APCE Thermodynamics & Thermochemistry 1 — Questions and Answers
Question 1: What does the first law of thermodynamics state?
- Energy can be created and destroyed.
- Energy is conserved in all processes. (Correct answer)
- Energy is lost as heat.
- Energy can only be transferred by work.
Correct answer: Energy is conserved in all processes.
The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed in an isolated system. It can only be transferred or transformed from one form to another. This means the total energy of the universe remains constant.
Question 2: What is enthalpy (ΔH)?
- The total energy of the system.
- The heat content of a system. (Correct answer)
- The change in pressure.
- The energy released during a chemical reaction.
Correct answer: The heat content of a system.
Enthalpy (ΔH) is a thermodynamic property that represents the total heat content of a system at constant pressure. While it is often used to describe the heat released or absorbed during a chemical reaction (enthalpy change), it fundamentally refers to the internal energy of the system plus the product of its pressure and volume.
Question 3: What is the difference between exothermic and endothermic reactions?
- Exothermic reactions absorb heat, and endothermic reactions release heat.
- Exothermic reactions release heat, and endothermic reactions absorb heat. (Correct answer)
- Both reactions release heat.
- Both reactions absorb heat.
Correct answer: Exothermic reactions release heat, and endothermic reactions absorb heat.
Exothermic reactions are chemical processes that release energy, typically in the form of heat, into the surroundings, causing the temperature of the surroundings to rise (ΔH < 0). Conversely, endothermic reactions absorb energy from the surroundings, usually as heat, leading to a decrease in the surroundings' temperature (ΔH > 0).
Question 4: What is Gibbs free energy (ΔG)?
- The total heat energy of a system.
- The energy available to do work at constant temperature and pressure. (Correct answer)
- The change in pressure of a system.
- The energy released during an exothermic reaction.
Correct answer: The energy available to do work at constant temperature and pressure.
Gibbs free energy (ΔG) is a thermodynamic potential that measures the "useful" or process-initiating energy obtainable from an isothermal, isobaric thermodynamic system. It determines the spontaneity of a process: a negative ΔG indicates a spontaneous reaction, while a positive ΔG indicates a non-spontaneous reaction under those conditions.
Question 5: What is the standard enthalpy of formation (ΔHf°)?
- The heat released when a substance is burned.
- The heat change when one mole of a compound is formed from its elements in their standard states. (Correct answer)
- The heat required to break a bond.
- The heat released during a chemical reaction.
Correct answer: The heat change when one mole of a compound is formed from its elements in their standard states.
The standard enthalpy of formation (ΔHf°) is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements, with all substances in their most stable physical states under standard conditions (1 atm pressure, 298 K). This value is a fundamental thermodynamic property used to calculate enthalpy changes for other reactions.
Question 6: What is the second law of thermodynamics?
- The total energy of the universe is constant.
- Entropy of an isolated system decreases over time.
- Entropy of an isolated system increases over time. (Correct answer)
- Energy cannot be transferred as heat.
Correct answer: Entropy of an isolated system increases over time.
The second law of thermodynamics states that the total entropy (disorder or randomness) of an isolated system can only increase over time, or remain constant in ideal reversible processes. It implies that natural processes tend towards a state of greater disorder and that heat cannot spontaneously flow from a colder body to a hotter body.
Question 7: What is the relationship between temperature and entropy?
- As temperature increases, entropy decreases.
- As temperature increases, entropy increases. (Correct answer)
- As temperature decreases, entropy increases.
- Temperature has no effect on entropy.
Correct answer: As temperature increases, entropy increases.
Entropy is a measure of the disorder or randomness of a system. As temperature increases, particles gain more kinetic energy, leading to increased molecular motion and a greater number of possible microstates. This increased freedom of movement and distribution of energy results in a higher degree of disorder, hence an increase in entropy.
Question 8: What is the relationship between enthalpy change (ΔH) and heat in a reaction?
- Enthalpy change is unrelated to heat.
- Enthalpy change represents the heat released or absorbed at constant pressure. (Correct answer)
- Enthalpy change is the energy required to break bonds.
- Enthalpy change represents the temperature change in a reaction.
Correct answer: Enthalpy change represents the heat released or absorbed at constant pressure.
Enthalpy change (ΔH) is a direct measure of the heat exchanged between a chemical system and its surroundings when the reaction occurs at constant pressure. A negative ΔH indicates heat is released (exothermic), while a positive ΔH indicates heat is absorbed (endothermic). It is a crucial thermodynamic quantity for characterizing chemical reactions.
Question 9: What is the heat of reaction?
- The total heat content of reactants.
- The heat absorbed or released during a reaction, represented as ΔH. (Correct answer)
- The energy required to break bonds.
- The energy required to form products.
Correct answer: The heat absorbed or released during a reaction, represented as ΔH.
The heat of reaction, denoted as ΔH (enthalpy change), quantifies the total heat absorbed or released during a chemical reaction. A negative ΔH indicates an exothermic reaction where heat is released, while a positive ΔH signifies an endothermic reaction where heat is absorbed. This value represents the difference in enthalpy between the products and reactants.
APCE Thermodynamics & Thermochemistry 1
What does the first law of thermodynamics state?