Chemistry- Gases

Gas definition

a state of matter with no fixed shape or volume, diffuses to fill the container where its placed

What are the four variables that describe gases?

• temperature

• pressure

• volume

• quantity

What do gas laws do

explain the relationship between the variables

Assumptions of the Kinetic Theory of Matter

• all matter is made of tiny, invisable particles

• tiny particles are always in motion

• temperature affects movement of particles

• temperature can cause a change in state

• no attractive force between gas molecules

• pressure in gases

Movement of particles in gases

random, haphazard and constant

Pressure in gases

explains how gases exert pressure, by colliding with each other and the walls of their container

What evidence is there for the Kinetic Theory of Matter

Brownian motion and diffusion of gases

What is the Brownian motion

random, irregular, zigzag motions of the tiny particles in a liquid or gas, resulting from the collision of the particles with the molecules of liquid or gas

What is diffusion

the movement of particles from an area of high concentration of particles to an area of lower concentration

How does diffusion of gases prove the kinetic threory of matter?

evidence of diffusion means that particles must be in constant motion, colliding with each other and spreading out to occupy all space available, as stated in the theory

What is temperature

a measure of hotness, measure of average kinetic energy a system contains

what are the two scales of temperature (both names obvss)

Celcius (centigrade) and Kelvin (absolute)

What are the fixed points of celcius

• water freezes at 0

• water boils at 100

At what temperature was it proposed that the energy of a molecule reaches 0?

-273.15 celcius (0K)

What is the standard temperature?

0 degrees celcius/ 273.15K

What is pressure

• the force a gas exerts on a unit area of its container

• exerted when particles collide with each other and with walls of container, more collisions means more pressure

What is pressure measured in?

Newtons/m² / Pascals (Pa)

what is the standard pressure

1 × 10⁵ Pa

What is a kilopascal

1000 Pa (kPa)

What is the normal atmospheric pressure in kilopascals

100kPa

What is the volume of a gas

The same as the volume of the container its held in

What is volume measured in

cm³ /L

What is the litre the same as

a cubic decimeter (dm³)

What is a cubic meter the same as

10⁶ cm³

How does temperature affect movement of particles

• increase temp means increased kinetic energy, particles move faster

• decrease temp means decreased kinetic energy, particles move slower

Temperature causing change in solid state

• when solid heated, particles vibrate vigourously

• more heat = particles break free and slide over each other= liquid

• some go directly to gas (subimation) eg. solid carbon dioxide (dry ice)

Temperature causing change in liquid state

• liquid heated, particles gain energy and move faster

• break away from liquid state and form gas

• certain temp= gas particles throughout liquid, not just surface (boiling)

Evaporation definition

changing of a liquid to a gas at the surface of the liquid

Boiling point definition

temperature at which a liquid changes to a gas throughout the liquid

Temperature causing a change in state in gases

• when gas cooled, particles slow and liquid is formed

• particles lose energy as gas changes to liquid (condensation)

Condensation definition

changing of a gas to a liquid

Freezing point definition

temperature at which a liquid changes to a solid

Forces between particles

• solids: particles held tightly together by strong forces and give definite shape

• Liquids: forces are weaker and allow particles to flow over each other so liquid flows

• Gases: forces very weak and gases diffuse to fill all space available

How does the Brownian motion prove the Kinetic Theory of Matter

• random, zigzag, irregular movement of visible particles observed because they are being bombarded by smaller, invisible molecules of water or gases in the air.

• Brownian motion continuously taking place, smaller particles must be continuously in motion

How to get from kPa to Pa

x 1000

How to get from Pa to kPa

divide by 1000

How to get from 1m3 to Litres

x 1000

How to get from 1L to cm3

x1000

How to get from m3 to cm3

x 10⁶

Boyle’s law

• at a constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure

• If volume of container is decreased, more collisions occur, so more pressure

Formula for Boyle’s law (and rewqritten version)

• V ∝ 1/p

• pV = k

Charles’ Law

• as temperature of gas increases, energy of gas particles also increase

• particles move more rapidly and collide more frequently

• larger volume of container means pressure remains constant

Formula for charles’ law

V ∝ T or V/T=k

What is the combined gas law formula

Gay-Lussac’s Law of combining volumes

• in a reaction between gases, volumes of reacting gases and volumes of any gaseous products are in ration of small whole numbers, provided the volumes are measured at the same temperature and pressure

Avogadro’s law

• states that equal volumes of gases contain equal numbers of molecules, under same conditions of temperature and pressure

• at stp one mole of any gas occupies 22.4L

• water molecle contained two hydrogen atoms for each oxygen atom

What is the molar volume at room temperature and pressure

24.0L

Kinetic theory of gases

• volume of gas molecules is negligible compared to volume of container they occupy

• gases are composed of molecules in constant, rapid, random motion, change direction when colliding with walls or each other

• collisions are elastic and no energy is lost

• no attractive or repulsive forces between gas molecules

• average kinetic energy of gas molecules are proportional to temperature (in K)

Ideal gases

• obey all assumptions of kinetic theory under all temperatures and pressures

• dont exist

What behaves closest to an ideal gas

hydrogen at low pressure and high temperature

How do real gases differ from ideal gases

• forces of attraction and repulsion exist

• volume of molecules is not negligible

• collisions not always elastic

Why do real gases behave LEAST like ideal gases at low temperature

less kinetic energy and move slower, intermolecular forces become more significant

Why do real gases behave LEAST like ideal gases at high pressre

high pressure means molecules are closer, volume is not negligible, and intermolecular forces are more significant

Ideal gas equation

PV = nRT

what does P stand for in the formulas?

pressure in Pascals

what does V stand for in the formulas?

volume in m³

what does n stand for in the formulas?

number of moles

what does R stand for in the formulas?

universal gas constant (8.31 Jmol^-1K^-1 )

what does T stand for in the formulas?

Temperature in Kelvin