MadSci Network: Chemistry

Re: Why does calcium chloride become warm when added to water?

Date: Tue Oct 10 22:48:40 2000
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Chemistry
ID: 970491900.Ch

Greetings, Cindy:

In Chemistry, there are reactions, and there are interactions.  I'll say
right up front that the distinction is not often pointed out; frequently
the two words are considered synonymous.  Nevertheless, we CAN make a
distinction between them, if we wish, and that is what I shall attempt to
do here, as a preliminary to Answering your Question.

Let me start off with an example of an interaction:  Think about ordinary
table salt, sodium chloride, being added to water.  Part of what happens
during the dissolving of ANY solid substance is that molecules break off
and insert themselves into the spaces between the molecules of the
solvent.  However, since water is a "polar" solvent, and because salt
happens to be a strongly ionic compound, its molecules can also THEMSELVES
break apart, into sodium ions and chloride ions.

Now, it takes energy to break up the salt molecules like that.  If we were
to call this a "chemical reaction", we could then remember the well-known
fact that some reactions release energy, and some reactions absorb energy,
and see no problem.  However, we might wonder why, after this particular
"reaction" is done, and the salt is dissolved, do we still think about the
salt as being salt, and we still think about the water as being water?
Isn't a chemical reaction supposed to turn the reactants into different
substances?  So that is why I think it might be more appropriate to label
events such as this as "interactions".  It certainly wouldn't be wrong to
think of them as being a special sub-category of the generic idea of
"reactions".  This makes it easy to keep remembering that interactions,
also, may cause energy to be either released or absorbed.  In any case,
there is a gray area that can make it tough to decide whether an event is
a reaction or an interaction -- and we will be entering that gray area

The energy to accomplish the particular interaction of common table salt
dissolving in water comes from the heat in the water:  as salt dissolves,
the temperature of the water will go down.  This happens to be a modest
drop that you probably won't notice in casual circumstances.  However, if
you were ever involved in making ice cream at home, you might have had to
add quite a lot of salt to ice-water.  In this special circumstance, the
salt both lowers the freezing point of water AND makes it notably colder!
(Please recall that impure water almost always has a lower freezing point
than pure water, and, obviously, when salt is added to water, the water
becomes impure!)

In general, for interactions involving the simple dissolving of one
substance in another, there is a "heat of solution".  This is merely the
name given to the amount of heat energy that is either released or
absorbed, during the interaction.

Next, here is another, different type of interaction:  Think about pure
liquid water being frozen.  In this case all that is available to
interact are water molecules, with each other.  One thing to pay close
attention to, as the temperature of the water is dropped, is the amount of
heat energy that the water must give up along the way.  For a specified
amount of liquid water, and for each degree of temperature, until we reach
the freezing point, this amount of heat is constant.  But once the
freezing point is reached, one will notice that AS ice forms, the
temperature HOLDS STEADY, even as a great deal of heat is being removed
from the freezing water.

In this situation of water becoming ice, molecules are interacting with
each other in a way that removes them from the liquid state, and binds
them to the solid state.  A weak kind of bond, known as a "hydrogen bond",
comes into existence between every neighboring pair of water molecules.
It is not a true chemical bond, but it does lead to a reasonably sturdy
alignment of molecules.  This interaction, involving hydrogen-bond-
formation, releases heat!  And this is the source of the "great deal of
heat" mentioned above, that must be removed from the liquid water, before
it can entirely freeze.  Another word for "bind" is "fuse", and the formal
name given to all that heat being removed from the freezing water energy
is "heat of fusion".

For ANY substance that we cool from the liquid state to the solid state,
there is a heat of fusion.  Water is notable for having one of the
largest values of all, for its heat of fusion.  This is entirely due
to its hydrogen bonds; most other molecules have no equivalent.

As you might expect, when one takes solid ice and tries to melt it, that
same heat of fusion must be added back.  Even though the water in this
case is melting and not freezing, we STILL use the name "heat of fusion"
to describe the extra energy being added at the melting point, to cause
the ice to liquify -- to break many, but not all, of those hydrogen
bonds.  (I think "heat of liquification" is sometimes used, but do we
really need different names to describe the same amount of energy?)

Furthermore, when we take water and heat it all the way to the boiling
point, there is a "heat of vaporization" that must be added AT the boiling
point, to cause the liquid to turn into a gas.  Water is again notable for
having one of the largest values of all, for its heat of vaporization (all
the rest of the hydrogen bonds must be broken).  The name "heat of
vaporization" can also be used to refer to the heat that must be removed
from water vapor, to cause it to condense, although you may sometimes
encounter the name "heat of condensation".  (The heat released by
condensing water, as hydrogen bonds begin to form, is the power behind
hurricanes;  THAT'S how much heat we are talking about here!  Also,
because of the historical importance and widespread use of steam in
industry, this is why there are two common terms for this particular
amount of energy.)


It should be easy to see that the interactions as just described can be
very distinct from ordinary chemical reactions.  The term "reaction"
is most often in Chemistry restricted to events that affect molecular
composition.  Even so, there is a "gray area" in which some events may be
considered to be either an interaction or a reaction.  An easy example is
found in every bottle or can of soda-pop, where carbon dioxide is mixed
with water under pressure.  A chemical reaction is often described:

  CO2 + H2O -> H2CO3 or "carbonic acid"
(sorry I can't do subscripts here for the numbers).

Nevertheless, there is so little energy involved in this reaction that it
is easily reversed, and as soon as you open the soda-pop, the carbon
dioxide promptly begins bubbling out of the water.  If you let the soda
stand long enough, it becomes "flat" -- and this is because only a tiny
amount of that tangy carbonic acid remains in the water.

Now, consider for a moment the amount of energy it takes to make or break
EITHER carbon dioxide or water molecules.  Then wonder about just HOW the
water and carbon dioxide molecules actually link up to form carbonic acid;
it seems like a not-easy thing!  Perhaps their linking is just another
kind of hydrogen bond!


At last we begin to reach the topic of adding calcium chloride and
magnesium sulfate to water...  In these cases we have more of an 
interaction going on, than a reaction.  Furthermore, I can describe TWO
interactions going on at the same time!

First, there is a type of molecule called a "hydrate", which is a
combination of water with some other molecule.  HOWEVER, in any hydrate
the individual water molecules continue to be whole water molecules, while
the other molecule also remains whole.  Yes, in the combination-molecule
known as a hydrate, I am talking about hydrogen bonds for sure!  And so I
dare to say that "interaction" is a more accurate term than "reaction",
with respect to the formation of hydrates.

There are so many types of molecules that can form hydrates that I will
use the generic X here.  What any individual hydrate has is a definite
ARRANGEMENT of water molecules associated with Molecule X.  Sometimes
there are two Molecules X associated with each water; usually there is one
or more water molecules associated with each Molecule X.  Frequently
Molecule X can form more than one type of hydrate; each type consists of
a different number of water molecules associated with one Molecule X. thing that can happen when the pure substance of Molecules X is
added to water, is the linking of them with water molecules, hydrogen
bonds coming into existence as hydrates form.  A key part of this process
is the RATE at which it occurs.  If a lot of hydrogen bonds form at once,
a lot of heat will be released quickly.  But if a lot of hydrogen bonds
form slowly, then that same amount of heat will be released slowly, and
someone merely holding a test tube may not notice it.  And since for two
different substances X, such as calcium chloride and magnesium sulfate,
different number of water molecules will likely be involved as hydrates
form, it follows that different numbers of hydrogen bonds will form.  So
between the sheer numbers of hydrogen bonds, and the rate at which they
form, THIS is part of the Answer to your Question -- because both calcium
chloride and magnesium sulfate DO form hydrates!

And the other interaction that is going on?  It is the actual dissolving
of the calcium chloride or magnesium sulfate in the water.  Just like
table salt, the process absorbs energy.  How much energy depends on the
type of molecule:  In simplest terms we might say that the more bulky and
massive a molecule is, the more energy it takes to break one away from a
solid grouping of them.  If you compare the two molecules in question
here, you will find that calcium chloride consists of 3 atoms and has a
molecular weight of about 111 units, while magnesium sulfate consists of 6
atoms and has a molecular weight of about 120 units.  So it would figure
that it takes more energy to cause magnesium sulfate to dissolve, than it
does calcium chloride.

In the final analysis, there is the formation of hydrates, which releases
energy, and there is the dissolving of the substances, which absorbs
energy.  Each substance is different, and each interaction is associated
with different amounts of energy.  What you noticed in your science class
was the NET gain or loss of energy, as each substance was dissolved in
water.  Calcium chloride consumed less energy to dissolve, and released
more energy faster during hydrate formation, than did magnesium sulfate.

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