Ask A Scientist Chemistry Archive

name         Asma
status       student
age          18

Question -   why is it essential for the formation of ion and ionic
bond that the sum of the energies
released in the second and third steps must be bigger then what is
required in the first step
example of NaCl
steps
1) sodium has one valence electron. the loss of this valence electron
requires 495KJ/mol of energy

2) chlorine has seven valence electrons.it needs one more electron to
complete its octet. chloring gaisn this electron and release 348
KJ/mol energy.

3) the third step involves the formation of crystal lattice and the
released enrgy is equal to
788 KJ/mol.
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HERE GOES!! SIMPLIFIED : For a chemical reaction to occur, there is a
quantity called the "free energy" delta G (the "delta means the change in or
difference) which must be negative for a reaction to occur. This term is
made up of two quantities: delta H, and delta S in the following way:
         delta G = delta H - T* delta S
T is the absolute temperature in kelvins.

The delta H term is the effect of heat absorbed or given off by the
reaction. This is the reason that most spontaneous reactions (consider
combustion) give off heat.
The delta S term measures the randomness of the reaction. Any reaction that
increases randomness tends to occur spontaneously also.

The reaction that you describe has a large exothermic component due to the
formation of the crystal, and hence occurs spontaneously.

You can find clear and readable details in the book "General Chemistry" by
Linus Pauling. He is a pretty good authority.

Vince Calder
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The deciding quantity in whether a chemical reaction will proceed
spontaneously is the entropy.  The total entropy change associated with the
combined steps of the process must be positive.  However, it is usually more
convenient to think in terms of quantities that depend only on the reacting
system, rather than the surrounding environment as well.  That is where the
concept "free energy" comes from, which I presume you are using.  The way
free energy is derived, if the change in free energy of the system
associated with some process is negative, the entropy of the system +
surroundings for the same process is positive.  This means that the process
will occur spontaneously.

So the answer to your question is that the first step of the process
described above is NOT spontaneous because its free energy change is
positive.  With the second and third steps included, however, the total
process is associated with a negative free energy change (energy goes down),
which is spontaneous.  It is like putting two people on one end of a seesaw
and one on the other - the weight of the two going down will pull the one on
the other side up, against the force of gravity.

Richard E. Barrans Jr., Ph.D.
Assistant Director, PG Research Foundation
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