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For part (a), we will compare the standard electrode potentials of zinc and cadmium, since this will determine the change in cell emf. Let's find out the standard electrode potentials for zinc and cadmium. Standard electrode potential for zinc: Zn -> Zn^2+ + 2e-; \(E^0_{Zn}\) = -0.76 V Standard electrode potential for cadmium: Cd -> Cd^2+ + 2e-; \(E^0_{Cd}\) = -0.40 V Now, let's determine the change in cell emf when using cadmium instead of zinc.

First, we need to find the difference between the standard electrode potentials of zinc and cadmium. Difference in standard electrode potentials: \(ΔE^0 = E^0_{Cd} - E^0_{Zn}\) \(ΔE^0 = -0.40 V - (-0.76 V) = 0.36 V\)

Since the standard electrode potential of cadmium is less negative than that of zinc, substituting zinc with cadmium will make the cell emf more positive (increase the cell emf) by 0.36V. This means that the battery using cadmium will have a higher cell emf than the one using zinc.

For part (b), discussing the environmental advantage of using nickel-metal hydride (NiMH) batteries over nickel-cadmium (NiCd) batteries: The primary environmental advantage of using NiMH batteries instead of NiCd batteries is that NiMH batteries don't contain toxic cadmium. Cadmium is highly toxic and can cause various environmental issues. Cadmium can contaminate the soil and water, which leads to its presence in plants, animals, and humans, causing serious health issues. When NiCd batteries are not disposed of properly, they can leak cadmium into the environment, making it essential that they are recycled properly. On the other hand, NiMH batteries, which use a metal hydride instead of cadmium, are considered more environmentally friendly due to the absence of toxic cadmium. While still requiring proper disposal and recycling, the overall environmental impact of NiMH batteries is less severe than that of NiCd batteries.