Ammonium and Lead

Transcript

0:00Introduction

Hello everyone! Here is a small mystery from the salt analysis lab. You add dilute hydrochloric acid to a salt solution, and a white precipitate appears. You boil it with water, and the precipitate disappears completely. Then you let it cool, and the white solid comes back, this time as needles. So which cation plays this disappearing trick? Hold that thought; we will catch it in this video. Today we meet two cations, ammonium and lead, and their tests are a regular part of JEE practical chemistry. So, let's begin.

0:36Ammonium: warm the salt with sodium hydroxide

Let us start with the ammonium ion, N H four plus. The small four written low is a subscript, four hydrogens, and the plus written up top is its charge. Warm the salt with sodium hydroxide, N a O H; the plus sign on screen means we add it. The hydroxide ion, O H minus, takes a hydrogen ion from ammonium, and the arrow means gives: we get ammonia, N H three, and water. Okay? So a gas comes off, but you cannot see it. How will you know that it really is ammonia, and not some other gas? We use three simple checks. First, ammonia is recognised by its smell. Second, ammonia is a basic gas, so it turns moist red litmus paper blue; the paper is kept moist, because ammonia acts as a base once it dissolves in water. Right? Third, bring hydrochloric acid, H C l, near the gas, and white fumes appear. These fumes are ammonium chloride, N H four C l, formed when ammonia meets hydrogen chloride. With me?

1:43Nessler's reagent: the brown test for ammonia

Now the special reagent for ammonia, Nessler's reagent. It is an alkaline solution of potassium tetraiodomercurate. On screen, K two stands for two potassiums, and the square brackets hold mercury, H g, with four iodines; the square brackets mark one complex ion. Clear? Pass ammonia into Nessler's reagent, and you get a brown precipitate. Sometimes you see just a brown colour instead. Brown with Nessler's reagent means ammonia, and so the ammonium ion. Okay? And what is this brown solid? Its formula is on screen: mercury oxide, then a dot, then a second mercury carrying an N H two group, in round brackets, and an iodine. The dot joins the two parts into one compound. Its name is the iodide of Millon's base. Got it?

2:33Trap: why test ammonium on the original salt?

Now here is a trap. Suppose you finish the whole group separation first, and only then test the solution for ammonium. Is that a good plan? That plan fails, because you would find ammonium every single time. The Roman numerals on screen are group numbers: groups three to six all add ammonium compounds, such as ammonium chloride and ammonium hydroxide, as reagents. So the ion would always be found later, even if the salt never had it. Right? So the ammonium ion is tested on the original salt, before any group reagent goes in. That is why ammonium is called group zero. Clear?

3:15Lead: white lead chloride and hot water

Now the second cation, lead, P b two plus; the raised two plus is its charge. Add dilute hydrochloric acid, and each lead ion joins two chloride ions, C l minus. We get lead chloride, P b C l two, a white precipitate. Okay? Here is its special property. Boil the precipitate with water, and the lead chloride dissolves. Keep this in mind, because it is a big clue for our mystery from the start. With me? And when the solution cools, lead chloride comes back, as needles. Dissolving in hot water and returning on cooling is how lead chloride gives itself away. Clear? Now another trap. Lead chloride is a precipitate, so group one removes all the lead from the solution, right? Not completely, and that is the trap. Lead chloride is sparingly soluble, not completely insoluble, so some lead stays dissolved in the filtrate. The rest of the lead comes down in group two, as lead sulphide, P b S on screen. Got it?

4:20Confirming lead: three tests on the hot solution

To confirm lead, we take the hot solution of lead chloride and divide it into three parts. To the first part, add potassium iodide, whose formula on screen is K I: K for potassium, and the letter I for iodine. We get a yellow precipitate of lead iodide, lead with two iodines. Okay? Now a lovely test to watch. Boil the yellow precipitate, and it dissolves. Let it cool, and lead iodide comes back as golden spangles, which means glittering golden yellow plates. Once you see it, you will never forget it. Clear? To the second part, add potassium chromate, K two C r O four. We get yellow lead chromate, P b C r O four, and it dissolves in sodium hydroxide. With me? So a yellow precipitate with potassium chromate always means lead, right? Careful here, because barium chromate, B a C r O four, is yellow too. Sodium hydroxide tells them apart: lead chromate dissolves in it, and barium chromate does not. Got it? And to the third part, add dilute sulphuric acid, H two S O four. We get white lead sulphate, P b S O four, which dissolves in ammonium acetate. So that is three confirmations for lead. Okay?

5:42Example 1: white precipitate that dissolves on boiling

Now, Example one, and it is our mystery from the start. Here, a white precipitate forms with dilute hydrochloric acid, dissolves on boiling, and reappears on cooling. Which cation is indicated? Pause the video and try it first. Done? Okay, let's check. Dilute hydrochloric acid is the group one reagent, so the white solid is a chloride. Okay? Next, it dissolves on boiling and comes back on cooling. So this chloride is much more soluble in hot water than in cold. Right? That is exactly lead chloride, so the cation is lead, P b two plus. Mystery solved: lead was playing the disappearing trick all along. Clear?

6:28Example 2: volume of NH3 at STP from 1.07 g NH4Cl

Now a numerical, a little harder. What volume of ammonia at STP is evolved when one point zero seven grams of ammonium chloride is warmed with excess sodium hydroxide? The small g on screen means grams, and the molar mass, capital M, is fifty three point five. STP means standard temperature and pressure. Pause the video and try it first. Done? Okay, let's check. Ammonium chloride and sodium hydroxide give sodium chloride, ammonia and water. It is the same ammonium test we learnt today, now with numbers. Okay? Now, the question says excess sodium hydroxide. Does more sodium hydroxide give more ammonia? No, and that is exactly the trap. All the nitrogen of the ammonia comes from ammonium chloride, so one mole of ammonium chloride gives one mole of ammonia; m o l on screen is short for mole. The excess sodium hydroxide gives no extra ammonia; it only makes sure all the ammonium chloride reacts. Right? So the number of moles, small n, is one point zero seven over fifty three point five, which gives zero point zero two zero mole. With me? At STP, one mole of any gas occupies twenty two point four litres; capital L stands for litres. Clear? So the volume, capital V, is zero point zero two zero times twenty two point four, which gives zero point four four eight litres. In millilitres, m L on screen, that is four hundred and forty eight millilitres of ammonia, and that is our answer.

8:10Quick revision: Nessler's test, PbCl2, PbI2 and PbCrO4

So, quick revision. Ammonium: warm the salt with sodium hydroxide, and the ammonia turns moist red litmus blue and gives a brown precipitate with Nessler's reagent. Lead: white lead chloride dissolves in hot water and returns as needles on cooling. And the hot solution gives yellow lead iodide, yellow lead chromate and white lead sulphate. Now go and try the four practice problems on this page. They are short, and you are ready for them. All the best!