Redox 2 Flashcards
what are decomposition redox reaction
Decomposition reactions are the opposite
of combination reactions. Precisely, a
decomposition reaction leads to the breakdown
of a compound into two or more components
at least one of which must be in the elemental
state.
what are combination redox reactions
A combination reaction may be denoted in
the manner:
A + B → C
Either A and B or both A and B must be in
the elemental form for such a reaction to be
a redox reaction. All combustion reactions,
which make use of elemental dioxygen, as well
as other reactions involving elements other
than dioxygen, are redox reactions.
what is a spectator ion
it is an ion that is present on the reactants and products side with the same oxidation state.
It may carefully be noted that there is no
change in the oxidation number of hydrogen
in methane under combination reactions
and that of potassium in potassium chlorate
in reaction (7.28). T
are all decomposition reacns redox reacns
all decomposition reactions are not
redox reactions. For example, decomposition
of calcium carbonate is not a redox reaction.
CaCO3 (s) —> CaO(s) + CO2(g)
what are displacement redox reacntins
In a displacement reaction, an ion (or an
atom) in a compound is replaced by an ion
(or an atom) of another element. It may be
denoted as:
X + YZ → XZ + Y
Displacement reactions fit into two
categories: metal displacement and non-metal
displacement.
what is metal displacement
A metal in a
compound can be displaced by another metal
in the uncombined state.
Metal displacement
reactions find many applications in
metallurgical processes in which pure metals
are obtained from their compounds in ores.
CuSO4(aq) + Zn (s) → Cu(s) + ZnSO4 (aq)
V2O5 (s) + 5Ca (s) 2V (s) + 5CaO (s)
TiCl4 (l) + 2Mg (s) Ti (s) + 2 MgCl2 (s)
Cr2O3 (s) + 2 Al (s) Al2O3 (s) + 2Cr(s)
the reducing metal is a
better reducing agent than the one that is
being reduced which evidently shows more
capability to lose electrons as compared to
the one that is reduced.
what is non metal displacement
The nonmetal displacement redox reactions include
hydrogen displacement and a rarely occurring
reaction involving oxygen displacement.
All alkali metals and some alkaline earth
metals (Ca, Sr, and Ba) which are very good
reductants, will displace hydrogen from cold
water.
Less active metals such as magnesium and
iron react with steam to produce dihydrogen gas.
Many metals, including those which do not
react with cold water, are capable of displacing
hydrogen from acids. Dihydrogen from acids
may even be produced by such metals which
do not react with steam. Cadmium and tin are
the examples of such metals( fe gives FeCl3)
uses of non metal displacement
Reactions (7.37 to 7.39) are used to
prepare dihydrogen gas in the laboratory.
Here, the reactivity of metals is reflected in
the rate of hydrogen gas evolution, which is
the slowest for the least active metal Fe, and
the fastest for the most reactive metal, Mg.
Very less active metals, which may occur in
the native state such as silver (Ag), and gold
(Au) do not react even with hydrochloric acid.
what is the reactivity series of halogens
activity series also exists
for the halogens. The power of these elements
as oxidising agents decreases as we move
down from fluorine to iodine in group 17 of the
periodic table. This implies that fluorine is so
reactive that it can replace chloride, bromide
and iodide ions in solution. In fact, fluorine is
so reactive that it attacks water and displaces
the oxygen of water :
+1 –2 0 +1 –1 0
2H2O (l) + 2F2 (g) → 4HF(aq) + O2(g)
explain the mechanism behind layer test
It is for this reason that the displacement
reactions of chlorine, bromine and iodine
using fluorine are not generally carried out in
aqueous solution. On the other hand, chlorine
can displace bromide and iodide ions in an
aqueous solution as shown below:
Cl2 (g) + 2KBr (aq) → 2 KCl (aq) + Br2 (l)
Cl2 (g) + 2KI (aq) → 2 KCl (aq) + I2 (s)
As Br2 and I2 are coloured and dissolve in CCl4,
can easily be identified from the colour of the
solution. The above reactions can be written
in ionic form as:
Cl2 (g) + 2Br–(aq) → 2Cl– (aq) + Br2 (l) (7.41a)
Cl2 (g) + 2I–(aq) → 2Cl–(aq) + I2 (s) (7.42b)
Reactions (7.41) and (7.42) form the basis
of identifying Br–
and I–
in the laboratory
through the test popularly known as ‘Layer
Test’. It may not be out of place to mention
here that bromine likewise can displace iodide
ion in solution
how to recover halogen from halide
The recovery
of halogens from their halides requires an
oxidation process, which is represented by:
2X– → X2 + 2e–
(7.44)
here X denotes a halogen element. Whereas
chemical means are available to oxidise Cl–
,
Br– and I–
, as fluorine is the strongest oxidising agent; there is no way to convert F–
ions to F2
by chemical means. The only way to achieve
F2 from F–
is to oxidise electrolytically,
what are disproportionation reaction
In a disproportionation
reaction an element in one oxidation state
is simultaneously oxidised and reduced.
One of the reacting substances in a
disproportionation reaction always contains
an element that can exist in at least three
oxidation states. The element in the form
of reacting substance is in the intermediate
oxidation state; and both higher and lower
oxidation states of that element are formed in
the reaction
2H2O2 (aq) → 2H2O(l) + O2(g)
explain mechanism of bleaching powder
Cl2 (g) + 2 OH– (aq) → ClO– (aq) + Cl–(aq) +H2O (l)
The reaction (7.48) describes the
formation of household bleaching agents.
The hypochlorite ion (ClO–
) formed in the
reaction oxidises the colour-bearing stains
of the substances to colourless compounds.
can flourine disproportionate
The reaction that
takes place in the case of fluorine is as follows:
2 F2(g) + 2OH–(aq) → 2 F–(aq) + OF2(g) + H2O(l)
(It is to be noted with care that fluorine in
reaction (7.49) will undoubtedly attack water
to produce some oxygen also). This departure
shown by fluorine is not surprising for us as
we know the limitation of fluorine that, being
the most electronegative element, it cannot
exhibit any positive oxidation state. This
means that among halogens, fluorine does not
show a disproportionation tendency
