1.7  Electrochemistry

Electrolysis, as I'm sure you would have guessed has something to do with electricity combined with a chemical reaction. We'll come up with a better definition later. Let's first review some of the old concepts we learnt before trying to understand electrochemistry.

We studied earlier that for a compound to conduct electricity it must have free ions or electrons that can transport charge around. Metals are very good conductors in any state since they have delocalized electrons. Ionic compounds are also good conductors but only in the molten state or when dissolved in water.  This is because in solid form their ions are packed together and are unable to move around. Finally, covalent compounds are very bad conductors of electricity since they don't have ions or free electrons. However, there are some exceptions where covalent compounds turn into ions. In this case they conduct electricity. An example is ammonia reacting with water to form an ammonium cation and hydroxide anion.  And off course don't forget about graphite which is the only giant covalent structure to conduct electricity.

Let's focus on ionic compounds. When they are dissolved or changed into the molten state, their ions are no longer packed. For example, if you take salt (sodium chloride) and dissolve it in water you get ions dispersed in water. The anions and cations are naturally scattered around in water since they willingly form bonds with water molecules. But what if you want to separate the components of the ionic compound so that they can be extracted?

 

One way to do this is to force the ions to separate by making them loose their charge. If they loose their +ve and -ve charges there can no longer be any electrostatic attraction between the ions. Which means that they will separate. But ions don't want to loose charge. Why? Because, as we discussed in the earlier chapters, atoms want to turn into ions to get full shells and stabilize. In other words this is not a spontaneous reaction. A spontaneous reaction is a reaction or change that occurs naturally without having to supply energy from the outside.

This is where electricity come into plays. If you add electricity to the molten salt you are supplying it with a source of electrons to drive the reaction and force NaCl to separate into into Na and Cl. This type of chemical reaction where a compound breaks down into its original components is known as decomposition. And the process of adding electricity to an ionic solution or molten ionic compound to cause decomposition is known as electrolysis. But why does electricity cause the ions to completely separate? 

The reason is that electricity is basically a flow of electrons, negatively charged particles. So if you connect a battery to a bulb, what's happening is that electrons are flowing from the negative terminal of the battery through the wires, through the filament of the bulb and back into the battery through the positive terminal to create a closed loop. This flow of electrons supplies energy to the system causing the bulb to light up. If you break anywhere in the loop the electrons can no longer flow and there is no energy. If instead of the bulb you send electricity through an ionic solution, using graphite rods as shown below, you can create an area of solely +ve charge and another area of solely -ve charge. By creating these areas of separate charges the ions of opposite charge can be attracted to these areas and thus separate by discharging.

Let's try to understand the mechanism of electrolysis by taking molten NaCl (NaCl melts at around 800C).

So we have a battery that has a +ve terminal and -ve terminal. The -ve terminal has an excess of electrons and for this terminal to stabilize and become neutral it has to drive all its electrons to the +ve terminal of the battery. So once you connect the -ve terminal to +ve terminal you open the gates and allow the electrons to flow.

The electrons will first flow into the graphite rod (remember graphite is a good conductor because it has free electrons)  connected to the negative terminal of the battery. Therefore, the number of electrons in this rod is increasing. As a result, it also becomes negatively charged. Now the battery is losing electrons but it desperately needs to make sure that these lost electrons make it back into the battery through the +ve terminal or else the system wont stabilize. So what happens is that the graphite rod connected to the +ve terminal of the battery will have to pump out its electrons into the +ve terminal to make up for the electrons the battery pumped out. In doing so this electrode loses electrons and becomes positively charged.

OK so now an equal amount of charges leaving the battery make it back into battery. This means the battery is balanced. But now we have an excess of electrons in one graphite rod and a lack of electrons in the other.  Therefore, the system is still unstable. This is where the ions become useful. 

The Na+ ions in molten NaCl are attracted towards the -vely charged graphite rod. This is why this rod is called a cathode. It attacts cations. Here, the sodium cations pull an electron from the -vely charged rod and adds it into its shell as shown in the equation below. Thus loosing their +ve charges to become neutral atoms. Sodium is naturally a solid therefore you start seeing little deposits of sodium around the cathode. 

The Cl- ions of the solution are attracted to the +vely charged graphite rod. This rod is called an anode because it attracts anions. Here, since the anode is pumping out electrons into the battery it needs more electrons. Cl- anions will donate an electron each as shown in the equation below. Thus loosing their -ve charge to become neutral atoms. Since chlorine is naturally a gas you can see it bubbling away at the anode.

All of the above mechanisms need to happen simultaneously for the electrons to flow and make a closed loop. It can only happen with molten and aqueous ionic compounds because the ions need to be able to move towards the graphite rods that they are attracted to. If not the reaction will not happen and NaCl will not decompose.

You may wonder why we use heat to transfer energy into this system. This is to keep the NaCl in molten liquid state.

It is also important to note that the more reactive an element is the more it will resist changing back into an neutral atom. For example, if you have silver cations and copper cations in the same solution then silver will loose its charge while copper will stay an ion. This is because copper is more reactive than silver. The image below shows the reactivity series with potassium being the most reactive and gold being the least. I recommend memorizing this series as it will be very useful when predicting how metals react in displacement reactions later on. This series is further discussed in the 'Reactivity Series' chapter in the 'Inorganic Chemistry' section.

Before moving on lets look at the technical terms used to identify the components in an electrolysis system. 

  • Electrolyte is the ionic compound, which has been molten or dissolved in a solution, that will undergo the reaction.

  • The electrodes are the two rods used to transfer electricity into the electrolyte. It should be good conductor of electricity and also be nonreactive so that it does not contaminate the reaction.

Now, we know how molten ionic compounds undergo electrolysis. What about an ionic compound dissolved in water to form an aqueous solution. Is the electrolysis mechanism any different in this case? Yes it is. In fact it becomes a bit more complicated since water is involved.

 

Although water is a covalent compound some of its molecules turn into ions during electrolysis. Therefore, the process is different from molten compounds.

Lets take NaCl as the example again but this time it is dissolved in water. Since water is present in the reaction we are going to have to assume that some (very few) of the water molecules will ionize. This means H2O will ionize into H+ and OH- ions as shown in the equation below. The (l) written next to H2O in the equation is used to show that water is in liquid state. (aq) is used to show that H+ and OH- are aqueous which means that they are dissolved in water. 

Since NaCl is dissolved in water it will dissociate into ions as shown in the equation below. You may be wandering why we are using 2 molecules of water and 2 molecules of NaCl. This is just to make it easier for us to understand how all the molecules and ions will balance out to give the final products at the end of the reaction. 

Now what happens next in the reaction is basically the same as what happened in the reaction with molten sodium chloride. The difference is that now we have H+ ions from water and also Na+ ions from NaCl in the same solution. As stated earlier, when there are two different cations in the same solution only the least reactive ion of the two will gain an electron and loose its charge. The more reactive ion will stay an ion. If we look at the reactivity series above we can see that sodium is way more reactive than hydrogen. Therefore, in this case H+ will be selected over Na+ to receive an electron at the graphite cathode (-vely charged rod). This is shown in the equation below. Na+ will take the position of H+ and combine with OH- to form sodium hydroxide (NaOH) solution.

Chlorine will undergo the same reaction it did in the previous case. Cl- will loose an electron to become Cl as shown below. Since in the equation above we are using 2 molecules of NaCl we will say that 2 Cl- ions will accept 2 electrons to form Cl2.

Now at the graphite anode we have chlorine gas bubbling off just like in the previous case. However, at the cathode instead of Na deposits we have hydrogen gas bubbling off.

 

We can combine all the above equations into one that we can use to represent the whole reaction. Lets try to do that:   

As you can see above, we can cancel out components if they are in both the left hand side (reactants) and right hand side (products) of the equations. The cancellations have been color coded to make it easier to understand. Now if we combine all the remaining components we get the equation shown below:

Na+ and OH- can also be written together to simplify the equation further. The final equation is shown below:

It is important to note that if hydrogen had been more reactive than sodium, then we would have had deposits of sodium at the cathode.

What are the uses of electrolysis? This is a very useful reaction when done on a large scale.

 

For example the electrolysis of aqueous NaCl, also known as brine solution, produces 3 very useful products. 

  • The chlorine produced is used in pesticides, bleach, textiles, antiseptics, plastics, paints and solvents.

  • Sodium hydroxide is used to make soaps, drain and oven cleaners, in water treatment, dyes, etc.

  • Hydrogen is used to make fuel cells, nylon, hydrogenate vegetable oil, etc.

It is also used in electroplating. Electroplating is a process that uses electricity to thinly coat something with a metal. In this process the inert rods are swapped with rods that are made out of the metal that will be used to plate the material. If you want to coat with gold then you use rods that are made out of gold.

In the system illustrated above a steel ring is being coated in gold. The battery is transferring its electrons into the steel ring, which is the cathode in this case. This causes it to be negatively charged. When gold cyanide is dissolved in water the ions are mobile. Therefore, since the steel ring is -vely charged it will attract the positively charged gold ions in the solution. These ions will receive an electron from the cathode and discharge into gold atoms by bonding onto the surface of the ring. Since now the solution has lost some gold ions, it will need more ions to be dissolved in it for the system to balance. Therefore, the gold atoms in the anode will give away one of their electrons each and transfer them into the battery through the positive terminal. As a result, the gold atoms in the anode will turn into cations, break away from the rod and dissolve in the aqueous solution. This cycle will repeat until the ring is completely coated in gold. Gradually the size of the gold anode will reduce. The equations are shown below. 

The gold in the anode will give away an electron and turn into a cation. It will then dissolve in the solution.

The gold ions in the solution will accept an electron from the cathode and discharge to form sold Au. It will deposit itself on the cathode.

The same method can be used to extract and purify a metal that has impurities. Lets take copper as an example. The impure copper sample is connected to the positive terminal of the battery. A pure rod of copper is connected to the negative terminal of the battery. Both rods are placed in an aqueous copper solution. Lets assume it copper sulphate solution. The reaction is similar to electroplating.

The copper cations in the solution will bond onto the pure copper cathode. To make up for the lost copper ions in the solution copper from the impure rod will loose two electrons and turn into cations. These cations dissolve in the solution. The impurities do not ionize therefore they do not dissolve. They just sink to the bottom. Gradually the anode will disintegrate and the cathode increases in size as pure copper is bonded onto it. The equations are shown below:

At the anode:

At the cathode:

End

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