Video Transcript
Consider the following reaction:
Cl2 aqueous and two KI aqueous react to form I2 aqueous and two KCl aqueous. Which of the following statements
is correct? (A) Iodide ions are oxidized and
gain electrons. (B) Iodide ions are oxidized and
lose electrons. (C) Chlorine is reduced and loses
electrons. (D) Potassium ions are oxidized and
lose electrons. (E) Potassium iodide is oxidized
and gains electrons.
In this question, we need to
identify which statement correctly describes the reaction of chlorine and potassium
iodide. Let’s first define some of the
terms found in the answer options. When a chemical species is
oxidized, we say it undergoes oxidation. Oxidation is a type of chemical
reaction where a chemical species loses electrons. When a chemical species is reduced,
we say it undergoes reduction. Reduction is a type of reaction
where a chemical species gains electrons. We can remember these terms by the
acronym OIL RIG. Oxidation is the loss of electrons,
and reduction is the gain of electrons.
To get some practice, we will first
look at an example of a reaction that undergoes oxidation and reduction, and then we
will look at the equation given to us in the question.
In this example, solid calcium
metal reacts with gaseous diatomic oxygen molecules. Solid calcium oxide is formed. Both reactants are in their
elemental form. They are neither oxidized nor
reduced. They have not lost nor gained
electrons. The calcium oxide product consists
of two ions. It consists of the positively
charged calcium two plus ion. Positive ions are also called
cations. And it consists of the negatively
charged oxide two minus ion. A negatively charged ion is called
an anion.
When calcium metal atoms are
converted to calcium two plus cations, each atom loses two electrons. This is oxidation. When oxygen atoms are transformed
into oxide two minus anions, they each gain two electrons. This is reduction.
Now that we have practiced
identifying substances which are oxidized and reduced, let’s have a look at the
equation in the question.
Here is the chemical equation. Let’s show the charges for the
substances which consist of ions. Potassium iodide consists of K+
cations and I− anions. And potassium chloride consists of
K+ cations and Cl− anions. We can see that each atom of
elemental chlorine, which is neither oxidized nor reduced, gains one electron and so
is reduced. Notice that potassium ions are
unchanged. They do not gain electrons, nor do
they lose electrons. They remain as K+ cations. And each iodide ion, I−, in the
reactants loses one electron as it is transformed to elemental iodine, which is not
charged. Elemental iodine is neutral. So iodide ions are oxidized.
We can see that the only answer
option which correctly describes the processes shown in the equation is answer
(B). Iodide ions are oxidized and lose
electrons.
