Thermodynamics is a part of science that explains why eggs
can't uncook themselves, why explosives are just waiting to explode, and why an
explosion could never go backwards. Why on Earth should a reaction go
backwards, you might ask. Well, some reactions do.
From a teacher’s perspective, a frustrating thing about reversible
reactions is that they are uncommon in everyday life. Reversible processes are very common, such as ice
melting into water and refreezing back into ice, but that is not a chemical
reaction. A reaction is when one or more substances are transformed into
different substances. For example, when you burn a candle, the wax reacts with
oxygen to produce carbon dioxide and water. This process is not reversible. You
cannot gather up all the carbon dioxide and water and cobble them back into
candle wax and oxygen molecules.
Carbon dioxide and water can be made to react together
though, and when they do the reaction is reversible. If you breathe carbon
dioxide into water for long enough, the pH will decrease. It becomes acidic as
carbonic acid is formed. You have may have heard that fizzy drinks like
Coca-Cola® are acidic. One reason for this is that they contain carbonic acid.
This is because of the carbon dioxide dissolved in the drink to make it fizzy,
which reacts with the water to make carbonic acid. The good news is that as the
drink goes flat, there will be less and less carbonic acid, because the
carbonic acid turns back into carbon dioxide and water. The reaction is
reversible.
The question now, is why can some reactions go backwards
when others can’t? This is where thermodynamics comes in. The laws of
thermodynamics state which reactions are possible and in which conditions.
Think of this, everything is made of chemicals. They are not just bubbling
potions made by mad scientists, nor harmful man-made ingredients added to food.
You are made of chemicals, the vitamins in the vegetables you eat are chemicals
and even your beloved mobile phone is made of chemicals. So, if everything is
made of chemicals, why don’t we see chemical reactions happening all the time? If
your mobile phone is made of chemicals, why doesn’t it react with the chemicals
that your trouser pocket or bag are made of? The reason is thermodynamics.
What does "thermodynamics" mean anyway? Dynamics
means how things move around in response to forces, for example, if we consider
how a football moves in response to forces, it will be briefly compressed when
we apply a kicking force from our leg before flying upwards. If it leaves the
ground, it will move upwards and away from the kicker. Gravity will immediately
slow down the speed of its upwards movement, eventually bringing it to a stop
in midair before pulling it down. During this time, it will still be moving
away from the kicker. Its speed in this direction will be slowed down by air
resistance and this will also cause the ball to get slightly warmer. This is
how various forces affect the movement of the ball and this is an example of
how we can consider the ball's dynamics. As for the "thermo" bit, you
probably recognise that already - it is a reference to heat. So thermodynamics
basically means how heat affects the dynamics of certain types of objects and
those objects are atoms and molecules.
There are three key factors that decide whether or not a
reaction will “go”. These are the temperature, the energy transfer and
something called entropy. Temperature is a fairly straight forward issue so we
will say no more about that for now. Instead, let’s consider energy transfer
and entropy.
Energy transfer actually relates quite neatly to something
called the first law of thermodynamics.
What this law states is that energy can never be created or destroyed. Instead,
one type of energy is transferred into another type of energy. Returning to the
candle, the candlewax is an example of a fuel, and the point of a fuel is to
release heat energy when it is burned. This heat comes from the reaction
between candlewax and oxygen. The reaction is called exothermic because it releases heat energy. However, there are
other examples of reactions that are endothermic,
meaning they absorb heat energy. Earlier we saw that the carbon dioxide and
water produced when a candle burns cannot be made to react back into candlewax.
However, they can be reacted together to make glucose, which is what happens
during photosynthesis. This natural process is very endothermic, meaning that a
lot of energy has to be provided to make the two compounds react. Plants get
this energy from the sun, but that energy is not enough on its own. The special
chemicals inside the plant also play a huge role. So, reactions can be
endothermic, meaning they take in heat energy, or they can be exothermic, meaning
they give out heat energy.
The second law of thermodynamics states that the universe
constantly gets less tidy. Put another way, the entropy of the universe always increases.
So, what's entropy? Entropy is a measure of disorder. What the second law is
saying is that disorder is always increasing overall in the universe. What do
we mean by ordered? Well, if you consider the particles in a solid, they are
arranged in nice, neat, regular rows. They are ordered, whereas in a gas the
particles are zinging around every which way and there is very little order at
all. It's like the difference between soldiers standing on parade and a group
of children playing in a park. The soldiers will be in neat, orderly rows
whereas the children will be running around in whichever direction takes their
fancy at any particular moment.
When it comes to chemical reactions, the second law means
that a reaction that produces a very ordered substance, such as a perfect
crystal, is less likely to happen than a reaction that produces disordered
substances, like gases. This turns out to be a statistical inevitability. Imagine
you make a glass of orange squash. You add the thick squash to the bottom of
the glass then fill it with water to the top. The squash molecules immediately
mix up with the water molecules. Now, what are the odds that they would
separate themselves into two layers of squash and water? First of all, if a
glass of squash is left for a long time, some of the squash molecules can sink
to the bottom, but this is not two layers. The top of the squash will still be
orange, just a paler orange than the bottom of the glass. That’s not what I
mean. I mean a layer of pure orange squash and a layer on top of it of pure
water.
Why on Earth should the two layers separate? Well, in
theory, they could. A glass of orange squash might look very still but
actually, all of the molecules are constantly moving around, from the top to the
bottom and from left to right. They are moving all over the place. So, there is
a statistical chance that all of the orange squash molecules would all randomly
move to the bottom of the glass at the same time. However, this chance is so
fabulously small, that it will basically never happen. What is infinitely more
likely to happen, and which is what we see happen, is that all of the squash
and water molecules will stay mixed up. Shuffling cards is the same. No matter
how long you shuffled a deck of cards for, the chance that you would randomly
shuffle it into perfect numerical and suit order is so astonishingly low that
it will not happen.
Chemical reactions are just the same as the squash and the cards.
A reaction is much less likely to take place if the products are more ordered
than the reactants. There are exceptions to this, and this is where we tie the
different thermodynamic components together.
In order to accurately predict whether a reaction is
possible, you need to consider the temperature, the energy transfer and the
entropy. An exothermic reaction – one that gives off heat – that also leads to
an increase in disorder can definitely happen, provided the temperature is
suitable. A burning candle is an example: it gives off heat and the ordered,
solid particles turn into disordered, gaseous particles. On the other hand, an
endothermic reaction that leads to a decrease in disorder can basically never
happen (except in special circumstances). This is why diamonds are so rare.
They are produced from graphite but because the process is endothermic – takes in
energy – and leads to a decrease in disorder, it is incredibly difficult to get
it to happen. (Of course diamonds exist so it must be possible to make them. It
is done by increasing the pressure.) Finally, what if the reaction gives off
energy but leads to more order? Or vice versa? These are the reversible
reactions, and it is how they get to be reversible. One factor will favour the
forwards reaction while the other factor favours the reverse, which is why the reaction
ends up going forwards and backwards at the same time. The role that
temperature plays in all this is complicated but all reactions have a cut off
temperature. Some reactions can’t go above the cut off point, while others can’t
go below it.
So, why can’t an explosion go backwards? First of all,
explosions are always exothermic reactions, so for the products to turn back
into reactants would be an endothermic reaction. If the products did react back
into reactants, they would have to take in heat. But, an explosion also causes
an increase in disorder. Usually neatly packed solid particles turn into
disordered gas particles. So for the reaction to go backwards, there would have
to be a decrease in disorder. You can have a decrease in disorder, or a
reaction that takes in energy, but you can’t have both at the same time. And
that’s how thermodynamics shows that an explosion can’t go backwards.
