Problem 77 - Chemistry: The Central Science in SI Units, Global Edition

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

Substituting cadmium for zinc in an alkaline battery would result in a lower cell emf, due to the higher standard reduction potential of cadmium (\(-0.403 V\)) compared to zinc (\(-0.763 V\)). The environmental advantage of using nickel-metal hydride (Ni-MH) batteries over nickel-cadmium (Ni-Cd) batteries is the less toxic nature of their components, reducing the environmental impact related to cadmium pollution and health risks when properly managed and disposed of.

Step 1: (a) Background on Alkaline Batteries)

Alkaline batteries are based on the electrochemical reaction that occurs between zinc (Zn) and manganese dioxide (MnO2) in an alkaline medium, typically containing potassium hydroxide (KOH). In this context, zinc acts as the anode, where its oxidation occurs, while the manganese dioxide serves as the cathode where the reduction half-reaction takes place.

Step 2: (a) Finding the Standard Reduction Potentials)

To analyze the effect of substituting cadmium (Cd) for zinc as the anode, we must compare their standard reduction potentials, which is a measure of their tendency to gain electrons and undergo reduction. The standard reduction potential of cadmium is approximately \(-0.403 V\), while that of zinc is \(-0.763 V\).

Step 3: (a) Comparing Standard Reduction Potentials)

Since the standard reduction potential of zinc \(-0.763 V\) is lower than that of cadmium \(-0.403 V\), this means that zinc is more prone to oxidation than cadmium. As a result, if an alkaline battery is manufactured using cadmium metal rather than zinc, it will have a lower cell emf because the difference between the cathode and anode potentials will be smaller, leading to a weaker driving force for the oxidation-reduction reaction.

Step 4: (b) Environmental Impact of Nickel-Cadmium and Nickel-Metal Hydride Batteries)

The environmental advantage of using nickel-metal hydride (Ni-MH) batteries over nickel-cadmium (Ni-Cd) batteries is mainly due to the less toxic nature of nickel-metal hydride batteries components. Cadmium is a known toxic heavy metal that poses environmental and health risks when not properly managed, especially when nickel-cadmium batteries are not properly disposed of at the end of their life cycle. Cadmium can leach into soil and water, entering the food chain and causing significant harm to humans and wildlife. In contrast, nickel-metal hydride batteries contain less toxic metals and offer similar performance characteristics to nickel-cadmium batteries, making them a more environmentally friendly choice for many applications. By using nickel-metal hydride batteries, the environmental impact related to cadmium, such as pollution and health risks, can be reduced.

Key Concepts

Alkaline Batteries

Alkaline batteries are a common type of dry cell battery, often used in everyday electronics like remote controls and flashlights. They operate on a simple electrochemical mechanism. Zinc serves as the anode (a crucial part of alkaline batteries), where oxidation takes place. Manganese dioxide acts as the cathode, undergoing reduction. The two components work in an alkaline medium, typically potassium hydroxide (KOH).

These batteries are preferred because they provide a more stable voltage and a longer shelf life compared to other non-rechargeable batteries. The zinc-manganese dioxide chemical reaction helps generate the electricity needed to power devices consistently. This makes alkaline batteries a reliable choice for many applications.

Standard Reduction Potentials

Standard reduction potentials are vital in understanding how batteries work. They measure how readily any given species can gain electrons (be reduced) during a chemical reaction. For example, cadmium has a standard reduction potential of about \(-0.403 V\), while zinc's standard reduction potential is \(-0.763 V\).

The difference in these potentials can influence a battery's effectiveness. An alkaline battery using zinc as the anode will have a higher cell emf than one using cadmium because zinc is more easily oxidized. With a lower standard reduction potential, zinc provides a bigger "push" for electrons to move, driving the electrochemical reaction more effectively. This means that the battery can deliver more power before running out.

Environmental Impact of Batteries

The environmental aspects of batteries are becoming increasingly important as we look for sustainable solutions. Batteries can cause environmental issues if not disposed of properly. Nickel-cadmium batteries, for example, contain cadmium, a toxic heavy metal. If these batteries are not handled and disposed of correctly, cadmium can enter the ecosystem, posing risks to both wildlife and human health.

On the other hand, nickel-metal hydride (Ni-MH) batteries offer a more environmentally friendly option. They do not contain cadmium and still provide similar performance levels. This makes them a better choice when considering the entire lifecycle of batteries from production to disposal. By opting for batteries with less hazardous materials, we can reduce our environmental footprint.

Nickel-Metal Hydride Batteries

Nickel-metal hydride (Ni-MH) batteries have grown in popularity due to their suitability in various applications without the environmental drawbacks of nickel-cadmium batteries. These batteries use a different internal chemistry that does not rely on toxic cadmium, making them much safer for the environment.

Ni-MH batteries retain many performance benefits, such as offering a good energy density and being rechargeable, making them ideal for modern electronics like cameras and some hybrid vehicles. They are particularly notable for having a significantly lower self-discharge rate than other rechargeable batteries. This means they can hold their charge longer when not in use.

• Less toxic than Ni-Cd batteries
• Rechargeable and efficient
• Suitable for a range of electronic devices

Choosing Ni-MH batteries is often a responsible decision for users looking to minimize their environmental impact while still prioritizing performance.