Question

The heat of formation is the change in enthalpy accompanying the formation of a substance fromits elements at \(298 \mathrm{~K}\) and 1 atm pressure. Since the enthalpies of elements in their most stable state are taken to be zero, the heat of formation of compound is (1) always negative (2) always positive (3) standard heat enthalpy of that compound (4) zero

Short answer

The heat of formation of a compound is the standard heat enthalpy of that compound (Option 3).

Step-by-step solution

Understand the Concept of Heat of Formation

The heat of formation, also known as enthalpy of formation, is the change in enthalpy that results when one mole of a compound is formed from its constituent elements in their standard states at 298 K and 1 atm.

Identify Standard States of Elements

The standard enthalpy of formation of elements in their most stable forms (like O2, H2, N2, etc.) is defined to be zero. This is an essential concept to understand in this context.

Analyze the Given Options

Evaluate each option based on the definition of the heat of formation:1. Always negative - Incorrect, as the heat of formation can be either negative or positive.2. Always positive - Incorrect, for the same reason above.3. Standard heat enthalpy of that compound - Correct, as the standard enthalpy of formation is precisely what it measures.4. Zero - Incorrect, the enthalpy of formation is not zero except for elements in their standard state.

Conclusion

The correct answer is the one that matches the definition of the heat of formation, which is the standard heat enthalpy of the compound.

Key concepts

enthalpy change

Enthalpy change refers to the heat absorbed or released during a chemical reaction at constant pressure. It is a vital concept in chemical thermodynamics, as it helps us understand how energy flows in and out of a system. We use the symbol \( \Delta H \) to denote changes in enthalpy. For exothermic reactions, where heat is released, \( \Delta H \) is negative. For endothermic reactions, where heat is absorbed, \( \Delta H \) is positive. In the context of heat of formation, the enthalpy change represents the energy change when forming a compound from its elements.

standard states

Standard states are specific sets of conditions under which we measure enthalpy changes. Typically, these conditions are 298 K (25°C) and 1 atm pressure. For instance, the standard state of oxygen is \(O_2(g)\), and for hydrogen, it is \(H_2(g)\). Importantly, the enthalpies of elements in their standard states are defined as zero. This makes it easier to calculate the enthalpy of formation for compounds, as you only need to account for the changes from these reference points.

chemical thermodynamics

Chemical thermodynamics is the study of energy transformations in chemical processes. It combines principles from thermodynamics with chemical reactions to understand how energy and matter interact. Key concepts include enthalpy, entropy (\(S\)), and Gibbs free energy (\(G\)). These help predict whether reactions occur spontaneously and how much energy is involved. For instance, in heats of formation, we are interested in the enthalpy changes that occur when new compounds are formed from their elements.

mole concept

The mole concept is a fundamental principle in chemistry used to quantify the amount of substance. One mole corresponds to Avogadro's number (approximately \(6.022 \times 10^{23}\) entities, such as atoms or molecules). This concept helps in measuring and relating masses of substances in chemical reactions. When discussing heat of formation, we are usually referring to the formation of one mole of a compound from its elements. It's crucial to use the mole concept to make accurate calculations and understand the stoichiometry of reactions.