Anions Detected with Dilute Acid

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle from the lab. Your friend adds dilute sulphuric acid to a white salt, and it fizzes. She passes the gas into lime water, and the lime water turns milky. Carbonate, she says. But she keeps the gas bubbling, and slowly the lime water turns clear again. So, did her test fail? Hold that thought. Tests for anions come up often in JEE practical chemistry, and one dilute acid sorts out three of them. So, let's begin.

0:34One dilute acid, three anions

So, here is the first test on a salt. Take a little of the salt, add dilute sulphuric acid, H two S O four, and warm it gently. The small numbers written low are subscripts that count the atoms, and the plus sign means we add the acid to the salt. Okay? If a gas comes off, then of the eight anions we study, the salt has one of three: carbonate, sulphide or nitrite. But the acid is only dilute. So why does it push a gas out of these salts? Because these anions belong to weak or unstable acids, okay? Even a dilute strong acid sets these acids free, and they give off a gas. Then the gas, its smell and its colour tell us the anion. Clear?

1:21Carbonate: brisk fizz, milky lime water

Let's start with carbonate. The two minus written up top is the charge on the carbonate ion, and H plus is the hydrogen ion from the acid. Carbonate, C O three, two minus, takes two H plus, and the arrow means gives: water and carbon dioxide, C O two. Okay? What do we see? Brisk effervescence, a quick, strong fizz. The gas is colourless, and it has no smell at all. Got it? To confirm, we pass the gas into lime water, which is a solution of calcium hydroxide; the brackets around O H with a two outside mean two hydroxide groups. Carbon dioxide gives calcium carbonate, which does not dissolve, so the lime water turns milky. With me? Now, our puzzle from the start. When more and more carbon dioxide is passed, the milkiness clears. So why does it clear? Did the test fail? No, the test did not fail. The extra carbon dioxide, with water, turns insoluble calcium carbonate into calcium hydrogencarbonate, which is soluble. So the clear solution is exactly what a carbonate should give. Right? So, milky lime water always means carbon dioxide, every single time, right? Careful, because that is a trap. Sulphur dioxide, S O two, also turns lime water milky. But sulphur dioxide has a suffocating smell of burning sulphur, while carbon dioxide has no smell. So always check the smell. Clear?

2:51Sulphide: rotten eggs, black lead acetate paper

Next, sulphide, S two minus. It takes two H plus and gives hydrogen sulphide, H two S, okay? The gas is colourless, but you will know it at once, because it smells of rotten eggs. That is a strong clue, but we still want proof. With me? For proof, we hold a paper dipped in lead acetate in the gas. Lead acetate is the acetate group, in brackets, taken twice, with lead, P b on screen. It reacts with hydrogen sulphide to give P b S and two molecules of acetic acid. So what colour does the paper turn? It turns black, yes, exactly! The black substance is lead sulphide, P b S on screen. So a rotten-egg smell and black lead acetate paper together mean sulphide. Got it?

3:40Nitrite: a colourless gas, yet brown fumes

Now the third anion, nitrite. The acid sets free nitrous acid, HNO two, which is unstable. Three molecules of it break down into nitric acid, HNO three, two molecules of nitric oxide, N O, and water. And nitric oxide is colourless. Okay? But in the lab, we see brown fumes. So if nitric oxide is colourless, where does the brown come from? Yes, exactly, the brown comes from the air. At the mouth of the tube, nitric oxide meets oxygen, O two, and two N O plus O two give two N O two, nitrogen dioxide, which is brown. Right? To confirm, hold starch–iodide paper in the gas, and it turns blue. The oxides of nitrogen oxidise iodide to iodine, and iodine gives a blue complex with starch. Clear? Now, brown fumes with acid. That always means nitrite, right? Careful, because nitrate also gives brown fumes. But nitrite reacts with dilute sulphuric acid, even in the cold, while nitrate responds only to concentrated sulphuric acid, on heating. So the strength of the acid tells them apart. Got it?

4:53Why sulphuric acid, and not nitric acid?

One last question before the examples. Why do we use dilute sulphuric acid here, and not dilute nitric acid, HNO three? Because nitric acid is an oxidising agent. It would oxidise sulphide to sulphur, and it would give brown fumes of its own, which we could mistake for nitrite. Both tests get confused, so we use dilute sulphuric acid. Okay?

5:19Example 1: odourless gas turns lime water milky

Now, Example one. We have a white salt that gives brisk effervescence with dilute sulphuric acid in the cold. The gas has no smell, and it turns lime water milky. Which anion is present? Pause the video and try it first. Done? Okay, let's check. Brisk effervescence with dilute acid tells us a gas was set free, so the anion is carbonate, sulphide or nitrite. Got it? The gas has no smell. So it is not hydrogen sulphide, which smells of rotten eggs, and not sulphur dioxide, which smells of burning sulphur. Right? And an odourless gas that turns lime water milky is carbon dioxide. So the anion is carbonate, C O three, two minus. See how the smell check from our trap did half the work?

6:11Example 2: volume of CO2 at STP from 1.06 g of Na2CO3

Now a numerical, a little harder. What volume of carbon dioxide at STP is given by one point zero six grams of anhydrous sodium carbonate with excess dilute acid? STP means standard temperature and pressure, and anhydrous means the salt has no water in it. Pause the video and try it first. Done? Okay, let's check. Start from the carbonate equation. Look at the two in front of H plus. Does that mean one carbonate gives two moles of carbon dioxide? No, and that is exactly the trap. The two belongs to the hydrogen ions. Carbonate has only one carbon, so one mole of carbonate gives one mole of carbon dioxide; m o l on screen is short for mole. Okay? Next, capital M stands for the molar mass of sodium carbonate. Two sodiums at twenty three each, one carbon at twelve, and three oxygens at sixteen each. Forty six plus twelve plus forty eight gives one hundred and six grams per mole; the minus one written up top means per. With me? So the number of moles, small n, is one point zero six over one hundred and six, which gives zero point zero one zero mole of carbonate. And so, the same number of moles of carbon dioxide. Clear? At STP, one mole of any gas occupies twenty two point four litres, the value we use unless a question says otherwise; capital L stands for litres. Right? So the volume, capital V, is zero point zero one zero times twenty two point four, which gives zero point two two four litres. In millilitres, m L on screen, that is two hundred and twenty four millilitres of carbon dioxide, and that is our answer. Well done if you got it!

8:00Quick revision: carbonate, sulphide and nitrite tests

So, quick revision. Carbonate gives a brisk fizz of odourless carbon dioxide, which turns lime water milky, and the milkiness clears with excess gas. Sulphide gives hydrogen sulphide, with the smell of rotten eggs, and lead acetate paper turns black. Nitrite gives brown fumes, and starch–iodide paper turns blue. Now go and try the practice problems on this page. You will find them easy. All the best!