Batteries and Fuel Cells

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Imagine a car battery running low. To check how much charge is left, a mechanic can simply test how dense the liquid inside is. The density of a liquid, telling you about electric charge? Strange, right? Today we open up everyday cells: the dry cell, the mercury cell, the car battery and the fuel cell. These come up often in JEE questions. So, let's begin.

0:29Dry cell: the everyday primary cell

So, first, the most common cell: the dry cell, or Leclanché cell. It is a primary cell, used once and not recharged. It gives about one point five volts; capital V means volts. Okay? The anode is zinc, Z n. The cathode is manganese dioxide, M n O two, with carbon; the small two below means two oxygen atoms. It sits in a paste of ammonium chloride, N H four C l, and zinc chloride, Z n C l two. Right? Now think. Zinc is the anode. Is it oxidised, or reduced? Oxidised, yes, exactly! The anode is always where oxidation happens. Zinc gives zinc two plus, plus two e minus. The arrow means gives, the two plus up top is the ion's charge, and e minus is an electron. With me? At the cathode, manganese dioxide takes the electron, with an ammonium ion, N H four plus, and gives M n O, O H in brackets, plus ammonia, N H three. Notice: ions take part here. Keep that in mind. Clear?

1:37Mercury cell: a voltage that holds steady

Now the mercury cell, about one point three five volts. Its reaction: Z n, H g in brackets, plus H g O, gives Z n O, plus H g. H g is mercury, and Z n, H g in brackets, is zinc amalgam: zinc dissolved in mercury. Okay? Its special feature: this voltage stays constant all its life. Why? Recall the Nernst equation at two hundred and ninety eight kelvin: E cell equals E naught cell, minus zero point zero five nine one over n, times log Q, where the little circle means standard, n is the number of electrons, and Q is the reaction quotient. So, why does this voltage never change? Yes, exactly! Every species in this overall reaction is a solid or a liquid; no ion at all. Pure solids and liquids count as one in Q, so Q stays fixed, and so does the voltage. Got it? But in the dry cell, ions like N H four plus and Z n two plus take part; square brackets mean concentration. These change as the cell is used, so Q changes, and the voltage falls.

2:46Lead storage battery: charge it again and again

Right? Now the car battery: the lead storage battery. It is a secondary cell, so we can recharge it again and again. Each cell gives about two volts. Okay? The anode is lead, P b, the cathode is lead dioxide, P b O two, and the electrolyte is about thirty eight percent sulphuric acid, H two S O four. Right? On discharge, while it gives current: P b, plus P b O two, plus two H two S O four, gives two P b S O four, plus two H two O, water. Both electrodes turn into lead sulphate. With me? Now look at the sulphuric acid here. As the battery discharges, what happens to it? Yes! It is used up, and water forms. So the acid gets weaker, and its density falls. Puzzle solved! Lower density means less charge left. Charging reverses this. Lead sulphate turns back into lead at one electrode and lead dioxide at the other, and the acid comes back. While charging, the battery acts as an electrolytic cell. Clear? Lithium-ion and nickel cadmium cells are rechargeable too; C d is cadmium, N i is nickel. C d, plus two N i O H in brackets, three, gives C d O, plus two N i O H in brackets, two, plus water. Read it carefully: the three and the two after the bracket count the O H groups. Got it?

4:15Fuel cells: energy straight from the fuel

Last, the fuel cell. It converts the energy of a fuel directly into electrical energy. The common one is the hydrogen oxygen cell, H two, O two, with potassium hydroxide, KOH, as the electrolyte. Okay? Now think. One gas is oxidised at the anode, and the other reduced at the cathode. Which gas is oxidised? Hydrogen, correct! It is the fuel. Anode: two H two, plus four O H minus, gives four H two O, plus four e minus; O H minus is the hydroxide ion. Cathode: O two, plus two H two O, plus four e minus, gives four O H minus. With me? Add them: electrons, hydroxide ions and two waters cancel. Overall: two H two, plus O two, gives two H two O, just water! E naught is one point two three volts, at two hundred and ninety eight kelvin and one bar. Clear? The maximum, or thermodynamic, efficiency is delta G over delta H, okay? The triangle, capital delta, means change: delta G is the Gibbs energy change, delta H the enthalpy change. Right?

5:29Example 1: moles of H2SO4 used per faraday, lead battery

Now, Example one. How many moles of sulphuric acid are consumed in a lead storage battery, per faraday delivered? One faraday is the charge on one mole of electrons. Pause the video, and try it first. Done? Okay, let's check. The reaction uses two moles of acid; m o l means moles. So the answer is two moles per faraday, right? No, no, careful! First count the electrons. Here the arrow means changes to: lead goes from zero to plus two, and the lead in lead dioxide from plus four to plus two. So two electrons move per reaction. With me? Two moles of electrons is two faradays, written two capital F, and they use two moles of acid. Okay? So per faraday: two over two, one mole of sulphuric acid. Count the electrons first, then divide. That's it!

6:25Example 2: H2-O2 fuel cell, 83% efficiency and E0 = 1.23 V

Now a harder one. For hydrogen gas plus half oxygen gas giving liquid water, delta G naught is minus two hundred and thirty seven point one, and delta H naught is minus two hundred and eighty five point eight, k J mol to the power minus one, kilojoules per mole. The g and l in brackets mean gas and liquid. Find the maximum efficiency of this fuel cell, and its E naught. Pause and try. Done? Okay, let's check. The fuel cell converts energy directly, so it is one hundred percent efficient, right? Oh no, careful! The best possible is delta G naught over delta H naught; eta, the Greek letter, is efficiency. Two hundred and thirty seven point one over two hundred and eighty five point eight; the minus signs cancel. The wavy equals means about: zero point eight three, so eighty three percent. Okay? Now E naught, from delta G naught equals minus n F E naught, where capital F is the Faraday constant. Each hydrogen atom goes from zero to plus one, and there are two, so n is two. Right? One check before we divide. Delta G naught is in kilojoules. What must we do first? Change to joules, exactly! Two hundred and thirty seven point one kilojoules is two hundred and thirty seven thousand one hundred joules. Forget this, and your answer is a thousand times too small. Clear? So E naught is two hundred and thirty seven thousand one hundred, over two times ninety six thousand five hundred, the Faraday constant in coulombs per mole: one point two three volts. Exactly the value we saw earlier!

8:06Quick revision: dry, mercury, lead storage and fuel cells

So, quick revision. Dry cell, one point five volts, falls; mercury cell, one point three five volts, constant, because no ion appears in its overall reaction. Lead storage: two volts per cell, and discharge uses up the acid, so its density falls. Hydrogen oxygen fuel cell: E naught one point two three volts, and maximum efficiency delta G over delta H, okay? Now go and try the practice problems on this page. All the best!