Outline of method:
· Set up a conical flask placed into an ice bath.
· Weigh 6.9g of solid NaNo2 and dissolve in 50cm3 of distilled water.
· Pour solution into conical flask inside ice bath.
· Cool down to a temperature range of (0 - 5)0C in ice bath.
· Add 50cm3 of 2M HCl to conical flask
· Place apparatus in fume cupboard
· Carry out tests with the conical flask placed in ice bath.
Procedure: 3cm3 of Nitrous acid was used to react in all cases to 3cm3 and small portions of reactant solutions and solids . A table of results was set up to summarise observations of colour changes, fumes and result of tests.
3
Reactions:
Acid - base
Magnesium
A metal plus an acid will result in the formation of a metal salt and hydrogen gas as shown in the equation.
2HNO2(aq) + Mg(s) ® Mg(NO2)2 (aq) + H2 (g)
(magnesium nitrite)
Prediction: An exothermic reaction will occur as there will be effervescence of hydrogen. Magnesium solid dissolves.
Sodium hydroxide
This acid -base reaction should produce salt and water as a result of neutralization.
HNO2(aq) + NaOH(aq) ® NaNo2 (aq) + H2O (l)
(sodium nitrite)
Prediction : A neutralization reaction should occur producing a salt. A neutral pH should indicate a green colour using a universal indicator.
Sodium Carbonate
A reaction involving carbonates produces a metal salt, water and carbondioxide gas.
HNO2(aq) + NaCO3(s) ® NaNo2(aq) +CO2(g) + H2O(l)
(sodium nitrite)
Prediction: The chemical equation suggests effervescence of the carbondioxide gas and a neutral pH for the metal salt formation of a green colour using the universal indicator.
Solution should turn cloudy with limewater (CaOH2) producing white precipitates of CaCO3(s)
CO2(g) + CaOH2 (aq) ® CaCO3(s) + H2O(l)
(calcium carbonate)
4
Oxidation
Pottasuim Iodide
The anticlockwise rule applies that I -(aq) will be oxidised to I2(s) according to the equation:
E
I2(s) +2e ---- 2I-(aq) + 0.54V
2HNO2(aq) + 2H+(aq) + 2e ----- 2NO(g) + H2O(l) +0.99V
Full equation
2HNO2(aq) + 2H+(aq) +2I-(aq) ® I2(s) + 2NO(g) + 2H2O(l)
Prediction: Iodine will be formed and its presence detected by a blueblack colour from a starch indicator. Brown fumes given off, due to NO2 gas formation from
2NO(g) + O2(g) ® 2NO2(g)
(air) (brown fumes)
Iron (II) sulphate
E
Fe3+(aq) + e ---- Fe2+(aq) +0.76V
HNO2(aq) + H+(aq) + e ---- NO(g) + H2O (l) +0.99V
Full equation
HNO2(aq) + H+ (aq) + Fe2+(aq) ® Fe3+(aq) + NO(g) + H2O(l)
Prediction: Fe3+ will be produced as brown precipitates of Fe(OH)3(s) ,when the solution mixture is reacted with sodium hydroxide.
Fe3+(aq) + OH-(aq) ® Fe(OH)3(s)
(brown ppt)
Sulphur dioxide
E
SO42-(aq) + 4H+(aq) +2e ---- SO2(aq) +2H2O(l) +0.17V
2HNO2(aq) + 2H+(aq) +2e ----- 2NO(g) +2H2O(l) +0.99V
Full equation
2HNO2(aq) + SO2(aq) +2H+ (aq) + 2H2O(l) ® SO42-(aq) + 4H+(aq) + 2NO(g) + 2H2O(l)
Prediction: SO42-(aq) will be produced as white precipitates of BaSO4(s), when the solution mixture is reacted with Barium Chloride.
Ba2+(aq) + SO42-(aq) ® BaSO4(s)
(white ppt)
5
Reduction
Bromine water
E
NO3-(aq) + 3H+(aq) +2e ---- HNO2(aq) + H2O(l) +0.94V
Br2(l) + 2e ----- 2Br-(aq) +1.07V
Full equation
HNO2(aq) +Br2(l) + H2O(l) ® NO3-(aq) + 3H+(aq) +2Br-(aq)
Prediction: Bromine water should be reduced to bromide ions and the positive test of creamy precipitates of silver bromide should be observed after adding a portion of AgNO3(aq) to the solution.
Br-(aq) + Ag+(aq) ® AgBr(s)
(creamy ppt).
Pottasuim manganate(VII)
E
5NO3-(aq) +15H+(aq) +10e ----- 5HNO2(aq) + 5H2O(l) +0.94V
2MnO42-(aq) + 16H+ (aq) +10e --- 2Mn2+(aq) + 8H2O(l) +1.52V
Full equation
5HNO2(aq) + 2MnO42-(aq) + 16H+(aq) +10e ---5NO3-(aq) +2Mn2+(aq) +15H+(aq) +8H2O(l).
Prediction: Purple Mn7+(aq) according to the equation will be reduced to colourless Mn2+(aq)
Sodium dichromate(VI)
E
NO3-(aq) + 3H+(aq) + e ---- HNO2(aq) + H2O(l) +0.94V
Cr2O72-(aq) + 14H+(aq) + e --- 2Cr3+(aq) + 7H2O(l) +1.33V
Full equation
HNO2(aq) +Cr2O7 2- (aq) +14H+(aq) + H2O(l)® 2Cr3+(aq) +NO3-(aq) +3H+(aq) + 7H2O(l)
Prediction: The anticlockwise rule applies that Nitrous acid in this case will reduce orange Cr6+(aq) to green Cr3+(aq).
6
Results demonstrating properties of Nitrous Acid.
Acidic properties
Conclusions :
The reactions occurred as anticipated producing conclusive sets of results. With the magnesium, a' pop' test was carried out to confirm H2 gas produced. Brown fumes given off by the system was unexplained in the predictions, but a qualitative reason was looked on further in the discussion. A change in pH during the NaOH(aq) test clearly indicated the formation of a salt, which had a neutral pH. The Carbonate reaction and test demonstrated that the metal salt NaNo2 was formed and carbon dioxide evolved.
Oxidising properties
Conclusions:
HNO2 has the tendency to act as an oxidant and oxidised the above reactants, approved by the observation and standard tests carried out. A detailed explanation of the mechanisms observed was given in the discussion.
Reducing properties
Conclusion:
Conclusive evidence from the above tests suggests that HNO2(aq) has the tendency to behave as a reducing specie.
8
Discussion
Using the anticlockwise rule had clearly with in effect suggested the mechanism in which the reactions occurred. A much negative E value increases the tendency for HNO2 to act as a reducing agent, and a more positive E value allows the tendency to act as an oxidising agent. The reason to which why these chemical reactions occur will be discussed in relation to Gibbs energy. The following data summarises the findings of the HNO2 reactions.
As an Acid , the results were obtained as predicted. Effervescence produced by the reaction with magnesium, warmed the test tube as it was an exothermic reaction. Brown fumes were however produced from this reaction without being predicted. The exothermic nature of this reaction could account for the oxidation of NO(g). A high confidence level in results was confirmed with the 'pop' test for hydrogen gas and with limewater in the carbonate reaction to check for CO2(g). Significantly the carbonate reaction had outlined HNO2 to be a stronger acid compared to carbonic acid as it had protonated the carbonate ion to release carbondioxide.
As an Oxidising agent, the E chart illustrates as to why the observations of the reactions fit into the prediction pattern. In this case HNO2 has more tendency to accept elactrons so has a more positive E than that of the reactants.Pottasuim iodide, ironIIsulphate and sulphur dioxide were oxidised by HNO2 which itself was reduced by release of NO2 fumes.When pottasuim iodide was used, the initial black precipitates seen was a result of iodine formation which was confirmed with starch to give the typical blueblack colour.
HNO2(aq) + H+(aq) + 2I-(aq) ® I2(s) + NO(g) + H2O(l)
(bluishblack (oxidised to brown fumes of NO2 )
with starch)
Nitrous acid reduced to nitrogen monoxide
Iodide oxidised to Iodine. (oxidation no.increase +1)
The reaction with ironIIsulphate produced a lime green coloured solution. After the addition of NaoH, the brown ppt. Produced was a positive test for the presence of Fe3+(aq).
HNO2(aq) +H+(aq) + Fe2+(aq)® Fe3+(aq) + NO(g) +H2O(l)
(green) (brown with NaOH)
Nitrous acid reduced to nitrogen monoxide
Fe2+ oxidised to Fe3+(oxidation no. increase +1)
The Sulphur dioxide reaction shows the formation of white ppt. of sulphate ions after the addition of barium chloride solution.
2HNO2(aq) +2H+(aq) + SO2(aq)+2H2O(l)® SO42-(aq) +4H+(aq) +2NO(g) +2H2O(l)
( white ppt with BaSO4 )
Nitrous acid reduced to Nitrogen monoxide
S4+ oxidised to S6+(oxidation no. increase +2)
In all the equations the oxidation number of nitrogen inHNO2 ( +3) had been reduced to (+2) in NO and so therefore HNO2 is an oxidising agent.
9
As a Reducing agent the E charts illustrates that from the anticlockwise rule that Nitrous acid is oxidised to nitrate and the corresponding reactants reduced as it has a more negative E value and more tendency to donate electrons.
The brown bromine water was reduced to colourless Br- in solution. Br- reacted with Ag+ to form AgBr(s).
HNO2(aq) + Br2(l) + H2O(l) ® NO3-(aq) + 2Br-(aq) + 3H+(aq).
(creamy ppt with AgNO3)
HNO2 oxidised to NO3-
Br2 reduced to Br- (oxidation no. decrease -1)
When KMnO4(aq) was reacted. A colour change from purple to 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 total
10
For 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 reactants
In 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.
-298
Pottasuim 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.
-298
Sodium 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.
-298
11
In the Oxidation reaction
Pottasuim 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.
-298
Iron (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 acid
Ka = [H+(aq)]eq[NO2-(aq)]eq Ka value = 4.7 x10-4
[HNO2(aq)]eq
Since very little HNO2 dissociates
[HNO2(aq)]aq = [HNO2(aq)]initial
Kc =
[ 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)
14
Chemical Equipment's and lists.
Equipments
Conical Flask
Spatula
Ice bath
Thermometer
Mass balance
Pipette
Solutions 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