Pressure is the physical force exerted by matter, resulting from the rapid collisions of molecules against a surface. This force is most noticeable in gases, where particles are highly energetic, but it also influences the behavior of liquids and solids. Understanding pressure allows chemists to predict how substances will behave, explaining the transition between different states of matter and the direction of chemical change.
The Molecular Basis and Standard Units of Measurement
Pressure is defined as the force exerted perpendicular to a surface divided by the area. At the molecular level, the Kinetic Molecular Theory explains pressure as the cumulative result of gas particles in continuous, random motion colliding with the container walls. The magnitude of pressure depends on the frequency and force of these molecular impacts.
Chemistry employs several standard units of measurement to quantify this force. The official SI unit is the pascal (Pa). Since the pascal is small, the kilopascal (kPa), equivalent to 1,000 pascals, is often used.
The atmosphere (atm) represents the average air pressure at sea level, where 1 atm equals 101.325 kPa. Other units include the millimeter of mercury (mmHg) or the torr, historically derived from barometers. One atmosphere is precisely equal to 760 mmHg or 760 torr. Standard pressure is defined as 1 atm or 101.325 kPa, providing a fixed reference point for comparing chemical data.
How External Factors Influence Gas Pressure
The pressure of a gas in a closed system is sensitive to changes in volume, temperature, and the amount of gas present.
Volume (Boyle’s Law)
The relationship between pressure and volume is inverse. If the volume of a container is halved while temperature is constant, the molecules have less space. This results in twice the number of collisions with the walls and a doubled pressure.
Temperature (Gay-Lussac’s Law)
The relationship between pressure and absolute temperature is direct. Increasing the temperature causes gas molecules to move faster, increasing their kinetic energy. These faster particles strike the walls more frequently and with greater force, leading to a proportional increase in pressure.
Quantity (Moles)
Pressure is directly proportional to the number of moles of gas present. Adding more gas molecules into a fixed volume increases the total number of particles available to collide with the walls. This higher collision frequency translates directly to an increase in measurable pressure.
These distinct relationships are consolidated into the Ideal Gas Law. This law demonstrates how these four variables are interconnected, allowing chemists to predict the state of a gas under various external conditions.
Pressure’s Role in Chemical Reactions and Phase Changes
Pressure plays a distinct role in both phase changes and chemical reactions.
Phase Transitions and Vapor Pressure
Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. As a liquid is heated, its vapor pressure increases because more molecules gain energy to escape the surface. The boiling point is reached when the vapor pressure equals the external atmospheric pressure. For example, water boils at a lower temperature at high altitudes because the lower external pressure requires a lower vapor pressure to achieve boiling.
Gas Mixtures (Dalton’s Law)
Dalton’s Law of Partial Pressures is used to analyze the total pressure of gas mixtures. This law states that the total pressure is the sum of the pressures that each individual gas would exert if it were alone in the container. The partial pressure of a specific gas is directly related to its mole fraction in the mixture.
Chemical Equilibrium (Le Chatelier’s Principle)
Pressure influences the outcomes of reversible chemical reactions involving gases. If a reaction at equilibrium is subjected to an increase in pressure, the system shifts its equilibrium position to the side of the reaction with the fewer total moles of gas. By favoring the formation of fewer gas molecules, the system reduces the stress of the increased pressure and establishes a new equilibrium.