PRESSURE

CHEMISTRY MODELING 2013

PRESSURE MACRO-SCALE• Pressure is the amount of force exerted over a given area

• The force exerted is caused by particles that collide with the surface of a “container”

• Familiar unit is “pounds per square inch” or psi (tire pressure)

• This is how many pounds of force something pushes on every 1 square inch of surface contact it has with an objectPressure can be increased by:

1. Increasing the force

2. Decreasing the amount of surface area

ATMOSPHERIC PRESSURE• Atmospheric pressure is the amount of pressure exerted

on an object by all the atmosphere directly above that object

• Remember that air is made of particles, which have mass. That massive amount of atmosphere is pushing down on you at all times

• What affects atmospheric pressure?

MEASURING PRESSURE• Units

• Pounds per square inch (psi)• Kilopascals (kPa)• Atmospheres (atm)• Millimeters of mercury (mmHg)

MEASURING PRESSURE• Kilopascals:

• The standard metric unit for measuring pressure.• 101.3 kPa is pressure at sea level at 0°C

• Atmospheres:

• A convenient unit where the pressure at 0°C at sea level is equal to 1.00 atm.

• mmHg:

• A unit based on the amount of liquid mercury that can be pushed up a tall column by atmospheric pressure.

• 760 mmHg is pressure at sea level at 0°C

STPStandard Temperature and Pressure

• 273 K (0° Celsius) and 1 atm pressure

DEVICES THAT MEASURE PRESSURE• Manometer

• A device that uses a fluid in the bottom of a U-shaped tube to measure differences in pressure.

Gas sealed in a container with an unknown pressure that we want to measure.

Fluid in the U-shaped tube.

Fluid level on the side containing the gas of unknown pressure (height in units of mm).

Fluid level on the side containing the gas of unknown pressure (height in units of mm).

End open to the air. Pressure is equal to atmospheric pressure.

DEVICES THAT MEASURE PRESSURE

ΔP

When pressure is greater on one side of the fluid, that side of the fluid is pushed down and the fluid on the side with less pressure goes up.

The difference between the two heights is equal to the difference in pressure.

If we know the atmospheric pressure and the pressure difference (ΔP), we can calculate the unknown pressure of the gas sealed in the container using the following formula:P1 = P2 + ΔP, where ΔP = h2 – h1.

h1

h2

DEVICES THAT MEASURE PRESSURE

Consider the following example:

• What is the pressure of the gas in the sealed part of the manometer?

P1 = ?

P2 = 730 mmHg

h1 = 84 mmHg

h2 = 127 mmHg

1. We know the gas inside the sealed bulb has a greater pressure than the atmospheric pressure because it is pushing the fluid down on its side of the U-tube.

2. The change in height is equal to h2 – h1. SoΔP = 127 mmHg – 84 mmHg = 43 mm Hg.

3. The pressure of the gas in the sealed glass bulb is equal to the atmospheric pressure plus the change in pressure. So this means that P1 = 730 mmHg + 43 mmHg = 773 mmHg