Which of the following is the
measure of the change in disorder or randomness of a system? A) Enthalpy change, B) entropy
change, C) standard enthalpy of formation, D) activation energy, or E) specific heat
In this question, we’re being asked
to identify a property that tells us about the change in disorder or randomness in a
system. So let’s take a look through our
answer choices to see which one best fits this description.
Mathematically, enthalpy is equal
to the internal energy plus the product of the pressure times volume. A change in enthalpy tells us how
much thermal energy was exchanged between the system and the surrounding environment
as the result of a process or chemical reaction. If energy was transferred to the
system from the surroundings, that indicates a positive change in enthalpy. And if energy was transferred to
the surroundings from the system, that indicates a negative change in enthalpy. Enthalpy doesn’t tell us about
changes in disorder or randomness. So this isn’t the answer that we’re
Entropy has to do with the number
of arrangements of particles in the system. If there are more ways to arrange
the particles in a system, that means that that system has a greater entropy.
For example, let’s compare a
substance that’s a solid to a substance that’s a gas. When the substance is a solid,
there’s more order to how the particles are arranged. After all, when the substance is a
solid, the particles have to be stuck together in order to be a solid. If the particles were arranged
randomly in the container, the substance wouldn’t be a solid anymore.
The opposite is true for a gas. In a gas, the particles can be
arranged in any way in the container. And the substance will still be a
gas. This means that a gas has more
arrangements of the particles. We could draw a few of these
arrangements for our system of seven gas particles. But there would be many, many
more. We could come up with more than one
way to arrange the particles of our solid. But because there’s more order in
the solid and the particles all have to be stuck together, there’s going to be more
ways to arrange the gas particles in general.
Because there’s more ways to
arrange the gas particles, a gas will usually have more entropy than a solid. Let’s say that we have a process
where a substance goes from a solid to a gas, such as the sublimation of CO₂. We could calculate the change in
entropy for this process. Because gases have more entropy
than solids do, we would expect this process to have a positive change in
Entropy isn’t technically a measure
of the change in disorder or randomness. It’s closer to a measurement of
probability. But changes in disorder or
randomness are certainly reflected by changes in entropy, as there’s more ways to be
disordered or random.
When the system was a more ordered
solid, there was fewer ways to arrange the particles. So that system had less
entropy. When the system was the more random
and disordered gas, there were more ways to arrange the particles. So that system had more
entropy. So it seems answer choice b, an
entropy change, is the answer that we’re looking for. But let’s look through our other
answer choices so we understand what they are.
The standard enthalpy of formation
is the energy that’s needed to form a substance from its constituent elements. For example, we could form carbon
dioxide from its constituent elements, carbon and oxygen. The standard enthalpy of formation
for carbon dioxide is negative 393.5 kilojoules per mole.
The activation energy is the energy
that’s needed to start a reaction. For example, let’s imagine that we
have a general reaction of the form A reacts to form B. The energy of our reactants is
higher than the energy of our products. So we expect this to be a
spontaneous reaction. But there’s still some amount of
energy that’s needed to start the reaction, to get over this hump in the
diagram. And that amount of energy is the
The specific heat capacity is the
energy that’s needed to increase the temperature of a substance per one gram of
substance per one degree Celsius. Different substances have different
specific heat capacities. For example, water has a very high
specific heat capacity, which means it takes a lot of energy to increase the
temperature of water. This is why on a hot day your pool
can be a relatively stable and comfortable temperature. But the surrounding concrete will
be hot enough to burn your feet.
In this question, we needed to
identify which was the measure of the change in disorder or randomness of a
system. This is the change in entropy
because entropy tells us the number of ways that we can arrange the particles in our
system. And there are more ways to arrange
particles in a disordered or random manner than there are in ordered one.