydroxide
oCu + 4HNO3 -> Cu(NO3)2 + 2NO2 + 2H2O
Reaction notes:
It is SPONTANEOUS
We carry this out in the fume hood and swirl the reaction mixture to
remove any gases trapped in the solution
oCu(NO3)2 + 2NaOH -> Cu(OH)2 + 2NaNO3
Reaction notes:
It is SPONTANEOUS
The solution should be basic (alkaline) after the addition
Cu(OH)2, the product, is a gelatinous precipitate
Cu(OH)2, the product, is BLUE
Part 2: Synthesis of Copper(II) Oxide
oCu(OH)2 –Δ-> CuO + H2O
Reaction notes:
It is BY HEATING
We want to convert Cu(OH)2 to CuO because it is a LESS GELATINOUS
precipitate than Cu(OH)2 is, and thus easier to isolate
The BLUE Cu(OH)2 becomes BLACK CuO
If heating doesn't do the trick we add MORE NaOH
oFiltration
Filter with a suction filter flask and a Buchner funnel
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Wash the CuO with water both to get it out of the beaker and because it is wet
with a solution which contains NaNO3 and NaOH, and we want to get rid of it
Part 3: Synthesis of Copper(II) Sulfate
oCuO + H2SO4 –Δ-> CuSO4 + H2O
Reaction notes:
It is BY HEATING
As the CuSO4 forms, it dissolves into Cu + SO4, and the Cu ion gets
hydrated to become Cu(H2O)42+
The BLACK CuO will dissolve into a BLUE solution
Part 4: Synthesis of Copper
oCuSO4 + Zn -> ZnSO4 + Cu
Reaction notes:
It is SPONTANEOUS
Zinc is more chemically active than copper and displaces copper(II)
ions from solutions, meaning that it is better at combining with SO4
than Cu is
The solid visible consists of unreacted zinc metal and copper metal
(the product)
The BLUE solution will turn WHITE/CLEAR
oZn + 2HCl -> ZnCl2 + H2
Reaction notes:
It is SPONTANEOUS
The purpose of this is to remove excess zinc metal from the previous
reaction
We know this reaction is over when we don't see anymore bubbles,
because that is the formation of H2 happening
Questions to Understand
Why would we allow the last traces of water to evaporate slowly rather than rapidly expelling the
water by intense heating?
o"At the end of the experiment, the final traces of water were allowed to evaporate
instead of intensely heating the copper because we wanted to prevent any heat-
catalyzed oxidation reactions from occurring which would have converted the metallic
copper to oxides (for example, CuO), thus introducing an impurity into our sample."
Experiment 2
Wednesday, November 30, 2005
9:51 AM
Need to Know
Be able to calculate molecular weights as done for your report
Know what substances (and their phases) are present at each stage of the experiment
Understand what errors may have occurred and how specific errors will affect the calculated
molecular weight
Purpose
To determine the molecular weight of volatile liquids using the Dumas Method
Theory
A volatile liquid is one that evaporates easily and allows us to use the Dumas Method to
determine its molecular weight
oThe Dumas Method assumes that the vapor obeys the Ideal Gas Law: PV=nRT
oThis means that liquids with WEAKER intermolecular forces will be more accurate than
those with stronger ones (why? I don't know…)
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The idea here is that we will place a volatile liquid into an Erlenmeyer flask where the boiling
point of the liquid is above room temperature (so it won't boil spontaneously) but below water
(so that we can put it in a boiling water bath and make it boil)
oWe cover the flask with foil but prick a hole to allow gas to escape
oWhen we heat the liquid, it will evaporate and gas will escape from the flask until there
is only so much inside that the pressure inside the flask EQUALS the atmospheric
pressure of the lab outside the flask
oOnce we reach this point, we can use the PV=nRT equation because:
We know P: it is the atmospheric pressure of the lab
We know V: it is the volume of the flask because there is going to be just
enough gas left in there as needed to fill up the flask
We know R: it is a constant
We know T: the temperature inside the flask will be equal to the temperature of
the water bath outside it
oThus we can calculate the molar amount of gas
Then we weigh the beaker to find how much the gas weighs
Now we know the molar amount and the weight, and we can calculate
molecular weight!
Procedure
Set up the Erlenmeyer flask with the volatile liquid inside
Boil the water bath and then put the flask in at a 45o angle (if not, we will not be able to tell
when the liquid has evaporated!)
oThe water should be SLOWLY boiling (or else we'll lose it)
oIf any water gets into the flask, it's game over…we must re-start the experiment (think
about why)
As soon as all the liquid has disappeared, continue heating for 1 more minute and then remove
the flask
Let it sit for 15-20 minutes so that all the vapor and condense back into liquid form
Weigh the flask to find the weight of the liquid and we're good to go with the PV=nRT equation!
Questions to Understand
What substances and phases of the substances are present in the flask after the methanol is
added and the flask covered with foil?
oLiquid methanol
oGas methanol (there is always a LITTLE evaporation)
oAir (oxygen gas)
What substances and phases of the substances are present in the flask just before you remove it
from the hot water bath?
oGas methanol
NO liquid because it has all disappeared!
NO oxygen because the evaporation of the methanol has pushed it all out!
What substances and phases of the substances are present in the flask when it is weighed after
cooling?
oLiquid methanol (because it has condensed by now!)
oGas methanol (again, there is always a little evaporation)
oAir (because after all, the thing is open to the environment)
What factors may contribute to the lack of accuracy in your calculated molecular weights?
oI don't know, what do you think?
Experiment 3
Sunday, December 04, 2005
10:14 PM
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