Thermochemistry concept questions and answers

  1. What is thermochemistry? Thermochemistry is a branch of physical chemistry that studies the relationship between heat, work, and chemical reactions or with the physical changes of state within the confines of the laws of thermodynamics.

  2. What are the two laws of thermodynamics that are most important in thermochemistry? The first law of thermodynamics (Law of Conservation of Energy) and the second law of thermodynamics (Entropy increases in an isolated system) are the most important laws in thermochemistry.
  3. What is enthalpy (H), and how is it used in thermochemistry? Enthalpy is a state function that’s used to measure the total heat content of a thermodynamic system. It is used in thermochemistry to calculate the heat change in reactions occurring at constant pressure.
  4. What is the difference between exothermic and endothermic reactions? Exothermic reactions are reactions that release heat into the surrounding environment, whereas endothermic reactions are reactions that absorb heat from the surrounding environment.
  5. What does the heat of formation (ΔHf°) refer to in thermochemistry? Heat of formation, or enthalpy of formation, refers to the change in enthalpy when one mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions.
  6. What is Hess’s Law and how is it useful? Hess’s Law states that the total enthalpy change for a reaction is independent of the pathway or number of steps from the initial to the final state; it depends only on the initial and final states. It is useful for calculating enthalpies for reactions that are difficult to measure directly.
  7. How do we calculate the enthalpy change for a reaction using bond enthalpies? The enthalpy change for a reaction can be calculated using bond enthalpies by taking the sum of the bond enthalpies of the reactants and subtracting from it the sum of the bond enthalpies of the products.
  8. What does spontaneity of a reaction mean in thermochemistry? Spontaneity in thermochemistry refers to the inherent ability of a reaction to occur without the input of external energy. Spontaneous reactions often release energy, though this is not always the case. The Gibbs free energy change is used to predict the spontaneity of a reaction.
  9. What is Gibbs free energy (G) and how is it related to spontaneity and equilibrium? Gibbs free energy is a thermodynamic potential that measures the maximum reversible work a system can perform at constant temperature and pressure. A negative change in Gibbs free energy indicates a spontaneous process, while at equilibrium, the Gibbs free energy change is zero.
  10. How does entropy (S) contribute to the spontaneity of a reaction? Entropy, a measure of disorder or randomness, contributes to the spontaneity of a reaction through the second law of thermodynamics, which states that in any isolated system, the entropy tends to increase. If the change in entropy is positive for a reaction, it contributes to the spontaneity of the reaction.
  11. What is the role of temperature in determining the spontaneity of a reaction? The temperature can affect the spontaneity of a reaction because it is part of the calculation for Gibbs free energy. If the enthalpy change and entropy change of a reaction have the same sign, then temperature can be the determining factor for whether or not the reaction is spontaneous.
  12. What is a state function and why are state functions important in thermochemistry? A state function is a property of a system that depends only on its current state and not on the path taken to reach that state. Enthalpy, entropy, and Gibbs free energy are all state functions. They are important in thermochemistry because they allow us to calculate the change in these quantities for a reaction without having to know the details of the process.
  13. What is calorimetry? Calorimetry is the process of measuring the heat of chemical reactions or physical changes. It is used to determine the enthalpy changes in reactions.
  14. How do specific heat capacity and molar heat capacity differ? Specific heat capacity is the amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius, while molar heat capacity is the amount of heat needed to raise the temperature of 1 mole of a substance by 1 degree Celsius.
  15. How is the standard enthalpy change of a reaction (ΔH°) calculated? The standard enthalpy change of a reaction is calculated by subtracting the sum of the standard enthalpies of formation of the reactants from the sum of the standard enthalpies of formation of the products.
  16. What does a positive or negative ΔH value indicate about a reaction? A positive ΔH value indicates that the reaction is endothermic, meaning it absorbs heat from the environment. A negative ΔH value means that the reaction is exothermic, meaning it releases heat to the environment.
  17. What is the standard state in thermochemistry? The standard state in thermochemistry is a reference condition of 1 atm of pressure and a specified temperature, usually 298.15 K. The properties measured under these conditions are known as standard-state properties.
  18. What does the term ‘system’ refer to in thermochemistry? In thermochemistry, a ‘system’ refers to the part of the universe being studied, often the reactants and products of a chemical reaction. Everything outside the system is known as the ‘surroundings’.
  19. How are enthalpy and internal energy related? Enthalpy (H) and internal energy (U) are related through the equation H = U + PV, where P is the pressure and V is the volume. This equation reflects that the total heat content (enthalpy) of a system is the sum of its internal energy and the product of its pressure and volume.
  20. How can the principle of ‘conservation of energy’ be applied in thermochemistry? The principle of ‘conservation of energy’ states that energy cannot be created or destroyed, only transferred or converted from one form to another. In thermochemistry, it implies that the total energy change in a closed system is always zero, meaning the total energy of the products of a reaction is equal to the total energy of the reactants.

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