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IB DP Physics Study Notes

5.3.1 Chemical Cells

Diving deep into the realm of electricity and magnetism, chemical cells stand out for their intricate processes that convert chemical reactions into electrical energy. This blend of chemistry and physics has vast real-world applications and forms the foundation of many devices we rely on daily.

Understanding Chemical Cells

A chemical cell, or electrochemical cell, is a device in which chemical energy is converted into electrical energy through spontaneous chemical reactions. The process revolves around two half-reactions that take place at two separate electrodes of the cell. For more on how chemical energy relates to electrical potential, see our notes on electric potential energy.

Types of Electrochemical Cells

There are primarily two types of electrochemical cells based on their functionality:

1. Galvanic Cells (or Voltaic Cells): These are spontaneous and generate electricity as a result of the redox reaction occurring in the cell. Common batteries are often based on this principle.

2. Electrolytic Cells: These are non-spontaneous and require an external source of energy to drive the redox reaction in the opposite direction.

Delving into Electrode Reactions

In the context of a basic galvanic cell, we generally deal with two distinct reactions:

1. Oxidation at the Anode

This is the process in which a substance loses electrons. Taking the classic example of a zinc-copper cell:

  • Zinc metal is oxidised to form zinc ions.

Zn -> Zn2+ + 2e-

This release of electrons is what facilitates the flow of electricity in the external circuit. Understanding the electric field strength can help in grasping how these electrons move.

2. Reduction at the Cathode

Here, a substance gains electrons. In our zinc-copper cell:

  • Copper ions accept the electrons released by the zinc and get reduced to form copper metal.

Cu2+ + 2e- -> Cu

The Mechanism of Energy Conversion

Understanding how chemical energy morphs into electrical energy in a chemical cell is central to appreciating its applications.

  • The Role of Electrolyte: An electrolyte, which can be a liquid, gel, or solid, facilitates the movement of ions inside the cell. It plays a crucial role in maintaining the cell's charge neutrality. For a deeper understanding, refer to EMF vs Terminal Voltage.
  • Flow of Electrons: The spontaneous redox reaction in the cell propels electrons from the anode (where they are released) to the cathode (where they are consumed). This flow constitutes an electric current.
  • Work from Electron Flow: When an external circuit is connected, these flowing electrons perform work on any device attached, like illuminating a bulb or powering a device. This concept can be compared to how a gravitational field operates in physics.

Factors Affecting a Cell’s Performance

The efficiency and life span of a chemical cell can be influenced by several factors:

  • Electrode Material: The choice of electrode material can significantly affect the potential difference and overall efficiency of the cell.
  • Electrolyte Concentration: The concentration of the electrolyte can influence the rate of electrode reactions, potentially affecting the cell's voltage and lifespan.
  • Temperature Impact: Elevated temperatures can accelerate chemical reactions but might also disrupt the balance within the cell, affecting its performance or life expectancy.

Practical Challenges in Chemical Cells

  • Cell Reversibility: Some cells can undergo a reverse reaction when external voltage is applied, allowing for recharging. This isn’t always desirable or efficient for all cell types. The principles of Lenz's Law are relevant in understanding these reversible reactions.
  • Depolarisers: Over time, unwanted secondary substances can accumulate at the electrodes, affecting cell performance. Depolarisers are used to counteract this phenomenon.
  • Leakage Issues: Especially with cells containing liquid electrolytes, there's a risk of leakage, leading to reduced performance and potential hazards.

Real-world Applications of Chemical Cells

Chemical cells' profound impact on modern society is evident through their diverse applications:

  • Batteries: Multiple cells can be connected in series or parallel to produce batteries. These configurations either boost the total voltage output or increase the battery's capacity.
  • Emergency Power Solutions: In scenarios where power failure is not an option, like in hospitals, chemical cells provide a reliable backup source of energy.
  • Portable Devices: From mobile phones to laptops, the compact nature and energy efficiency of chemical cells have made the miniaturisation of numerous devices possible.


In a galvanic cell, the salt bridge plays a pivotal role in maintaining the flow of electrons and the overall function of the cell. It is a U-shaped tube filled with a salt solution, facilitating the movement of ions between the two half-cells. This ion migration is essential to counterbalance the charge buildup that occurs due to the electron flow in the external circuit. Without the salt bridge, the accumulation of positive and negative charges would disrupt the redox reaction, effectively halting the cell's operation. Additionally, the salt bridge minimises the direct mixing of solutions, preventing unwanted reactions between the half-cell components.

Chemical cells, owing to their portable energy storage capability, have found their way into a myriad of applications. On a daily basis, we rely on them to power our portable electronic devices, from mobile phones and laptops to digital cameras and flashlights. On a larger scale, chemical cells, particularly in the form of batteries, are being used in electric and hybrid vehicles, promoting sustainable transport. In the realm of renewable energy, these cells store energy harvested from solar panels or wind turbines, ensuring a consistent power supply even during non-optimal conditions. The medical field also greatly benefits from chemical cells, with devices like hearing aids, insulin pumps, and life-saving pacemakers relying on them.

Concentration cells stand apart from other galvanic cells because they operate based on an ion concentration gradient rather than differences in electrode materials. In these cells, the electrodes are chemically identical, but the solutions surrounding them have different ion concentrations. As the cell functions, ions migrate to try and equilibrate the concentration difference, creating a potential difference in the process. As this equilibration progresses, the cell's potential diminishes. In contrast, typical galvanic cells derive their potential difference from the innate chemical differences between electrode materials, leading to electron transfer.

Rechargeable batteries, while a marvel of modern technology, aren't immune to wear and tear. With every charge and discharge cycle, chemical reactions ensue inside the battery. Over multiple cycles, side reactions can produce unwanted by-products that accumulate on the electrode surfaces. These by-products can hinder the flow of ions between the electrodes, effectively reducing the battery's capacity. Physical changes also pose challenges; conductive dendrites might grow and potentially short-circuit the battery, or the electrolyte may degrade over time. Structural changes in the electrode materials due to the constant stress of charging and discharging further contribute to capacity loss. While contemporary research aims to enhance battery life, the inherent chemistry and physics dictate a limited lifecycle for rechargeable batteries.

The potential difference, or cell potential, between two half-cells in a galvanic cell is measured using a voltmeter. This instrument is connected across the two half-cells without allowing them to come into direct contact. The voltmeter registers the voltage, or electromotive force (EMF), representing the maximum potential difference due to the varying tendencies of the half-cells to lose or gain electrons. These tendencies drive the spontaneous redox reaction when the circuit is complete, providing insights into the energy changes involved in the reaction and the feasibility of the chemical processes in the cell.

Practice Questions

In the context of a galvanic cell with zinc and copper electrodes, describe the processes taking place at the anode and the cathode. Explain how these processes lead to the flow of electricity in the external circuit.

In a zinc-copper galvanic cell, the anode is made of zinc and undergoes oxidation. This means that the zinc metal loses electrons to form zinc ions: Zn -> Zn2+ + 2e-. The released electrons from the anode flow through the external circuit. On the other side, the cathode, made of copper, witnesses a reduction process. Copper ions accept the electrons flowing from the anode to get reduced to form copper metal: Cu2+ + 2e- -> Cu. This flow of electrons from the anode to the cathode in the external circuit constitutes the electric current, which can be used to power devices or perform work.

Briefly discuss the significance of electrolytes in chemical cells and mention one factor that can influence the efficiency of a chemical cell.

Electrolytes in chemical cells play an essential role in facilitating the movement of ions. They maintain the cell's charge neutrality, ensuring a balance between positive and negative ions, which is crucial for the cell's proper functioning. An electrolyte can be in the form of a liquid, gel, or solid. One significant factor that can influence the efficiency of a chemical cell is the concentration of the electrolyte. It can impact the rate of electrode reactions, which, in turn, might affect the cell's voltage output and overall lifespan. Proper maintenance of electrolyte concentration ensures optimum cell performance.

Dr Shubhi Khandelwal avatar
Written by: Dr Shubhi Khandelwal
Qualified Dentist and Expert Science Educator

Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.

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