Behaviour Of Gases - Chemistry

Behaviour Of Gases:

    The assumptions of kinetic theory account for most of the properties associated with gases and we can obtain better understanding of gas behaviour. 

Diffusibility:

    The distribution or spreading of gas molecules through out the vessel is  known as the diffusion. Unlike liquids or solids, the gases diffuse very rapidly. A drop of perfume for instance, slowly evaporates out the fragrant gas announces the presence of wearer. It is due to the diffusion of perfume through the air.

    In terms of kinetic theory, diffusion is explained as follows. The molecules of a gas are widely separated and there are large empty spaces due to which they are free to move. Due to this free movement the molecules of gases intermingle and spread out easily throughout the vessel. The opposite of diffusion is effusion in which a gas passes through the pores or tiny holes in the vessel, the air effuses from the tire as a result of which the tire loses pressure gradually.

Compressibility: 

    In contract to liquids or solids, all the gases are easily compressed or squeezed. In terms of "Kinetic theory" gases are easily compressed due to large empty spaces. By applying pressure, the molecule come closer. Air, for instance is squeezed into automobile tires. When the tire is punctured, the air rushes out. It is the reverse of compressibility - the expansibility, thus volumes of gases are highly affected by the changes in pressure.  Volume is measured in cubic decimetres, dm3 or cubic centimetres, cm3 (1 dm3 =1000 cm3).

Pressure: 

    All the gases exert pressure. It may appear surprising but it is a fact that we are being pressed upon by an enormously heavy blanked of atmosphere. The mass of atmosphere on our body at steal level and at 0 c is 14.7 psi (Pounds per square inch) or there is about 20 tons total pressure on our bodies.  

    When a gas is confined in a closed container, it exerts pressure on the walls of the container which is due to the collisions of gas molecules with the walls. The tires of automobiles are filled with air until the gauge shows the pressure of about 28 psi. This means that the pressure inside the tire is 28 psi greater than the outside pressure since the external atmospheric pressure is 14.7 psi, hence the total pressure inside the tire is 28 + 14.7 = 42.7 psi.

     Since pressure is defined as a force pushing on a unit of area, therefore pressure may be measured in psi, kilo grams per square meter (kg / m2). The unit is however newtons per square meter (N / m2 or pascals, Pa). Since units of a newton of force are kg. m/s , the S.I units of pressure are :

Pressure= Force/Area = newton/m2 = Kg m s2/ m2 = kg/ ms2

Normal atmospheric pressure at sea level at 273 K is expressed in several ways.

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Significant Figures - Chemistry

  The students is carrying out additions, subtractions, multiplications and divisions are confronted with the problems of the number of digits retain in the answer, specially the last figure in a number when measured on a scale is usually an estimated one. Actually all measuring instruments such as meter sticks, clocks and balances have scales which are sub divided into various units and subunits. Suppose, for example a glass rod 'A' whose length is to be measured by two different scales which can be seen in the given Scale (1) is divided only in centimetres while scale (2) is divided in millimetres also.

      In Scale (1) the length of the glass rod 'A' is bout 3.6 cm. The length of 'A' on scale (2) however is 3.62 cm. Thus the value 3.6 cm contains two significant digits while the value 3.62 cm contains three significant digits. This show that one factor which is very important in determining the number of significant digits is the accuracy of the measuring instrument and the second factor which also counts is the size of the object to be measured.

Rules For Determining Significant Figures: 

    Below are given the rules that will help students to determine the number of significant figures:

• Non zero digits are all significant; for example 363 has three significant figures and 0.68 has only two significant figures.
• Zero between non zero digits are significant, for example, 5004 has four significant figures and like wise 20.4 has three significant figures.
• Zero locating the decimal point in numbers less than one are significant for example. 0.062 has two significant figures and 0.001 has only one significant figure.
• Final zeros to the right of the decimal point are significant for example 2.000 has four significant figures and 506.40 has five significant figures.
• Zeros that locate the decimal point in numbers larger than one are not necessarily significant for example 40 has one significant and 2360 has three significant figures.

     Thus significant figures by definition are the reliable digits in a number that are known with certainly. The last digit of a number is generally considered uncertain by 1 in the absence of qualifying information. For example the mass of an object can be expressed as 0.0112 g or as 11.2 mg without changing the uncertainly of the mass or the number of significant figures. The mass is still uncertain by 1 in the last digit, this can be expressed as 0.0112 0001 g or as 11.2 0.1 mg. 

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