Video Transcript
The photochemical decomposition reaction of silver bromide is also an example of a
redox reaction. Which ions will be reduced and which ions will be oxidized during the reaction?
This question is asking us to determine which ions will be reduced and oxidized
during the photochemical decomposition of silver bromide. Let’s begin by writing a balanced chemical equation for this decomposition
reaction. First of all, the reactant of this reaction is the ionic compound silver bromide.
The charge of a silver ion is typically one plus in most compounds, and the charge of
a bromide ion is one minus. The silver bromide compound must have one silver ion per one bromide ion for the
compound to have no net charge. Therefore, its chemical formula is AgBr.
Now, we need to determine what substances form when silver bromide decomposes. We know that silver bromide is composed of only two elements: silver and bromine. So the products should be solid elemental silver and diatomic bromine, which is a
liquid at room temperature.
Now, we’re ready to balance the chemical equation. We notice that there is one bromide ion in the reactant silver bromide and two
bromine atoms in diatomic bromine. Therefore, we need to place a stoichiometric coefficient of two in front of silver
bromide. On the reactant side, there are now two silver ions. And on the product side, there is one silver atom. Therefore, if we place a coefficient of two in front of silver, we will have a
balanced chemical equation.
Now that we have a balanced chemical equation, let’s discuss what a redox reaction
is. A redox reaction is a chemical reaction that involves the transfer of electrons
between chemical species. To help us keep track of the transfer of electrons in our reaction, let’s identify
the oxidation number of all chemical species. The oxidation numbers of the silver and bromide ions in silver bromide are equal to
their ionic charges. So silver ions have an oxidation number of plus one, and bromide ions have an
oxidation number of minus one.
On the product side of the equation, silver and bromine are in their elemental
form. This means they both have an oxidation number of zero. We can see that the oxidation number of silver decreased from positive one to
zero. During the reaction, each silver ion gained one electron. On the other hand, the oxidation number of bromine increased from negative one to
zero. During the reaction, each bromide ion loses one electron.
Reduction is a process in which a chemical species gains electrons during a chemical
reaction. So, silver ions undergo reduction during the reaction. Another way to say this is that Ag+ ions will be reduced during the reaction.
Oxidation is a process in which a chemical species loses electrons during a chemical
reaction. So, bromide ions undergo oxidation during the reaction. Another way to say this is that Br− ions will be oxidized during the reaction.
In conclusion, during the photochemical decomposition of silver bromide, Ag+ ions
will be reduced and Br− ions will be oxidized.
