Problem 79 - Chemistry The Central Science

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

In short, (a) using cadmium metal instead of zinc in an alkaline battery would result in a higher cell emf by 0.36 V due to its higher standard reduction potential. (b) The environmental advantage of using nickel-metal-hydride (NiMH) batteries over nickel-cadmium (NiCd) batteries lies in the absence of toxic heavy metals (like cadmium) in NiMH batteries, making them less harmful to the environment and more environmentally-friendly.

Step 1: Part a: Effect on cell emf

To analyze the effect of using cadmium instead of zinc in an alkaline battery, we first need to compare their standard reduction potentials. The standard reduction potential for zinc is: \( Zn^{2+} + 2e^− \rightarrow Zn\); \(E^0 = −0.76 V\) And for cadmium: \( Cd^{2+} + 2e^− \rightarrow Cd\); \(E^0 = −0.40 V\) Higher standard reduction potential means higher tendency for the reduction to occur. Therefore, cadmium has a higher tendency to get reduced as compared to zinc. Now, let's calculate the difference in the cell emf if we were to replace zinc with cadmium. We can do this by subtracting the standard reduction potentials of zinc and cadmium: \(ΔE^0 = E^0_{Cd} - E^0_{Zn} = -0.40 V - (-0.76 V) = 0.36 V\) Since the difference is positive, the cell emf will be higher when using cadmium metal instead of zinc (by 0.36 V).

Step 2: Part b: Environmental advantage

Nickel-metal-hydride (NiMH) batteries and nickel-cadmium (NiCd) batteries are both types of rechargeable batteries. The main difference between them is the materials used for their negative electrodes; NiCd batteries use cadmium while NiMH batteries use a metal alloy for their negative electrode. The environmental advantage of using NiMH batteries over NiCd batteries comes from the fact that cadmium is a toxic heavy metal. The disposal of NiCd batteries can lead to environmental pollution and harm, as cadmium can contaminate soil and water sources, posing a threat to both human health and wildlife. NiMH batteries, on the other hand, do not contain toxic heavy metals like cadmium. This makes them a more environmentally-friendly choice compared to NiCd batteries.

Key Concepts

Standard Reduction Potential

In the world of chemistry, the standard reduction potential is a crucial concept. It helps us understand how likely a substance is to gain electrons, which is called reduction. Each element or compound has its unique standard reduction potential, denoted by \( E^0 \).
For instance, zinc has a standard reduction potential of \( -0.76 \, V \) while cadmium's is \( -0.40 \, V \). This means that cadmium has a greater tendency to be reduced than zinc. The higher the value, the easier it is for the reduction process to take place.

• A more positive standard reduction potential indicates a stronger oxidizing agent, meaning it can more readily accept electrons.
• This property is essential when designing batteries as it influences the voltage the battery can provide.

Understanding standard reduction potentials helps us predict and manipulate reactions, especially in electrochemistry.

Cell EMF

The electromotive force (EMF) of a cell, often simply called cell voltage, indicates the maximum potential difference between two electrodes in a battery when no current flows. It’s a measure of the energy provided by the battery per charge as it moves through the complete circuit.
EMF is calculated using the standard reduction potentials of the two electrodes involved.

For the equation:\[ \Delta E^0 = E^0_{cathode} - E^0_{anode} \]
In our example, to find the change in EMF when replacing zinc with cadmium, we observe the following steps:

• Identify the \( E^0 \) values for both metals.
• Subtract the anode's standard reduction potential \( (E^0_{anode}) \) from the cathode's \( (E^0_{cathode}) \).
• A positive change in \( \Delta E^0 \) means the battery will have a higher voltage output.

This change in EMF helps us understand battery performance and efficiency.

Environmental Impact of Batteries

Batteries can vary significantly in their environmental impact. This depends largely on the materials they consist of. Some batteries, like the nickel-cadmium (NiCd), use heavy metals that are harmful to the environment.
Heavy metals like cadmium pose risks because they are toxic and can leach into soil and water systems upon disposal, leading to long-term environmental harm.
NiMH (nickel-metal-hydride) batteries present a more environmentally sustainable option than NiCd batteries.

• NiMH batteries don’t contain toxic heavy metals, reducing the risk of contamination.
• This makes them safer to produce, handle, and dispose of, lowering their overall environmental footprint.

Choosing battery types wisely and understanding their impact can lead to a safer and more eco-friendly practice in everyday technology use.