Problem 75 - 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-metalhydride batteries over the nickel-cadmium batteries?

Step-by-Step Solution

Answer

Using cadmium instead of zinc in an alkaline battery would result in a higher cell emf, as the standard electrode potential for cadmium (-0.403 V) is less negative compared to zinc (-0.763 V). The main environmental advantage of using nickel-metal hydride (NiMH) batteries over nickel-cadmium (NiCd) batteries is the absence of toxic cadmium, which can cause harm to the environment and human health. NiMH batteries use a metal alloy, making them a more environmentally friendly choice compared to NiCd batteries.

Step 1: Understand the Nernst equation

The Nernst equation relates the cell emf with the standard electrode potentials and the concentration of the involved species. It is given by: \[E = E^\circ -\frac{RT}{nF}\ln Q\] where \(E\) is the cell emf, \(E^\circ\) is the standard cell emf, \(R\) is the gas constant, \(T\) is the temperature in Kelvin, \(n\) is the number of electrons transferred, \(F\) is the Faraday constant, and \(Q\) is the reaction quotient.

Step 2: Obtain the standard electrode potentials

We need to find the standard electrode potential for the reactions involving zinc and cadmium. The values are already experimentally determined and can be found in tables. For zinc and cadmium, \[E^\circ_{Zn} = -0.763\;V\] \[E^\circ_{Cd} = -0.403\;V\]

Step 3: Analyze the cell emf for the given battery

From the standard electrode potentials, we can see that the emf will be different for a cell with cadmium than for a cell with zinc. The cell emf will be higher for the cadmium cell as the potential is less negative compared to the zinc cell. Therefore, using cadmium instead of zinc will result in a higher cell emf. Now let's move to the second part of the exercise.

Step 4: Understand the differences between nickel-metal hydride and nickel-cadmium batteries

Nickel-metal hydride (NiMH) batteries and nickel-cadmium (NiCd) batteries are two different types of rechargeable batteries. The main difference between them is the material used for the negative electrode. In NiCd batteries, cadmium is used as the negative electrode, while in NiMH batteries, a metal alloy is used.

Step 5: Identify the environmental advantage of using nickel-metal hydride batteries over nickel-cadmium batteries

The main environmental advantage of using NiMH batteries over NiCd batteries is the absence of toxic cadmium. Cadmium is a hazardous heavy metal that can cause harm to the environment and human health when not disposed of properly. On the other hand, NiMH batteries use a metal alloy, which is less harmful to the environment. This makes NiMH batteries a more environmentally friendly choice compared to NiCd batteries.

Key Concepts

Nernst Equation

The Nernst equation is a foundational principle in electrochemistry, crucial for understanding how electrochemical cells function. It mathematically relates the voltage (also known as electromotive force or emf) of an electrochemical cell to the standard electrode potential, temperature, and the concentrations of the reactants and products involved in the reaction.

In its most common form, the Nernst equation is expressed as:
\[E = E^\circ - \frac{RT}{nF}\ln Q\]
where:\

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• \(E\) represents the cell emf under non-standard conditions,\
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• \(E^\circ\) is the standard emf of the cell,\
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• \(R\) is the ideal gas constant (8.314 J/mol K),\
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• \(T\) is the temperature in Kelvin,\
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• \(n\) is the number of moles of electrons exchanged,\
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• \(F\) is the Faraday constant (roughly 96500 coulombs/mol), and\
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• \(Q\) is the reaction quotient, which reflects the ratio of product activities to reactant activities.\
\

Understanding this equation is vital for predicting how changes in concentration or temperature will affect cell voltage, and for solving complex problems related to battery design and efficiency.

Standard Electrode Potentials

Standard electrode potentials are intrinsic properties of electrodes that determine their tendency to lose or gain electrons — a characteristic known as reduction potential. Measured under standard conditions (1 M concentration, 1 atm pressure, and a temperature of 298 K), these potentials are crucial for predicting the direction of electron flow in electrochemical cells and hence the viability and voltage of batteries. They are often tabulated for reference.

For instance, different metals have different standard electrode potentials; in the provided example, zinc \(E^\circ_{Zn}\) has a potential of -0.763V and cadmium \(E^\circ_{Cd}\) has a potential of -0.403V. This disparity in potentials implies that when used in electrochemical cells, different metals will confer different cell voltages. Understanding these potentials aids in the selection of appropriate materials for constructing batteries, ensuring optimal performance and energy output.

Nickel-Metal Hydride Batteries

Nickel-metal hydride (NiMH) batteries represent a significant advance in battery technology, especially over older nickel-cadmium (NiCd) batteries. A key feature of NiMH batteries is the metal hydride used as the negative electrode instead of the toxic cadmium in NiCd batteries.

Benefits of NiMH Batteries

They offer several advantages like higher energy density, which means they can store more energy for longer durations. NiMH batteries also suffer less from memory effect, an issue where NiCd batteries lose their maximum energy capacity when repeatedly recharged after being only partially discharged. Moreover, NiMH batteries are more environmentally friendly due to using less harmful materials. This shift in the battery industry towards materials with lower environmental impact reflects a broader commitment to sustainability in technology development.

Environmental Impact of Batteries

Batteries have an undeniable environmental impact, from their manufacturing process to disposal. Batteries use various heavy metals and chemicals which, if not properly managed, can lead to soil and water pollution. These contaminants pose risks to ecosystems and human health.

The transition from NiCd to NiMH batteries highlights an endeavour to reduce toxic waste in the environment, as cadmium is particularly hazardous. Nickel-metal hydride batteries use a combination of nickel, a metal alloy, and a hydrogen-absorbing alloy for the negative electrode, which is comparatively less harmful. However, even NiMH batteries must be recycled properly to mitigate their environmental footprint.

Recycling and Proper Disposal

Proper recycling and disposal systems are critical to managing the environmental impact of batteries. Comprehensive recycling programs help in recovering valuable materials and preventing toxic substances from entering the environment. Thus, environmentally conscious use and disposal of batteries are crucial for sustainable development and for protecting our planet for future generations.