Video Transcript
The equation X− gas plus an
electron produces X2− gas shows the second electron affinity of an element. Will this process result in a
positive or a negative change in energy?
This question is about second
electron affinity. Second electron affinity is defined
as the energy released per mole when a one minus ion gains an electron to become a
two minus ion. To apply the general equation given
in the question to a specific situation, let’s consider the equation where an oxygen
one minus ion gains an electron to become an oxygen two minus ion. The second electron affinity of
oxygen is negative 844 kilojoules per mole. In other words, when we carry out
this reaction, negative 844 kilojoules of energy per mole is released.
This question is specifically
asking about the change in energy. Electron affinity is always
reported as the energy released when an electron is added to a species. By convention, we indicate that
energy is released from the system to the surroundings using a negative sign. Here, a negative amount of energy
is released. If a negative amount of energy is
released, we run into a double negative.
Another way to say this is that
energy is being absorbed. If energy is being absorbed, then
the energy level of the system is increasing. In other words, there’s a positive
energy change to the system.
To better understand the difference
between a positive and a negative energy change, let’s also take a look at the
formation of an O− ion. This equation where we add an
electron to a neutral oxygen atom represents the first electron affinity of
oxygen. The first electron affinity of
oxygen is positive 141 kilojoules per mole. Once again, this is the amount of
energy that is released, which we indicate with a negative sign. No double negative exists here, so
we can simply say that energy is being released. When energy is released from the
system, the amount of energy decreases. In other words, there’s a negative
energy change.
For the first electron affinity of
oxygen and many other elements, there’s a negative energy change. However, the second electron
affinity for oxygen, and in fact the second electron affinity for every other
element, involves a positive energy change.
What causes this difference? Well, for second electron affinity,
we’re combining two negatively charged particles, the ion and the electron. Since they both have a negative
charge, they repel one another. In order to overcome the repulsion,
energy needs to be added to the system in order to push these two particles
together. This results in a positive change
in energy.
In conclusion, the second electron
affinity of an element is a process that results in a positive change in energy.
