Question

(a) What happens to the emf of a battery as it is used? Why does this happen? (b) The AA-size and D-size alkaline batteries are both 1.5-V batteries that are based on the same electrode reactions. What is the major difference between the two batteries? What performance feature is most affected by this difference?

Short answer

(a) As a battery is used, its emf decreases due to the reduction in the concentration of chemicals inside the battery and the increase in the internal resistance. This is because the potential difference between the electrodes decreases as the concentration of reactants decreases and internal resistance causes the electrical energy to be wasted as heat. (b) The major difference between AA-size and D-size alkaline batteries is their size, with D-size having a larger volume than AA-size. Both batteries produce 1.5 Volts using the same electrode reactions, but the D-size battery contains a larger amount of reactants. Consequently, the performance feature most affected by this size difference is the capacity. A D-size battery can store more energy, providing power for a longer time or supplying a higher current for a given time, resulting in a longer lifespan and more energy delivery compared to an AA-size battery.

Step-by-step solution

a) Emf change and reason as battery is used

As a battery is used, its emf decreases. This happens due to the reduction in the concentration of chemicals inside the battery and the increase in the internal resistance. Initially, a battery has a certain concentration of chemicals inside it. As these chemicals react to produce a flow of electrons (current), the concentration of reactants decreases, and the concentration of products increases. As a result, the potential difference between the electrodes decreases, causing a decrease in emf. Additionally, as the battery is used, some of its components may undergo irreversible changes, leading to the development of internal resistance. This internal resistance causes a part of the electrical energy to be wasted as heat, further reducing the effective emf.

b) Major difference between AA-size and D-size alkaline batteries

The major difference between AA-size and D-size alkaline batteries is their size, i.e., D-size has a larger volume than AA-size. Both batteries use the same electrode reactions to generate 1.5 Volts, but the D-size battery contains a larger amount of the chemical reactants due to its increased size.

b) Performance feature affected by the size difference

Due to the larger amount of reactants in the D-size battery, the performance feature most affected by the size difference between AA-size and D-size batteries is the capacity. The energy storage capacity of a battery is directly related to the amount of reactants it contains. A D-size battery can store more energy than an AA-size battery, allowing it to provide power for a longer time, or supply a higher current for a given time, depending on the specific application. Essentially, the D-size battery has a longer lifespan and can deliver more energy overall compared to the AA-size battery.

Key concepts

Electromotive Force (EMF)

The electromotive force (EMF) of a battery is a fundamental concept in electrochemistry. EMF is the measure of the energy provided by a battery per unit charge, essentially the voltage output when no current is flowing. However, it's important to note that the EMF is not the voltage you typically measure when the battery is in use. As a battery operates, its EMF can decrease due to several factors.

• Decrease in Chemical Concentration: The chemical reactions within a battery lead to a consumption of reactants, which decreases their concentration. This reduces the potential difference, thereby lowering the EMF.
• Development of Internal Resistance: Over time, as the battery is used, materials inside the battery might degrade or change in a way that increases resistance. This resistance uses some of the potential energy for itself, decreasing the total EMF available for external work.

Understanding EMF is crucial for determining the overall performance and lifespan of a battery as it is the driving force for the energy output.

Internal Resistance

Internal resistance is a critical factor that can significantly affect a battery's performance and efficiency. It refers to the opposition to current flow within the battery itself. Several factors contribute to internal resistance:

• Material Degradation: As a battery is used, its components, such as the electrodes, may degrade, leading to increased resistance.
• Temperature Fluctuations: Extreme temperatures can also change internal resistance. High temperatures may lower resistance, while low temperatures can increase it.
• Age of the Battery: Older batteries tend to have higher internal resistance due to accumulated wear and irreversible chemical changes.

The increase in internal resistance not only causes a reduction in the effective EMF but also results in energy being wasted as heat, reducing the battery's efficiency. Managing and minimizing internal resistance is key to maintaining a battery's performance over its lifespan.

Battery Capacity

Battery capacity is a measure of the total amount of energy a battery can store and deliver. It is closely linked to the size of the battery and the amount of reactants it contains. Battery capacity is usually measured in ampere-hours (Ah).

• Influence of Battery Size: Larger batteries, like D-size compared to AA-size, contain more reactants, leading to a higher capacity. This means they can deliver power for longer periods or support higher current draws.
• Impact on Performance: High-capacity batteries last longer, providing more energy before needing replacement. This means fewer interruptions and potentially lower costs over time.
• Application Specificity: Different devices may require varying battery capacities, influencing the choice between larger and smaller batteries.

Choosing the right battery capacity ensures efficient power delivery and matches the energy needs of different applications, whether for long-term use or quick bursts of energy.

Electrochemical Reactions

Electrochemical reactions are the heart of battery functionality. These reactions involve the transfer of electrons from one substance to another, producing electric current as a byproduct. In batteries, these reactions often take place through oxidation and reduction processes.

• Oxidation: The anode in the battery undergoes oxidation, losing electrons. This loss is what generates the flow of electrons through the external circuit.
• Reduction: Meanwhile, at the cathode, reduction occurs where electrons are gained from the external circuit, completing the circuit.
• Reaction Efficiency: The efficiency and rate of these reactions dictate the energy output and longevity of the battery. Highly efficient reactions deliver consistent power and extend battery life.

Understanding these reactions helps in designing better batteries with improved energy outputs and lifespans. Advances in materials and techniques aim to enhance these electrochemical processes, making batteries more efficient and environmentally friendly.