Endothermic and Exothermic Reactions Many chemical reactions give off energy. Chemical reactions that release energy are called exothermic reactions. Some chemical reactions absorb energy and are called endothermic reactions. In this lab students studied one exothermic and one endothermic reaction. In Part I, we studied the reaction between citric acid solution and baking soda. An equation for the reaction is: H3C6H5O7(aq)
+ 3 NaHCO3(s) In Part II, we studied the reaction between magnesium metal and hydrochloric acid. An equation for this reaction is: Mg(s) + 2 HCl(aq)
Freezing temperature, the temperature at which a substance turns from liquid to solid, and melting temperature, the temperature at which a substance turns from a solid to a liquid, are characteristic physical properties. In this experiment, the cooling and warming behavior of a familiar substance, water, was investigated. By examining graphs of the data, the freezing and melting temperatures of water was determined and compared. Melting and freezing behavior are among the characteristic properties that give a pure substance its unique identity. As energy is added, pure solid water (ice) at 0°C changes to liquid water at 0°C . In this experiment, students determined the energy (in joules) required to melt one gram of ice and from this data they then determined the molar heat of fusion for ice (in kJ/mol). Excess ice was added to warm water, at a known temperature, in a Styrofoam cup. The warm water was cooled down to a temperature near 0°C by the ice. The energy required to melt the ice was removed from the warm water as it cooled. To calculate the heat that flows from the water, you can use the relationship q = Cp•m•D T where q stands for heat flow, Cp is specific heat, m is mass in grams, and DT is the change in temperature. For water, Cp is 4.18 J/g°C.
In many laboratory investigations, a primary purpose will be to find the mathematical relationship between two variables. For example, you might want to know the relationship between the pressure exerted by a gas and its temperature. In another experiment you might be asked to determine the relationship between the volume of a confined gas and the pressure it exerts. A very important method for determining mathematical relationships in laboratory science makes use of graphical methods. In this exercise, students used a computer and the Logger Pro program to help them determine several of these relationships.
In this experiment, students use Hess's Law to determine a heat of reaction that would be difficult to
obtain by direct measurement—the heat of combustion of magnesium ribbon. The
reaction is represented by the equation:
(4)
Mg(s)
+ 1/2 O2(g) This equation can be obtained by combining equations
(1), (2), and (3):
(1)
MgO(s) + 2 HCl(aq)
(2)
Mg(s) +
2 HCl(aq)
(3)
H2(g) +
1/2 O2(g)
Boyle’s Law: Pressure-Volume Relationship in Gases
In this experiment students investigated the chemical significance of Avogadro's hypothesis. They determined the volume of hydrogen gas evolved in a reaction between magnesium metal and hydrochloric acid, and from their results determined the mass of H2 produced. Their experimental results were then compared to the results predicted by Avogadro's hypothesis. Pictures from this Lab
In this experiment students encounted two types of organic compounds—alkanes and alcohols. The two alkanes are pentane, C5H12, and hexane, C6H14. In addition to carbon and hydrogen atoms, alcohols also contain the -OH functional group. Methanol, CH3OH, and ethanol, C2H5OH, are two of the alcohols that we used in this experiment. Students examined the molecular structure of alkanes and alcohols for the presence and relative strength of two intermolecular forces—hydrogen bonding and dispersion forces. Pictures from this LabProperties of Solutions: Electrolytes and Non-Electrolytes In this experiment, students discovered some
properties of strong electrolytes, weak electrolytes, and non-electrolytes by observing
the behavior of these substances in aqueous solutions. When the conductivity probe is
placed in a solution that contains ions, and thus has the ability to conduct electricity,
an electrical circuit is completed across the electrodes that are located on either side
of the hole near the bottom of the probe body. This results in a conductivity value that
can be read by the computer. The unit of conductivity used in this experiment is the
microsiemens, or µS.
Rate Law Determination of the Crystal Violet Reaction In
this experiment, students observe the reaction between crystal violet and sodium
hydroxide. One objective is to study the relationship between concentration of
crystal violet and the time elapsed during the reaction. The equation for the
reaction is shown here:
A
simplified (and less intimidating!) version of the equation is: CV+ +
OH–
The rate law for this reaction
is in the form: rate = k[CV+]m[OH–]n,
where k is the rate constant for the
reaction, m is the order with respect
to crystal violet (CV+),
and n is the order with respect to the
hydroxide ion. Since the hydroxide ion concentration is more than 1000 times as
large as the concentration of crystal violet, [OH-]
will not change appreciably during this experiment. Thus, you will find the
order with respect to crystal violet (m), but not the order with respect to
hydroxide (n). As
the reaction proceeds, a violet-colored reactant will be slowly changing to a
colorless product. Using the green (565 nm) light source of a
computer-interfaced Colorimeter, students monitor the absorbance of the crystal
violet solution with time. Because absorbance is proportional to the
concentration of crystal violet (Beer’s law), absorbance is used in place of
concentration in plotting the following three graphs:
Once the order with respect to
crystal violet is determined, students also are able to find the rate constant,
k, and the half-life for this reaction.
NaCl(s)
Acid
Dissociation Constant, Ka In this experiment you will: The acid to be used is acetic acid, HC2H3O2, and its dissociation equation is: HC2H3O2(aq)
The procedure for this lab involves the making of solutions of various concentrations of acetic acid from a 2.00M stock solution and then measuring the pH of each solution using the pH probe. From the measured pH the [H+] can be calculated, then from the above equation [C2H3O2–] and [HC2H3O2] and be obtained. Substituting these calculated values into the Ka expression the students then calculate the value of Ka for acetic acid.
Using Conductivity to Find an Equivalence Point In this experiment, students monitored conductivity during the reaction
between sulfuric acid, H2SO4, and barium hydroxide,
Ba(OH)2, in order
to determine the equivalence point. From this information, they found the concentration of
the Ba(OH)2 solution.
They also saw the effect of ions, precipitates, and water on conductivity. The equation
for the reaction in this experiment is: Ba2+(aq) + 2 OH-(aq) + 2 H+(aq) + SO42-(aq)
As 0.0200 M H2SO4 was slowly added to Ba(OH)2 of unknown concentration, changes in the conductivity of the solution were monitored using a conductivity probe. When the probe was placed in a solution that contains ions, and thus has the ability to conduct electricity, an electrical circuit was completed across the electrodes that are located on either side of the hole near the bottom of the probe body. This resulted in a conductivity value that can be read by a computer. The unit of conductivity used in this experiment is the microsiemens, or µS. Pictures from this Lab
In this experiment students reacted the following combinations of strong and weak acids and bases: o Hydrochloric acid, HCl (strong acid), with sodium hydroxide, NaOH
(strong base)
One objective of this lab is to observe differences in shapes of titration curves when various strengths of acids and bases are combined. Students also learn about the function and selection of appropriate acid-base indicators in this experiment. A titration is a process used to determine the volume of a solution needed to react with a given amount of another substance. In this experiment,students titrated hydrochloric acid solution, HCl, with a basic sodium hydroxide solution, NaOH. The concentration of the NaOH solution was given and students determined the unknown concentration of the HCl. Hydrogen ions from the HCl react with hydroxide ions from the NaOH in a one-to-one ratio to produce water in the overall reaction: H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)
In this experiment, students used a computer to monitor pH as they titrated. The region of most rapid pH change was then used to determine the equivalence point. The volume of NaOH titrant used at the equivalence point was used to determine the molarity of the HCl.
By comparing the voltage values obtained for several pairs of half-cells, and by recording which metal made contact with the (+) and (–) leads, students established the reduction potential sequence for the five metals in this lab.
Electrical resistance in metals arises because electrons propagating through the solid are scattered due to deviations from perfect translational symmetry. In a superconductor below its transition temperature there is no resistance because these scattering mechanisms are unable to impede the motion of the current. The superconductive state is another physical state. Zero resistance is hard to measure, and the most definitive evidence for superconductivity arises form DC magnetic measurements. Persistent currents on the surface of the superconductor make it perfectly diamagnetic -- expelling all manetic flux. A magnet placed above the superconductor will levitate because of the expulsion of the magnet’s flux by the superconductor. This phenomenon is known as the Meissner Effect. In a class demonstration a ceramic material was cooled below its transition temperature with liquid nitrogen. Then a small magnet was placed above the superconducting ceramic. Last Updated on
07/18/2010 |
3 CO2(g) + 3 H2O(l)
+ Na3C6H5O7(aq)
The primary
objective of this experiment is to determine the relationship between the pressure and
volume of a confined gas. The gas we use was air confined in a syringe connected to a
pressure sensor (see Figure 1). When the volume of the syringe was changed by moving the
piston, a change in the pressure exerted by the confined gas resulted. This pressure
change was monitored using a pressure sensor interfaced to a computer. We assumed that
temperature will be constant throughout the experiment. Pressure and volume data pairs
were collected during this experiment and then analyzed. From the data and graph, students
should be able to determine what kind of mathematical relationship exists between the
pressure and volume of the confined gas. Historically, this relationship was first
established by Robert Boyle in 1662 and has since been known as Boyle’s law.
In
this experiment, temperature probes were placed in various liquids. Evaporation occurs
when the probe is removed from the liquid’s container. Evaporation is an endothermic
process that results in a temperature decrease. The magnitude of a temperature decrease
is, like viscosity and boiling temperature, related to the strength of intermolecular
forces of attraction. In this experiment, students studied temperature changes caused by
the evaporation of several liquids and related the temperature changes to the strength of
intermolecular forces of attraction. They used the results to predict, and then measure,
the temperature change for several other liquids.
In this experiment, students studied the effect of increasing the
concentration of an ionic compound on conductivity. Conductivity was measured as
concentration of the solution was gradually increased by the addition of concentrated NaCl
drops. The same procedure was be used to investigate the effect of adding solutions with
the same concentration (1.0 M), but different numbers of ions in their formulas: aluminum
chloride, AlCl3, and calcium chloride, CaCl2.
A computer-interfaced conductivity probe was be used to measure conductivity of the
solution. Conductivity was measured in microsiemens (µS).
Before reacting, Ba(OH)
When an HCl solution is titrated with an NaOH solution, the pH of the acidic
solution is initially low. As base is added, the change in pH is quite gradual until close
to the equivalence point, when equimolar amounts of acid and base have been mixed. Near
the equivalence point, the pH increases very rapidly, as shown in Figure 1. The change in
pH then becomes more gradual again, before leveling off with the addition of excess base.
A voltaic cell utilizes a spontaneous oxidation-reduction reaction to produce
electrical energy. Half-cells are normally produced by placing a piece of metal into a
solution containing a cation of the metal (e.g., Cu metal in a solution of CuSO4 or Cu2+). In this micro-version of a voltaic cell,
the half cell will be a small piece of metal placed into 3 drops of solution on a piece of
filter paper. The solution contains the cation of the solid metal. Figure 1 shows the
arrangement of half-cells on the piece of filter paper. The two half-reactions are
normally separated by a porous barrier or a salt bridge. Here, the salt bridge will be
several drops of aqueous NaNO3 placed on the filter paper between
the two half cells. Using the computer as a voltmeter, the (+) lead makes contact with one
metal and the (–) lead with another. If a positive voltage is recorded on the screen,
you have connected the cell correctly. The metal attached to the (+) lead is the cathode
(reduction) and thus has a higher, more positive, reduction potential. The metal attached
to the (-) lead is the anode (oxidation) and has the lower, more
negative, reduction potential.