Question

The enthalpies of elements in their standard states are taken as zero. Thus, the enthalpy of formation of a compound (a) will always be positive (b) will always be negative (c) may be positive or negative (d) will always be zero

Short answer

The standard enthalpy of formation of a compound may be either positive or negative (option (c)), depending on if the reaction is endothermic (absorbs heat) or exothermic (releases heat).

Step-by-step solution

Comprehend the definition of Standard Enthalpy of Formation

It is important to know that the standard enthalpy of formation, ΔH⁰f, is the amount of energy absorbed or released when one mole of a substance is formed from its constituent elements in their most stable form at standard conditions (1 bar or 1 atm pressure and 298K).

Understanding the sign convention

The sign of ΔH⁰f depends on whether the process absorbs or releases heat. If heat is absorbed during the reaction (endothermic process), ΔH⁰f is positive. Conversely, if heat is released (exothermic process), ΔH⁰f is negative.

Recognize Various Scenarios

Most compounds have either positive or negative ΔH⁰f, not zero, because they require or release energy when forming from their constituent elements. So the ΔH⁰f is not always positive, always negative, or always zero. Hence, the ΔH⁰f of a compound may be positive or negative based on the nature of the reaction whether it's endothermic or exothermic.

Key concepts

Enthalpy

Enthalpy is a fundamental concept in thermodynamics that refers to the total heat content of a system. It's often symbolized by the letter "H" and is used to describe energy changes in chemical reactions.
Understanding enthalpy is crucial when discussing reactions and their energy dynamics. It's defined as the sum of the internal energy of the system plus the product of the pressure and volume of the system:\[ H = U + PV \] where \( U \) is the internal energy, \( P \) is the pressure, and \( V \) is the volume of the system.
Enthalpy changes are particularly important in understanding how heat is exchanged with the surroundings when a chemical reaction occurs at constant pressure. This is expressed as:\[ \Delta H = \Delta U + P\Delta V \]- **Positive ΔH (Endothermic):** The system absorbs heat from the surroundings.- **Negative ΔH (Exothermic):** The system releases heat to the surroundings.

Endothermic Process

An endothermic process is one where the system absorbs energy from the surroundings in the form of heat. This is a characteristic of certain chemical reactions and physical changes. - For example, melting ice is an endothermic process because heat is absorbed from the environment to change the ice into water.
This absorption leads to a positive change in enthalpy (\( \Delta H > 0 \)), indicating that energy is needed for the process to proceed. In chemical reactions, bonds in the reactants must absorb energy to break before new bonds can form in the products. As such, the standard enthalpy of formation for a compound formed endothermically will have a positive \( \Delta H^0_f \).
If you're analyzing whether a reaction is endothermic, consider these points:

• Energy is consumed during the reaction.
• The surroundings become cooler as energy is absorbed.
• Entropic changes and temperature variables can influence the overall energy dynamics.

Exothermic Process

Opposite to endothermic processes, exothermic processes involve the release of energy in the form of heat to the environment. Most common chemical reactions tend to be exothermic, such as combustion reactions.
When a reaction is exothermic, it results in a negative change in enthalpy (\( \Delta H < 0 \)), signifying that energy is released as the process progresses. This energy release happens because the formation of product bonds releases more energy than is required to break the bonds in the reactants.
Understanding exothermic processes can be simplified with these characteristics:

• Energy is released during the reaction.
• The surroundings become warmer as energy is released.
• Exothermic reactions tend to be spontaneous due to the release of energy.

Such reactions often have a standard enthalpy of formation that is negative, which indicates that the formation of the compound from its elements releases heat.