Electrode Potentials and Reactivity

Transcript

0:00Introduction

Hello everyone! Let us start with a small experiment. Drop zinc into dilute hydrochloric acid, and hydrogen bubbles out. Now drop copper into the same acid. This time, nothing happens at all. Both are d block metals, side by side in the periodic table. So why does copper refuse to give hydrogen? Today, electrode potentials will answer that. They come up often in JEE, so let's begin.

0:29The M²⁺/M potentials across the series

First, let us read the symbol. Capital E with a small raised circle is read E naught, and the circle means standard. In the bracket we have M two plus and M, joined by a slash, where capital M stands for the metal. The raised two plus is the charge on the ion, and the slash pairs the ion with its metal, which we call a couple. It is a reduction potential, measured against the standard hydrogen electrode, SHE, taken as zero volts; capital V means volts. Now the values, from titanium to iron. Titanium is minus one point six three, vanadium minus one point one eight, chromium minus zero point nine zero, manganese minus one point one eight, and iron minus zero point four four. Careful, this capital V in the list is vanadium, not volts! Right? Then cobalt is minus zero point two eight, nickel minus zero point two five, copper plus zero point three four, and zinc minus zero point seven six, all in volts. Okay? Now, one rule from electrochemistry. The more negative E naught is, the stronger the metal is as a reducing agent: it gives away electrons more easily. Clear? So look at all nine values, and tell me: how many of them are positive?

1:47Copper: the only positive value

Yes, exactly, there is just one! Only copper is positive, at plus zero point three four volts. Copper is the odd one out, the metal from our puzzle. With me? But why is copper so different? To turn solid copper into copper two plus ions in water, which steps cost energy, and which step gives energy back? Very good, let us count them. Breaking the solid into atoms costs the enthalpy of atomisation, and pulling off two electrons costs the ionisation enthalpies. For copper, both are high. Hydration of copper two plus gives energy back, but not enough to recover these costs. So copper's potential ends up positive. Got it?

2:34Mn, Ni and Zn: more negative than the trend

Now, across the series, the values generally become less negative, okay? Now, manganese lies between chromium, at minus zero point nine zero, and iron, at minus zero point four four. So its value should fall in between, right? No, no, careful, it is not in between at all! Manganese is minus one point one eight, more negative than chromium. The reason is that manganese two plus is d five, the five written small and raised: five d electrons, a stable configuration. So manganese forms this ion readily. Clear? Nickel, at minus zero point two five, is also more negative than the trend suggests, because nickel two plus has a high hydration enthalpy. Okay? And zinc, at minus zero point seven six, is far below copper. Zinc two plus is d ten, the ten raised: a completely filled, stable set. And zinc has a low atomisation enthalpy. Both push its value more negative.

3:35The M³⁺/M²⁺ couples

Right? Now a second set, the M three plus, M two plus couples. Titanium is minus zero point three seven, vanadium minus zero point two six, and chromium minus zero point four one. Then manganese jumps to plus one point five seven, iron is plus zero point seven seven, and cobalt plus one point nine seven volts. Okay? Use our rule here. Chromium's value is negative. So, is chromium two plus keen to lose an electron, or to gain one? Yes, it is keen to lose one, exactly! Chromium two plus is a strong reducing agent; it becomes chromium three plus, which is d three. That is the stable t two g three arrangement: t with a small two g below names a set of d orbitals, and the raised three means three electrons in it. With me? Now manganese, plus one point five seven, large and positive. Which ion is the strong oxidising agent, manganese three plus or manganese two plus? It is manganese three plus, that is correct! It grabs an electron eagerly, because it becomes manganese two plus, the stable d five ion. Got it? And cobalt three plus, at plus one point nine seven volts, is higher still. The O two, H two O couple, with each two written small below, is the oxygen and water couple, at plus one point two three volts. Cobalt is above it, so cobalt three plus oxidises water. Okay?

5:06Example 1: why copper gives no hydrogen with dilute HCl

Now, Example one, our opening puzzle. Why does copper not liberate hydrogen from dilute H C l, hydrochloric acid? Pause the video and try it first. Done? Okay, let's check. Copper's standard potential, for the copper two plus, copper couple, is plus zero point three four volts. Right? And what is E naught for the hydrogen couple? Zero volts, exactly, for the H plus, H two couple, since the standard hydrogen electrode is the reference. So copper's couple lies above it, and H plus can oxidise only a metal whose couple lies below its own. So H plus cannot oxidise copper. Clear? So copper's reduction potential is positive, and it gives no hydrogen. Zinc, at minus zero point seven six volts, is below zero, so zinc does give hydrogen. And that solves our opening puzzle. Okay?

6:05Example 2: is Cu+ stable in water? Disproportionation

Now, Example two, a harder one. Is copper one plus stable in water? Copper two plus, copper one plus is zero point one five volts, and copper one plus, copper is zero point five two volts. Pause the video and try it first. Here is a hint: can copper one plus react with itself? Done? Okay, let's check. Copper one plus can go up to copper two plus, or down to copper metal. So we test this change: two copper one plus ions give copper two plus and copper; the arrow means gives. The same ion oxidised and reduced together is called disproportionation. Right? We need a cathode and an anode. Which change here is the reduction, at the cathode? Yes, exactly! Copper one plus going to copper gains an electron, so it is the reduction, at the cathode, with zero point five two volts. Okay? And copper one plus going to copper two plus loses an electron, so it is the oxidation, at the anode. Here we still use the reduction potential of its couple, copper two plus, copper one plus, which is zero point one five volts. Clear? Now, a tempting shortcut. Both values are positive, so copper one plus must be stable, right? No, no, careful, the signs alone do not decide it! We need E naught cell, with cell written small below, equal to cathode minus anode. Zero point five two minus zero point one five gives zero point three seven volts, and the greater than sign says it is above zero. With me? Positive E naught cell means the reaction is spontaneous. So copper one plus is not stable in water: it disproportionates. Got it? And here is the reason behind it. The hydration of copper two plus releases a large enthalpy, which more than repays its second ionisation enthalpy. So in water, copper two plus wins. Okay?

8:06Quick revision: Cu, Mn, Ni, Zn, Cr2+, Mn3+, Co3+ and Cu+

So, quick revision. First, only copper has a positive E naught for the M two plus, M couple, so it gives no hydrogen with dilute acids. Second, stable d five and d ten ions make manganese and zinc more negative, and nickel has a high hydration enthalpy. And third, chromium two plus reduces, manganese three plus oxidises, cobalt three plus oxidises water, and copper one plus disproportionates. Now try the two practice problems on this page. All the best!