colourless? was shown the colour change
showed that Mn2+ was formed. 5HNO2(aq) + 2MnO42-(aq)
+16H+(aq)®
5NO3-(aq) + 15H+(aq) +2Mn2+(aq) +
4H2O(l) ??????????????????????? (purple)??????????????????????????????????????????????????????????? (colourless)HNO2 oxidised to NO3- Mn7+ reduced to Mn2+ (oxidation
no. decrease ?5)The Na2Cr2O7
reaction showed that Cr3+ was formed, the colour change from
orange to green was observed. HNO2(aq)
+Cr2O72-(aq) +14H+(aq) +H2O(l)® 2Cr3+(aq)
+NO3-(aq) +3H+(aq)+7H2O(l) ??????????? ?????? (orange)??????????????????????????????????????? (green)HNO2
oxidised to NO3- Cr6+
reduced to Cr3+ (oxidation no.decrease ?3)??????????? In all the above equations, Nitrous
acid was oxidised to nitrate ion. The
oxidation number of nitrogen in HNO2 (+3) increased to (+5) in NO3-. HNO2? is a reducing agent.??????????? Related studies as to why the redox
reactions of Nitrous acid occur could be viewed by the entropy changes in the
reactions. The study by the American scientist Willard Gibbs reviews that the
relationship between the total entropy of a chemical reaction and the electromotive
force of the corresponding cell is??????????????????????? ∆S total
= -zFEcell ??????????????????????????????????? ?????????? T As
chemists are only concerned with the reaction inside the test tube excluding
the surroundings the Gibbs free energy?
∆G was used and expressed as??????????????????????? ?∆G = -T∆S total10For a
change, in a reaction to have taken place of its own accord, For a spontaneous
change ?∆S
must be positive. It therefore follows that for a spontaneous
change? ∆Gmust be negative. A
reaction is usually described as going to completion if kc =1010 or
greater ; this corresponds to a value of ∆G of
about ? 60kJmol-1 or a greater negative value.? ∆G = ∆G
products? – ∆G
reactantsIn the
Reduction reaction Bromine
water HNO2(aq)
+Br2(l) +H2O(l) ®
NO3-(aq) +3H+(aq) +2Br-(aq) (-37.2)?????????
0.0?? (-237.2)???? ?
(111.3)?????????? 0.0?????? ?
(-104.0)?∆G = [ 2(-104.0) ? (113.0)] ?
[(-237.2)-(37.2)] ? ? = -44.9 kJmol-1?∆S total =-44.9 x 1000? = +150Jmol-1k-1? , thus the reaction is feasible. ??????????? ????
-298Pottasuim
maganate(VII) 5HNO2(aq)
+ 2MnO42-(aq) + 16H+(aq) + 5H2O(l)® 5NO3-(aq)
+ 2Mn2+(aq) +15H+(aq)+8H2O(l) (-37.2)???????????? (-447.2)?????????????????????? ? 0.0???? ?
(-237.2)???????? (-111.3)??????? (-228.0)? ????? 0.0???????
(-237.2)? ∆G? = [5(-111.3) +2(-228.0) +8(-237.2)] ? [5(-37.2) + 2(-447.2) +
8(-237.2)]?? = -643.6 kJmol-1?? ∆S total = -643.6 x 1000
= +2159 Jmol-1k-1 , thus the reaction is feasible. ??????????????????????? -298Sodium
dichromate(VI) HNO2(aq)
+ Cr2O72- (aq) +14H+(aq) +H2O(l)
®2Cr3+(aq)
+NO3-(aq) +3H+(aq) +7H2O(l) (-37.2)????
(-1301.2)??? ????? 0.0????? (-237.2)???? ?? (-204.9)?????? (-111.3)?????? 0.0??????????? ????????? (-237.2)? ∆G = [7(-237.2) ? 111.3 +
2(-204.9)] ? [-37.2 ? 1301.2 ? 237.2]?= -605.5 kJmol-1? ∆S total = -605.5 x 1000
= +2031.9 Jmol-1k-1 , thus the reaction is feasible. ??????????????????????? -29811 In the
Oxidation reactionPottasuim
iodide HNO2(aq)
+ 2H+(aq) + 2I-(aq)®I2(s)
+ 2NO(g) +2H2O(l) (-37.2)??????????? 0.0?????? (-51.6)????? 0.0?????? (86.6)?????? (-237.2)?∆G = [2(-237.2) =2(86.6)] ?
[2(-51.6) + 2(-37.2)]?= -123.6 kJmol-1?∆S total = -123.6 x 1000
= +414.7 Jmol-1K-1, thus the reaction is feasible. ??????????????????????? -298Iron (II)sulphate HNO2(aq) + Fe2+(aq) + H+(aq)
® Fe3+(aq)
+NO(g) +H2O(l) (-37.2)???????????? (-78.9)???? 0.0?????????????? (-4.6)?? ???
(86.6)???? (-237.2)?∆G =
[86.6 ? 4.6 ? 237.2] ? [-78.9 ? 37.2]= -39.1 kJmol-1? ∆S total
= -39.1 x 1000 = +131.2 Jmol-1k-1 , thus the
reaction is feasible. ??????????????????????? -298?The
Sulphur dioxide data was unavailable.The pH of 2M Nitrous acidKa = [H+(aq)]eq[NO2-(aq)]eq???????????? Ka value = 4.7 x10-4 ??????????? ??? [HNO2(aq)]eqSince very little HNO2? dissociates [HNO2(aq)]aq = [HNO2(aq)]initialKc = ?????????????????
[ H+(aq)]eq x [H+(aq)]?? ??????????? ???????? ??{HNO2(aq)]eq??????? 4.7
x10-4 = [H+(aq)]2 ??????????????????????? ??? 2.0 ?????????? Ö9.4 x10-4
= [H+(aq)], ??????????? 3.0
x 10-2 = [H+(aq)]. pH = -log10[3.0 x 10-2] ??? = 1.5 12 Nitrous acid is a weak acid and will only ionize
little in water. The Kc value is therefore small due to incomplete ionization.
The? ∆G, E and ∆S total
values have demonstrated that the reactions were feasible.EVALUATION: ?The
technique and procedures applied to the test was of good measures, as most of
the predictions were observed. To raise the confidence levels in the results if
this investigation was to be repeated a larger range of reactants would be
used. As with the acidic properties, a buffer solution of Nitrous acid and
sodium nitrite could be set up to observe the buffering effect of the acid, and
its reactions with oxides to look for a pattern. In the redox reactions,
vanadium compounds could be used as the variable oxidation numbers of vanadium
are easily distinguished in simple reactions by the colours produced by its
ions.13 Bibliography ??????????? ??????????? Relevant
references were obtained from the following sources.Nuffield Advanced Science Chemistry student book (third edition)????????????????????????????????????????????????????????????????????????? (longman
publishers)Chemistry in context (third edition)????????????????????????? P.W.
Atkins ??????????????????????????????????????????????? M.J.
Glugston ??????????????????????????????????????????????? M.J.
Fazer ??????????????????????????????????????????????? RAY
Jones. ??????????????????????????????????????????????????????????????????????? Advanced Chemistry (first edition) ??????????????????????????????????????????????? Michael
Clugston ??????????????????????????????????????????????? Rosalind
Flemming. ??????????????????????????????????????????????????????????????????????????????????????????????? (Oxford
Publishers)14Chemical
Equipment?s and lists. Equipments?? ??????????????????????????????????????????????????????????????????????? Conical Flask Spatula Ice bath Thermometer
Mass balance PipetteSolutions and Materials Distilled water Iron(II)sulphate Pottasuim iodide Pottasuim manganate Sodium dichromate Magnesium Solid Hydrochloric acid Sodium nitrite Sodium hydroxide Sodium carbonate Sulphur dioxide Bromine water? ?????????????????????????????????????????????????????????? ? Indicators ?Universal
indicator Starch Silver nitrite Barium Chloride. 15