Unit 3 Chem (chapter 5 Gases)

general properties of Gas:

1) Gases expand to fill their containers. Because gas particles are separated by relatively large distances, they do not interact with one another as much as particles in the liquid or solid phase do. Consequently, the attractions keeping the particles of solids and liquids close to each other are mostly absent in gases.

2) Gases can be compressed. Because the particles in a gas are spread out, they can be pushed closer together when a force is applied.

3) Gases have relatively small densities compared to solids and liquids. Because density is a measure of compactness, density gets smaller as particles become more spread out.

4) Gases are miscible in all proportions, which means they can be mixed to form homogenous mixtures.

5) Gases have low viscosities, or resistance to flow.

Gas particles are constantly moving and frequently collide with one another and the surfaces surrounding them. Consider a gas contained inside a balloon. Although each collision with the surface of the balloon exerts only a tiny force, the total force of all the collisions with the balloon’s inner surface is significant and results in a property called pressure

Gas pressure. Pressure is a result of collisions between gas particles and the surface of their container.

Pressure is the force applied over a specified area: P= Force/Area

The pressure of a gas depends on a number of different factors, including the volume of the container, the number of gas particles present, and the temperature.

The SI unit for pressure is the pascal (Pa), which is defined as 1 newton (N) per square meter: 1 Pa=1N/m²

A newton is the SI unit for force and is equal to the product of mass (in kg) and acceleration (in m/s2). A pascal can be written in fundamental SI units as 1 kg/(m · s²): 1 Pa= 1N/m² = 1kg(m/s²)(1/m²) = 1kg/m(s²)

1kg(m/s²) = force, (1/m²) = 1/area

many general chemistries resources report pressure in a unit called the standard atmosphere (atm), which is defined as precisely 101,325 Pa: 1atm=101,325 Pa

measuring gas pressure

A barometer measures atmospheric pressure. The classic mercury barometer uses a glass tube filled with mercury, inverted in a mercury basin. Atmospheric pressure pushes mercury up the tube; the height of the mercury column indicates the pressure. Higher pressure means a higher mercury column, and vice versa.

The pressure exerted by a fluid column in a barometer is given by: P=dhg

where P is the pressure in Pa, or kg/(m · s2), d is the density of the fluid in the barometer in kg/m3, h is the height of the column in meters, and g is the gravitational constant (9.807 m/s²).

If the density (ρ) of the fluid increases, the height (h) of the fluid column decreases for a given pressure P, because a denser fluid will exert more pressure per unit height. Conversely, if the fluid density is lower, the height of the fluid column needs to be taller to balance the same atmospheric pressure. So, the fluid’s density and the height of the fluid column in a barometer have an inverse relationship for a given pressure.

A closed-end manometer measures gas pressure relative to a vacuum (zero pressure). It consists of a U-shaped tube, one end sealed and the other open to the gas container.

The U-tube is filled with a liquid, typically mercury. Gas exerts pressure on the liquid column, causing it to move. The difference in liquid height between the two arms of the U-tube indicates the gas pressure. Because one end is closed to a vacuum, the pressure difference directly translates to absolute gas pressure.

If the gas pressure is higher than atmospheric pressure, the liquid level on the side connected to the gas will be lower. If the gas pressure is lower than atmospheric pressure, the liquid level on the side connected to the gas will be higher.

Gauge pressure is the pressure of a system above atmospheric pressure. It’s the measurement you get from most pressure gauges, which doesn’t include atmospheric pressure.