From Names to Formulae

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. There is a compound known as Prussian blue, and its full name is iron three hexacyanidoferrate two. Look at that name carefully. Iron appears twice, once as iron and once as ferrate, and with two different Roman numerals! So can you write its formula, and say how many of each ion it needs? Hard to say right now, isn't it? By the end of this video, you will write it in under a minute. Writing formulae from names comes up often in JEE, and it is a scoring skill once you know the steps. So, let's begin.

0:38Step 1: read the name piece by piece

So, how do we turn a name into a formula? We read the name piece by piece, like a recipe. First, for a salt, the name works just like a simple salt: the cation is named first, then the anion. Okay? Next, the prefixes tell you how many ligands there are. Di means two, tri means three, tetra four, penta five, and hexa six. Easy so far, right? But some ligand names already have a prefix inside them, like ethane one two diamine, which already contains di. For such ligands we count with bis for two, tris for three, and tetrakis for four, and the ligand name goes in round brackets, as in tris, ethane one two diamine, on screen. Clear? Now the ligand names. Ammine is ammonia, N H three; the small three written below means three hydrogen atoms. Aqua is water, H two O, and both of these are neutral. Chlorido is the chloride ion, C l minus, and cyanido is the cyanide ion, C N minus, both anionic. The small raised minus is the charge, one minus each. Okay? Then comes the Roman numeral in round brackets, right after the metal. It gives the oxidation state of the metal. So three in Roman numerals means plus three. With me? Now let me ask you something. Suppose the name of the metal ends in ate, like nickelate or ferrate. What does that ending tell you about the complex? Yes, exactly, the complex is an anion! When the metal name ends in ate, the complex carries a negative charge. And iron takes its Latin root here, so iron in an anion becomes ferrate. Got it?

2:25Step 2: write the formula

Now step two, writing the formula. The whole complex goes inside square brackets. The metal is written first, then the ligands, in alphabetical order of their symbols or abbreviations. Okay? Ligands with more than one atom, and short forms like e n, go in round brackets. Here e n is the short form of ethane one two diamine. Look at the example on screen: open square bracket, C o, then e n in round brackets with a small three below, close square bracket, and a raised three plus outside. That is three e n around cobalt, with a total charge of three plus. Right? Now think carefully. The Roman numeral gives the oxidation state of the metal. So is the charge on the complex always equal to that Roman numeral? No, not always, and this is where students lose marks! The charge of the complex equals the oxidation state of the metal, plus the charges of all the ligands. Neutral ligands like ammine, aqua and e n add zero. But every chlorido or cyanido adds one minus. Clear? Finally, the charge is written outside the square bracket. And in a salt, the cation comes first, and the counter ions make the whole compound neutral. That's it, just two steps. With me?

3:46Watch one: a cationic complex

Let me do one with you, straight from the course. Tris, ethane one two diamine, chromium three, chloride. Let's read it piece by piece. Okay? Tris means three e n ligands, and each e n is neutral. Chromium three means chromium in the plus three state. Right? So the charge is plus three, plus three times zero, which is plus three. The complex is a cation. Clear? Chloride ends the name, so it is the anion, C l minus. To cancel the plus three, we need three chlorides. So the formula is open square bracket, C r, e n in round brackets with a small three, close square bracket, then C l three. Got it?

4:29Example 1: formula of potassium tetracyanidonickelate(II)

Alright, Example one, and now it is your turn. Write the formula of potassium tetracyanidonickelate two. Read it piece by piece, just as we did. Pause the video and try it first. Done? Okay, let's check. Tetracyanido means four cyanide ions, C N minus. And nickel two means nickel in the plus two state. Right? Now here is the trap. It says nickel two, so the complex carries a charge of two plus, doesn't it? No, no, wait, look at the ending, nickelate! The ate tells us the complex is an anion. The charge is plus two, plus four times minus one, which gives minus two. Clear? So the complex ion is open square bracket, N i, then C N in round brackets with a small four, close square bracket, and a raised two minus outside. Potassium is named first, so it is the cation, K plus, and we need two of them to cancel the two minus. Okay? So the answer is K two, open square bracket, N i, C N four, close square bracket. Cation first, then the complex anion. That's it! With me?

5:44Example 2: Prussian blue formula, Fe4[Fe(CN)6]3

Now, remember our puzzle from the start? This is Example two. Write the formula of iron three hexacyanidoferrate two, the compound known as Prussian blue. Pause the video and try it first. Done? Okay, let's check. Iron three comes first in the name, so it is the cation. That is F e three plus, iron in the plus three state, with the raised three plus as its charge. Okay? Now the second part, hexacyanidoferrate two. Ferrate is built from ferrum, the Latin name for iron, with ate because this part is an anion. So the anion has a charge of two minus, right? Oh no, careful, it is the same trap again! The two is the oxidation state of iron, not the charge. Six cyanides bring six minus. Plus two, plus six times minus one, gives minus four. Got it? So the anion is open square bracket, F e, then C N in round brackets with a small six, close square bracket, with a raised four minus outside. Right? Now the last step. The cation is three plus, and the anion is four minus. How many of each do you need so that the charges cancel? Very good, four and three! Four iron three ions give four times plus three, which is twelve. And three anions give three times four, which is also twelve, but as negative charge. So the charges cancel. Clear? So the formula is F e four, open square bracket, F e, C N six, close square bracket, with a small three outside. Four irons outside the bracket, and three complex anions. See, our puzzle is solved, in under a minute! With me?

7:35Quick revision: read the name, find the charge, balance ions

So, quick revision. First, read the name piece by piece: count the ligands, find the oxidation state, and watch for the ate ending. Second, the charge of the complex is the oxidation state plus the ligand charges, not just the Roman numeral. And third, write the cation first, and let the counter ions make the compound neutral. Now go and try the practice problems on this page. You will find them easy. All the best!