Gases

Collage shows gases in shape of balloon. Big orange colour balloon contains six balloons. Each balloon gives us important information about gases and gas laws . Which we will be studying today.

You must have seen a hot air balloon? Had a soda bottle spray all over you!

Air is a gaseous state of matter, is filled in balloon

The upper middle balloons shows

Ideal gas particles have mass ,but no volume no attractive force between particles. Most gases behave as ideal gas except under the condition of very high pressure and low temperature

one can predict the behavior of gases based on the following properties:

Pressure,volume,amount(moles),temperature

Question is what is a gas law

Simple mathematical relation between volume, pressure ,temperature and amount of gas is known as Gas Law

If you’ve ever inflated a tire, you’ve probably made a pressure measurement in pounds (force) per square inch (area).

What is a pressure of gas?

Pressure is the amount of force applied to an area

P = F/A

Where P = Pressure , F = Force, A = Area of surface

Atmospheric Pressure is the weight of air per unit of area

Unit of pressure is

Pascal : 1Pa = 1N/m²

Bar 1 bar = 10⁶ Pa

Figure below shows 1atm pressure at surface of earth.

Figure above shows units of pressure, 1.00 atm =760 torr or mm Hg

Units of pressure

1.mm Hg or torr

2.Atmosphere

These units can be visualized as the difference in the heights measured in mm(h) of two connected colums of mercury.

1.00 atm =760 torr or mm Hg

Unlike liquids and solids gases expand to fill their containers ,are highly compressible

And have extremely low densities

Lower middle balloon

Pressure is defined as the force the gas exerts on a given area of the container in which it is contained. The SI unit for pressure is the Pascal, Pa.

Volume is the three-dimensional space inside the container holding the gas. The SI unit for volume is the cubic meter, m³. A more common and convenient unit is the liter, l.

Amount of substance. SI unit for amount of substance is the mole/mol. Since we can’t count molecules, we can convert measured mass (in Kg) to the number of moles, n, using the molecular or formula weight of the gas.(shown in figure below).

By definition, one mole of a substance contains approximately 6.022 x 10²³ particles of the substance. One can understand why one use mass and moles!

Temperature is the measurement with which one , probably most familiar (and the most complex to describe completely). For this collage lessons, we will be using temperature measurements in Kelvin/ K. Temperature is related to average kinetic energy of particle. KE=(1/2. )mv^2, where m is mass and v is velocity of particle.

Some relationships of gases may be easy to predict. Some are more subtle.

Now that we understand the factors that affect the behavior of gases, we will study how those factors interact

Upper left baloon in collage , introduces Boyle’s Law, which describes the relationship between pressure and volume of gases at constant temperature.

Boyle determined that for the same amount of a gas at constant temperature,

P* V = constant

This defines an inverse relationship between P and volume: when one goes up, the other

comes down.

The plot of P vs V , V vs 1/P and plot of PV vs P is shownin figures below.

The figure above shows the apparatus for studying the relationship between Pressure P and Volume V of a Gas, at constant temperature.

With constant pressure and amount of gas, one can use these relationships to predict changes in temperature and volume.

Figure below show graph of t⁰C vs V at constant pressure, P

Figure above shows :T(K) = t(⁰C)+273.15

.Charles law is shown in upper right side of collage in the form of gas baloon

This law is named for Jacques Charles, who studied the relationship volume, V, and temperature, T(K), at constant P around the turn of the 19^th century.

He determined that for the same amount of a gas at constant pressure,

V / T = constant

This defines a direct relationship: an increase in one results in an increase in the other.

Figure above show experimental set to study effect of temperature T on volume of gas ,V.

A plot of V vs T(K) for different gases ,a straight line is shown in figure below.

Figure below show Relation between ⁰C and K scale

Lower right side of collage is another balloon which depicts the Gay –Lusccas law.

Which states that if in gas sample n and V are constant then

P ά T

Means P is directly propotional to T ie as P goes up T goes up at (constant n,V)

P1/T1 =P2/T2

Figure below shows a plot of P vs T(K) at constant (n,V) , a straight line.

Figure below gives a sketch of flow diagram showing Boyles, Charles , Avogadros, Ideal gas Law respectively.

Lower left side of collage gives the summary of three important laws of gases which are shown as balloons .

Avogadro's law (sometimes referred to as Avogadro's hypothesis or Avogadro's principle) is a gas law named after Amedeo Avogadro who, in 1811, hypothesized that two given samples of an ideal gas, at the same temperature, pressure and volume, contain the same number of molecules. Thus, the number of molecules or atoms in a specific volume of gas is independent of their size or the molar mass of the gas.

Avogadro's Law states that for a gas at constant temperature and pressure the volume is directly proportional to the number of moles of gas,

According to Avogadro's Law:

V / n = constant, Plot of V vs n will be a straight line.

Mathematically ,this means V = kn

Figure below depicts the Avogadro's Law

The Ideal Gas Law is a combination of Boyle's Law, Charles' Law and Avogadro's Law. It can be expressed by a single equation, PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the general gas constant, and T is the absolute temperature.

The value of the gas constant, R is 8.3143 J K^-1 mol^-1. Note that these are SI units, and therefore the volume used in this equation must be in m³and the pressure in N^-2. In all gas law calculations, absolute values of temperature (Kelvin) must be used.

This is one of the few equations in chemistry that one should commit to memory!

By remembering this single equation, one can predict how any two variables will behave when the others are held constant.

Summary

V∞ 1/P( At constant n,T)—Boyles Law

V∞ T( At constant n,P)--- Charles Law

V∞ n (At constant V,T)---Avogadros Law

V ∞ nT/P

PV =nRT, ( where R is gas constant.)----Ideal Gas Law

Take home message: this collage gives message that though gases are easiest state of matter but knowledge of their behavior based on properties like temperature, pressure and volume can take you to height of success in the same way as gas balloon when left alone goes up very high. The orange colour in collage is sign of bravery (according to hindu mythology).Hence knowledge of gas laws makes one brave for adventure of science.

References

http://hendrix2.uoregon.edu/~imamura/102/section2/chapter13.html

http://www.chemteam.info/GasLaw/Gas-Boyle-Data.html

http://www.indiana.edu/~geog109/topics/10_Forces&Winds/GasPressWeb/PressGasLaws.html

http://www.ausetute.com.au/charslaw.html

http://www.uccs.edu/vgcl/gas-laws/experiment-1-boyles-law.html

http://www.chemguide.co.uk/physical/kt/idealgases.html

http://www.chm.davidson.edu/vce/gaslaws/avogadroslaw.html

http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/imgkin/igascn.gif