Dissolving calcium salts
R. Raadsen & J.D.A. Lieshout
Het 4e Gymnasium Amsterdam
Received May 2011
Summary
This article describes the determination of the highest temperature increase dissolving
three different calcium salts. To determine the temperature increase, a simple
thermometer is used. The dissolution of calcium chloride proved to be most exothermic,
for its temperature increase was the least negative. This was also the salt having the
most considerable molar mass. From this questions arise: does a calcium salt with an
even bigger molar mass increase the temperature ever further? What is the relation
between molar mass and temperature increase and decrease?
Introduction
Reactions can be either endothermic or
exothermic.
These two sorts of
chemical reactions are basically
opposites. An endothermic reaction
needs warmth to happen, so it absorbs
warmth from the surrounding area. This
results in a temperature decrease.
There are three main endothermic
processes:
 Phase changes. For Example:
H2O (s)
H2O (g)
 Dissolution processes. For Example:
NaCl (s) + water (l)
Na+ (aq) + Cl- (aq)
 Chemical reactions which result in
the production of new substances,
like the reaction between hydrated
 barium hydroxide and ammonium
chloride (l).
These three processes are better
explained
as:
vaporizations,
dissolutions, and reactions in which
new substances are formed.
Vaporizations are the phase changes in
which a gas is formed from either a
solid or liquid reactant. Dissolutions are,
as the name says,
reactions in which a salt dissolute into
water, where the ions of which the salt
is made, separate and associate with
water.
Exothermic reactions, on the other
hand, increase the temperature of the
surrounding area, because it releases
energy in the form of light and/or heat.
In a chemical equation this is expressed
as:
reactants
products + energy
This means that the energy released is
more than required to start the reaction.
The most common form of an
exothermic reaction is combustion or
burning. Although the combustion
reactions do require a starting energy
(like a burning match), the energy
released from the reaction (for example
burning gasoline) is far greater.
However, there are more kinds of
exothermic reactions:
 Adding concentrated acid to
water
 Neutralization
reactions
(reactions of acid and base)
 Corrosion reactions
 Polymerization reactions
All of these reactions release more
energy than they require to commence.
But there are also exothermic
dissolution reactions.
Whereas dissolution of ammonium salts
result in a cooling effect, dissolution of
calcium salts result in a heating effect.
Knowing this about dissolution of
calcium salts, the question arises: does
the size/weight of the second ion in an
calcium salt have an effect on the extent
to which an exothermic reaction occurs ?
As noticeable in the figure below, the
molar mass of calcium chloride is more
considerable than the molar mass of
calcium nitrate.
-
Calcium
chloride
Molar mass 110,982
(g)
Calcium
nitrate
102,092
Table 1: the molar masses of calcium
chloride and calcium nitrate
Experimental procedure
Besides the use of calcium chloride and
calcium nitrate, another salt using a
second monovalent ion is used too:
calcium acetate.
First, three different measures are filled
with 0,3 liters of water and 0,1 mol of
each salt is weighted and pulverized, so
that the main structure of each salt is
the same.
Used
masses
Calcium Calcium Calcium
chloride nitrate acetate
11,1
10,2
9,91
Table 2: the masses of the three salts
being used
Then, two cubic centimeters of ice are
added to each measure and each
amount of a salt is added to the
measures too
and stirred, until all the salt is dissolved
into the water. All this happens at the
very same time.
From this moment the temperature of
the water in each measure is measured
every minute by using a lab
thermometer. After ten minutes the
whole measurement is stopped and
repeated another two times.
The chloride ion, as well as the nitrate
ion are monovalent. This means they
are charged by one negative value.
Consequently, two of both ions are
needed to make both salts neutral:
calcium chloride is noted down as
Ca2+(Cl-)2 and calcium nitrate as
Ca2+(NO3)-2.
Too, calcium chloride is more
exothermic1. This makes us presume the
truth of the following hypothesis: the
heavier the second part of the salt, the
more exothermic the reaction.
Figure 1: the test’s set-up
25
Data analysis
20
The measurements of the tests are
averaged and viewed graphically. On
top of that, the average increase or
decrease in temperature is shown in
order to make clear the differences
between the exothermic effects of the
salts.
15
Furthermore, another conspicuous
issue is the ratio between the
temperature
decreases
and
the
differences in molar mass.
The
difference between
the molar mass of calcium nitrate and
calcium acetate in proportion to the
difference between the molar mass of
calcium chloride and calcium acetate is
nearly the same as the same proportion
using average temperature decreases
instead of molar masses:
((102,09) – (99,12)) / ((110,98) –
(99,12))
= 0,2504
(6,5 – 5,0)/ (6,5 – 1,0) = 0,2727
Striking moreover, is the recur of our
hypothesis: calcium chloride has the
lowest temperature increase, while
having the most substantial molar mass.
Either the other two salts correspond
with the hypothesis in behavior.
Calcium
nitrate
10
5
Calcium
acetate
0
0 1 2 3 4 5 6 7 8 9 1011
Graph 1: the average measurement (X-axis =
time expressed in minutes, Y-axis = temperature
of the water in degrees (°C).
Results
As noticeable, the temperature first
decreases in general. At the end of the
testing procedure, all the temperatures
are increasing instead of decreasing,
which is caused by the salt and the
temperature of the environment (which
is considerably higher than the
temperature of the water including the
ice cube).
Calcium
chloride
Calcium
chloride
T (°C)
Calcium
acetate
Calcium
nitrate
1,0
5,0
Table 3: the average decrease of the
water’s temperature (during 11
minutes)
Conclusion and discussion
According to the last mentioned issue
about proportion, the mass of a salt’s
second ion could be direct proportional
to the extent to which an exothermic
reaction occurs. This should mean that
the presumed hypothesis is true.
However, the following issues obstruct
the acknowledge of the hypothesis:
1. The accuracy of our
measurements. For instance, a
thermometer was used that
could only measure wholes and
halves of a degree. This could
cause an error in our results and
it could lead to a wrong
conclusion. One measure could
be just stirred a tiny bit bettter,
which could influence the
process of solution.
2. Other factors, such as a salt’s
structure. Notwithstanding the
inaccuracy of our measurements
and in spite of other influences
from the environment the
course of a reaction is caused by
the salts structure. As noticeable
in the figures below, calcium
6,5
chloride as well as calcium
nitrate and calcium acetate
differ in the way the second ions
are built up.
We have investigated the relation
between the second (negative) ion of a
calcium salt and the increase of the
environmental temperature. Although
we are close to an evidence of what was
meant to be proved, we cannot conclude
the hypothesis.
In order to make a valid hypothesis, the
measurements need to be more
accurate. At second, more salts should
be investigated and tested in the same
way. In that case, patterns in, for
example, proportion, could be better
provable. Also, it’s better to use ions
that are built up from a single atom,
since then there is no doubt about the
salt’s structure: the only changing
factors are the masses of the protons
and neutrons in the atom core. In
addition, electrons in the shell could
have an effect either.
Bibliography
1. Dissolving Salts in Water,
internet
source
(http://www.teachersdomain
.org/resource/lsps07.sci.phys
.matter.dissolvesalt/)
2. G.
Verkerk,
J.Kranendonk,
Binas, the student handbook
for chemistry