CHEM 152 University of Arizona Ammonium Nitrate in Water Lab Report Hi there, this is lab report with guide assignment pdf & assignment pdf all uploaded Ch

CHEM 152 University of Arizona Ammonium Nitrate in Water Lab Report Hi there, this is lab report with guide assignment pdf & assignment pdf all uploaded Chemical Thinking
Guide for Lab Assignment 2 v1.1 | 1
How do we characterize thermochemical change?
In the study of thermochemistry, heat changes that accompany physical and/or chemical processes are measured
with a calorimeter, a closed insulated container designed for this purpose and equipped with a thermometer.
Constant-Pressure Calorimetry
A constant-pressure calorimeter is used to measure the heat exchange
between a system and the surroundings for a variety of processes such
as acid-base neutralizations, heats of solution and heats of dilution. The
constant-pressure calorimeter (Figure 1) consists of an insulated
vessel that contains the reaction mixture or a liquid of known specific
heat (usually water), a stirrer to ensure homogeneity in the liquid
phase, and a thermometer to indicate the liquid phase temperature.
While the vessel is closed to minimize heat loss, it is not sealed,
consequently the pressure within the vessel remains constant and
equal in magnitude to the ambient pressure.
Because the pressure is constant, the heat change for the process ( )
is equal to the enthalpy change (∆ ). In such experiments we consider
the reactants and products to be the system, and the water in the
calorimeter to be the surroundings. Typically, the small heat capacity
of the calorimeter is ignored.
Figure 1. Constant-Pressure Calorimeter
In the case of an exothermic reaction, the heat released by the system is absorbed by the water (surroundings),
thereby increasing its temperature. Knowing the mass ( ) of the water in the calorimeter, the specific heat ( ) of
the water, and the change in temperature (∆ ), we can calculate (heat at constant pressure) of the system by
= − ∆
where ∆ is defined as − . Note the minus sign makes a negative number if ∆ is positive (increasing
temperature). This is in keeping with the sign convention of ∆ . A negative ∆ or indicates an exothermic process,
whereas a positive ∆ or indicates an endothermic process.
Constant-Pressure Calorimetry Example: Determination of ΔHrxn
Assume we are interested in determining the ∆ for the acid-base neutralization between hydrochloric acid (HCl)
and the base sodium hydroxide (NaOH). We start with 49.11 mL of 1.01 M HCl and 50.01 mL of 1.03 M NaOH. Both
solutions are at room temperature, which we measure as 23.5 °C. The balanced chemical equation is:
HCl + NaOH → NaCl + H2O
Both solutions are poured into a constant-pressure calorimeter and the calorimeter is closed. As the reaction
proceeds, the temperature of the water increases as it absorbs energy released by the reaction. A maximum water
temperature of 30.3 °C is recorded.
Assuming the density and specific heat of the HCl and NaOH solutions to be the same as those of water (1.00 g/mL,
4.184 J/g °C), we calculate ( for the solution) as follows:
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= ∆
= (4.184

) (49.11 + 50.01 )(30.3 ℃ − 23.5 ℃) = 2.82 × 103
℃
Recall = − and that we consider = with = . Thus, assuming the
heat capacity of the calorimeter is negligible, we know that = − , and can write
= −2.82 × 103
This is the heat of reaction when 0.0496 mol HCl reacts with 0.0515 mol NaOH. To determine ∆ , we divide
by the number of moles of the limiting reagent (in this case HCl),
∆ =
∆ =

−2.82 × 103

= −5.69 × 104
= −56.9
0.0496

Constant-Volume Calorimetry
The heat of combustion is usually measured using constantvolume calorimetry. Typically, a known amount of sample is
placed in a steel container called a constant-volume bomb,
or simply a bomb, which is pressurized with oxygen (O2). The
closed bomb is then immersed in a known amount of water
within an insulated container, as illustrated in Figure 2.
Together, the steel bomb and the water in which it is
submerged constitute the calorimeter. The sample is ignited
electrically, and the heat released by the combustion of the
sample is absorbed by the bomb and the water and can be
determined by measuring the increase in the water
temperature. The special design of this type of calorimeter
allow us to assume that no heat (or mass) is lost to the
surroundings during the time it takes to carry out the
reaction and measure the temperature change. Therefore,
we call the bomb and the water in which it is submerged an
isolated system. Because no heat enters or leaves the
system during the process, the heat change of the system
overall is zero ( = 0) and we can write:
Figure 2. Constant-Volume Calorimeter
= −
where and are the heat changes for the calorimeter and reaction, respectively. Thus,
= −
To calculate we need to know the heat capacity of calorimeter ( ) and the change in temperature, that is,
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= ∆
And because = − ,
= − ∆
The heat capacity of the calorimeter ( ) is determined by burning a substance ( ) with an accurately known heat
of combustion ( ) for a given mass and measuring the temperature increase ∆ . Subsequently, the heat
capacity of the calorimeter is given by:
=

∆
Once has been determined, the calorimeter can be used to measure the heat of combustion of other substances.
Because a reaction in a bomb calorimeter occurs under constant-volume rather than constant-pressure conditions,
the measured heat change corresponds to the internal energy change (∆ ) rather than the enthalpy change (∆ ).
It is possible to correct the measured heat changes so they correspond to ∆ values, but the corrections are usually
quite small, so we will not concern ourselves with the details here.
Constant-Volume Calorimetry Example: Determining the Energy Content per Mass and Calories per
Serving
A sample of Post Grape-Nuts® cereal weighing 5.81 g is burned in a bomb calorimeter to determine its energy
content. The heat capacity of the calorimeter is 43.7 kJ/°C. During the combustion, the temperature of the water in
the calorimeter increases by 1.92 °C. Calculate the energy content (in kJ/g) and Calories per serving of Grape-Nuts®.
First, we determine the heat released by the combustion using
= − ∆
= − (43.7

) (1.92 ℃) = −83.9
℃
The negative sign in the result indicates heat is released by the combustion. To find the energy content per gram of
Grape-Nuts®, we divide the heat released by the mass of the Grape-Nuts® sample.
=
| |
83.9
=
= 14.4 /

5.81
(Note, because energy content must be a positive quantity, we write 83.9 kJ as a positive
value. In the above, we use the absolute value notation, | |, to remind us of this.)
According to the nutrition facts label on the Grape-Nuts® package, a serving size is 58 g. The
dietary Calorie (Cal) is 4.184 kJ. Hence, converting the energy per gram to Calories per
serving gives,
(
14.4
1
58
)(
)(
) = 2.0 102 /

4.184
Which, ignoring proper sig figs, is the value given on the Grape-Nuts® nutrition facts label.
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Lab Assignment 2 v1.1 | 1
Lab Assignment 2
Your name: ___________________________________ Your section: ________ GRADE ____ /25 p
All work must be very neat and organized. To receive credit, you must show all work in a legible, highly organized
manner. Responses that are illegible and/or lack organized supporting work will be scored a “zero.” Significant
figures must be reasonable, and correct units (where applicable) must be present.
Problem 1. Assume you are interested in exploring the use of ammonium nitrate (NH4NO3) as the active ingredient
in an inexpensive home-made cold pack. You decide that a practical cold pack should be able to depress the
temperature of 1 kg of human muscle tissue by 5 oC. Now you need to know the amount of NH4NO3 required to
achieve this. Ammonium nitrate is a highly water soluble solid. You believe the enthalpy of dissolution ∆ for
NH4NO3 must be determined. To this end, you add 2.339 g of NH4NO3 to 99.7 mL of water at 23.7 °C in a constantpressure calorimeter and close the calorimeter. The temperature of the water decreases to a minimum of 21.9 °C.
1a (5p). Assume the density and specific heat of water is 1.00 g/mL and 4.184 J/g °C, respectively. Using the data
above, determine the ∆ for ammonium nitrate in water. Assume the heat capacity of the calorimeter is
negligible. To receive credit, you must show all work in a legible, highly organized manner.
1b (2p). Is the dissolution of ammonium nitrate in water an endothermic or exothermic process? Briefly state your
reasoning.
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Lab Assignment 2 v1.1 | 2
1c (5p). The specific heat of human muscle tissue (smuscle) is 3.47 kJ/kg oC. Given this value, how many grams of
ammonium nitrate (NH4NO3) is required to depress the temperature of 1.00 kg of human muscle by 5.00 oC? Please
remember, you must show all work in a legible, highly organized manner.
Problem 2. You wish to determine the energy content per gram and Calories per serving of a chocolate chip cookie
and a banana bread slice from the Student Union to decide which is less “fattening.” The intact cookie and banana
bread slice you are analyzing weigh 72.501 g and 69.003 g, respectively. Before you can analyze the samples, the
calorimeter heat capacity ( ) must be determined. It is known that combustion of 1.000 g of benzoic acid releases
26.44 kJ of heat ( = 26.44 kJ). Thus, you combust 1.000 g of benzoic acid in a constant-volume calorimeter
and record a temperature increase of 2.52 °C.
2a (2p). From the data given above, determine the heat capacity of the constant-volume calorimeter ( ). To
receive credit, you must show all work in a legible, highly organized manner.
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2b (4p). Now you take a 5.688 g sample of the chocolate chip cookie and burn it in the same calorimeter. During the
combustion, the temperature of the water in the calorimeter increases by 11.81 °C. For the chocolate chip cookie,
calculate the energy content in kJ/g and the Calories per serving. Please remember, you must show all work in a
legible, highly organized manner.
2c (4p). Finally, you take a 4.904 g sample of the banana bread and run it in the same calorimeter. During the
combustion, the water temperature in the calorimeter increases by 12.29 °C. For the banana bread slice, calculate
the energy content in kJ/g and the Calories per serving. Don’t forget, you must show all work in a legible, highly
organized manner.
2d (3p). Based on the above Problem 2 data, is the chocolate chip cookie or banana bread slice less “fattening” in
terms of the energy content in kJ/g? What about in terms of the Calories per serving? In complete sentences, clearly
justify your reasoning citing the evidence.
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