Consider the equation shown. Cl2 aqueous plus 2Br− aqueous react
to form Br2 aqueous plus 2Cl− aqueous. (a) Which arrow shows
reduction? (b) Which arrow involves a loss of
electrons? And (c) which arrow involves the
gain of electrons?
The arrow in all three questions
refers to either A or B. In the equation, we can see the
chlorine molecule Cl2 reacting with the bromide ion Br−, forming bromine Br2, which
dissolves readily in water producing a memorable orange color, and chloride ions
Cl−. To make it easier, you can separate
the individual processes. So A refers to Cl2 transforming
into two Cl− and process B refers to two Br− transforming into Br2. Here, I’m ignoring the state
In process A, we see a molecule of
chlorine composed of two chlorine atoms, transforming into two chloride ions, each
with a charge of one minus. Therefore, this process reflects
the addition of two electrons, one to each of the chlorine atoms. And for B, we have two negatively
charged bromide ions, transforming into neutral bromine atoms in a bromine
molecule. Therefore, B corresponds to the
process of removing two electrons.
Question (a) refers to
reduction. Reduction is the process of gaining
electrons. This is easy to remember if we use
OIL RIG. Oxidation is loss; reduction is
gain of electrons. Therefore, the answer to part (a)
is arrow A. In arrow A, chlorine is being
transformed to chloride by the addition of electrons. This is reduction.
Part (b) asks us to indicate the
arrow that involves a loss of electrons. This is arrow B. In process B, we have two bromide
ions being transformed into bromine, losing two electrons. And finally, the answer to part (c)
is arrow A. In reaction A, chlorine is gaining
electrons to form chloride.