Gases: Properties, Pressure, and Conversions (General Chemistry Study Notes)

Gases

Introduction to Gases

Gases are one of the fundamental states of matter, characterized by their ability to fill any container and their unique physical properties. This unit covers the behavior, properties, and measurement of gases, as outlined in Sections 6.2–6.10 of Chemistry: A Molecular Approach.

Standard Temperature and Pressure (STP)

Definition and Conditions

• Standard Temperature (TSTP): 273.15 K (0°C)

• Standard Pressure (PSTP): 1 atm (760 mm Hg)

These conditions are commonly used as reference points for gas calculations and comparisons.

Elements That Are Gases at STP

• Diatomic gases: H2, N2, F2, O2, Cl2

• Noble gases: He, Ne, Ar, Kr, Xe, Rn

• No ionic compounds are gases at STP.

• Some molecular compounds are gases (e.g., CO, CO2, NH3, CH4), but most are solids or liquids at STP.

Properties of Gases

Physical Properties

• Indefinite shape: Gases take the shape of their container.

• Expand or compress:

  • If volume increases, gas expands to fill the larger volume.

  • If volume decreases, gas particles move closer together.

• Low density: Gases are approximately 1000 times less dense than solids and liquids, which is why their density is measured in g/L.

• Mixing: Gases mix uniformly in containers to form homogeneous mixtures.

• Pressure exertion: Gas particles in constant motion exert pressure on the walls of their container.

Pressure

Definition and Measurement

• Pressure: The force exerted by gas molecules colliding with surfaces.

• Atmospheric pressure: Pressure exerted by air molecules in the environment. It decreases as altitude increases because air becomes thinner.

• Barometer: An instrument invented by Torricelli to measure atmospheric pressure.

• Standard atmospheric pressure: Defined as the pressure exerted by a column of mercury (Hg) measuring 760 mm in height.

Pressure Units and Conversion Factors

• 1 atm = 101.3 kPa = 1.013 bar = 760 torr = 760 mm Hg = 14.7 psi

These conversion factors are essential for solving gas law problems.

Pressure Conversion Example

Problem: Tire Pressure Conversion

The tire pressure for most automobiles is about 32 psi. Express this pressure in atm, torr, and kPa.

• To atmospheres (atm): $32\ \text{psi} \times \frac{1\ \text{atm}}{14.7\ \text{psi}} = 2.2\ \text{atm}$

• To torr: $32\ \text{psi} \times \frac{760\ \text{torr}}{14.7\ \text{psi}} = 1.7 \times 10^3\ \text{torr}$

• To kilopascals (kPa): $32\ \text{psi} \times \frac{101.3\ \text{kPa}}{14.7\ \text{psi}} = 2.2 \times 10^2\ \text{kPa}$

Example Application: Converting pressure units is essential for using gas laws, as equations require consistent units.

Summary Table: Pressure Units and Conversions

Unit	Equivalent Value
Atmosphere (atm)	1 atm
Millimeters of Mercury (mm Hg)	760 mm Hg
Torr	760 torr
Kilopascal (kPa)	101.3 kPa
Bar	1.013 bar
Pounds per square inch (psi)	14.7 psi

Additional info: These notes cover the foundational concepts of gases, including their properties, measurement of pressure, and unit conversions, which are essential for further study of gas laws and behavior in General Chemistry.