Pressure is defined as the force exerted perpendicular to a surface divided by the area over which that force is distributed. In chemistry, this force often comes from the constant collisions of gas molecules. The concept of “standard pressure” is not fixed but depends on the specific context of the measurement or calculation. This reference point is necessary to ensure that experimental data from different laboratories can be accurately compared, though the specific value varies between historical and modern conventions.
Standard Temperature and Pressure (STP)
Standard Temperature and Pressure (STP) is the oldest and most recognized reference point, established primarily for comparing the properties of gases. Historically, the standard pressure component of STP was defined as exactly one atmosphere (\(\text{atm}\)). This value was chosen because it closely approximates the average atmospheric pressure found at sea level, and \(1 \text{ atm}\) is precisely equal to \(101.325\) kilopascals (\(\text{kPa}\)).
This pressure is paired with a standard temperature of \(0^\circ \text{C}\) (\(273.15 \text{ K}\)). These conditions are used in gas law calculations, such as determining the molar volume of an ideal gas. Under traditional STP (\(1 \text{ atm}\) and \(0^\circ \text{C}\)), one mole of an ideal gas occupies approximately \(22.4 \text{ liters}\). While the definition has been updated by international bodies, this older \(1 \text{ atm}\) standard remains a common fixture in many introductory chemistry textbooks and is frequently used in engineering and industrial calculations.
The Modern Standard State Pressure (IUPAC Convention)
The International Union of Pure and Applied Chemistry (IUPAC) introduced a modern standard to simplify calculations and align with the International System of Units (\(\text{SI}\)). In 1982, IUPAC redefined the standard pressure for general use, including some STP contexts, to exactly \(1 \text{ bar}\). This \(\text{bar}\) unit is precisely \(100\) kilopascals (\(\text{kPa}\)), which is a slight reduction from the older \(101.325 \text{ kPa}\) value. The shift to \(1 \text{ bar}\) provides a cleaner, more convenient number for calculations, particularly in physical chemistry.
The \(1 \text{ bar}\) pressure is also designated as the “Standard State” pressure for thermodynamic measurements. Standard state conditions are used to report values such as the standard enthalpy of formation (\(\Delta H^\circ\)) or the standard Gibbs free energy (\(\Delta G^\circ\)). These thermodynamic values are measured and reported under the \(1 \text{ bar}\) convention to ensure universal consistency across published data. This modern standard provides a rigorous and internationally accepted reference point for chemical data.
Understanding the Different Pressure Units
The existence of multiple standard pressures in chemistry is compounded by the variety of units used to express pressure measurements. The atmosphere (\(\text{atm}\)) is based on historical sea-level air pressure, and \(1 \text{ atm}\) is defined as \(101.325 \text{ kPa}\). The kilopascal (\(\text{kPa}\)) is derived from the SI unit for pressure, the Pascal (\(\text{Pa}\)), where \(1 \text{ kPa}\) equals \(1000 \text{ Pa}\).
The \(\text{bar}\) is a metric unit defined as exactly \(100 \text{ kPa}\), making \(1 \text{ atm}\) approximately \(1.013 \text{ bar}\). Another common unit is the millimeter of mercury (\(\text{mmHg}\)), also known as the \(\text{torr}\). This unit originated from mercury barometers, and \(1 \text{ atm}\) is precisely equal to \(760 \text{ mmHg}\) or \(760 \text{ torr}\). The choice of unit often depends on the field; \(\text{kPa}\) and \(\text{bar}\) are favored in modern scientific literature, while \(\text{atm}\) is common in introductory gas calculations.