Gases & Atmospheric Chemistry

Unit 5

States of MatterState PropertiesSolid •Definite shape and volume

•Are virtually incompressible•Do not flow easily

Liquid •Assume the shape of the container but have a definite volume•Are virtually incompressible•Flow readily

Gas •Assume the shape and volume of the container•Are highly compressible•Flow readily

Solid, Liquid & Gas

Forces Holding Solids Together The forces that are holding a solid

together are very strong

Forces: Ionic Covalent Some intermolecular forces in some

substances

Forces Holding Solids TogetherExplains why solids: Have a definite shape

Strong bonds holding molecules together (rigid)

Do not flow readily In order to be able to flow particles have to

slip past one another, strong bonds do not allow this

Cannot be compressed Strong bonds mean that there are few

empty spaces between the particles

Forces Holding Liquids Together The forces that are holding a liquid

together are not as strong as ionic or covalent bonds

Forces: intermolecular

Bonds hold molecules closely together but do not lock them into place

Liquids can spread out and take the shape of the container while keeping a constant volume

Because gases have NO definite shape or volume there appears to be an absence of forces between the molecules in a gas

No limit to the diffusion of gas molecules into the atmosphere (a very large container)

Gases – Lack of Forces

The Kinetic Molecular Theory Kinetic Molecular Theory = the idea

that all substances contain particles that are in constant, random motion

Particles are continually moving & colliding

Explains:1. Diffusion2. Evaporation

Diffusion Example: food colouring is added to

water will slowly spread out

Explanation from Kinetic Molecular Theory: molecules of food colouring and molecules of water are moving and colliding with each other which causes them to mix

Evaporation Example: water in an open container

slowly decreases as some of the water evaporates

Explanation from Kinetic Molecular Theory : some water molecules in the open container obtain sufficient energy from collision to escape from the liquid

3 Types of Motion A particle an exhibit 3 type of motion:

1. Vibrational = back-and-forth motion of atoms within a molecule

2. Rotational = spinning

3. Translational = straight line

3 Types of Motion

Motion in Relation to State Solid – mainly vibration due to

restriction of the strong bonds Particles stay together in a relatively

ordered state Liquid – some of all 3 types of motion

Less orderly state than solid Gas – rotate and vibrate but

translational (straight-line) motion is the most significant Most disordered state with no organization

Properties of Gases1. Gases are compressible: When pressure is

increased, the volume of a gas decreases. When pressure is decreased, the volume of a gas increases. The volume of a liquid and a solid remain constant during changes in temperature because their particles cannot move independently of one another like the gas particles can.

2. Gases expand as the temperature increases (much more than water and solid).

3. Gases have very low viscosity (they flow fast). 4. Gases have much lower densities than solids or

liquids. 5. ALL Gases are miscible (some liquids are miscible

yet some are immiscible).

Earth’s Leaky Atmosphere? Many of the gases that make up Earth’s

atmosphere and those of the other planets are slowly leaking into space.

Hot gases, especially light ones, evaporate away

chemical reactions and particle collisions eject atoms and molecules

and asteroids and comets occasionally blast out chunks of atmosphere

The Atmosphere

Measurement of Gas Pressure Pressure = force

exerted on an object per unit of surface area Unit = Pa (pascal)

Atmospheric Pressure = the force per unit area exerted by air on all objects Standard Atmospheric

Pressure: 101.3 kPa or 760 mm

Hg or 760 torr one standard atmosphere

(1atm)

Units of PressureUnit of pressure Symbo

lInstruments that use the unit

1) Millimetres of Mercury: mm of Hg.

mmHg Blood pressure meters

2) 1 Torr torr Vacuum pumps

3) Pascal (Pa) the SI unit of pressure. 1 kPa = 1000 Pa

Pa Pressure sensors in pipelines

4) Bars: 1 bar bar Pressure sensors in scooba gear

5) Atmospheres (atm) atm Gas compressors

6) Pounds per square inch Psi Hydraulic pumps

Conversion: 1 atm = 760 mm of Hg = 101.325 kPa = 1.01325 bar = 760 torr = 14.7 psi

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Pressure Conversions

A. What is 475 mm Hg expressed in atm?

760 mm Hg = 1 atm

475 mm Hg = x

x = 475/760 = 0.625 atm

B. The pressure of a tire is measured as 10 kPa. What is this pressure in mm Hg?

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Pressure Conversions

A. What is 2 atm expressed in torr?

STP & SATP STP = Standard Temperature &

Pressure Exactly 0°C (273K) 1atm or101.325kPa

SATP = Standard Ambient Temperature and Pressure exactly 25°C (298K) 100kPa

Gases Moving Gases naturally move from areas of

high pressure to low pressure, because there is empty space to move into

Examples of Spray Cans: whipped cream, hair spray, paint

a propellant forces the product out

Gas Law – Boyle’s LawRelationship: Pressure & Volume

As pressure on a gas increases, the volume of the gas decreases

Pressure and Volume Relationship As pressure increases volume decreases

Gas Law – Boyle’s LawRelationship: Pressure & Volume

Boyle’s Law = as the pressure on a gas increase, the volume of the gas decreases proportionally

p1v1 = p2v2

Provided that the temperature and amount of gas are constant

The volume and pressure of a gas are inversely proportional

Graphically: P 1 (inverse relationship) V

Robert Boyle (1627-1691). Son of Early of Cork, Ireland.

Boyle’s LawSample Problems: 1. A 350 mL sample of air at 125 kPa is reduced to a

volume of 250 mL. Calculate the new pressure.(ans: 180 kPa)

2. A 55 mL sample of helium at 525 torr is compressed to a certain volume at 2.5 atm. What is the new volume, in litres?(ans: 0.015 L)