Oxides of Nitrogen and Nitric Acid

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Drop copper into dilute nitric acid: a colourless gas comes out, yet brown fumes appear where it meets the air. In concentrated nitric acid, the gas is brown right away. Same metal, same acid, so why different gases? This topic comes up often in JEE, so let's find out.

0:25Six main oxides of nitrogen: +1 to +5

So, nitrogen forms six main oxides. First, N two O, where the small two written low means two nitrogen atoms, and N O, with one of each. The numbers in brackets are oxidation states, plus one and plus two. Both are colourless, neutral gases: they form no acid with water. Okay? Now you try one. In N two O five, what is the oxidation state of nitrogen? Yes, exactly, it is plus five. Let x be that oxidation state: two times x, plus five times minus two, equals zero, so x comes out as plus five. Right? The other four, N two O three at plus three, N O two and N two O four at plus four, and N two O five at plus five, are all acidic. Got it? N O two is a brown gas. Cool it, and two molecules join into colourless N two O four. The two half arrows, one each way, mean the change is reversible. With me?

1:25Structures: N–N bonds and odd electrons

Now, structures. Three oxides have a nitrogen nitrogen bond: N two O, N two O three and N two O four. In the brackets, a dash is a single bond and two lines a double bond. Okay? Now, N two O five also has two nitrogen atoms. So it must have a nitrogen nitrogen bond too, right? Oh no, careful, it does not! Its two N O two groups are joined through an oxygen atom. And solid N two O five is ionic: N O two plus and N O three minus, in square brackets, the raised signs being charges. Clear? Next, N O and N O two are odd-electron molecules, with one unpaired electron, so they are paramagnetic. But liquid N O is diamagnetic. Why do you think that is? Yes, exactly! In the liquid and the solid, N O molecules pair up loosely into N two O two dimers, so the odd electrons get paired. Got it? Last, N two O, laughing gas, is linear, with two resonance structures obeying the octet rule. The double-headed arrow means resonance, three lines mean a triple bond, and in both, the central nitrogen carries the plus charge, while the minus sits on an end atom. With me?

2:38Nitric acid: the Ostwald process

Now, nitric acid, HNO three, made by the Ostwald process. Four N H three, ammonia, plus five O two gives four N O plus six H two O; the arrow means gives. Above it: a platinum and rhodium catalyst, P t slash R h, five hundred kelvin, the capital K, and nine bar. Okay? Then two N O plus O two give two N O two, reversibly, and three N O two plus water give two HNO three, plus N O again. So what should we do with that N O? Yes, it is recycled, fed back in. Distillation then gives sixty eight percent acid, and concentrated H two S O four, sulphuric acid, removes water to reach ninety eight percent. Right? In the laboratory, N a N O three, sodium nitrate, plus H two S O four gives N a HSO four plus HNO three. Clear?

3:31Nitric acid, a strong oxidising agent

Now, the HNO three molecule is planar, and nitric acid is a strong acid and a strong oxidising agent. So why does the concentrated acid turn yellow on standing? It slowly decomposes: four HNO three gives four N O two, plus O two, plus two H two O, and that brown N O two dissolves in the acid, turning it yellow. Got it? Now metals. Zinc, Z n, with dilute acid: four Z n plus ten HNO three gives four zinc nitrate, the small two outside the bracket meaning two nitrate groups, plus N two O, plus five H two O, with me? With concentrated acid, Z n plus four HNO three gives zinc nitrate, plus two N O two, plus two H two O, so the gas depends on the concentration and the metal. Okay? With very dilute acid, zinc gives N H four N O three, ammonium nitrate. But chromium, aluminium and iron, C r, A l and F e, are made passive by the concentrated acid. Right?

4:33The brown ring test for nitrate

Next, the brown ring test for nitrate. To the nitrate solution add F e S O four, then pour concentrated H two S O four down the side, so it forms a lower layer. Got it? Where the layers meet, F e two plus reduces nitrate to N O: N O three minus, plus three F e two plus, plus four H plus, gives N O, plus three F e three plus, plus two H two O, with me? Then N O replaces one water in the iron complex: F e H two O six, two plus, plus N O gives F e H two O five N O, two plus, plus water. That complex is the brown ring. Okay?

5:09Example 1: copper with dilute and concentrated nitric acid

Now, example one, which finishes our puzzle. Write equations for the action of dilute and of concentrated nitric acid on copper, and identify the gas in each case. Pause the video and try it first. Done? Okay, let's check. Dilute acid: three C u, copper, plus eight HNO three gives three copper nitrate, plus two N O, plus four H two O, and the nitrogens check out: eight on the left, six plus two on the right. Right? Concentrated acid: C u plus four HNO three gives copper nitrate, plus two N O two, plus two H two O, clear? So the gas is N O with dilute acid, and N O two with concentrated. And colourless N O meets the air at once: two N O plus O two gives two N O two, the brown fumes. Puzzle solved: the concentration decides the gas. Got it?

6:07Example 2: balance I2 with conc. HNO3, mol NO2 per mol I2

Now a harder one. Balance the oxidation of iodine by concentrated nitric acid, and find the moles of N O two per mole of I two. Here is a hint: iodine becomes HIO three. Pause the video and try it first. Done? Okay, let's check. Iodine starts at zero in I two. In HIO three, what is its oxidation state? It is plus five, because one, plus x, minus six, equals zero. So each iodine goes from zero to plus five, losing five electrons, and I two, with two atoms, loses ten. With me? Now nitrogen goes from plus five in nitric acid to plus four in N O two, gaining one electron each. So, how many N O two do we need for one I two? Yes, exactly, ten: ten electrons lost by iodine, ten gained by nitrogen. So, ten moles of N O two per mole of I two, okay? Now the equation. Ten HNO three bring ten hydrogens; two go into HIO three, and eight make four H two O, while the oxygens give thirty on the left, six plus twenty plus four on the right. Balanced, right?

7:20Quick revision: N oxides, Ostwald process, copper with HNO3

So, quick revision. N two O and N O are neutral, N O and N O two are paramagnetic, and N two O five has no nitrogen nitrogen bond. Ostwald: ammonia to N O, to N O two, to nitric acid, with N O recycled. And copper gives N O with dilute acid, N O two with concentrated. Now try the practice problems on this page. All the best!