In some applications nickel–cadmium batteries have been replaced by nickel–zinc batteries. The overall cell reaction for this relatively new battery is: 2 H₂O(l) + 2 NiO(OH)(s) + Zn(s) → 2 Ni(OH)₂(s) + Zn(OH)₂(s) (c) A single nickel–cadmium cell has a voltage of 1.30 V. Based on the difference in the standard reduction potentials of Cd²⁺ and Zn²⁺, what voltage would you estimate a nickel–zinc battery will produce? (d) Would you expect the specific energy density of a nickel–zinc battery to be higher or lower than that of a nickel–cadmium battery?

Verified step by step guidance

1. Step 1: Understand the concept of specific energy density. Specific energy density is the amount of energy stored in a battery per unit volume or weight. It is usually measured in watt-hours per kilogram (Wh/kg) or watt-hours per liter (Wh/L).
2. Step 2: Consider the reactants and products in the given reaction. The reaction involves the conversion of NiO(OH) and Zn to Ni(OH)2 and Zn(OH)2. The specific energy density of a battery depends on the energy changes associated with these reactions.
3. Step 3: Compare the molar masses of the reactants and products. The molar mass of Zn is less than that of Cd. Therefore, for the same volume or weight of battery, more moles of Zn can be accommodated compared to Cd. This means that more reactions can occur, potentially leading to a higher energy output.
4. Step 4: Consider the standard reduction potentials of the metals involved. The standard reduction potential of Zn is less than that of Cd, which means that Zn is more easily oxidized than Cd. This could potentially lead to a higher voltage for the battery, and hence a higher energy density.
5. Step 5: Based on the above considerations, one might expect the specific energy density of a nickel-zinc battery to be higher than that of a nickel-cadmium battery. However, the actual energy density also depends on other factors such as the efficiency of the battery design and the specific conditions under which the battery is used. Therefore, experimental data would be needed to confirm this expectation.

Key Concepts

Specific Energy Density

Specific energy density refers to the amount of energy stored in a battery per unit mass, typically expressed in watt-hours per kilogram (Wh/kg). It is a crucial metric for evaluating battery performance, as higher specific energy density indicates a battery can store more energy for a given weight, making it more efficient for applications requiring lightweight power sources.

Electrochemical Reactions

Electrochemical reactions involve the transfer of electrons between chemical species, which occurs in batteries during discharge and charge cycles. Understanding the half-reactions and overall cell reactions, such as those in nickel–zinc and nickel–cadmium batteries, is essential for comparing their energy outputs and efficiencies, as these reactions dictate the voltage and capacity of the batteries.

Battery Chemistry

Battery chemistry refers to the materials and reactions that occur within a battery, influencing its performance characteristics. Nickel–cadmium (NiCd) and nickel–zinc (NiZn) batteries utilize different chemical components, which affect their energy density, cycle life, and environmental impact. Analyzing these differences helps in understanding why one battery type may outperform another in specific applications.