CHEN 4501W Lecture Notes - Lecture 20: Inta, Bacteriophage, Heat Exchanger

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Tcw,in = 90 of (worst-case secenario for hot humid day, conservative) Tcw,out = 120 of (corrosion and scale formationif we go higher, we"ll have minerals that will precipitate out) Can only have 30 degrees rise in t, so then we find out how much water we need. Refrigeration is a process in which work is done to move heat from one location (cool) to another (hot) Heat naturally goes from hot to cold, so we have to apply work. Heat transferred via phase change of a working fluid in a flowing cycle is the most common basis for refrigeration. Heat engine: uses heat flow to produce work. Refrigeration cycle: uses work to produce heat flows (opposite to their normal direction, from cold to hot) Most efficient heat cycle = carnot cycle --> let"s start with that. Operate within the vapor-liquid region of a phase diagram (before, when we learned, we considered everything to be gas) critical temperature liquid vapor.

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Â© Division of Chemical Educatio

â¢

www.JCE.DivCHED.or

â¢ Vol.

86 No.

11 November

2009 â¢

Journal of Chemical Educatio

1277

In the Classroom

Self-heating or self-cooling containers for meals and bever

ages are excellent examples of chemistry in action for the every

day life of consumers. Such containers consist of dual chambers

where the food is usually contained in the internal chamber

while the chemical process that would heat or cool the food or

beverage occurs in the other. A common cooling system consists

of ammonium nitrate and water while a common heating system

consists of the reaction of calcium oxide in water. The heating

or cooling chamber requires the reagents to be separated until

ready to use.

These hydration methods are simple and do impart heat

effects; however, they present some limitations such as heat

ing times of 5â15 minutes (required to generate the necessary

temperature increases) and the need for large amounts of the

reagents because of the low heat energ y yield, which then must

occupy a considerable space in the containers. Emerging tech

nologies are making self-heating foods more accessible to the

general public. One such technolog y is the self-propagating

high-temperature synthesis (SHS) that involves the oxidation of

a mixture of aluminum and other metals by iron oxide, Fe

The change in enthalpy of this reaction is greater than 3 kJ/g

of reactants, making it more than 4 times higher than the heat

energ y evolved when limestone reacts with water

(1).

This tech

nolog y ensures heating a beverage from 2â3 Â°C to the boiling

point in less than 90 seconds and cutting the heating time for

food to less than four minutes.

In Spain and other European countries self-heating bever

ages are known as âautocalentablesâ and are easily found at gas

stations, airports, and highway rest stops. A variety of these

products are available including different types of coffee (black,

with milk, or cappuccino), chocolate, and tea. In the United

States commercialization of these food products has been tar

geted toward outdoors enthusiasts and the military; however,

companies such as Starbucks and Wolfgang Puck are advancing

into this market. In the 1980s the U.S. Army took the lead to

further develop the technolog y required to enhance the Meals,

Ready to Eat or MREs that were used a decade earlier by the U.S.

Space Program. One of these advancements included the Flame

less Ration Heater (FRH) that allows military troops in combat

to have a hot meal. These MREs, which include snacks, main

entrees, and desserts, are now sold through a number of online

sites and are available individually or in packages of âemergency

supplyâ or âdisaster preparednessâ units

(2).

These products provide an excellent means to promote

interest in chemistry. The activity described in this article uses

these commercial products to study the chemistry that produces

the self-heating mechanism. Concepts such as stoichiometry,

enthalpy of reaction, enthalpy of solution, heat transfer, and

density of liquids are the core principles involved in these reac

tions. Creative ways to use these products may also be discussed.

For example, the FRH of the MREs have been used in combat

situations to warm intravenous fluids before administering them

to patients as deployed medical units often do not have means to

heat these fluids and by doing so they may prevent hypothermia

in patients

(3).

Methodology

We have used this activity with two different method

ologies in the classroom: as the foundation for problem-based

learning (PBL) and as the framework for inquir y-g uided

instruction (IGI). Even though this activity could be easily

performed in the laboratory, it has been designed, tested, and

implemented as a classroom activity where calculations and

evaluation of results take precedence to data collection. In the

PBL methodolog y the acquisition of skills and knowledge arise

from the need to solve a problem related to the background

or experience of the students. The expected learning depends

primarily on the collective reflection of a group of students.

The role of the instructor is to meet with the group of students

primarily as a listener and when necessary pose questions to

lead students in the right direction. In PBL the problem is

loosely constructed but the goal is clearly stated. PBL has

received prominent emphasis in health-related careers

(4â6),

but its benefits have been documented in a variety of disciplines

(7â10)

and academic levels of instruction

(11â14).

IGI adopts

a more structured approach where student learning is promoted

through the use of questions meant to initiate the curiosity of

the students to get them started on finding answers to their own

questions

(15, 16).

Both methodologies rely on group work

where students collectively think, reflect, and provide ways

by which to solve a problem. The instructor facilitates these

processes rather than informing students what needs to be done

next. This activity is presented using both methodologies so that

instructors may choose the one that better fits their pedagogical

goals or teaching styles.

The Chemistry of Self-Heating Food Products

An Activity for Classroom Engagement

Maria T. Oliver-Hoyo*

Department of Chemistry, North Carolina State University, Raleigh, NC 27695; *

[email protected]

Gabriel Pinto

E.T.S. de Ingenieros Industriales, Universidad PolitÃ©cnica de Madrid, 28006 Madrid, Spain

Juan Antonio Llorens-Molina

E.T.S. del Medio Rural y EnologÃa, Universidad PolitÃ©cnica de Valencia, 46010 Valencia, Spain

1278

Journal of Chemical Educatio

â¢ Vol.

86 No.

11 November

2009 â¢

www.JCE.DivCHED.or

â¢

Â© Division of Chemical Educatio

In the Classroom

The design of this activity fulfills the common elements

these methodologies share along with what Duch identifies as

practical criteria for a âgoodâ problem

(17)

â¢ âhookingâ

the student

to spark

interest

and

motivation

preferably within a real-life context

â¢ requiring

reflection

as students

recognize

and explain

how

to proceed, discerning which information is relevant and

necessary at each stage of the solving process

â¢ relying

on group

thinking

rather

than

dissecting

tasks

among group members

In addition we designed this activity to depend on analysis of

data, evaluation of results, and extension of concepts of a real-life

chemistry application.

Activity

The same principles and rationale apply to the two ex

amples provided: self-heating beverage and MRE (or the FRH

system). Instructors may use the example that students are more

familiar with so that chemistry principles become more tangible

to them. The aim is to propose a real-life chemistry problem for

which students will need to calculate the heat produced by the

chemical reaction or the dissolution process, the accompany

ing theoretical change in temperature, and finally compare the

theoretical change to the temperature observed. With either

methodolog y, PBL or IGI, the instructor shows the class the

container, carries out a demonstration, and gives necessary data

or appropriate information resources. The activity is designed

to use five minutes of class time and allow students to work in

groups, outside the classroom, to solve the problem posed.

Self-Heating Beverage

In class, the instructor asks a student to follow the direc

tions on the label of the self-heating chocolate beverage, while

the instructor does the demonstration. The temperature reached

inside the beverage container is recorded. The beverage is passed

around the classroom for students to feel the warmed container.

Students are given the masses of the different substances. In this

particular example these are CaCl

(54.31

g ), water (60.45

g ),

chocolate (93.68

g and a volume of 75 mL), container (21.22 g

polypropylene and 8.39

g Al). The instructor may choose to

adopt PBL methodolog y when presenting this activity to the

students and the problem could be similar to the one presented

in Figure 1. If the methodolog y adopted is IGI, a homework

set may be distributed that includes the following guiding

questions:

Draw a scheme that describes the container (materials

and design). Based on this design could you propose the

chemical process involved in heating the beverage?

Suggest a procedure by which the instructor was able to

determine the masses given.

Because the dissolution of CaCl

in water is an exother

mic process, how would you estimate the heat (in kJ)

liberated during the process and the temperature the

beverage should reach? Does the estimated temperature

match the temperature claimed by the manufacturer of

the container? Provide explanations and state possible

assumptions.

Explain the rationale for the following recommendations

given in the product label:

i) Shake upside down for 40 seconds.

ii) Do not perforate or cut the container.

iii) Self-heating will occur only once.

iv) Do not warm the container by any other means such

as microwave or oven.

Comment on the advantages or disadvantages of this type

of container and suggest improvements.

f )

Decide whether this type of container could be used to

cool a beverage rather than to heat it. Propose a chemical

process that could achieve that.

g )

With the data provided in this exercise, could you

compute the density of the chocolate? Using values of

the enthalpy of salt dissolution in water and other ther

mochemical data, could you calculate the lattice energ y

of the salt?

The proposed solutions to each question can be found in the

online material.

It is worth pointing out the rationale for including each of

these questions in the homework is because the methodolog y

is as important as the exercise. When students are required to

examine the information provided by manufacturers they realize

that without a chemical equation to describe the process they

Figure

1. Scheme

of the self-heating

beverage container and PBL problem.

Product A:

Self-Heating Beverage

PBL Problem:

Currently there are commercial

products that claim to heat its contents

based on the dissolution process of

a salt, in our case, calcium chloride.

We need to warm the 76 mL of the

beverage in this container to 60 Â°C.

To do so we need to know how much

salt and water are required in this

container.

Seperation lab: Will rate question

Separation lab - will choose best answer

Seperation lab: will choose best answer

Separation of a Mixture Laboratory

In this experiment we will separate the different components of a mixture made up of sand (SiO2) and sodium chloride (NaCl, commonly known as salt). Both of these compounds are solid white crystals, so it is difficult to tell them apart with the naked eye. However, each component exhibits different physical properties that can be used to separate one from the other. There are several physical separation techniques that can be used to determine the percent composition of the original mixture.

Materials & Apparatus:

Sand/salt mixture provided in your Lab Kit

2 - Erlenmeyer Flasks

Scale

Water

Graduated Cylinder

Background:

It is important to know how to separate mixtures because many reactions carried out in the laboratory produce mixtures of different compounds. It is often necessary to separate the components of these mixtures, which can be done using several techniques. We will explore a few of these techniques: sublimation, filtration, evaporation and decantation.

Sublimation is the process of a substance passing from the solid to the gaseous state without first passing through the liquid state. Not all substances possess this characteristic. In fact, most substances to NOT sublime under normal conditions, but must be placed under low pressure and low temperature. Ammonium chloride (NH4Cl) and iodine (I2) are both substances that do sublime under normal conditions.

Filtration is the process used to separate the components of a mixture of solid substances that differ in solubility in a certain solvent. You can separate a mixture of solid substances by adding a liquid to the original mixture in which only some of the components of the mixture will dissolve. The resulting mixture, which contains both the dissolved and undissolved substances, is then poured through a filter paper--a porous paper that allows liquids to pass through while retaining any solids. The liquid that has passed through the filter paper is called the filtrate; the solid on the paper is the residue.

Evaporation can also be used to separate the solvent from the solution. A solution can be slowly heated in an Erlenmeyer flask, and the solute will remain after the solvent has evaporated.

Decantation is the process of separating a liquid from a solid (sediment) by gently pouring the liquid from the solid so as not to disturb the solid.

For this experiment, a practical plan is as follows: First combine the solid mixture of the two compounds, sodium chloride (NaCl) and sand (SiO2), with water. This will dissolve the sodium chloride. The sand will not dissolve into the water. We will "filter" the entire mixture, which allows the water in which the sodium chloride is dissolved to pass through, but not the sand. We will now have separated the sodium chloride and the sand. Then, we must evaporate off the water so that we weigh only the two original solids and not the water we added.

Procedure:

(NOTE: Do NOT dispose of any part of your lab until you are 100% positive you do not need it again. It is good practice to keep all parts of your lab until you are finished.)

Record the letter (A, B, C, or D) of your mixture (on the bag contents label) on the line above Data Table 1 below.

Obtain an Erlenmeyer flask [label it 1] and weigh and record the mass.

Add the entire contents of your unknown mixture bag to the flask and weigh and record the mass of the Erlenmeyer flask and the mixture.

Add 10 mL of water (preferably distilled) to the solid in the Erlenmeyer flask [Erlenmeyer flask 1] and stir gently for 5 minutes.

Weigh a new clean, dry Erlenmeyer flask [Erlenmeyer flask 2].

Decant the liquid from Erlenmeyer flask 1 into Erlenmeyer flask 2. This may work better if you place a utensil against the lip of the pouring flask to encourage a slow steady stream of liquid into the second flask.

Add another 10 mL of water to the solid in Erlenmeyer flask 1, stir for 1-2 minutes, and decant this liquid into the Erlenmeyer flask 2 as before.

Repeat with still another 10 mL of water. This process extracts (dissolves) the soluble sodium chloride from the insoluble sand.

Allow the two flasks to dry. Let them dry until the sand appears dry and freely moves when you shake the Erlenmeyer flask a little and the sodium chloride looks dry.

To speed up the process, you can heat the flasks slowly over a burner set on low on your stove top or in the oven (set at 125-200ËF). I would suggest heating it on top of another pan (NOT nonstick coated) Use a low setting and watch closely as you donât want it to boil over.

Allow the Erlenmeyer flasks to cool to room temperature (if they were heated) and weigh the Erlenmeyer flasks and their contents separately. Record their masses in the appropriate sections of the table.

To ensure that there is no water remaining, heat for another 5-10 minutes oven or 1-2 minutes on the stove. Allow to cool to room temperature and reweigh the Erlenmeyer flask. If it is within 0.3 g of your initial weighing you can stop. If the second weighing is not within 0.3 g of your initial weighing, reheat again, cool, and reweigh. The difference between this mass and the mass of the empty Erlenmeyer flask is the mass of NaCl.

Unknown Mixture Label (A, B, C, or D) ___________

Data Table 1

Measure or calculate and record the following (in grams):
mass of the empty Erlenmeyer flask 1:
mass of the Erlenmeyer flask 1 plus the mixture sample:
mass of the mixture sample:
mass of the Erlenmeyer flask 1 and the remaining contents after drying of sand:
mass of Erlenmeyer flask 1 and dry sand (2nd weighing)
Subtract (a) from (e) to obtain the mass of sand in the sample:
Measure or calculate and record the following (in grams):
mass of an empty Erlenmeyer flask 2:
mass of Erlenmeyer flask 2 and dry NaCl (1st weighing)
mass of Erlenmeyer flask 2 and dry NaCl (2nd weighing)
mass of Erlenmeyer flask 2 and dry NaCl (3rd weighing) if necessary
measured mass of dry NaCl:

Assignment Questions (all work must be shown for all calculations):

Using the results of your lab, complete all the questions below.

Upload one picture that is representative of your experiment with this report to the appropriate D2L Brightspace Assignments folder.

Complete Data Table 1 located in the Procedure. Make sure to include all your work for all the calculations performed.

The amount of sodium chloride in the original mixture may also be determined indirectly by subtracting the SiO2 mass from the initial sample mass. Calculate the expected mass of NaCl using this method.

Calculate the percent difference of NaCl (Note: This is not % Error, but it is a similar calculation. This is an error calculation that checks precision between the results of the two methods from the following equation: (Note: the unit for this calculation is â% differenceâ)

% difference=measured mass-mass by subtractionmass by subtraction x 100

What is the mass percent of each component in your mixture: SiO2, and NaCl? (Note: the units for this calculation are â% SiO2â and â% NaClâ)

mass percent=mass of componenttotal mass of sample x 100

Error Analysis: Was your % difference from Assignment Question 4 within 10%? Why or why not? Did your two mass percentages in Assignment Question 5 add up to 100%? Why or why not? What were your biggest sources of error in this experiment? What would you do differently if you were to repeat this experiment? Explain.

What type of properties (physical or chemical) are we using to separate the mixture in this laboratory. Explain your answer.

A mixture is found to contain 0.69 g SiO2, 1.05 g of cellulose, and 2.17 g of calcium carbonate. What is the mass percentage of SiO2 in this mixture?

Assume you have a mixture of NH4Cl, sand, and salt and access to a regular chemistry laboratory. How would you separate the three?Write out what your procedure would be in numbered steps. Please research the answer to this question and properly cite your source. It is alright to research using the internet. However, Wikipedia, Yahoo Answers, Ask.com, and other non-scientific sources cannot be used.

CHEN 4501W Lecture Notes - Lecture 20: Inta, Bacteriophage, Heat Exchanger

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