Cathodes and anodes are two types of electrodes within an electrical cell denoting the point where electricity both moves into the cell and where it leaves from.
Cathode vs Anode
The main difference between anode and cathode is that electrons will move away from the anode, whereas they move into the cathode of the cell.
The cathode and anode locations are not fixed within a cell and can change depending on what is happening at any given time. For example, when recharging a rechargeable battery.
Anodes and cathodes in the context of a battery can be confusing, as the labeling of the positive and negative sides of the battery often does not match their respective charges.
Comparison Table Between Cathode and Anode (in Tabular Form)
| Parameter of Comparison | Cathode | Anode |
|---|---|---|
| Electron movement | Flows into the cathode | Flows out of the anode |
| Net charge | Negative (electrolytic cells), positive (galvanic cells) | Positive (electrolytic cells), negative (galvanic cells) |
| Attracts | Cations | Anions |
| Reaction taking place | Reduction | Oxidation |
| What happens in charging | Creation of electrons | Consumption of electrons |
What is Cathode?
A cathode refers to an electrode inside an electrical cell (either galvanic or electrolytic), which has a flow of negatively charged electrons entering into it.
The cathode is almost always referred to as the positive electrode, despite actually having a net negative charge.
This is because there has been a positive increase in the number of electrons at that side, hence why the cathode side will always be marked positive on a battery, rechargeable or not.
This increase in the number of electrons is known as a reduction reaction, as this refers to a reduction in the oxidation state of the element.
For example, in a zinc-manganese dioxide battery, which is the most common chemical construct of a household battery, we see the electrons move from the negatively charged zinc molecules to the positively charged manganese.
This means that the manganese dioxide side of the battery is our cathode and that as the battery “charge” is used up, the zinc ions become positively charged cations which are attracted to the cathode end of the battery and migrate across.
It is important to note that in some instances, for example when charging a battery, the anode and cathode change ends.
The electrons at the positive end of the cell are consumed, meaning that there is a decrease in the number of electrons, as they are moving away, which means this end is now the anode.
What is Anode?
An anode is an electrode inside an electrical cell from which electrons depart and head towards other molecules in different parts of an electrical cell, or external to the cell.
Almost always, for example in a household battery, the anode is called the negative side of the cell, despite having a positive charge from the electrons moving away.
This decrease in the number of electrons is known as an oxidation reaction and gives the molecules on the anode side of the cell a positive charge, turning them into cations.
In our battery example, the zinc side is the anode, as the electrons are moving from the zinc to the manganese dioxide.
Once the manganese dioxide molecules have received their electrons from the zinc molecules, their negative charge will draw them across to this side of the battery as anions.
When charging a battery, just as with the cathode, the location of the anode will be swapped around.
During charging, the anode material will be oxidized and electrons created, while at the same time being removed from the other end of the battery.
This means that electrons are now moving into the electrical cell via the negative side of the cell, meaning that this side is now the cathode whilst the battery is being charged.
This redistributes the electrons to the positions they were before (although not quite to the same extent) and allows the battery to provide a charge once again.
Main Differences Between Cathode and Anode
- Electrons will flow into an electrical cell or system through the cathode electrode, whereas they will leave from the anode electrode. These will change location under certain circumstances, i.e. battery recharging.
- The cathode will have a net negative charge in electrolytic cells, such as a disposable battery, and a positive charge in galvanic cells, such as a battery being recharged. Anode electrodes will undergo the opposite.
- Once the movement of electrons has begun in the electrical cell (discharging), the resulting anions will be attracted to the anode end of the cell, whereas the cations will be attracted to the cathode end.
- The process of electrons flowing into the cathode is known as reduction, as it results in a negative charge and a reduction of the molecule’s oxidation number. Whereas with the anode we will see a positive charge from the electrons leaving, the process is known as oxidation.
- During charging, at the cathode end, we will see the creation of electrons through the oxidation of the anode material, whereas we will see uptake and reduction of the electrons at the anode end.
Conclusion
Cathode and anode are important classifications of electrodes inside an electrical cell and help to give electrochemists an insight as to what and where electrochemical reactions are taking place.
The terms can get a little confusing, as the location of each can change within the same cell depending on whether the cell is being used to power a device, or is itself being recharged.
The main takeaway from all this is that electrons flow into the cathode electrode of an electrical cell, and flow out of the anode electrode
It is also important to note that their positions inside a cell are not fixed, and can change depending on what is happening to the whole cell.
References
- https://www.nature.com/articles/srep02671
- https://aip.scitation.org/doi/abs/10.1063/1.1710038