PressureChemistry Modeling 2013

Pressure Macro-Scale• Pressure is the amount of force exerted over a

given area The force exerted is caused by particles

collisions with the surface of a “container” is a PRESSURE EVENT

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 object

Changes and effects of Pressure are only observed in gases (not liquids or solids)

Atmospheric Pressure• Atmospheric pressure is the amount of

pressure exerted on an object by all the particles in 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? Elevation Temperature Amount of particles in the gas (water,

particulate)

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• Pounds per square inch (psi)

The unit of pressure used my many industries in the US

14.7 psi is the pressure at sea level at 0°C

What does this mean?• 760 mmHg = 101.3 kPa = 1 atm = 14.7 psi

For every 1 atmosphere there is 760 mmHgWhat are some other “for every” statements

you could write?

How to convert between pressure units• 3.10 atm kPa

Use equalities to solve.What do you know?

Given 3.10 atmWhat equalities can you use?

1 atm = 101.3 kPaSet up ratios, cross multiply & solve

3.10 atm = 1 atm x 101.3 kPa (cross multiply to solve for x) The answer is 314 kPa

STP Standard Temperature and Pressure

a standard set of conditions for experimental measurements established to allow comparisons to be made between different sets of data.

Temperature = 273 K (0° Celsius, 32° Fahrenheit)

Pressure = 1 atmosphere of pressure (often expressed as “at sea level”)

KMT – The Kinetic Molecular TheoryParticles of a gas are in Constant Motion,

moving Randomly until they Collide with another particle or a container wall.

Particles experience Elastic collisions.The Speed of the particles is directly related

to the TemperaturePressure of a gas is related to the Number

and the Strength of the collisions with the sides of the container wall.