Problem 77 - 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

(a) Using cadmium instead of zinc would result in an increase in the cell emf by 0.36 V due to the difference in their standard reduction potentials, as \(\Delta E^0 = E^0_{Cd} - E^0_{Zn} = 0.36 V\). (b) The environmental advantage of using nickel-metal hydride (NiMH) batteries over nickel-cadmium (NiCd) batteries is the absence of toxic cadmium in NiMH batteries. Cadmium can cause significant environmental damage when not properly disposed of or recycled. In comparison, the metal hydride used in NiMH batteries has a lower environmental impact and toxicity, making them a more environmentally friendly option.

Step 1: (a) Compare standard reduction potentials of cadmium and zinc

To determine how using cadmium instead of zinc would affect cell emf, we need to compare the standard reduction potentials of the two metals. For zinc, the standard reduction potential is as follows: \[Zn^{2+}(aq) + 2 e^- \rightarrow Zn(s) \quad E^0 = -0.76 V\] For cadmium, the standard reduction potential is as follows: \[Cd^{2+}(aq) + 2 e^- \rightarrow Cd(s) \quad E^0 = -0.40 V\]

Step 2: (a) Calculate the change in cell emf due to cadmium use

The difference in the standard potentials of cadmium and zinc can now be calculated as: \[\Delta E^0 = E^0_{Cd} - E^0_{Zn}\] Plug in the values: \[\Delta E^0 = -0.40 - (-0.76)\] Now, calculate the value: \[\Delta E^0 = 0.36 V\]

Step 3: (a) Interpret the effect of using cadmium on cell emf

Since the difference in standard reduction potentials is positive, we find that using cadmium instead of zinc would result in an increase in the cell emf by 0.36 V. This means that the alkaline battery would provide more electrical potential when using cadmium as compared to using zinc.

Step 4: (b) Compare nickel-metal hydride and nickel-cadmium batteries

Now, let's discuss the environmental advantage of using nickel-metal hydride (NiMH) batteries over nickel-cadmium (NiCd) batteries. Both NiMH and NiCd batteries share similar positive electrodes (nickel hydroxide) and similar alkaline electrolytes. However, the negative electrodes of NiMH batteries contain a metal hydride instead of cadmium as in NiCd batteries.

Step 5: (b) Assess the environmental advantage of using NiMH batteries

The primary environmental advantage of using NiMH batteries over NiCd batteries lies in the absence of toxic cadmium in NiMH batteries. Cadmium is a heavy metal that can be toxic to humans, animals, and the environment. When not properly disposed of or recycled, cadmium can leach into soil and water systems, resulting in significant and long-lasting environmental damage. On the other hand, the metal hydride used in the negative electrode of NiMH batteries has a relatively lower environmental impact and toxicity compared to cadmium. This makes NiMH batteries a more environmentally friendly option than NiCd batteries.