Properties of Gases

3. GASES 3.1 STATES OF MATTER Matter exists in four states i.e., solid, liquid, gas and plasma. The simplest form of matter is the gaseous state and most of matter around on is in the solid state. Liquids are less common than solids, gases and plasmas. The reason is that the liquid state of any substance can exist only within a relatively narrow range of temperature and pressure. Let us look at the general properties of gases, liquids and solids. Kinetic molecular theory of gases can help us understand their properties. 1. Gases don't have a definite volume and occupy all the available space. The volume of a gas is the volume of the container. 2. They don't have a definite shape and take the shape of the container just like liquids. 3. Due to low densities of gases, as compared to those of liquids and solids, the gases bubble through liquids and tend to rise up. 4. Gases can diffuse and effuse. This property is negligible in solids but operates in liquids as well. 5. Gases can be compressed by applying a pressure because there are large empty spaces between their molecules. 6. Gases can expand on heating or by increasing the available volume. Liquids and solids, on the other hand, do not show an appreciable increase in volume when they are heated. 7. When sudden expansion of gases occurs cooling takes place, it is called Joule Thomson effect. 8. Molecules of gases are in a constant state of random motion They can exert a certain pressure on the walls of the container and this pressure is due to the number of collisions. 9. The intermolecular forces in gases are very weak. 3.1.2 Properties of Liquids 6. Liquids don't have a definite shape but have a definite volume. Unlike solids they adopt the shape of the container. 7. Molecules of liquids are in a constant state of motion. The evaporator and diffusion of liquid molecules is due to this motion. 8. The densities of liquids are much greater than those of gases but are close to those of solids. 9. The spaces among the molecules of liquids are negligible just like solids. 10. The intermolecular attractive forces in liquids are intermediate between gases and solids. The melting and boiling points of gases, liquids and solids depend upon the strength of such forces. 11. Molecules o f liquids possess kinetic energy due to their motion. Liquids can be converted into solids on cooling i.e., by decreasing their kinetic energy. Molecules of liquids collide among themselves and exchange energy but those of solids can not do so. 31.3 Properties of Solids 1. The particles present in solid substances are very close to each other and they are tightly packed. Due to this reason solids are non-compressible and they cannot diffuse into each other. 2. There are strong attractive forces in solids which hold the particles together firmly and for this reason solids have definite shape and volume. 3. The solid particles possess only vibrational motion. 3.1.4 Units of Pressure: The pressure of air that can support 760 mmHg column at sea level, is called one atmosphere. It is the force exerted by 760mm or 76cm long column of mercury on an area of 1cm° at 0°C.It is the average pressure of atmosphere at sea level 1mmHg=1torr. The S.I. unit of pressure is expressed in Nin'. One atmospheric pressure i.e 760 Corr is equal to 101325 Dim 1pascal=1 '.50, 760 tort= 101325Pa = 101.325 kilopascals (kpa is another unit of pressure) The unit pounds per square inch (psi) is used most commonly in engineering work, and 1 atm = 760 torm14.7 pounds inch'. The unit millibar is commonly used by meteorologists. 3.2 GAS LAWS It is a matter of common observation that when external conditions of temperature and pressure are changed, the volume of a given quantity of all gases is affected. This effect is nearly the same irrespective of the nature of the gas. So gases show a uniform behaviour towards the external conditions. The gas laws describe this uniform behaviour of gases. The relationships between volume of a given amount of gas and the prevailing conditions of temperature and pressure are called the gas laws. Different scientists, like Boyle, Charles, Graham and Dalton have given their laws relating to the properties of gases. 3.2.6 Derivation of Absolute Zero In order to derive absolute zero of temperature, consider the following quantitative definition of Charles's law. At constant pressure, the volume of the given mass of a gas increases or decreases by 1/273 of its original volume at 0°C for every 1 °C rise or fall in temperature respectively. In order to understand the above statement, look at the Table (3.1) of temperature volume data of a hypothetical gas. At 0 °C the volume of the gas taken is 546 cm' It is twice 273cm', and is being supposed for the sake of convenience of understanding. At 273 °C, the volume of the gas has doubled (1092 cm') and it should become practically zero at -273°C. The general equation to know the volumes of the gas at various temperatures is V,=V°(1+273(3) Where V,= volume of gas at temperature T = Volume of gas at 0°C t= Temperature on centigrade or celsius scale If a gas is warmed by 1°C, it expands by of its original volume at 0°C. Since original volume is 546 cm ',so, for 1°C rise in temperature, 2 cm' increase in volume will take place. 2cm' is the L of 546 cm'. Similarly, for 100 °C rise in temperature, a change of 200 cm' will take place. The273 Table (3.1) shows that the volume does not increase corresponding to increase in temperature on celsius scale. The two sides of equation are not equal. So, Charles's law is not being obeyed when temperature is measured on the Celsius scale. For this reason a new temperature scale has been developed. It starts from 273 °C more precisely -273.16 °C)which is called zero Kelvin orzero absolute. Let us nowexplain how the new temperature scale has been developed. The best way is to plot a graph between the variables of Charles's law. 3.4 AVOGADRO'S LAW According to this law, ''equal volumes of all the ideal gases at the same temperature and pressure contain equal number of molecules". This statement is indirectlythe same as has been used for evaluating the general gas constant R i.e., one mole of an ideal gas at 273.16K and one atm pressure has a volume of 22.414 dm'. Since one mole of a gas has Avogadro's number of particles, so 22.414 dm' of various ideal gases at S T P will have Avogadro's number of molecules i.e. 6.02 x 1033. 22.414 dm3 of a gas at 273.16 K and one atmospheric pressure has number of molecules = 6.02 01 0.. In other words, if we have one dm' of each of He, N2, 02 , and CO in separate vessels at STP, then the number of molecules in each will be 2.68 x 1033 This is obtained by dividing 6.020 10. with 22.414 dm'. Similarly, when the temperature or pressure are equally changed for these four gases, then the new equal volumes i.e. each will have the same number of molecules i.e. 2 . 6 8 x 113.. No doubt, one dm3 of H3 at STP weighs approximately 0.0899 grams and one dm' of 02 at STP weighs 1.4384 & but their number of molecules are the same. Although, oxygen molecule is 16 times heavier than hydrogen, but this does not disturb the volume occupied, because molecules of the gases are widely separated from each other at STP One molecule is approximately at a distance of 300 times its own diameter from its neighbour at room temperature. 3.9 LIQUEFACTION OF GASES 3.9.1 General Principle of Liquefaction The conversion of a gas into a liquid requires high pressure and low temperature. High pressure brings the molecules of a gas close to each other. Low temperature deprives the molecules from kinetic energy and attractive forces start dominating. For every gas there exists a temperature above which the gas cannot be liquefied, no matter how much pressure is applied. The highest temperature at which a substance can exist as a liquid, Is called Its critical temperature (T,). There is a corresponding pressure which is required to bring about liquefaction at this critical temperature (Tc). This is called critical pressure (Pd. The critical temperature and the critical pressure of the substances are very important for the workers dealing with the gases. These properties provide us the information about the condition under which gases liquefy. For example, 0, has a critical temperature 154.4 K (-118.75 °C). It must be cooled below this temperature before it can be liquefied by applying high pressure. Ammonia is a polargas. Its critical temperature is 405.6 K (132.44°C), so it can be liquefied by applying sufficient pressure close to room temperature.