Nitrogen(V) oxides and oxoanions

 Key points: The nitrate ion is a strong but slow oxidizing agent at room temperature; strong acid and heating accelerate the reaction.

Figure 15.6 Frost diagram for the elements of the nitrogen group in acidic solution. The species with oxidation number 3 are NH₃, PH₃, and AsH₃, and those with oxidation numbers 2 and 1 are N₂H₄ and NH₂OH, respectively. The positive oxidation states refer to the most stable oxo or hydroxo species in acidic solution, and may be oxides, oxoacids, or oxoanions.

The most common source of N(V) is nitric acid, HNO₃, which is a major industrial chemical used in the production of fertilizers, explosives, and a wide variety of nitrogen-containing chemicals. It is produced by modern versions of the Ostwald process, which make use of an indirect route from N₂ to the highly oxidized com pound HNO₃ via the fully reduced compound NH₃ . Thus, after nitrogen has been reduced to the -3 state as NH3 by the Haber process, it is oxidized to the +4 state:

The NO₂ then undergoes disproportionation into N(II) and N(V) in water at elevated temperatures:

All the steps are thermodynamically favourable. The byproduct NO is oxidized with O2 to NO2 and recirculated. Such an indirect route is used because the direct oxidation of N2 to NO2 is thermo dynamically unfavourable, with ∆_r G^O (NO₂, g) = +51 kJ mol 1. In part, this endergonic character is due to the great strength of the N≡N bond. Standard potential data imply that the NO3 ion is a mod erately strong oxidizing agent. However, its reactions are generally slow in dilute acid solution. Because protonation of an O atom promotes NO bond breaking, concentrated HNO₃ (in which NO₃ is protonated) undergoes more rapid reactions than the dilute acid (in which HNO₃ is fully deprotonated). It is also a thermodynamically more potent oxidizing agent at low pH. A sign of this oxidizing character is the yellow colour of the concentrated acid, which indicates its instability with respect to decomposition into NO₂:

This decomposition is accelerated by light and heat. The reduction of NO^-₃ ions rarely yields a single product, as so many lower oxidation states of nitrogen are available. For example, a strong reducing agent such as zinc can reduce a substantial proportion of dilute HNO₃ as far as oxidation state 3:

A weaker reducing agent, such as copper, proceeds only as far as oxidation state +4 in the concentrated acid:

With the dilute acid, the +2oxidation state is favoured, and NO is formed:

Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid, which is yellow due to the presence of the decomposition products NOCl and Cl₂. It loses its potency as these volatile products are formed:

Aqua regia is Latin for ‘royal water’ and was so called by alchemists because of its ability to dissolve the noble metals gold and platinum. Gold will dissolve to a very small extent in concentrated nitric acid. In aqua regia the Cl ions present react immediately

with the Au3 ions formed to produce [AuCl4]^- and thereby remove Au3 from the product side of the oxidation reaction:

The anhydride of nitric acid is N₂O₅. It is a crystalline solid with the more accurate formula [NO₂⁺] [NO₂⁻] and can be prepared by dehydration of nitric acid with P₄O₁₀:

Thesolid sublimes at 320ºC and the gaseous molecules dissociate to give NO₂ and O₂. The compound is a strong oxidizing agent and can be used for the synthesis of anhydrous nitrates:

اخر الاخبار

اخبار العتبة العباسية المقدسة

معهد القرآن الكريم النسويّ يعقد الجلسة الرابعة من مسابقة فضة القرآنية

العتبة العباسية المقدسة تهيّئ الصحن الشريف استعدادًا لمهرجان رحمة للعالمين الدولي الأوّل

بمشاركة عددٍ من الطلبة.. المجمع العلمي ينظّم دورة مختصّة بالتجويد في محافظة الديوانية

العتبة العباسية المقدسة تنشر لافتات احتفائية بذكرى المولد النبوي الشريف

المزيد

الآخبار الصحية

هل من الآمن شرب القهوة على معدة فارغة؟

تحذير: مرض السكري يسرق طاقة قلبك

حقيقة صادمة.. لماذا يجب أن تتوقف عن شراء الجزر الصغير؟

بعد عمر الستين.. توقيت وجبة الإفطار مؤشر على صحتك

المزيد

الآخبار التكنلوجية

تركيب مقعد الطفل بأمان في سيارتك: دليل كامل وسهل

مليار مليار عملية في الثانية.. ألمانيا تدشن الحاسوب الفائق

أخيرا.. إنستغرام يصل رسميا إلى آيباد

حل يثير جدلا طبيا.. حقنة قادرة على إخفاء الشيب من رأسك ؟!

المزيد

مواضيع ذات صلة

Condensed phosphates

Oxoanions of phosphorus, arsenic, antimony, and bismuth

Oxides of phosphorus, arsenic, antimony, and bismuth

Oxides and oxoanions of nitrogen

Oxohalides

Halides of the heavy elements

Phosphine, arsane, and stibine

Hydrazine and hydroxylamine

Ammonia

Phosphides

Azides

Nitrides