The voltaic cell and the electrolytic cell are both electrochemical cells which are at the heart of electrochemistry. In this ScienceStruck article, we learn about the individual workings of each of these cells, and then do a voltaic cell vs. electrolytic cell comparison by listing out their similarities and differences.
Did You Know?
A voltaic cell is also known as a galvanic cell. These names have been derived from Alessandro Volta and Luigi Galvini respectively, both of whom pioneered this technology.
Before attempting to study an ocean, one must first study the local pond. The same is true in the world of electrochemistry. To understand the working of large batteries, we first need to analyze and study the working of a single cell.
First brought to use in the late 1800s, the electric cell has evolved to being more than just a component within a battery. Today, along with powering our mobile phones, laptops, cars, inverters, etc., electrochemical cells have also found applications in various specialized processes, such as electrolysis and electroplating.
At the grass root level, cells can be split into two basic types – the voltaic cell and the electrolytic cell. The former gives the basis of working of modern-day batteries, while the latter helps us understand how cells can be used in specialized applications. In the following sections, we examine both these types of cells in greater detail.
The Voltaic Cell
A voltaic cell is an example of what is known as an electrochemical cell. It gets this classification owing to the fact that, within it, a chemical reaction takes place which results in the generation of proportional electrical energy. The voltaic cell forms the basis on which modern-day batteries are built.
The following is the working principle of a basic voltaic cell.
When two dissimilar metal plates are simultaneously immersed in an electrolytic solution, the one having higher reactivity with it, chemically combines with the electrolyte. This causes it to dissolve in it as positively charged ions, leaving behind free electrons on the plate. Therefore, that metal plate becomes negatively charged.
The other metal plate which has a lower reactivity, will tend to attract these positive ions present in the electrolyte which will get deposited on its surface, making it positively charged.
Now, if externally, a conductor is connected between these two metal plates, a current starts flowing through it. This current is directly proportional to the amount of positive and negative charge present on the respective metal plates.
Simple Voltaic Cell: Construction and Working
The above diagram shows the main parts used to construct a simple voltaic cell. A sulfuric acid (H2SO4) solution taken in a container acts as the electrolyte for the reaction. Two metal electrodes are immersed in this solution. One electrode is made of zinc (Zn), while the other is made of copper (Cu). An external conductor is connected between these two electrodes, as shown in the diagram.
Voltaic Cell Working
Of the two metals used, zinc has a higher reactivity with sulfuric acid. Therefore, it gets slowly dissolved in the electrolyte solution in the form of positive zinc ions, leaving behind free electrons on the zinc electrode. This reaction continues spontaneously until an equilibrium is established between the number of zinc ions that are formed and the number of zinc ions returning to the electrode. At this point, the voltage on the zinc electrode is (- 0.62V). The following equation represents this reaction.
At Zinc Electrode: Zn –> Zn++ + 2 e—
The positive zinc ions break the molecular bond between hydrogen and sulfur in (H2SO4) and react with the negative SO4 ions of the electrolyte. This results in the formation of ZnSO4. The following is the chemical reaction that takes place at the zinc electrode.
In the Sulfuric Acid Solution: H2SO4 –> 2H+ + SO4—
Zn++ + SO4— –> ZnSO4
The positively charged hydrogen ions which are freed from the sulfuric acid get dispersed in the electrolytic solution. When they reach the copper electrode they react with it, extracting electrons to form hydrogen gas which bubbles out of the solution. This reaction is shown below.
At Copper electrode: 2H+ + 2e– –> H2gas
Over time, the loss of electrons from copper ceases, because the positive potential that develops on the copper electrode repels the positive hydrogen ions. At this point, the positive voltage on the copper plate is 0.46V.
Thus, the total potential difference between the two electrodes becomes [0.46 – (0.62)] = 1.08V. This is called the voltaic cell’s electromotive force (emf). When a conductor is externally connected across the two electrodes, due to the above potential difference, current starts flowing through it when the switch is closed, and the bulb turns on.
An electrolytic cell is another example of an electrochemical cell. However, it works in exactly the opposite manner to that of the voltaic cell. Instead of generating electricity through a chemical reaction, an electrolytic cell drives a useful chemical reaction with the help of an externally applied potential difference.
The following is its working principle.
An electrolytic cell is used to perform electrolysis. The suffix -lysis in Greek means to split up. The term electrolysis, therefore, is the process in which a compound is split up into its constituent elements with the help of electrical energy.
In electrolytic cells, the compound to be split is converted to liquid and used as an electrolyte. Two inert electrodes are immersed in this electrolyte solution, and externally, a potential difference is created between them using an emf source. This causes one electrode to become positively charged, while the other one becomes negatively charged.
Due to this emf, the elements of the liquid electrolyte get split into positive and negative ions, with the positively charged ions getting attracted and deposited on the negative electrodes, while the negative ions getting deposited on the positively charged electrode. Thus, electrolysis, or splitting up of a compound using electrical energy is achieved. This is the basic working principle of an electrolytic cell.
Simple Electrolytic Cell: Construction and Working
In our example, the compound to be split is plain salt (NaCl). It is first liquefied by heating it till the salt crystals melt. This liquid is then pored in an electrolysis container. Two inert electrodes are immersed in this liquid NaCl solution, and using a battery, potential difference is applied between them, as shown in the figure above.
When potential difference is applied between the two inert electrodes, one becomes positively charged, while the other becomes negatively charged. The positively charged electrode attracts the negative chlorine (Cl–) ions, while the negatively charged electrode attracts the positive sodium (Na+) ions.
When the (Na+) ions come in contact with the negatively charged electrode, they acquire electrons from it and convert to sodium (Na) metal, which is deposited on the electrode. This is represented by the following equation.
At the Cathode: Na++ + e– –> Na
Similarly, when (Cl–) ions reach the positively charged electrode, they lose electrons and get oxidized to chlorine gas, which bubbles out of the solution. This is shown in the equation below.
At the Anode: 2Cl– –> + Cl2 + e–
Thus effectively, the compound NaCl gets split into its constituent elements of Na and Cl in an electrolytic cell.
Electrolysis of Salt: 2 NaCl –> 2Na + Cl2 (gas)
Note: The potential required to oxidize (Cl–) ions to Cl2 is 1.36V, while that required to reduce (Na+) ions to sodium metal is (- 2.71V). Thus, the battery required to carry out this reaction needs to be at least [1.36 – (- 2.71)] = 4.07V.
Similarities Between Voltaic Cell and Electrolytic Cell
1) Reactions at Anode and Cathode
In both voltaic and electrolytic cells, oxidation occurs at the anode, while reduction occurs at the cathode. Thus, both these cells exhibit redox reactions.
2) Direction of Electron Flow
In both these cells, electrons flow from the anode to the cathode through the externally connected conductor.
Differences Between Voltaic Cell and Electrolytic Cell
Electric Energy and Chemical Reaction
In the voltaic cell, a chemical reaction is used to generate electrical energy. In an electrolytic cell, a potential difference applied externally is used to drive a chemical reaction.
In voltaic cells, the two electrodes need to be made out of two different metals, with one being more reactive to the electrolytic compound as compared to the other. Typically, in electrolytic cells, both the electrodes used are made of the same inert metal (graphite or platinum).
Conversion of Energy
In voltaic cells, chemical energy is converted to proportional electric energy, while in the electrolytic cells, electric energy is converted to chemical energy.
Anode and Cathode Polarities
Voltaic cells have a negatively charged anode and a positively charged cathode. Electrolytic cells have a positively charged anode and a negatively charged cathode.
Spontaneity of Reaction
In voltaic cells, the chemical reaction leading to the generation of electrical energy occurs spontaneously. Conversely, in an electrolytic cell, an external emf source has to be used to drive the chemical reaction.
Thus, the voltaic cell and the electrolytic cell are two different kinds of electrochemical cells that are used for distinct and separate applications. Each of them represents a basic type, through which many modern-day batteries and other electrochemical applications have been designed and built.