Which of the following is a condition for measuring the standard enthalpy change of a substance?

The standard enthalpy change of a substance is specifically defined under particular conditions to ensure consistency and comparability of measurements. One of these key conditions is that the measurement should occur at a standard pressure of 1 bar. This standardization allows scientists to report and compare enthalpy changes in a uniform way.

The concept of standard enthalpy change encompasses various scenarios, such as standard enthalpy of formation or standard enthalpy of combustion, which are all defined at this pressure. This pressure is chosen rather than atmospheric pressure, which can vary slightly depending on location and weather conditions.

In contrast, the other conditions listed are not suitable for standard enthalpy measurements. While enthalpy changes can technically be measured at any temperature, they may not yield standard enthalpy values unless they adhere to specific conditions, including the referenced pressure. Additionally, measuring at temperatures above 100°C or solely at room temperature would not fulfill the requirement of standard conditions. These variations could introduce significant discrepancies in enthalpy values, making the data less reliable and less representative of standard states.