Video Transcript
A halogen displacement reaction is
shown in the following equation: X2 plus 2Br− aqueous makes Br2 gas plus 2X−
aqueous. What is the color change that
occurs when the bromide solution is converted to bromine gas? What halogen could X2 be? (A) Colorless to purple, chlorine;
(B) colorless to brown, iodine; (C) colorless to brown, chlorine; (D) brown to
colorless, iodine; and (E) brown to colorless, chlorine.
In this question, we see that we
are starting with a solution of bromide ions. Bromide ions are halide ions. And all halide ion solutions are
colorless unless there’s a positively charged cation present that gives the solution
a color. So, the bromide ions solution
starts off as a colorless solution. In the reaction, bromine gas is
being displaced from bromide ions and another halide ion X− is being formed. The bromine gas displaced will have
an orange-to-brown color, and the X− ions will be colorless.
So, the color change expected would
be colorless to orange or brown. We can, therefore, eliminate
responses (D), (E), and (A) as these do not contain the correct color changes
expected. The halogen or X2 that displaced
the bromine must be more reactive than the bromine itself. This is the halogen that was added
to the bromide ions in the reaction at the start. The only halogen commonly found in
the lab that is more reactive than bromine is chlorine. Response (B) suggests that iodine
was used to displace the bromine, and this will not work as it’s less reactive than
bromine. We can, therefore, reject response
(B). The only response remaining is (C),
which describes the correct color change and suggests chlorine a halogen that is
more reactive than bromine as the displacing agents. This is the correct answer.
