Energy in systems (thermal, chemical, electrical) Transitional Reasoning Download PDF

Questions

1. 1.

   An endothermic synthesis is conducted inside a sealed reaction vessel immersed in a liquid bath. During the reaction, an internal element delivers 80 kJ of electrical energy to the mixture, while heat transfer from the bath supplies an additional 30 kJ of thermal energy across the vessel boundary. Temperature measurements confirm that the internal thermal energy of the mixture is identical at the start and conclusion of the process. Based on the conservation of energy, which statement correctly determines the change in the stored chemical energy of the system?

   • A) It increases by 50 kJ because the absorbed thermal energy offsets part of the electrical energy required to drive the reaction.
   • B) It increases by 110 kJ because both the electrical energy supplied and the thermal energy absorbed are converted into chemical energy.
   • C) It increases by 80 kJ because only electrical work can alter chemical bond energy, whereas thermal energy only maintains temperature.
   • D) It decreases by 110 kJ because the reactants must release internal chemical energy to absorb the incoming electrical and thermal energy.
2. 2.

   An electrochemical battery stores 360 kJ of chemical energy that can be transformed during discharge. In a low-current test, the battery delivers 324 kJ of electrical energy to an external circuit, while the remaining energy is dissipated as thermal energy within the battery casing. In a high-current test using an identical battery, internal resistance causes the thermal energy dissipated by the battery to increase to 72 kJ. Assuming all stored chemical energy is consumed, how much electrical energy is delivered to the external circuit in the high-current test, and how does the energy distribution explain this change?

   • A) 288 kJ; a greater fraction of the stored chemical energy is transformed into internal thermal energy, reducing the electrical energy transferred to the external circuit.
   • B) 288 kJ; the high current destroys a portion of the chemical energy, so less total energy is conserved across the system.
   • C) 432 kJ; the additional thermal energy produced inside the battery is added to the chemical energy to supply extra electrical energy to the circuit.
   • D) 216 kJ; the electrical energy delivered decreases by twice the internal thermal dissipation because thermal and chemical energies oppose each other.
3. 3.

   An electrically preheated catalytic reactor in an automotive research facility is tested to evaluate its energy balance during cold-start operation. An internal resistance element draws 50 kJ of electrical energy to preheat the catalyst, after which an exothermic oxidation reaction releases 120 kJ of stored chemical energy from fuel vapors inside the chamber. During the operating cycle, the internal thermal energy of the reactor structure and exhaust gas increases by 145 kJ, while the remaining energy is transferred as heat to the surrounding laboratory. Applying the principle of conservation of energy to the reactor, how much thermal energy is dissipated to the surroundings, and how are the input energies transformed?

   • A) 70 kJ, because the electrical energy is consumed solely to activate the catalyst, leaving only the chemical energy to warm the system.
   • B) 95 kJ, because only the electrical energy directly supplies heat, while the chemical energy remains trapped in the exhaust gas molecules.
   • C) 25 kJ, because both the supplied electrical energy and the released chemical energy are transformed into thermal energy within the reactor.
   • D) 315 kJ, because the energy dissipated to the surroundings equals the combined sum of the electrical input, chemical energy, and internal thermal increase.
4. 4.

   A stationary micro-cogeneration unit in an eco-home operates at steady state by consuming fuel gas to simultaneously supply electricity and domestic hot water. During a test run, the incoming fuel provides 250 MJ of chemical energy, producing 100 MJ of useful electrical energy and delivering 115 MJ of thermal energy to a circulating water circuit. Assuming all non-recovered energy is dissipated as heat from the unit's casing to the surrounding utility room, which statement correctly determines the dissipated heat and the total thermal energy produced?

   • A) The casing dissipates 15 MJ of thermal energy to the room, and the total thermal energy generated by chemical reactions within the system is 115 MJ.
   • B) The casing dissipates 50 MJ of thermal energy to the room, and the total thermal energy generated by chemical reactions within the system is 165 MJ.
   • C) The casing dissipates 135 MJ of thermal energy to the room, and the total thermal energy generated by chemical reactions within the system is 235 MJ.
   • D) The casing dissipates 35 MJ of thermal energy to the room, and the total thermal energy generated by chemical reactions within the system is 150 MJ.
5. 5.

   A portable cooler uses a chemical battery to operate an electric thermoelectric cooling module. In one cycle, the battery consumes 120 kJ of stored chemical energy, delivering 100 kJ of electrical energy to the module while 20 kJ is lost as heat directly from the battery casing. The cooling module absorbs 50 kJ of thermal energy from the insulated storage compartment and transfers heat to the surroundings through an external radiator. Applying the principle of conservation of energy to the cooling module, how much thermal energy does the radiator discharge to the surroundings?

   • A) 50 kJ, because the radiator discharges only the thermal energy extracted from the storage compartment.
   • B) 150 kJ, because the radiator must discharge both the electrical energy delivered to the module and the thermal energy extracted from the storage compartment.
   • C) 70 kJ, because the radiator discharges the total chemical energy consumed by the battery minus the thermal energy extracted from the storage compartment.
   • D) 170 kJ, because the radiator must discharge the entire chemical energy consumed by the battery in addition to the thermal energy extracted from the storage compartment.
6. 6.

   To determine the energy content of a fuel sample, an investigator places the fuel and excess oxygen inside a rigid, thermally insulated bomb calorimeter and supplies a measured electrical energy input $E_{\text{elec}}$ to an ignition wire. The electrical energy ignites the sample, allowing it to undergo complete combustion and release stored chemical energy. After the reaction stops and the system returns to internal thermal equilibrium, the thermal energy of the calorimeter and its contents has increased by $\Delta E_{\text{th}}$. Assuming no energy is exchanged with the external environment, which statement correctly expresses the energy balance of this system?

   • A) The increase in thermal energy $\Delta E_{\text{th}}$ is equal to $E_{\text{elec}}$, because chemical bonds only store energy and cannot contribute to thermal changes without external mechanical work.
   • B) The decrease in the chemical potential energy of the reactants equals $\Delta E_{\text{th}} + E_{\text{elec}}$, because electrical ignition reduces the total energy available to become heat.
   • C) The increase in thermal energy $\Delta E_{\text{th}}$ equals the electrical energy input $E_{\text{elec}}$ plus the decrease in the chemical potential energy of the reactants.
   • D) The electrical energy input $E_{\text{elec}}$ must equal the sum of $\Delta E_{\text{th}}$ and the chemical potential energy released, because electrical work must balance all internal thermal changes.
7. 7.

   A high-power charging station delivers electrical energy to recharge a depleted battery pack in an electric utility vehicle. During the rapid charging cycle, temperature sensors detect that the battery cells warm up, prompting an active cooling system to dissipate heat to the surrounding air. Based on the principle of conservation of energy applied to the battery as the system, which statement correctly describes the relationship between the energy pathways?

   • A) The total electrical energy supplied is less than the gain in stored chemical energy because ambient heat is absorbed to drive the charging process.
   • B) The supplied electrical energy is completely converted into chemical potential energy, while the measured thermal energy arises solely from mechanical friction in the cooling unit.
   • C) The net increase in stored chemical energy equals the total electrical energy delivered plus the thermal energy transferred to the cooling air.
   • D) The total electrical energy supplied to the battery equals the increase in its stored chemical energy plus the total thermal energy transferred to the surroundings.
8. 8.

   An industrial water-electrolysis unit receives a continuous supply of electrical energy to split liquid water into hydrogen and oxygen gases. Due to internal electrical resistance and electrochemical overpotentials, the electrolyte solution heats up and continuously transfers heat to an active cooling system. According to the principle of conservation of energy applied to this system, which statement accurately describes the energy pathways and balance?

   • A) The electrical energy supplied is entirely converted into chemical potential energy because bond breaking in endothermic reactions cannot release thermal energy.
   • B) The chemical potential energy gained by the gases must equal the thermal energy removed by the cooling system because electrical energy acts only as an activation trigger.
   • C) The increase in chemical potential energy of the produced gases exceeds the supplied electrical energy because additional thermal energy from the surroundings is absorbed into the molecular bonds.
   • D) The electrical energy supplied to the unit equals the net increase in chemical potential energy of the produced gases plus the total thermal energy transferred to the cooling system and surroundings.
9. 9.

   An off-grid field station uses a thermoelectric generator mounted between a biomass stove and an external heatsink to recharge a lithium-ion battery pack. As the fuel burns at a steady rate, the resulting temperature difference across the module drives an electric current through the charging circuit. Considering the energy transformations across the complete system, which statement correctly describes the energy pathways and conservation involved in this process?

   • A) The electrical energy delivered to the battery pack exceeds the thermal energy conducted across the module because the thermoelectric material acts as an active energy multiplier.
   • B) Chemical energy from the biomass is directly converted into electrical energy without generating thermal energy, allowing complete conservation without environmental heat loss.
   • C) Chemical energy from the biomass is released as thermal energy, part of which is converted into electrical energy and stored as chemical energy in the battery, while the rest dissipates as thermal energy.
   • D) The chemical energy accumulated in the battery pack is exactly equal to the chemical energy lost by the consumed biomass because conversions between identical energy forms are inherently lossless.
10. 10.

    In a well-insulated laboratory calorimeter, a student connects a chemical cell to a submerged heating resistor to warm a sample of pure water. Over a five-minute period, the cell discharges steadily and the temperature of the water rises, while heat exchange with the surrounding room remains negligible. Which statement correctly traces the primary energy transformations and conservation in this system?

    • A) Chemical energy in the cell is converted into electrical energy, which is then transformed into thermal energy, keeping the total energy of the system constant.
    • B) Electrical energy initially stored in the cell is directly converted into thermal energy, producing a net increase in the total energy of the calorimeter.
    • C) Chemical energy is converted directly into thermal energy without an electrical transfer step, causing the cell's potential energy to vanish from the system.
    • D) Thermal energy gained by the water exceeds the chemical energy lost by the cell because electrical current multiplies energy during conduction.