Copper

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle from the salt analysis lab. Take some blue copper sulphate solution, and add potassium iodide to it. Out comes a white precipitate, sitting in a brown solution. But wait a moment, copper two plus salts are usually coloured, so how can a copper compound be white? And where does that brown colour come from? By the end of this lesson you will answer both, and even count the substance behind that brown colour. Copper has five tests in our course, each with its own colour, and they do come up in JEE practical chemistry. So, let's begin.

0:37Group II: black copper sulphide

Let us start with test one. Copper is in group two, the Roman two in the heading, so the reagent is hydrogen sulphide, H two S, in dilute hydrochloric acid, H C l; the small two below the H is a subscript. Copper comes down as a black precipitate of copper sulphide, C u S on screen. Okay? Next, we bring it back into solution. Hot fifty per cent nitric acid dissolves the black copper sulphide, and we get a blue solution. Right? Three copper sulphide and eight nitric acid give three copper nitrate, two nitric oxide, three sulphur and four water. The arrow means gives, and in copper nitrate the nitrate group, N O three, sits in round brackets, taken twice. So the sulphide turns into sulphur, and copper goes into solution. With me? Now let me ask you this. Why do we dissolve copper sulphide in nitric acid, and not in hydrochloric acid? Because the K s p of copper sulphide, its solubility product, is tiny; the small s p below the K is just a label. So H plus from the acid, the raised plus is its charge, cannot by itself lower the concentration of sulphide ion, S two minus, enough; the square brackets mean concentration, and the raised two minus is the charge on the sulphide ion. Clear? Nitric acid, HNO three, does something extra. It oxidises the sulphide to sulphur, and so removes it, and that is why the copper sulphide dissolves. Got it?

2:13Ammonia: pale blue, then deep blue

Test two uses ammonium hydroxide, N H four O H on screen. Add a little to the copper solution, and a pale blue precipitate forms. Okay? Now think about this. What happens if we keep adding more and more ammonia? Does the precipitate simply grow? It does not grow at all. In excess, the pale blue precipitate dissolves, and we get a deep blue solution. The colour comes from this ion: copper with four ammonia molecules, N H three, inside square brackets, and the raised two plus outside is the charge of the whole ion. Right? Its name is tetraamminecopper two; the Roman two in round brackets is the oxidation state of copper. And its shape is square planar. Clear? Now, here is a trap. You add excess ammonia and you see a blue solution. So the cation must be copper, right? That is exactly the trap, because nickel two plus also gives a blue solution with excess ammonia. So the blue colour alone does not prove copper. With me? So check another test too. Copper gives a black sulphide in acid. Nickel is not precipitated by hydrogen sulphide in acid, and it gives a red precipitate with dimethylglyoxime instead. Got it?

3:31Potassium ferrocyanide: chocolate-brown

Now for test three, we take the deep blue solution, make it acidic with acetic acid, and then add potassium ferrocyanide. Okay? And down comes a chocolate-brown precipitate. Its formula shows two copper ions with one ferrocyanide group: an iron with six cyanide groups, C N in round brackets, all inside square brackets. Right? Now, why add acetic acid first? Why not pour the ferrocyanide straight into the deep blue solution? Because in the deep blue solution, copper is held inside the ammine complex. The acid destroys that complex and sets copper two plus free, and only then does it give the chocolate-brown precipitate. Clear? In the ammoniacal solution itself, the precipitate does not form. So skip the acetic acid, and you could miss copper altogether.

4:22Potassium iodide, and the flame

With me? Now test four, our opening puzzle. Add potassium iodide, K I on screen, to the copper solution. We get a white precipitate, a copper iodide with two copper and two iodine atoms in its formula, sitting in a brown solution of iodine. Okay? Here is the equation: two copper two plus ions and four iodide ions give the white copper iodide and one iodine molecule; the raised minus on iodide is its charge. Now, which species is oxidised here, and which is reduced? Yes, exactly, iodide is oxidised to iodine. And copper two plus is reduced to copper one plus, which comes down as the white precipitate. So the brown colour in our puzzle is simply the iodine that is set free. Right? Now the other half of the puzzle. Copper two plus salts, like blue copper sulphate crystals, are usually coloured. So why is this copper one compound white? Think about the d electrons. Copper two plus is three d nine; the raised nine counts the electrons in the three d orbitals, leaving one unpaired electron, so its hydrated salts and its solutions are coloured. Copper one plus is three d ten, with no unpaired electron, so compounds like this iodide are white. That solves our puzzle. Clear? And test five is the flame test. Copper gives a green flame with a blue centre. Got it?

5:50Example 1: black sulphide and deep blue with ammonia

Alright, example one, an easy one. Some blue solution gives a black precipitate with hydrogen sulphide in dilute acid, and a deep blue colour with excess ammonia. Which cation is present? Pause the video and try it first. Done? Okay, let's check. The first clue is a black sulphide with hydrogen sulphide in dilute acid, which means group two. And nickel is not precipitated by hydrogen sulphide in acid, so it is not nickel. Okay? The second clue is the deep blue colour with excess ammonia, and that is our tetraamminecopper two ion, in square brackets. Right? So the cation present is copper two plus. The blue colour alone could fool you, as we saw with nickel, but the two clues together point to copper. Clear?

6:38Example 2: millimoles of iodine from 2.5 g CuSO4.5H2O

Now example two, with numbers, back to the potassium iodide test. Two point five grams of copper sulphate pentahydrate is treated with excess potassium iodide. The dot in the formula joins five water molecules to each copper sulphate unit, and the molar mass, capital M, is two hundred and forty nine point five. How many millimoles of iodine are set free? Pause the video and try it first. Done? Okay, let's check. Each formula unit carries one copper two plus ion, so we find its moles; the small n means number of moles. Two point five over two hundred and forty nine point five moles is approximately ten point zero millimoles; the wavy equals sign means approximately. Okay? Now for the iodine, many students write ten point zero millimoles as well, one for one. Is that right? That is the trap, so look at the equation from test four. Two copper two plus ions give only one iodine molecule. With me? So the iodine is half the copper: one half times ten point zero gives five point zero millimoles. Very good if you got that! That is the brown iodine from our opening puzzle, now counted. See how simple that was?

7:57Quick revision: CuS, deep blue, ferrocyanide, KI and flame

So, quick revision. First, in group two, copper gives black copper sulphide, which dissolves in hot fifty per cent nitric acid to a blue solution. Second, excess ammonia gives a deep blue solution, and acetic acid with potassium ferrocyanide gives a chocolate-brown precipitate. Third, potassium iodide gives a white precipitate in brown iodine, and the flame is green with a blue centre. Now go and try the four practice problems on this page. All the best!