Video Transcript
In black-and-white photography,
light results in the decomposition of tiny amounts of silver bromide on the
photographic film. What is the chemical equation for
this reaction, including state symbols?
This question is asking us to
determine the chemical equation for the decomposition of silver bromide. To answer this question, we need to
know the chemical formula of silver bromide. We also need to determine the
chemical formulas and state symbols of the reaction products. First, we can determine the
chemical formula of silver bromide. Since it is a halogen, the bromine
element forms one minus ions. The silver element can form one
plus or two plus ions. But the one plus ion is much more
common. We cross the charge values of one
and drop the plus and minus signs. This gives the formula Ag1Br1. We can leave out subscripts of one
since they are implied.
Silver bromide has no overall
electrostatic charge since there is one positive silver one plus ion for every one
negative bromide one minus ion. We can deduce that silver bromide
is a solid since it is present on photographic film, which contains no liquids or
gases.
Now, let’s consider what the
decomposition products of silver bromide are. We know that silver bromide only
contains two elements, silver and bromine. Now, a decomposition reaction is a
reaction in which a compound is broken down into two or more simpler substances. So we can deduce that the most
likely product substances are silver and bromine. But remember, bromine exists in its
elemental form as diatomic Br2 molecules.
What are the physical states of
silver and bromine at room temperature? Elemental silver is a solid under
normal conditions and bromine is a liquid. So silver has the s state symbol,
and bromine has an l state symbol. We are asked for the chemical
equation. However, we must always balance
equations, as a balanced equation reflects the correct proportions of each species
taking part and being formed in a reaction. There is one bromine ion on the
left-hand side of the equation and two bromine atoms on the right-hand side. So, to balance the bromines, we put
a two in front of silver bromide. Then, we can balance the number of
silver ions on the left, which is now two, with the number of silver atoms on the
right. We place a two in front of the
silver metal. The equation is now balanced.
We have determined the chemical
equation for the decomposition of silver bromide. The equation is two AgBr solid
reacts in the presence of light to produce two Ag solid plus Br2 liquid.
