General Chemistry II Lab Manual

Troy University Chemistry Faculty

Fall, 2018

Licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Table of Contents

Spontaneity and Reversal of Reactions

Molecular Geometry

Resonance Structures, Formal Charges, and Polarity

Determination of Absolute Zero on the Celsius Scale

Determining the Molar Mass of a Volatile Liquid by the Dumas Method

Molar Mass by Freezing Point Depression

Determining How Much Aspirin Is in Aspirin Tablets by Spectroscopy

Value of the Equilibrium Constant for the Reaction of Iron(III) with Thiocyanate

Le Châtelier’s Principle

Determining the Ka of an Acid from pH Measurements

Change in pH on Adding Acid or Base to a Buffer

Spontaneity and Reversal of Reactions

Troy University Chemistry Faculty

Licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Objective

Distinguish spontaneous and non-spontaneous reactions.
Demonstrate that a non-spontaneous reaction can be made to take place if work is done on the reaction system.

Introduction

A spontaneous reaction is one that takes place without any work having to be done. For example, manganese metal will spontaneously react with nickel(II) ion:

Mn(s) + Ni(NO3)2(aq)  Mn(NO3)2(aq) + Ni(s)

Some spontaneous reactions can be made to go backwards by doing work.This lab uses a hand-held DC generator (Figure 1) to do the work. When the crank is turned, electricity is produced. Electricity can, of course, do work; this electrical work can cause a reaction to go in the opposite direction:

Work + Mn(NO3)2(aq) + Ni(s)  Mn(s) + Ni(NO3)2(aq)

The chemical changes taking place are easier to see if these reactions are written as net ionic equations. (You are asked to write some of those for this lab.) Reminder: in a net ionic equation, all aqueous species are written with the ions separated; all phases are shown, and charges are given for all ions. The ionic equation for the spontaneous reaction above is:

Mn(s) + Ni2+(aq) + 2NO3–(aq)  Mn2+(aq) + 2NO3–(aq) + Ni(s)

Cancelling the spectator ions gives

Mn(s) + Ni2+(aq)  Mn2+(aq) + Ni(s)

A net ionic equation for the non-spontaneous reaction can written as

Work + Mn2+(aq) + Ni(s)  Mn(s) + Ni2+(aq)

Procedure

Suppose that two compounds, A and B, spontaneously react to form C and D.

A + B  C + D

Now, consider what would happen if C and D were mixed. Because the reaction goes spontaneously from left to right; the reverse reaction will not spontaneously occur, so if C and D are mixed, nothing will happen, so they must be the products of the reaction.

The object of the next two experiments is to determine which pair of compounds is reactants, and which pair is products for a spontaneous reaction. Using that information, you can then write a net ionic equation for the reaction that is spontaneous.

Equipment Check

Connect the light bulb to the hand-cranked generator by connecting the alligator clips from generator’s leads to the alligator clips on the light bulb. (It does not matter which color lead is connected to which.) Turn the generator crank, and confirm that the light bulb lights up. If it doesn’t, the instructor can provide a replacement bulb.

A Spontaneous Reaction

In some of the following reactions a black powder will appear on an electrode. The black powder is tiny particles of the element. Although in large amounts copper and zinc are not black, as very tiny particles, these metals may appear as a black powder.

Adding Cu(s) toZnSO4(aq)

Fill a test tube with ZnSO4(aq) solution, and place the tube in a test tube rack. Bend a strip of Cu(s) metal into a hook shape and place it in the solution, as shown in Figure 2. After a short time—20 seconds is sufficient—remove the copper strip, examine it, and record your observations in Table 1. Be sure to note any color changes in the Cu(s) strip or in the solution. Does it appear that a chemical reaction has spontaneously occurred?

Note that, since sulfate has a 2– charge, the charge on zinc in ZnSO4 is Zn2+. This is the only commonly found charge on zinc ions, so, if Zn is present as an ion, it will have a 2+ charge.

Adding Zn(s) toCuSO4(aq)

Fill a test tube with CuSO4(aq) solution, and place the tube in a test tube rack. Bend a strip of Zn(s) metal into a hook shape and place it in the solution, as shown in Figure 3. After a short time—20 seconds is sufficient—remove the zinc strip, examine it, and record your observations in Table 1. Be sure to note any color changes in the Cu(s) strip or in the solution. Does it appear that a chemical reaction has spontaneously occurred?

Write a balanced net ionic reaction for the spontaneous reaction involving the four substances used above.

A Non-spontaneous Reaction

Sensation of Doing Work

In the following sections, we are going to sense the work we do with the generator and make the non-spontaneous reaction take place by doing work on the system.

The ease of turning the generator crank depends on how much work the generator is doing. A qualitative comparison of the amount of work required toturn the crank will be made under the following conditions:

with nothing connected (load-free).
with a 6 V light bulb connected to the leads.
with the leads connected to power an electrochemical process (the reaction of Zn(s) and Cu+2(aq)).

Generator under No Load

Obtain a hand cranked DC generator. To test the frictional resistance of the generator, separate the alligator clip ends of the wires that are plugged into the generator. Make sure the clips are not touching one another, and turn the generator crank in a clockwise direction. Note the resistance the generator offers to your turning the crank, and record your impression of this level of resistance under the “Task 1” row in the “Load Level” column of Table 2.

Generator Loaded with a Light Bulb

Attach the alligator clips on the end of the leads from the generator to the leads attached to a flashlight light bulb. Turn the generator crank clockwise until the light bulb filament starts to glow. Continue turning the generator crank until you have a firmly established sensation of how hard it is to turn the crank. This assessment gives you an impression of the work needed to keep a small light bulb lighted. Record your impressions of the level of work required for this task under “Task 2” in Table 2.

Generator Loaded with Chemical Reaction

Prepare a test tube as shown below. Use a large test tube, and partition it into two chambers with a strip of cardstock paper cut to size. Fill the test tube to within 1 cm to 2 cm of the top of the tube with a solution of ZnSO4 (the previous ZnSO4 solution may be used here). Place the tube in a test tube rack. It should resemble the left portion of Figure 4. The right portion of the figure shows the test tube after the two copper strips have been inserted into the two respective partitions that the cardstock creates in the test tube.

The cardstock serves to keep the two copper strips separated and prevents a short circuit that their contact would create. Make sure that the two Cu(s) strips are completely separated during the following process.

Attach the generator’s lead’s alligator clips to each of the Cu(s) strips in the test tube illustrated above. Then turn the crank of the generator in a clockwise direction. Note the force the new load has created for the power source (you!) driving the generator and record your observation concerning the load in “Task 3” of Table 2. Also record any noticeable color changes on either of the strips and in the solution. Continue turning the crank until you are certain that any chemical change that will occur does occur… this may take up to two minutes.

Disconnect the generator leads from the copper strips. Next, adjust the voltmeter to display voltage in volts. This is done by setting the maximum voltage the meter will experience; a setting of 20 DCV (direct currentvolts) will do the job. The voltmeter electrode tips are metallic; if they are not visible, remove their plastic covers. Press the two voltmeter electrodes down onto the two copper strips (this is easiest to do if the electrodes presseach strip down onto the glass lip of the test tube). Recordthe voltage (it will fluctuate a bit) and your observations in “Task 4” of Table 2. Switch the voltmeter electrodes. What happens to the reading? What is another name for this apparatus? (Hint: it is portable, and is used in cell phones.)

Electrode Cleanup

The Cu(s) strips need to becleaned for use in other lab sections. To clean them, obtain a large test tube, fill it to within 2-4 cm of the top with 1 MHCl(aq). Place the Cu(s) electrodes in the HCl. Note any visible reaction that takes place in Table 3. This reaction may take a few moments to begin.

When the reaction appears to be complete, pour the acid and the Cu(s) into a 150 ml beaker, remove the Cu(s) with tongs, and rinse the Cu(s) well under tap water. Return the Cu(s) to your desk area, and discard the acid in the spent acid disposal container provided. (All waste for General Chemistry II Lab goes in the hood behind the hood at the front of the room.)

Remove the black on the zinc by just wiping with a paper towel. Record in Table 3 what the material on the Cu and Zn electrodes is most likely to be.

Turn in just the data sheets. Although you may work in pairs, each student should turn in a data sheet.

Name: ______Lab Day & Time: ______Date: ______

Data Sheet

A Spontaneous Reaction

Table I. Results of test of reaction of metals in solutions

Metal / Solution / Observations / Spontaneous?
Cu(s) / ZnSO4(aq) / Yes No
Zn(s) / CuSO4(aq) / Yes No
Write the net ionic equations for each spontaneous reaction observed:

A Non-spontaneous Reaction

Table II. Assessing the level of work required to operate a DC Generator

Task # / Task Description / Load Level
(low, medium, or high) / Comments/Observations
1 / Load Free – Clips not touching
2 / Load to Light 6 V Flashlight Bulb
3 / Load to Drive Reaction of Cu(s) in ZnSO4(aq) / Describe Color Change
Cu(s) #1
Cu(s) #2
Solution
4 / Voltmeter / Initial Reading:
Reading after switching electrodes:
What this apparatus represents:

Table III. Electrode Clean-up

Observations on adding HCl(aq) to the dirty Cu electrode:
What was the material on the copper strip?
What was the material on the zinc strip?

Post-Lab Questions:

What is a “spontaneous” chemical reaction?

How is work related to whether a chemical reaction is spontaneous or nonspontaneous?

Give the net ionic equation for the reaction that takes place when the copper electrode is cleaned.

Spontaneity and Reversal of Reactions

Molecular Geometry

Troy University Chemistry Faculty

Licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Chemists can look at the formula of a simple compound and picture the compound in their minds. This ability is developed by making and examining models. This lab project will help you develop this ability. You will start by drawing a Lewis structure of a species, then make a model of it, and finally decide if the molecule is polar.

Read the material in chapter 9 of your chemistry textbook about bonding and geometry before doing this experiment. Bringyour textbook to the lab for this experiment.

Procedure

For each species in the following table, give the number of valence electrons in the species, draw the Lewis structure, and give the names of the electronic and molecular geometry of each species. Also, construct a model of each species and draw a sketch of the model.

Your sketches should have bonds in the plane of the paper drawn as plain lines, bonds coming out of the paper drawn as wedges, and bonds going behind the paper drawn as dashes. For example, CH4 could be drawn like this:

Your sketches may include or exclude double and triple bonds, as you choose. However, please omit lone electrons. Obtain your instructor’s signature on the lab pages before turning in the lab. The instructor will not sign the pages unless he or she has observed that the models were actually made.

Molecular Geometry

Molecular Shape

Number of
Electron Groups / Number of
Lone Pairs / Electronic Geometry / Molecular Geometry
1 / 0 / linear / linear
2 / 0 / linear / linear
3 / 0 / trigonal planar / trigonal planar
3 / 1 / trigonal planar / bent (angular)
4 / 0 / tetrahedral / tetrahedral
4 / 1 / tetrahedral / trigonal pyramidal
4 / 2 / tetrahedral / Bent
5 / 0 / trigonal bipyramidal / trigonal bipyramidal
5 / 1 / trigonal bipyramidal / see-saw
5 / 2 / trigonal bipyramidal / T-shaped
5 / 3 / trigonal bipyramidal / linear
6 / 0 / octahedral / octahedral
6 / 1 / octahedral / square pyramidal
6 / 2 / octahedral / square planar

From

Name: ______Lab Day & Time: ______Date: ______

Data Sheet

Species / Valence Electrons / Lewis Dot Structure / Electronic Geometry / Molecular Geometry / Sketch
CH4 / C4
4H4
8 / / tetrahedral / tetrahedral /
ICl
CO2
BF3
SiI4
NCl3
H2O
HCl / No central atom, so skip this box.
SbCl5
SF6
XeF2
XeF4
BrF5
CH2O (C is central atom)
ClF3
SiO32-(Just 1 reson-ance form)
IO4–
H2O2
NH4+
H3O+
NO2+
O3
(Just 1 reson-ance form)

Resonance Structures, Formal Charges, and Polarity

Troy University Chemistry Faculty

Licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Introduction

Lewis structures are models representing covalent bonding between atoms. These structures use dots around atoms to signify electrons and lines to signify bonds between atoms. Lewis structures that differ only in the arrangement of electrons are called resonance structures. The most likely Lewis structures is determined using formal charge, which is the charge of an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms. Below are resonance structures of the carbonate anion, CO32-.

Procedure

For each species in the worksheet:

Draw a Lewis structure. Since this lab is about resonance, nearly every structure will have a double bond.
Determine the formal charges on each atom of the Lewis structure.The formal charge are calculated using the formula:

Draw a structure with the most favorable formal charges. This may involve moving the double bond around, or converting a single bond to a double bond, or converting something like X=M=X to X–MX or XM–X. The dominant structure has

a) the lowest formal charges, and

b) any negative formal charges on the most electronegative element.

Atoms in row 3 and beyond can have more than an octet. For such atoms, consider converting single bonds to double bonds to lower the formal change.

Draw all possible res1onance structures for this most likely structure.
Include formal charges on all of your structures. Formal charges of 0 do not need to be shown.
Identify the electronic and molecular geometry. The tables on the following pages may help.
Construct a model of the probable structure using the model kit and show to the instructor.
Draw a sketch of the model. Your sketches should have bonds in the plane of the paper drawn as plain lines, bonds coming out of the paper drawn as wedges, and bonds going behind the paper drawn as dashes. (The attached table of VSEPR geometries has examples you can use.)
In the column labeled polarity, indicate if the molecule is polar or nonpolar. If everything attached to the central atom is the same, it is nonpolar; otherwise, it is polar. For example, CH4 is tetrahedral and non-polar; H2O has two lone pairs and two bonds, the things around it are not identical, so it is polar. Polar and nonpolar can even be assigned to ions.
Wait to leave until your instructor has finished checking your report to ensure that you understand everything, and that you get full credit on this lab report.

Electronegativity

Resonance Structures, Formal Charges, and Polarity

Molecular Shape

From

Resonance Structures, Formal Charges, and Polarity

Name: ______Lab Day & Time: ______Date: ______

Data Sheet

Species / Resonance Structures Including Formal Charges / Electronic Geometry / Molecular
Geometry / Sketch / Polarity
NO3-
SO42-
Species / Resonance Structures Including Formal Charges / Electronic Geometry / Molecular
Geometry / Sketch / Polarity
NO2-
SO2
N2O
(N is
central
atom)
Species / Resonance Structures Including Formal Charges / Electronic Geometry / Molecular
Geometry / Sketch / Polarity
SCN-
SO3
(S2CO)2-
(C is the
central
atom)
Species / Resonance Structures Including Formal Charges / Electronic Geometry / Molecular
Geometry / Sketch / Polarity
IO3-
ClO4-

Determination of Absolute Zero on the Celsius Scale

Troy University Chemistry Faculty

Licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Objectives

To give a feel for how gases behave when heated.

Introduction

The plot above shows how the value of absolute zero is determined experimentally. The volume and temperature of a fixed amount of gas at constant pressure are measured at two different temperatures, giving a pair of points: (Tlow, Vlow) and (Thigh, Vhigh). These two points of data are plotted. As temperature is lowered, the volume gets smaller. It looks like the volume would become zero if the temperature were made low enough. To determine at what temperature the volume would appear to go to zero, a line is drawn connecting the two data points, then the line is extended such that the volume gets less and less as the temperature gets lower and lower. Eventually, the line reaches a point where the volume would be zero. This occurs where the line touches the x-axis; the point is called the x-intercept (in contrast the y-intercept is where the line crosses the y-axis). This x-intercept corresponds to a particular temperature, which is called absolute zero. Getting colder than absolute zero makes no sense, because volume cannot be less than zero.