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Sulphur and its Compounds

 

Introduction

Air is a mixture of gases. The behaviour of gases is influenced by various conditions which include pressure, volume and temperature. The bahaviour of gases under different conditions are described by gas laws.

Air in a baloon

In form 1 we learnt that matter is made up of small particles that occupy space and has mass. There are three states of matter namely solids, liquids and gases.In this topic we will focus on the properties of matter in gaseous state.

States of matter

By the end of the lesson you should able to:
a) State Boyles law
b) Illustrate Boyle's law.
c) Carry out calculations involving Boyle's law.
 

In this lesson we will discuss Boyles law.

The following animation shows what happens to particles inside a container when the volume is increased and decreased. Click on the X button and Observe what happens carefully.

In containers A and B, the number of gaseous particles are the same and in continuous random motion. In container A the particles occupy a larger volume. They collide with each other and hit the wall of the container, therefore exerting pressure. In container B, the volume of the gas in the container is reduced, and the particles collide more frequently and therefore increasing the pressure.
From the above activity, we can see that when the volume of the a gas is increased, the pressure decreases and if the volume occupied by the gas is reduced, particles collide more frequently and therefore increasing the pressure.
Therefore the volume of the gas is inversely proportional to the pressure applied on the gas .

The following graph shows the pressure volume curve illustrating boyles law. Click on the points A, B, and C to observe the illustrations showing the relationship between volume and pressure at these different points.

Note that the volume of a gas is inversely proportional to the pressure applied and vice versa.The relationship is expressed in Boyle's law which states that the volume of a fixed mass of a gas is inversely proportional to its pressure at constant temperature.

Mathematical expression

Thus, for different conditions of temperature and pressure the relationship is

P1V1 = P2 V2

A plot of the graph of pressure against volume gives a curve as shown below.The graph generally shows that as volume increases, the pressure of the gas decreases.Both pressure and volume never reach zero hence curve does not touch both the x and y axis.

If a graph of pressure against I/volume is plotted, the graph will be a straight line.

Remember that according to Boyle's law.
Volume of a fixed mass of a gas is inversely proportional to its pressure at a constant temperature.


 

EXAMPLE 1
A sample of Nitrogen gas occupies a volume of 960cm3at a pressure of 760mmHg.Calculate the pressure of a gas if its compressed to a volume of 100cm3.Assume that temperature remains constant.


 

By the end of the lesson you should be able to:
i) State Charles' law
ii) Illustrate Charles' law graphically
iii) Carry out calculations involving Charles' law

In this lesson we will discuss the Charles law.

Observe the following animation and note:The increase in gas volume when heated at X.Particles moving randomly in the container.Gradual increase in gas particles movement and piston moving up to point B.The movement of the piston when the bunsen burner is put off. Note also what happens when the piston moves down to point A.Click the ENTER button to observe.

The volume of the gas increases as its temperature increases at constant pressure. However, if its temperature is decreased, its volume will consequently decrease. We say the volume of gas is directly proportional to its temperature.
The relationship between the volume of a gas and its temperature is summarized by the Charles's law,

Which states that ''the volume of a fixed mass of a gas is directly proportional to its absolute temperature at constant pressure''.
V a T (at constant pressure) .

Consider the results obtained from the experiment where the volume of helium gas was measured against temperature at a constant pressure of one atmosphere.

To draw a graph of volume against temperature, follow steps below.

Click on [Plot points] button to plot the points.

Click on [ join points ] button to obtain a line of best fit.

The result of this experiment shows that if the gas is cooled to -273 degree Celcius the volume would be zero. Practically real gas cannot cool to -273 degree Celcius because it liquefies before that temperature is achieved.
This temperature (-273 degree Celcius) is referred to as absolute zero temperature. The zero point on Kelvin (absolute) temperature scale.
To convert to degrees Celsius to Kelvin, add 273
For example 25 degree Celcius =273+25=298K

Click the enter button to follow up on a worked example.

By the end of this lesson you should be able to:

Use the combined gas laws in calculations.

In this lesson we will discuss combined gas law.

Combined gas law Equation

Remember: Charles law V/T=constant
Boyle's law PV=constant.By combining the two laws we obtain the combined gas law given as.P V/T=constant.When comparing the same quantity of a gas under different condition we obtain:
P1V1/T1=P2V2/T2=constant

By the end of the lesson you should be able to:
i. State Graham's law of diffusion.
ii. Explain diffusion in liquids and gases in terms of kinetic theory.
iii. Relate rate of diffusion to the relative molecular mass of the gas.
iv. Carry out calculations involving Graham's law of diffusion.

In this lesson, we will discuss Grahams law of diffusion.

William Graham

To investigate what happens to a crystal of potassium manganate (VII) in water.

Click to play the video and observe what happens.

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To investigate what happens to a crystal of potassium manganate (VII) in water.

Click to play the video and observe what happens.

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When potassium manganate (VII) is put in water, the water slowly turns purple.
The particles of potassium manganate (VII) spread from the bottom of the water in beaker towards the surface of water.

To investigate what happens when drops of Bromine liquid are in a gas jar.
CAUTION: The experiment should be done in fume chamber.

Click to play the video to observe what happens.

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When Bromine liquid is placed in a gas jar and another gas inverted over it, Brown fumes of Bromine vapour spread from the inverted gas jar after a short time.
CONCLUSION
From the two experiments, particles of gases and liquids spread from areas of high concentration to areas of low concentration through process called DIFFUSION.

To investigate the rate of diffusion of different gases.

Click to play the following video to observe how different gases diffuse.

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Hydrogen chloride and Ammonia gas diffuse from the opposite ends of the long glass tube.
When they meet, they react to form a white ring of Ammonium chloride.

CONCLUSION
Ammonia with the Relative Molecular Mass that is RMM of 17 is lighter and spreads to a longer distance then hydrogen chloride with RMM of 36.5.
The white ring of Ammonia chloride is formed nearer to the Hydrogen chloride at the end of glass tube.
Therefore the heavier the gas, the slower the rate of diffusion, and the lighter the gas the faster the rate of diffusion.

The behavior of gases when they diffuse is summarized by Graham's law which states that, at a constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density.
 

Remember Graham's law states that, at a constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density.

100cm3 of gas X diffuse from a porous plug in 10 secs, while 300cm3 of hydrogen gas diffuse from the same apparatus in 5 seconds .Under the same condition of temperature and pressure, calculate the relative molecular mass of X (H=1)

Calculate the time taken for a given volume of Nitrogen to diffuse through a porous plate if the same volume of carbon (IV) oxide takes 100 seconds to diffuse under the same condition given that the relative atomic mass of Nitrogen=14.0, Carbon =12.0, Oxygen=16.0)

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