Difference between primary and secondary cells

Key difference - primary vs. secondary cell

Batteries are cells that can generate electricity. A battery can have one or more electrochemical cells and consists of connectors that can be connected to external devices that use a power supply. Primary cells and secondary cells are two types of batteries. The main difference between primary and secondary cells is that primary cells can only be used once, while secondary cells can be used more than once.

Key areas covered

1. What are primary cells - definition, structure, chemical reactions 2. What are secondary cells - definition, structure, chemical reactions 3. What is the difference between primary and secondary cells - Comparison of the main differences

Key Terms: Battery, Primary Cell, Secondary Cell, Anode, Cathode, Electricity Difference Between Primary and Secondary Cells - Comparative Summary

What is a primary cell?

Primary cells or primary batteries are also called non-rechargeable batteries . This is mainly because these batteries cannot be recharged when they are exhausted and stop working. Although they are not reusable, primary cells are very useful for storing electricity for long-term use because their rate of self-discharge is comparatively low. Hence, their use in cardiac pacemakers can be seen in cardiac patients, smart counters and military campaigns where charging is impractical and impossible

Primary cells are not rechargeable because of the chemical reactions inside the battery, which are irreversible. The chemical reactions use all of the chemicals present in the cell, and when all of the chemical species are used the generation of electricity stops.

A primary battery or cell consists of an anode (positively charged end) and a cathode (negatively charged end). Graphite is usually used as the cathode and zinc as the anode. Oxidation reactions take place in the anode, where electrons are released into the circuit, and reduction reactions take place in the cathode, where electrons are taken in from the outside. There is also an electrolyte that is helpful in passing electrical current through. The electrolyte consists of electrically charged ions that can carry their charge between the cathode and anode.

Difference between primary and secondary cells

Figure 1: A primary battery with a zinc anode and a Cu cathode

A common example of a primary cell is the Leclanche cell . It consists of a zinc anode and a porous graphite cathode. The electrolyte in the battery is a moist mixture of NH 4 Cl (ammonium chloride), zinc chloride (ZnCl 2 ) and manganese dioxide (MnO 2 ). The chemical reactions that take place inside the cell can be specified as follows.

Anode: Zn (s) → Zn 2+ (aqueous) + 2e

Cathode: 2NH 4 + ( aqueous ) + 2e → 2NH 3 (g) + H 2 (g)

Two gases NH 3 and H 2 are produced in the cathode. But these gases also take part in reactions, as shown below.

2NH 3 (g) + Zn 2+ (aqueous) → [Zn (NH 3 ) 2 ] 2+ (aqueous)

2MnO 2 (s) + H 2 (g) → Mn 2 O 3 (s) + H 2 O (l)

What is a secondary cell?

Secondary cells are also known as secondary batteries or accumulators . These batteries can be used multiple times as they can be recharged when they stop working. The chemical reactions that take place in these batteries are reversible. This means that these reactions can be reversed by applying an electrical charge. However, unlike primary batteries, secondary batteries must be charged before use. Chargers are used for the charging process.

Different secondary batteries have different functions. Therefore, the user should know which type of battery should be used for a particular need.

These batteries also have a cathode and anode like primary batteries. Reduction reactions take place in the cathode, while oxidation reactions take place in the anode. A good example of secondary cells is the lead-acid battery. The reactions that take place within this cell are shown below.

Cathode: PbO 2 (s) + HSO 4 - (aq) + 3H + (aq) + 2e → PbSO 4 (s) + H 2 O (l)

Anode: Pb (s) + HSO 4 - (aq) → PbSO 4 (s) + H + (aq) + 2e

Main difference - primary and secondary cells

Figure 2: A schematic diagram of a lead-acid battery.

Difference between primary and secondary cells

definition

Primary cells: Primary cells are batteries that cannot be recharged or reused.

Secondary cells : Secondary cells are batteries that can be charged and reused.

Chemical reaction

Primary cells: irreversible reactions occur in primary cells.

Secondary cells: Reversible reactions occur in secondary cells.

purpose of use

Primary cells: Primary cells can only be used once.

Secondary cells : Secondary cells can be used more than once.

meaning

Primary cells: Primary cells can produce electricity immediately; therefore, they are used in portable devices.

Secondary cells : Secondary cells should be charged before use. So they can be used in cars.

Rate of self-discharge

Primary cells: Primary cells have lower self-discharge rates and can be used for long-term storage of energy.

Secondary cells : Compared to primary cells, secondary cells have a higher rate of self-discharge.

diploma

Batteries are very important in technological applications. All batteries consist of electrochemical cells. Primary and secondary cells are two types of batteries that are useful in daily life. The main difference between primary and secondary cells is that primary cells can only be used once, while secondary cells can be used more than once.

References:

1. "Primary and Secondary Batteries." Lead Acid INp, nd Web. Available here . June 12, 2017. Free texts. "Rechargeable batteries." Chemistry LibreTexte. Lyrics, January 22, 2017. Web. Available here . June 12, 2017.

Image courtesy:

1. “Hand-painted primary battery” by 脂肪 酸钠 - Own work (CC BY-SA 4.0) via Commons Wikimedia 2. “Unloading” (CC BY 3.0) via Commons Wikimedia

About the author: Madhusha

Madhusha is a BSc (Hons) graduate in Biological Sciences and is currently doing her Masters in Industrial and Environmental Chemistry. Her areas of interest for writing and research include biochemistry and environmental chemistry.