Paper
Contents Page
No.??????????????? . .Introduction?????????????????????????????????????????????????????????????????????????????????????????????????? 1. Plan
under this investigation was concerned with the acid base and redox reactions
of Nitrous acid. Nitrous acid is a weak acid of ka 4.7 x 10-4
formed from the reaction:NaNo2(aq)? + HCl(aq) ®
HNO2(aq)? +NaCl(aq). ?????????????????
??????????????????????(nitrous
acid) ??????????? Related
studies have shown that a pale blue solution is formed from this reactions a
result of an unstable oxide N2O3 and that HNO2
is an unstable compound because it decomposes very fast at room temperature. It
was hence used up immediately after being made from its constituent?s
reactants, Sodium nitrite and Hydrochloric acid. The instability of the compound means that it
disproportionates at standard room temperature .HNO2(aq)? —– HNO3(aq)?
+NO (g) ????????????
??????(nitric acid)?????? (nitrogen monoxide)??????????? Disproportionation
occurs when one of the reactant molecule in a reaction forms two products, one
product which is obtained by the oxidation of the reactant molecule and the
other by reduction. To limit this effect, Le Chatelier?s principle applied that
cooling the system will shift the equilibrium to the exothermic direction of
the equation to minimise the temperature decrease. In this case an ice bath was
used to drive the reaction towards the left-hand side. This inevitably limits
the dispropornation reactions.To demonstrate the acidic properties of Nitrous
acid, the three standard tests were set.Acid?
+? base/alkali??? ®??? Salt + waterAcid?
+? Metal??? ®????? Metal Salt?? + HydrogenAcid? +
Carbonate???? ®???? Salt?
+? Carbondioxide? +?
WaterThe redox reactions involve reductions and
oxidations. To question the view that HNO2 behaved as either a
reductant, oxidant or both, scientific theories and experiments were applied
based on the fact that it displays the characteristics set by the following
standards As a reducing
agent it reduces another specie by donating electrons to it. As an oxidising
agent it oxidises another specie by removing electrons from it. To practically demonstrate the acid base, redox
reactions of HNO2 the following reagents were used based on factors
concerned with cost, availability and the nature of their reactions.1Reagents: Acidic reactions Magnesium solid Sodium hydroxide solution Sodium Carbonate solidReduction reactions Bromine water Pottasuim manganate (VII) solution Sodium dichromate (VI) solutionOxidation reactions Pottasium iodide solution Iron (II) sulphate solution Sulphur dioxide solutionIn all the cases, the anticlockwise rule
was applied to make predictions on whether a reaction took place and possible
outcomes of the products. Appropriate tests were carried out in some of the
reactions to distinguish formation of products if any from that of a mixture of
the reactants. Prior to making an aqueous solution of HNO2,
consideration were given to the small scale laboratory procedure and the
following safety precautions: ·
Use standard laboratory concentrations of 2M. ·
Wear laboratory coats and goggles. ·
Avoid inhaling toxic nitrogen monoxide gas (use
fume cupboard)2 Plan :
Preparation of Nitrous acid ?An aqueous
solution of 2M HNO2 was made from cold acidified aqueous solution of
sodium nitrite from the equation.NaNo2(aq)? + HCl(aq)??
®? HNO2(aq)? +?
NaCl(aq)Calculations:Moles of NaNo2 and HCL, ratio 1:1 Amount needed = 100cm3 of 2M HNO2 Amount of HCl = 50cm3 of 2M HCl????? ????? 50 x 2??? = 0.1 moles ??????????? 1000Moles of NaNo2 = 0.1 moles Mass = 0.1?
x? (RMM? 69g) ??????????? ??????????? =6.9g??????????? 6.9g
of NaNo2 makes up 50cm3 of 2M solution.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)????? ?????? ??+0.99VFull 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)?????????????????????? ??????????? +0.99VFull 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)?????????????????????????????????????????? ??????? +1.07VFull 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)???????? +1.52VFull 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)?? ?????????? ????????????+1.33VFull
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). 6Results
demonstrating properties of Nitrous Acid. Acidic
properties ?Reagents Predictions Observations Magnesium Solid Exothermic reaction should occur as
hydrogen gas is given off. Magnesium solid dissolves. Vigorous reaction. Effervescence of H2 (g)
leaves test tube warm. Brown gas produced. Sodium hydroxide solution A neutralization reaction in this case should
result to a neutral pH. Colour should turn green with a universal indicator. Colourless
+indicator(purple)+ HNO2(green) Sodium carbonate solid The metal salt NaNo2 will be formed
with a neutral ph. Carbon dioxide and water produced. CO2 presence
detected using limewater to produce white CaCO3(s) Colourless+ indicator(blue) + HNO2 (green) Effervescence of CO2(g) With limewater white ppt produced. Carbonate dissolves. 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 ?Reagents Predictions Observations Pottasuim Iodide I2 will be formed and the presence
demonstrated by a blueblack colour with starch. Probable brown fumes of NO oxidation. ?Fizzing. Colourless
+HNO2(black ppt) +
starch (bluishblack) brown
fumes IronIISulphate Fe3+(aq) produces as brown ppt of
Fe(OH)3(s) when the mixture is reacted with NaOH(aq) Probable brown fumes of NO oxidation. Brown fumes Clear solution +HNO2 (darkgreen ppt) + NaOH(brown ppt) Sulphur dioxide Mixture of acid and SO2 reacts to
give White precipitates of BaSO4(s) with
a??? mixture of BaCl2 Mixture +BaCl2® white ppt. 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 ?Reagents Predictions Observations ?Bromine
water Br2(l) reduces to Br-(aq) Br-(aq) + Ag+(aq)® AgBr(s) creamy ppt Brown
+HNO2(clear) +
AgNo3(aq) creamy ppt. ?Pottasuim maganate(VII) Purple Mn7+ reduces to colourless Mn2+ Colour change Purple +HNO2(colourless) Sodium dichromate (VI) ?Anticlockwise rule applies that Orange Cr6+(aq)
reduces to greenCr3+ Orange+HNO2(green) 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 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 ?????????? Ц9.4 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
| ! |
Как писать рефераты Практические рекомендации по написанию студенческих рефератов. |
| ! | План реферата Краткий список разделов, отражающий структура и порядок работы над будующим рефератом. |
| ! | Введение реферата Вводная часть работы, в которой отражается цель и обозначается список задач. |
| ! | Заключение реферата В заключении подводятся итоги, описывается была ли достигнута поставленная цель, каковы результаты. |
| ! | Оформление рефератов Методические рекомендации по грамотному оформлению работы по ГОСТ. |
| → | Виды рефератов Какими бывают рефераты по своему назначению и структуре. |