Question #59ff2
1 Answer
Bromine is being oxidized and manganese is being reduced.
Explanation:
In this redox reaction, hydrobromic acid,
You can see what's going on here by assigning oxidation numbers to the atoms that take part in the reaction
#stackrel(color(blue)(+1))("H")stackrel(color(blue)(-1))("Br")_ ((aq)) + stackrel(color(blue)(+4))("Mn")stackrel(color(blue)(-2))("O") _ (2(aq)) -> stackrel(color(blue)(+2))("Mn")stackrel(color(blue)(-1))("Br")_ ((aq)) + stackrel(color(blue)(+1))("H")_ 2stackrel(color(blue)(-2))("O")_ ((l)) + stackrel(color(blue)(0))("Br")_(2(l))#
The oxidation number of bromine changes from
On the other hand, manganese's oxidation number goes from
The oxidation half-reaction will look like this
#2stackrel(color(blue)(-1))("Br")""^(-) -> stackrel(color(blue)(0))("Br"_2) + 2"e"^(-)#
Here every atom of bromine loses one electron, which means that two atoms will lose a total of two electrons.
The reduction half-reaction looks like this
#stackrel(color(blue)(+4))("Mn")"O"_2 + 2"e"^(-) -> stackrel(color(blue)(+2))("Mn")""^(2+)#
Since you're in acidic solution, you can balance the atoms of oxygen by adding water to the side that needs it, and the atoms of hydrogen by adding protons,
Add two water molecules on the products' side to get two atoms of oxygen
#stackrel(color(blue)(+4))("Mn")"O"_2 + 2"e"^(-) -> stackrel(color(blue)(+2))("Mn")""^(2+) + 2"H"_2"O"#
Add four protons on the reactants' side to get four atoms of hydrogen
#4"H"^(+) + stackrel(color(blue)(+4))("Mn")"O"_2 + 2"e"^(-) -> stackrel(color(blue)(+2))("Mn")""^(2+) + 2"H"_2"O"#
Notice that the number of electrons lost in the oxidation half-reaction is equal to the number of electrons gained in the reduction half-reaction, which means that you can go ahead and add the two half-reactions to get
#{(color(white)(aaaaaaaaaaaa)2"Br"^(-) -> "Br"_2 + 2"e"^(-)), (4"H"^(+) + "MnO"_2 + 2"e"^(-) -> "Mn"^(2+) + 2"H"_2"O") :}#
#color(white)(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa)/color(white)(aaaaaaaaaaaaaaa)#
#4"H"^(+) + 2"Br"^(-) + color(red)(cancel(color(black)(2"e"^(-)))) -> "Mn"^(2+) + 2"H"_2"O" + color(red)(cancel(color(black)(2"e"^(-)))) + "Br"_2#
which is equivalent to
#4"H"^(+) + 2"Br"^(-) + "MnO"_2 -> "Mn"^(2+) + 2"H"_2"O" + "Br"_2#
The balanced chemical equation that describes this redox reaction will thus be
#color(green)(|bar(ul(color(white)(a/a)color(black)(4"HBr"_ ((aq)) + "MnO"_ (2(aq)) -> "MnBr"_ (2(aq)) + 2"H"_ 2"O"_ ((l)) + "Br"_ (2(l)))color(white)(a/a)|)))#
So, in this reaction, hydrobromic acid acts as a reducing agent and manganese(IV) oxide acts as an oxidizing agent. In other words, bromine is being oxidized and manganese is being reduced.