© Division of Chemical Educatio
n
â¢
www.JCE.DivCHED.or
g
⢠Vol.
86 No.
11 November
2009 â¢
Journal of Chemical Educatio
n
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
2
O
3
.
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 °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 Politécnica de Madrid, 28006 Madrid, Spain
Juan Antonio Llorens-Molina
E.T.S. del Medio Rural y EnologÃa, Universidad Politécnica de Valencia, 46010 Valencia, Spain
1278
Journal of Chemical Educatio
n
⢠Vol.
86 No.
11 November
2009 â¢
www.JCE.DivCHED.or
g
â¢
© Division of Chemical Educatio
n
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)
:
⢠âhookingâ
the student
to spark
interest
and
motivation
preferably within a real-life context
⢠requiring
reflection
as students
recognize
and explain
how
to proceed, discerning which information is relevant and
necessary at each stage of the solving process
⢠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
2
(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:
a)
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?
b)
Suggest a procedure by which the instructor was able to
determine the masses given.
c)
Because the dissolution of CaCl
2
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.
d)
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.
e)
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 °C.
To do so we need to know how much
salt and water are required in this
container.
