Potassium Dichromate

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. Someone blows into a breath analyser. Inside it is acidified potassium dichromate, which is orange. If the breath carries ethanol, the alcohol in drinks, the orange turns green. So what exactly happened to the chromium? Interesting, right? Today we see how potassium dichromate is made, what its ion looks like, and why it is such a useful oxidising agent. It comes up often in JEE, so let's begin.

0:30Making K₂Cr₂O₇ from chromite ore

So, let us see how potassium dichromate, K two C r two O seven, is made. We start from chromite ore, F e C r two O four; the small numbers written low count the atoms, and the numbers in front balance the equation. Fuse it with sodium carbonate in air, which supplies the O two, oxygen. The arrow means gives: we get sodium chromate, along with F e two O three and carbon dioxide. Okay? Next, we acidify. H plus, with the plus small and raised, is the hydrogen ion. Sodium chromate turns into sodium dichromate, with sodium ions and water. With me? In the last step, we add potassium chloride, K C l, to get potassium dichromate and sodium chloride. But we already had a dichromate. So why swap sodium for potassium at all? Very good if you said solubility! The potassium salt is less soluble, so it crystallises out. Clear? And there is a bonus. Potassium dichromate is a primary standard, so we can weigh it accurately for volumetric analysis. Sodium dichromate is hygroscopic, it picks up water from the air, so it is not a primary standard. Got it?

1:43Chromate and dichromate in equilibrium

Now, a lovely equilibrium. C r O four two minus is the chromate ion; the raised two minus is its charge. Two chromate ions and two H plus give C r two O seven two minus, dichromate, and water; the two half arrows mean it goes both ways. Chromate is yellow, dichromate is orange. So if I add acid, which way does it shift? To the right, yes, exactly! More H plus pushes it forward, so yellow chromate turns into orange dichromate; this arrow means turns into. Right? Add a base, which removes H plus, and it shifts back, so orange turns yellow again. Whether this is a redox change is our Example one. Clear?

2:30Shapes of chromate and dichromate

Now the shapes, on the figure. On the left, chromate: chromium in the middle with four oxygens around it, a tetrahedron; the dashed bond points behind the screen. Now, dichromate has two chromium atoms but only seven oxygens, and two separate tetrahedra would need eight. So how do they manage with seven? They share one oxygen, exactly! Two tetrahedra share one corner, and that shared oxygen is the bridge in the middle of the figure. With me? The other six oxygens sit at the ends, so we call them terminal, and all six terminal chromium oxygen bonds are equivalent. Right? And the bridge is bent and symmetrical. The chromium, oxygen, chromium angle is about one hundred and twenty six degrees; the small circle is the degree sign. The two manganese ions on the right are permanganate and manganate, for our next lesson. Got it?

3:27Dichromate as an oxidising agent in acid

Now the key part: dichromate as an oxidising agent in acid. One dichromate ion, fourteen H plus and six electrons give two C r three plus ions and seven water molecules; e minus is the electron. E naught, with a small circle, is the standard electrode potential, and here it is one point three three volts, with V for volts. Okay? Now let me ask you this. Why exactly six electrons, and not three? Look at the chromium. Each one goes from plus six, which is orange, to plus three, which is green, so it gains three electrons. There are two chromium atoms, and two times three is six. So the n factor of dichromate in acid is six. Clear? Here is what it oxidises. Iron two plus to iron three plus, iodide, I minus, to iodine, I two, hydrogen sulphide to sulphur, and tin two plus to tin four plus. With me? Sulphur dioxide goes to sulphate, S O four two minus, so a paper soaked in acidified dichromate turns green with sulphur dioxide. Okay? And ethanol is oxidised to ethanoic acid. Now, back to our breath analyser: the ethanol on the breath reduces orange chromium plus six to green chromium three plus. That is what happened to the chromium. Got it?

4:50Three special reactions

Now three special reactions. First, the chromyl chloride test. We warm potassium dichromate with sodium chloride and concentrated sulphuric acid, H two S O four. We get C r O two C l two, chromyl chloride, with potassium and sodium hydrogen sulphates, and water. Okay? Chromyl chloride comes off as red vapours. Pass them into sodium hydroxide, N a O H, and you get yellow sodium chromate. So this is a test for chloride. Clear? Second, with hydrogen peroxide in acid, dichromate gives a deep blue compound, C r O five, extracted into ether. Oh no, careful here: five oxygens does not mean plus ten. Chromium is still plus six, because four of those oxygens form two peroxo groups. With me? And third, heat ammonium dichromate: N H four in brackets, taken twice, with C r two O seven; the triangle over the arrow means heat. It gives nitrogen, green C r two O three and water, like a little green volcano: nitrogen is oxidised and chromium reduced, in the same compound. Got it?

6:02Example 1: is chromate to dichromate a redox change?

Now, Example one, the question we kept aside. Is the conversion of chromate into dichromate a redox reaction? Hint: find the oxidation state of chromium in each ion. Pause the video, and try it first. Done? Okay, let's check. Many students say: the colour changes from yellow to orange, so something must be oxidised or reduced, right? No, no, wait, let us calculate. Let x be the oxidation state of chromium. In chromate, x plus four times minus two equals minus two, the charge on the ion. So x is plus six; the double arrow means so. Okay? In dichromate, two x plus seven times minus two equals minus two. So two x is twelve, and x is plus six again. Right? So chromium is plus six in both: nothing is oxidised or reduced, so the answer is no. Remember, a change of colour does not always mean redox. Got it?

7:05Example 2: ionic equation for Fe2+ oxidised by dichromate

Now, Example two, a little harder. Write the ionic equation for the oxidation of iron two plus by acidified dichromate. Hint: count the electrons. Pause the video, and try it first. Done? Okay, let's check. From the half equation, one dichromate ion gains six electrons. Each iron two plus loses just one electron and becomes iron three plus. So how many iron two plus ions do we need? Yes, exactly, we need six of them! Iron two plus gives iron three plus and one electron, taken six times: six electrons lost, and six gained. With me? Now add the two halves, and the six electrons cancel. One dichromate, fourteen H plus and six iron two plus give two chromium three plus, six iron three plus and seven water molecules. Right? And always check the charge. On the left, minus two, plus fourteen, plus twelve, gives twenty four. On the right, six plus eighteen is also twenty four. So it is balanced. Clear?

8:16Quick revision: preparation, equilibrium, n-factor 6

So, quick revision. First, chromite gives sodium chromate, then sodium dichromate, and then potassium dichromate. Second, yellow chromate in base and orange dichromate in acid, with chromium staying plus six. And third, in acid, dichromate goes to chromium three plus, orange to green, with an n factor of six. Now go and try the two practice problems on this page. All the best!