Electrochemistry: Crash Course Chemistry #36

Summary

This Crash Course Chemistry episode delves into the world of electrochemistry, exploring how redox reactions power devices like laptops and cars. It explains the concept of half reactions and their role in creating voltage, which is essential for energy transfer. The video also covers the workings of alkaline batteries and galvanic cells, demonstrating how they harness chemical reactions to generate electricity. Additionally, it touches on the applications of electrolysis and electroplating, highlighting the significant role chemistry plays in our daily lives.

Takeaways

• 🔋 Batteries work through electrochemistry, which involves reactions that produce or consume free electrons.
• 🔬 Redox reactions are crucial in electrochemistry, involving the exchange of electrons between substances.
• 🔌 Voltage, or electrical potential, is a measure of how much work electrons can do and is determined by the push or pull on electrons between reactants.
• 🔄 Half-reactions are used to describe electrochemical reactions, separating the processes where electrons are released and where they are accepted.
• 🛠️ Alkaline batteries, like those in the example, work by isolating half-reactions to build up charge in the cathode and create a vacuum in the anode.
• 🌐 The standard reduction potential is a measure of a substance's tendency to gain electrons, with values relative to the reduction of hydrogen ions.
• 🔄 In a galvanic cell, the anode is where oxidation occurs (loss of electrons), and the cathode is where reduction occurs (gain of electrons).
• ⚖️ The standard cell potential is calculated by summing the standard potentials of the two half-reactions involved in the cell.
• ⚡️ The electrical potential of a redox reaction is related to its equilibrium constant and can indicate the spontaneity of the reaction.
• 🛡️ Electrolysis is the process of using electricity to drive non-spontaneous chemical reactions, such as electroplating, where a metal is deposited onto a conductive surface.

Q & A

What is electrochemistry and why is it important?

Electrochemistry is a branch of chemistry that deals with reactions that produce or consume free electrons, typically involving oxidation or reduction reactions. It's important because it forms the basis for technologies like batteries, which power many devices in our daily lives.

What is the relationship between redox reactions and electrochemistry?

Redox reactions are a type of chemical reaction where electrons are transferred from one substance to another. Electrochemistry specifically deals with redox reactions that involve the transfer of electrons through an electrical circuit, which can be harnessed to perform work.

How does voltage relate to the work that can be done by electrons in a redox reaction?

Voltage, or electrical potential, is a measure of the energy available to do work per unit charge. In the context of redox reactions, a higher voltage means that each electron can do more work when it moves from one reactant to another.

What are half reactions in the context of electrochemistry?

Half reactions refer to the individual processes of oxidation and reduction that occur in a redox reaction. Electrochemists often consider these half reactions separately to understand and balance the flow of electrons in electrochemical cells.

How do batteries like alkaline batteries work?

Alkaline batteries work by isolating the half reactions of a redox process. This allows a buildup of electrons at the negative terminal (cathode) and a deficit of electrons at the positive terminal (anode). When connected, electrons flow through an external circuit, doing work.

What is the role of the salt bridge in a galvanic cell?

The salt bridge in a galvanic cell allows ions to move between the two half-cells to maintain electrical neutrality. It completes the circuit by facilitating the flow of ions while keeping the reactants in the half-cells separate.

How is the voltage of a redox reaction measured and what does it signify?

The voltage of a redox reaction is measured relative to the reduction of hydrogen ions to hydrogen gas, which is set at zero volts. This measurement, known as the standard reduction potential, signifies the electrical potential of each half reaction and is indicative of the reaction's ability to do work or release energy.

What is the significance of the standard cell potential in a galvanic cell?

The standard cell potential is the total electrical potential of a galvanic cell under standard conditions. It represents the maximum electrical energy that can be derived from a redox reaction and is calculated by summing the standard reduction potentials of the half reactions.

How does electroplating work and how is it different from a galvanic cell?

Electroplating is an electrochemical process where a metal is deposited onto a conductive surface from a solution containing ions of the metal. Unlike a galvanic cell, which generates electricity from a spontaneous redox reaction, electroplating requires an external power source to drive the process.

What is electrolysis and how is it used in various applications?

Electrolysis is a process that uses an electric current to drive a non-spontaneous chemical reaction. It's used in various applications such as electroplating, refining metals, separating metal ions, and even in the production of hydrogen and oxygen from water.

Outlines

00:00

🔋 Electrochemistry and Batteries

This paragraph introduces the concept of electrochemistry, which is the foundation of batteries. It explains that electrochemistry involves reactions that produce or consume free electrons, specifically oxidation and reduction (redox) reactions where electrons are exchanged. The paragraph emphasizes the importance of redox reactions in powering modern devices and how the flow of electrons through a conductor can perform work. It also introduces the concept of voltage, which is the electrical potential that drives the flow of electrons and is a measure of how much work can be done. The discussion then moves to the structure of a typical alkaline battery, describing the electrochemical reaction between zinc and manganese dioxide, and how batteries are designed to harness energy by isolating half-reactions, creating a potential difference that drives the current when the battery terminals are connected.

05:04

🔌 Understanding Electrochemical Cells and Electroplating

The second paragraph delves into the workings of electrochemical cells, specifically galvanic cells, which generate electrical energy from redox reactions. It provides an example of a galvanic cell where zinc is oxidized, and copper ions are reduced, explaining the concept of standard reduction potentials and how they are measured relative to the reduction of hydrogen ions. The paragraph clarifies that the standard cell potential is the sum of the potentials of the half-reactions and is indicative of the reaction's spontaneity and the amount of electrical energy that can be produced. It also touches on the relationship between voltage, equilibrium constants, and Gibbs free energy. The paragraph concludes by contrasting galvanic cells with electrolytic cells, which are used for processes like electroplating, where electricity is used to drive non-spontaneous redox reactions, such as depositing a metal coating onto an object. The broader impact of electrochemistry on daily life is highlighted, emphasizing its role in powering devices and enabling various industrial processes.

Keywords

💡Electrochemistry

Electrochemistry is the branch of chemistry that deals with the relationship between electricity, as a measurable and quantitative phenomenon, and chemical reactions. In the video, electrochemistry is central to the discussion of how batteries work, as it involves reactions that produce or consume free electrons, such as oxidation and reduction reactions. The video explains that the chemistry inside a battery is a form of electrochemistry, which is essential for powering devices like laptops and phones.

💡Redox Reactions

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons between chemical species. The video emphasizes that redox reactions are a type of chemical reaction that can produce or consume free electrons, which is crucial for the operation of batteries. Redox reactions are powerful because they are complex, involving at least two processes: the release of electrons and the demand for them.

💡Voltage

Voltage, in the context of electrochemistry, is the electrical potential difference between two points. It is described in the video as the 'push' or 'pull' on electrons between reactants, which determines the amount of work that can be done by the flow of electrons. The video explains that a high voltage means each electron can do more work, which is essential for the functionality of devices powered by batteries.

💡Half Reactions

Half reactions refer to the separate oxidation and reduction processes that occur in a redox reaction. The video uses the example of an alkaline battery to illustrate how a redox reaction can be broken down into two half reactions: one where zinc is oxidized and another where manganese dioxide is reduced. Understanding half reactions is key to grasping how batteries harness energy.

💡Cathode and Anode

In the context of a battery, the cathode is the negative terminal where excess electrons build up, and the anode is the positive terminal where an electron vacuum occurs. The video explains that by isolating the half reactions, the battery allows electrons to flow only when the cathode and anode are connected via a conductor, which can then be used to do work.

💡Alkaline Batteries

Alkaline batteries are a type of electrochemical cell that uses an alkaline substance, like potassium hydroxide, to facilitate the flow of electrons. The video describes how alkaline batteries work, with zinc reacting with manganese dioxide to produce manganese three oxide and zinc oxide. These batteries are designed to harness energy by isolating the half reactions.

💡Galvanic Cell

A galvanic cell, as mentioned in the video, is a device that generates electrical energy from a spontaneous redox reaction. The video provides an example of a galvanic cell where metal rods are suspended in a solution, and the flow of ions through a salt bridge completes the circuit. This concept is fundamental to understanding how batteries convert chemical energy into electrical energy.

💡Standard Reduction Potential

The standard reduction potential is a measure of the tendency of a substance to gain electrons, relative to the reduction of hydrogen ions to hydrogen gas, which is set at zero volts. The video explains that these values are used to calculate the overall voltage of a redox reaction, which is crucial for understanding the electrical potential of batteries and other electrochemical devices.

💡Electrolysis

Electrolysis is an electrochemical process that uses electricity to drive a non-spontaneous chemical reaction. In the video, electrolysis is contrasted with the spontaneous reactions in batteries, highlighting its use in processes like electroplating, where a metal is deposited onto a surface. The video explains that electrolysis involves the application of an electric current to cause a redox reaction in a solution.

💡Electroplating

Electroplating is a specific application of electrolysis where a thin layer of one metal is deposited onto the surface of another metal or object. The video describes how electroplating works, with an object being immersed in a solution containing the coating metal's ions, and an electric current causing the metal atoms to deposit onto the object. This process is used in various industries for coating and protecting surfaces.

Highlights

Electrochemistry involves reactions that produce or consume free electrons.

Redox reactions are the basis for electrochemistry, involving electron exchange.

The flow of electrons in redox reactions can be harnessed to do work.

Voltage, or electrical potential, is a measure of how much work electrons can do.

Batteries work by isolating half reactions to build up charge at terminals.

Alkaline batteries contain zinc and manganese dioxide, which react to produce energy.

Half reactions are balanced to show the transfer of electrons in redox reactions.

The design of batteries prevents spontaneous reaction to harness energy efficiently.

Galvanic cells generate electrical energy from redox reactions.

Electroplating uses electrochemistry to coat objects with a layer of metal.

Electrolysis is the process of using electricity to drive non-spontaneous chemical reactions.

Standard reduction potentials are used to calculate the voltage of electrochemical reactions.

The standard cell potential is the sum of the potentials of the half reactions in a cell.

The electrical potential of a redox reaction is related to its equilibrium constant.

High voltage in a redox reaction indicates more electrical energy can be produced.

Electrochemistry is fundamental to the operation of batteries and many other technologies.

The episode was created with the help of a team of writers, editors, and consultants.

Transcripts

Aaah, the controlled flow of electrons, making possible laptops and phones and cars and pacemakers.

Batteries, just like everything else in life, is just chemistry raised to the power of awesome.

The kind of chemistry that happens inside of a battery is called electrochemistry

because it involves reactions that produce or consume free electrons.

Specifically, they are oxidation or redox reactions, the ones where electrons are exchanged.

I've told you about redox reactions before and if you haven't seen that episode yet,

you should probably go watch that before you watch this.

Don't worry, I will still be here when you get back.

Now, when the flow of electrons in these kinds of reactions are sent through a conductor,

like a piece of metal, it can be used to do all sorts of work.

Like, for example, this kind of work.

The amount of work that can be done depends on how strong the push or pull on electrons is between the two reactants.

This is the reaction's electrical potential, but to its friends, it's known simply as voltage.

Basically, if the voltage is high, each electron can do a lot more work than if the voltage is low.

Many of the wonderful things in our modern lives are based on one simple premise:

putting a device between the two halves of just such a reaction;

the half that donates electrons, and the half that accepts them.

By harnessing that energy, a lot of the coolest things you've done today,

up to and including watching this episode of Crash Course Chemistry, has been made possible.

[Theme Music]

Part of what makes redox reactions so powerful, and powerfully excellent, is that they are complicated.

Because in each reaction, there's at least two things going on:

there's the part of the reaction where the electrons are being released

and another part where they're being eagerly demanded.

So when we deal with electrochemistry, we usually think of reactions in terms of half reactions.

Let's start with a typical redox reaction that happens in this alkaline battery as an example.

In here elemental zinc is going to react with manganese dioxide, also known as manganese

four oxide, to produce manganese three oxide and zinc oxide.

When you break this down in to half reactions,

first you have elemental zinc with an oxidation number of 0 being oxidized to zinc 2 ion.

At the same time manganese 4 is being reduced to manganese 3.

When we balance the half reactions,

we see that two electrons are released during the oxidation of each zinc atom,

and one electron is consumed by each manganese 4 atom.

The water and hydroxide ions, by the way, come from a solution of potassium hydroxide.

Which is a basic, or alkaline compound.

Which is why we call these things alkaline batteries.

Now if each of these half reactions occurred in contact with the other one,

they'd just spontaneously go to equilibrium releasing energy as a bunch of heat which wouldn't be very helpful.

So batteries are designed to harness that energy by isolating the half reactions from each other.

This allows excess electrons to build up in the negative terminal, called the cathode,

while an electron vacuum of sorts occurs in the positive terminal, the anode.

Electrons can then cross from one half reaction to the other,

only when we connect the cathode and the anode of the battery via conductors.

So the current can be used to do work. Which I can do by licking this 9 volt battery.

Ahhh! [laughs]

In these batteries, the zinc is in the center surrounded by a layer of cellulose that allows ions to pass through.

The manganese oxide is in the outer layer that surrounds the zinc core,

but the cellulose barrier doesn't allow the zinc and the manganese to mix.

Alkaline batteries are a type of galvanic cell.

Which is generally defined as an apparatus that generates electrical energy from a redox reaction.

Here's another example of a galvanic cell,

one where the interesting part is the flow of whole ions instead of the flow of electrons.

In this case, wires connect metal rods that are suspended in the solution.

They're are the anode and cathode here.

What's noteworthy, is that the metal atoms are actually consumed.

They're used up from the anode rod as they're oxidized, and the metal slowly wears away.

Meanwhile, the opposite happens at the cathode rod, where metal ions from the solution gain electrons

and precipitate on to the cathode as pure metal, gradually growing larger.

The circuit is completed by the wire, but also by a salt bridge,

which is also a U-shaped tube that contains a salt solution

that allows the metal ions to go from the anode to the cathode.

So, now you know how batteries work: developing and transferring a charge using electrochemistry.

But before any redox reaction can be used to text your boyfriend or girlfriend or whatever,

we need to know how much voltage it can generate.

Fortunately all of those amazing chemists who have come before us have done a lot of the work once again.

The voltage generated by many half reactions is already known and can be found in most textbooks or online.

And as I mentioned earlier, voltage is really just a way of expressing the electrical potential of each half reaction.

The difference between the chemical demand for the electrons in one half,

versus the tendency to lose them in the other.

These measurements are done at standard conditions, which we discussed in the enthalpy and entropy episodes.

And by convention they are written as if the substance is being reduced, not oxidized.

For this reason the value is known as the standard reduction potential of a substance.

To see how reduction potentials work in half reactions and combine in an overall reaction,

let's consider a galvanic cell where zinc is oxidized and copper ions are reduced.

Keep in mind that potentials are determined at standard state:

25 degrees Celsius and 1 molar solutions of the copper and zinc ions.

Our cell needs to be set up under the same conditions or the voltage will be a bit different than expected.

The half reactions show more clearly what the electrons are doing.

We can see that the zinc is oxidized and the copper is reduced.

Now all standard reduction potentials are measured relative to the reduction of hydrogen ions to hydrogen gas,

which is set at zero, just as a baseline.

When copper is reduced, for example, it generates 0.34 volts more than hydrogen does,

so we say its standard reduction potential is +0.34 volts.

The standard reduction potential for zinc is -0.76 volts,

but because zinc is oxidized in this reaction, we can't use the reduction potential directly.

Instead, as a general rule, when you convert a reduction half reaction to oxidation,

the sign of the voltage is simply reversed.

So, the -0.76 volts for the reduction potential of zinc

becomes +0.76 volts for the oxidation potential of zinc ions.

And the electrical potential for the whole reaction, called the standard cell potential,

is just the sum of the standard potentials of both half reactions.

In this case, that would be 1.1 volts.

Now I gotta point out here that the electrical potential of a redox reaction is related to its equilibrium constant.

There's actually a way to determine the equilibrium constant from a measured voltage, and vice versa.

Both of these constants have a lot to do with the energy the reaction can release, or its Gibbs free energy.

But in brief, the higher the voltage, the more electrical energy can be produced.

So if the voltage is positive, it means that under standard state conditions, the reaction will spontaneously go forward.

If the sign is negative, the reaction will proceed backward.

And it totally makes sense, when you think about it, that reactions like this are used to make battery cells.

The reactions in batteries need to be spontaneous

because their whole purpose is to release energy, not consume it.

So what if we don't want to power a phone or a laptop or a toy shuttle.

What if instead we want to plate an iron car bumper with chrome?

This cannot be done with a spontaneous reaction.

Instead, a different electrochemical process is needed, one that you've probably heard of: electroplating.

This is done by immersing an object in a solution that contains an excess of ions of the coating metal.

A bar of the coating metal, in this case chromium, is used as the anode,

and the item to be plated, the iron bumper, acts as the cathode

When an electric current is applied, a redox reaction occurs in the solution

and atoms of the coating metal are deposited on the cathode.

This is essentially the opposite of a galvanic cell, known as an electrolytic cell,

and it performs electrolysis, which uses electricity, electro-, to do the breaking apart, -lysis.

In this case molecules in the solution are being broken down so that the metal atoms can be deposited on the surface.

Electrolysis is used for lots of other things too, like coating jewelry or flatware with gold or silver,

refining metals or separating mixtures of metal ions.

Also, converting water into hydrogen gas and oxygen.

So if you didn't already understand how much impact chemistry has in our daily lives,

you certainly should be able to see it now.

Not only are all the materials around you made of chemicals,

but even the electrical devices that power our lives depend on the reactions of electrochemistry.

Thank you for watching this episode of Crash Course Chemistry.

If you were listening, you learned that electrochemical reactions are redox reactions

that we describe in terms of half reactions.

You learned how an alkaline battery works and what's inside of it,

what a galvanic cell is and how it can be set up

and how to calculate the voltage that can be generated by a half reaction, the standard reduction potential,

and by the overall reaction, the standard cell potential.

You also learned how electrolysis and electroplating work.

This episode was written by Edi Gonzalez and edited by Blake de Pastino.

Our chemistry consultant was Dr. Heiko Langner. It was filmed, edited and directed by Nicolas Jenkins.

Michael Aranda is our sound designer and Thought Cafe is our graphics team.