Galvanic Cells

Transcript

0:00Introduction

Hello everyone! Let's start with a small puzzle. Drop a zinc rod into copper sulphate solution, and copper metal comes out on the zinc. Each zinc atom hands two electrons straight to a copper ion. So, can this beaker light a bulb? No, it can't. The electrons jump straight across, and no current flows through any wire. So how do we make those electrons take the long road, through a wire? That is exactly the job of a galvanic cell. Cell questions come up often in JEE, so let's begin.

0:33The Daniell cell: a galvanic cell

So, here is the answer: the Daniell cell. On the left, a zinc rod in zinc sulphate solution. On the right, a copper rod in copper sulphate solution. Two separate beakers, joined by a wire on top, and by this upside-down U tube, the salt bridge. The circle with the V measures the voltage. The first line: a galvanic cell, also called a voltaic cell, turns a spontaneous redox reaction into electrical energy. The arrow means turns into. Okay? Now, in our puzzle, zinc gave electrons to copper ions. So here, which rod gives away electrons, zinc or copper? Zinc, yes, exactly! Each zinc atom loses two electrons and enters the solution as a zinc two plus ion. The two plus up top is its charge, and e with a small minus is an electron. Losing electrons is oxidation, and the electrode where oxidation happens is the anode. Right? The electrons travel to the copper rod. There, copper two plus ions from the solution take two electrons each and deposit as copper metal. Gaining electrons is reduction, and that electrode is the cathode. Clear? So, in the wire, electrons flow from the anode to the cathode, from zinc to copper. See the red arrows on top, marked e minus? That is the electron flow. Got it? Now, a trap. Many students hear the word anode and think positive. So in this cell, the anode is positive, right? No, no, careful! In a galvanic cell, the anode is the negative terminal, the minus in brackets, and the cathode is positive, the plus. It makes sense: the anode is where the electrons come from, so it is the negative end. In electrolysis, later in this unit, the anode is positive, but oxidation is still at the anode. With me? Now, a trick to remember this. An Ox, Red Cat. Anode, oxidation: both start with vowels. Reduction, cathode: both start with consonants. Remember it, and you won't mix them up. Okay?

2:41Cell notation: anode left, cathode right

Now, we don't draw beakers every time. We write the cell in one line. Read it with me: zinc solid, single bar, zinc two plus aqueous, double bar, copper two plus aqueous, single bar, copper solid. The s in brackets means solid, and the letters a q mean aqueous, dissolved in water. Right? Rule one: the anode goes on the left, and the cathode on the right. Zinc is the anode, so zinc comes first. Clear? The single vertical bar is a phase boundary, where the solid rod meets its solution. And the double vertical bar stands for the salt bridge between the two solutions. Got it? Now, the voltage of the cell. E cell, the E with the small word cell below it, equals E right minus E left. The cathode sits on the right, so this is E cathode minus E anode. But careful! Both E values here are reduction potentials. So don't flip the anode's sign and then subtract it again: a common slip. Okay?

3:43The salt bridge

Now, the salt bridge, and what goes inside it. Potassium chloride, K C l, or potassium nitrate, KNO three, or ammonium nitrate, N H four N O three, set in agar jelly. The small numbers below are subscripts: they count the atoms. With me? Now think. As the cell runs, zinc two plus ions keep entering the left beaker, and copper two plus ions keep leaving the right one. So what happens to the charge in each beaker? Yes! The left beaker gains positive charge, and the right one loses positive charge. So the anions of the bridge, the negative ions like nitrate, move to the anode side. And the cations, like potassium, move to the cathode side. Clear? This keeps each solution electrically neutral, and the bridge also stops the two solutions from mixing. Right? Now, a quick question. What happens to the voltage if we pull the salt bridge out? It drops to zero! The circuit is broken: charge builds up in each beaker at once, and the current stops. So the bridge is what completes the circuit. Got it? Now, two rules for choosing a salt bridge. First, its cation and anion should have nearly equal mobilities, so they move about equally fast. Okay? Second, it must not react with the cell solutions. So potassium chloride is not used with silver ions, A g plus, or lead ions, P b two plus. Keep this in mind, we need it very soon. With me?

5:18Example 1: cell notation for Mg and Ag+, why no KCl bridge

Alright, example one, an easy one. Write the cell for this reaction: magnesium solid, plus two silver ions, gives magnesium two plus ions and two silver atoms. Pause the video and try it. Done? Okay, let's check. Magnesium goes from zero to plus two: it loses electrons. So magnesium is oxidised. It is the anode, and it goes on the left. Okay? Silver goes from plus one to zero: it gains an electron. So the silver ion is reduced, and the silver rod is the cathode, on the right. Clear? Now, the equation has two silver ions. Does that two go into the cell notation? No, it doesn't! The notation only lists the species and their phases. So: magnesium, single bar, magnesium two plus, double bar, silver ion, single bar, silver. That's it! And one bonus. This cell has silver ions, so remember our rule: this salt bridge cannot be potassium chloride. See how the ideas connect?

6:27Example 2: cell reaction for H2 and Fe3+ with Pt electrodes

Now a harder one. Write the cell reaction for this cell. Read it with me: P t, platinum, single bar, H two gas, single bar, H plus aqueous, double bar, iron three plus and iron two plus, both aqueous, single bar, platinum. The g in brackets means gas. Pause the video, and try it. Done? Okay, two new things first. Platinum is inert: it does not react, it only carries the electrons. And the comma between iron three plus and iron two plus means both are in the same phase, the same solution. Okay? Now, the left side is the anode, so oxidation happens there. Hydrogen gas gives two H plus ions and two electrons. Clear? The right side is the cathode, so reduction. Iron three plus takes one electron and becomes iron two plus. With me? So, we just add the two, right? Hydrogen plus iron three plus gives two H plus ions and one iron two plus? Oh no, careful! The left side gives two electrons, but the right side takes only one. Electrons given must equal electrons taken. So multiply the right side by two; the little cross means times. Got it? Now add them. H two, plus two iron three plus, gives two H plus, plus two iron two plus. The electrons cancel out. Quick check of charge: on the left, two times three gives plus six. On the right, two plus four gives plus six again. Balanced. Perfect!

8:06Quick revision: anode, cathode, E cell and salt bridge

So, quick revision. Anode: oxidation, the negative terminal, written on the left. Cathode: reduction, the positive terminal, written on the right. E cell equals E cathode minus E anode, both as reduction potentials. And the salt bridge completes the circuit and keeps each side neutral. Now go and try the two practice problems on this page. You can do them. All the best!