Passage
An automated external defibrillator (AED) is a medical device used to send an electric shock to the heart after cardiac arrest. A key component of the AED is the power source, or battery. Batteries used in AEDs need to have a good charge-to-weight ratio; they must be safe and reliable as well as rechargeable.Two types of rechargeable batteries used in early models of AEDs are the lead storage (also called lead-acid) battery and the nickel-cadmium (NiCd) battery. These batteries consist of multiple electrochemical cells that are connected in series to deliver a potential between 9 V and 18 V. A capacitor allows the AED to accumulate charge so that it can deliver between 300 V and 1,000 V.Some AEDs use a sealed lead storage battery. Lead storage batteries are robust and hold a charge for a long time. However, they have a low energy-to-weight ratio. Each lead storage cell delivers approximately 2.0 V, and a battery of four cells weighing 1000 g can provide 30 W∙h of energy.
Figure 1 Electron flow in a lead storage battery when discharging and chargingThe half reactions for the anode and cathode of a lead storage battery in 4 M of sulfuric acid (H₂SO₄) are shown in Reactions 1 and 2:
Reaction 1
Reaction 2
NiCd batteries have a higher energy-to-weight ratio than lead storage batteries but cannot hold as much charge. Each NiCd cell delivers approximately 1.3 V, and a single-cell battery weighing 120 g can provide 7.2 W∙h of energy.The half reactions at the anode and cathode for a NiCd battery in KOH are shown in Reactions 3 and 4:
Reaction 3

Reaction 4
-If energy density is the amount of energy per unit mass, which of the following statements must be true?

Question

Passage
An automated external defibrillator (AED) is a medical device used to send an electric shock to the heart after cardiac arrest. A key component of the AED is the power source, or battery. Batteries used in AEDs need to have a good charge-to-weight ratio; they must be safe and reliable as well as rechargeable.Two types of rechargeable batteries used in early models of AEDs are the lead storage (also called lead-acid) battery and the nickel-cadmium (NiCd) battery. These batteries consist of multiple electrochemical cells that are connected in series to deliver a potential between 9 V and 18 V. A capacitor allows the AED to accumulate charge so that it can deliver between 300 V and 1,000 V.Some AEDs use a sealed lead storage battery. Lead storage batteries are robust and hold a charge for a long time. However, they have a low energy-to-weight ratio. Each lead storage cell delivers approximately 2.0 V, and a battery of four cells weighing 1000 g can provide 30 W∙h of energy.

Figure 1 Electron flow in a lead storage battery when discharging and chargingThe half reactions for the anode and cathode of a lead storage battery in 4 M of sulfuric acid (H₂SO₄) are shown in Reactions 1 and 2:

Reaction 1

Reaction 2
NiCd batteries have a higher energy-to-weight ratio than lead storage batteries but cannot hold as much charge. Each NiCd cell delivers approximately 1.3 V, and a single-cell battery weighing 120 g can provide 7.2 W∙h of energy.The half reactions at the anode and cathode for a NiCd battery in KOH are shown in Reactions 3 and 4:

Reaction 3

Reaction 4
-If energy density is the amount of energy per unit mass, which of the following statements must be true?

Quizplus · Accepted Answer

11ecba2d_11b8_7abc_a3bf_ab2bd1011a78_MD0008_00 The electromotive force E°_cell is the work that is done per unit of charge, or joules of work per coulomb of charge. The amount of work that a battery does is due to the conversion of chemical energy into electrical energy (joules) by an oxidation-reduction reaction. Energy density relates the E°_cell of a battery to its mass, in which a battery with a high energy density has a high energy output, a low weight, or both. According to the passage, one of the advantages of NiCd batteries is that they have a higher energy-to-weight ratio than lead storage batteries. Even though lead-acid batteries have a higher E°_cell, NiCd batteries have more energy per unit mass and are smaller in size. Therefore NiCd batteries have a higher energy density. The difference in energy density between the batteries can be verified by calculating the values from the energy output and battery mass information given in the passage. A 1000 g (1 kg) lead storage battery that provides 30 W∙h of power has an energy density of 30 W∙h/kg. 11ecba2d_11b8_7abd_a3bf_9db203f913bd_MD0008_00 However, a 120 g (0.12 kg) NiCd battery that provides 7.2 W∙h of power has an energy density of 60 W∙h/kg. 11ecba2d_11b8_7abe_a3bf_599fbe8ee6da_MD0008_00 (Choice B) Lead storage batteries provide about 2.0 V of charge whereas NiCd batteries provide 1.3 V. The amount of energy a cell provides is directly proportional to the potential E°_cell, so lead storage batteries produce more energy in absolute terms than NiCd batteries. (Choice C) Based on the oxidation-reduction half-reactions for the NiCd battery and the lead storage battery, both reactions produce 2 moles of electrons per 1 mole of Cd or Pb that is oxidized. (Choice D) Lead storage batteries have a lower energy-to-weight ratio compared to NiCd batteries, meaning lead storage batteries are heavier relative to the amount of energy they produce. Therefore, they do not have the same energy density. Educational objective: Energy density is the amount of energy produced by a battery per unit of mass. It is used to compare the energy output of batteries, or the work done by the battery. The electromotive force that a battery produces on discharge is the amount of work done per unit of charge.

Passage An Automated External Defibrillator (AED) Is a Medical Device Used