Problem 77 - Chemistry The Central Science In Si Units

Question

(a) Suppose that an alkaline battery was manufactured using cadmium metal rather than zinc. What effect would this have on the cell emf? (b) What environmental advantage is provided by the use of nickel-metal hydride batteries over nickel-cadmium batteries?

Step-by-Step Solution

Answer

Replacing zinc with cadmium decreases the battery's emf. NiMH batteries are environmentally safer than NiCd because they do not contain toxic cadmium.

Step 1: Understanding the effect of using cadmium

Zinc is commonly used in alkaline batteries because it has specific electrochemical properties that contribute to the battery's cell emf (electromotive force). When cadmium replaces zinc, the cell emf changes due to the different standard electrode potentials of cadmium compared to zinc. Zinc has a standard electrode potential of -0.76 V, while cadmium has a slightly higher potential of -0.40 V.

Step 2: Calculating the change in cell emf

To determine the effect on cell emf when switching from zinc to cadmium, the difference in their standard electrode potentials is considered. The equation for cell emf is based on the reduction potentials: \[E_{cell} = E_{ ext{cathode}} - E_{ ext{anode}}\] When comparing cadmium to zinc in the anode position, because cadmium's potential is less negative than zinc's, the overall cell emf of the battery would decrease.

Step 3: Explaining environmental advantages

Nickel-metal hydride (NiMH) batteries are favored over nickel-cadmium (NiCd) batteries due to their lesser environmental impact. Cadmium is a toxic heavy metal that poses environmental hazards when batteries are disposed of improperly. NiMH batteries do not contain cadmium and therefore reduce the potential for soil and water contamination.

Key Concepts

Cell EMF

The cell electromotive force (emf) is a measure of the energy provided by a battery or electrochemical cell. It represents the voltage generated by the cell before any external circuit is connected. This is an important concept because it determines how much electrical energy is available to do work. The cell emf is calculated from the standard electrode potentials of the electrodes in the cell.
To find the cell emf, you use the formula:
\[E_{cell} = E_{\text{cathode}} - E_{\text{anode}}\]The standard electrode potential is a property of the material that helps dictate its ability to donate or accept electrons. When comparing two different materials, such as zinc and cadmium in battery chemistry, their differing standard electrode potentials will affect the cell emf. In alkaline batteries, replacing zinc with cadmium changes the cell emf because cadmium has a different, less negative standard electrode potential than zinc. This results in a lower cell emf, meaning less energy is available.

Electrode Potential

Electrode potential is the voltage associated with a half-cell in an electrochemical cell. This potential measures the tendency of a material to gain or lose electrons.
Each electrode of a battery has a standard electrode potential, which is determined under specific conditions (1 M concentration, 1 atm pressure, and 25°C). For zinc, this potential is -0.76 V, while cadmium's is -0.40 V. The higher (less negative) potential of cadmium means it does not release electrons as readily as zinc in an electrochemical reaction.
Key points to remember about electrode potential:

• A more negative value signifies a stronger tendency to lose electrons (act as an anode).
• A less negative (or more positive) potential indicates a tendency to gain electrons (act as a cathode).
• Replacing one material with another in a battery alters the cell emf due to changes in electrode potentials.

Environmental Impact

The production and disposal of batteries can have significant environmental consequences. Different batteries pose different risks based on the materials they use. Nickel-cadmium (NiCd) batteries, for example, contain cadmium, a toxic heavy metal. Improper disposal can lead to soil and water contamination, posing health risks to humans and wildlife.
Nickel-metal hydride (NiMH) batteries, on the other hand, provide a more environmentally friendly alternative. They do not contain cadmium, which significantly diminishes the environmental hazards linked to battery disposal.
Benefits of NiMH batteries:

• Reduced potential for pollution and toxicity.
• Easier and safer disposal and recycling processes.
• Safer for landfills and ecosystems, reducing long-term environmental damage.

Nickel-Metal Hydride Batteries

Nickel-metal hydride (NiMH) batteries are popular for their safety and efficiency. Unlike older nickel-cadmium batteries, they do not rely on cadmium, which is hazardous to the environment.
Features of NiMH batteries:

• Higher energy density compared to NiCd, allowing them to store more energy in a smaller space.
• Reduced environmental impact due to the absence of toxic materials.
• Longer cycling life, meaning they can be recharged many times before losing capacity.
• Ability to withstand more charge-discharge cycles, making them suitable for renewable energy storage and electronic devices.

NiMH batteries are an excellent choice for those looking for a more sustainable power source without sacrificing performance.