Free First Law of Thermodynamics MCQs with Answers

1. The first law of thermodynamics is a statement of

• A. the conservation of energy
• B. the conservation of mass
• C. the increase of entropy
• D. the constancy of temperature

Explanation: Energy can be converted from one form to another but can neither be created nor destroyed, which is expressed as the change in internal energy equalling the heat added to the system plus the work done on it. It follows that a machine producing energy from nothing is impossible. The tendency of entropy to increase is the second law, an entirely separate statement.

2. When 1 mole of methane burns, the energy released is used to heat water. This illustrates that

• A. energy is created during combustion
• B. energy is destroyed during combustion
• C. energy is transferred from the system to the surroundings but the total remains constant
• D. the enthalpy of the system increases

Explanation: The chemical energy stored in the bonds of methane and oxygen is converted into heat, which passes to the water, so nothing is created or destroyed and the first law holds. The system loses enthalpy, which is why the value is negative, while the surroundings gain the same amount. Every combustion process is an example of this transfer.

3. The mathematical statement of the first law of thermodynamics is

• A. change in internal energy equals heat added plus work done on the system
• B. change in internal energy equals heat added minus temperature
• C. enthalpy equals internal energy divided by pressure
• D. heat equals work in every process

Explanation: The internal energy of a system can be raised either by heating it or by doing work on it, and the equation simply accounts for both routes. When the system does work on the surroundings, for example by expanding against the atmosphere, that term becomes negative. The sign conventions must be stated clearly, since they are the commonest source of error.

4. If 100 kJ of heat is absorbed by the system and 40 kJ of work is done on the system, what is the change of internal energy?

• A. -60 kJ
• B. +60 kJ
• C. -140 kJ
• D. +140 kJ

Explanation: By the first law, the change in internal energy is the heat added to the system plus the work done on it, so 100 plus 40 gives plus 140 kJ. Both inputs raise the internal energy, which is why they add rather than cancel. Had the system done 40 kJ of work on the surroundings instead, the answer would have been plus 60 kJ, which is the distractor offered.