Monday 20 April 2020

1st year Chemistry Chapter 3 Short Questions Notes

1st year Chemistry Chapter 3 Short Questions Notes
Looking for the 11th class Chemistry chapter 3 Gases notes? Here we have shared the 1st year Chemistry Chapter 3 Short Questions Notes.

01. The plot of PV versus P is a straight line at constant temperature and with a fixed number of moles of an ideal gas.
 When a graph is plotted between P on x-axis and PV on the y-axis at constant temperature and fixed “n” a straight line is produced. This straight-line shows that at constant high temperature, the volume increase and value of product PV should increase due to an increase of volume at the same pressure, but PV remains constant.

 02. The straight line in (above graph) is parallel to the pressure-axis and goes away from the pressure axis at higher pressures for many gases.
 When high pressure is applied to gas then repulsion, as well as attractive forces, become dominant in some gas molecules. So at this stage gas doesn’t behave ideally so the line goes away from the pressure axis at high pressure.

 03. H2 and He are ideal at room temperature but SO2 and Cl2 are not. Explain.
 Both SO2 and Cl2 have strong intermolecular forces in them due to greater size and greater polarizability.
 The force of attraction per unit volume is very high hence excluded volume (factor b) is also high. That is why they behave non-ideal at room temperature.

 04. Water vapors do not behave ideally at 273o C. Explain.
 The critical temperature of water vapors is 374o C. The temperature of 273o C is below the critical temperature.
 At this temperature, water vapors can be converted into liquids by applying pressure.

 05. SO2 is comparatively non-ideal at 273K and but behave ideally at 327oC.
 Gases behave ideally at high temperature and low pressure. When the temperature of SO2 is increased 273K to 327oC, the intermolecular forces decreased and gas becomes more ideal.

 06. Throw some light on the factor 1/273 in Charle’s law.
 The factor 1/273 in Charle’s law is very important. Because of the volume of a given mass of a gas increases or decreases by 1/273 of its original volume at 0°. The equation which is used to calculate the volume of a gas at any temperature is:
 Vt = Vo (1+t/273)

09. Why do we feel comfortable in expressing the densities of gases in the units of g dm−3 rather than g cm−3, a unit which is used to express the densities of liquids and solids?
 Gases have low density than liquids and solids. It is due to light mass and more volume occupied by the gas molecules. Gas density is expressed in g dm−3 while the density of liquids and solids expressed in g cm−3. the density of CH4 at 0°C and 1 atm is 0.71 g dm−3, but if it is expressed in g cm−3, then it is 0.00071. so if densities of gases are expressed in g dm−3 then, the answer is reasonable but when expressed in g cm−3 then, the answer is very small and may go to fourth place of decimal for some gases.
 10. Do you think that 1 mole of H2 and 1 mole of NH3 at 0°C and 1 atm-pressure will have Avogadro’s number of particles?
 1 mole of H2 and 1 mole of NH3 at 0°C and 1 atm-pressure should occupy 22.414 dm3 volume and so both of these gases should have the same Avogadro’s number of particles.
 11. How can you explain that plasma as a whole is neutral?
 Plasma can be created by ionizing the gas mixture, which consisting of ions, electrons and neutral atoms. Although plasma includes electrons and ions and conducts electricity it is macroscopically neutral. Because in measurable quantities the number of electrons and ions is equal.

12. Justify that 1 cm3 of H2 and 1 cm3 of CH4 at STP will have the same number of molecules when one molecule of CH4 is 8 times heavier than that of hydrogen.
 According to Avogadro’s Law: “Equal volumes of all the ideal gases at the same temperature and pressure contain an equal number of molecules”. So 1 cm3 of H2 and 1 cm3 of CH4 at STP will have same number of molecules. No doubt, 1 cm3 of CH4 is 8 times heavier than that of hydrogen, but this doesn’t disturb the volume occupy. Because at STP one molecule of the gas is at a distance of 300 times its own diameter from its neighbor molecules.
 13. Do you think that some of the postulates of the kinetic molecular theory of gases are faulty? Point out these postulates.
 There are two faulty postulates in the kinetic theory of gases.
 (i) The actual volume of gas molecules is negligible as compared to the volume of the vessel.
 (ii) There are no forces of attraction among the molecules of a gas.
 Both these postulates are correct at low pressure and high temperature and these postulates become wrong at low temperatures and high pressure.
 It is necessary to account for the actual volume and mutual attractions of molecules. This job is done by Van der Waal.
 14. All gases can be liquefied by Linde’s Method except Hydrogen and Helium, explain why?
 Hydrogen and helium can’t be liquefied by Linde’s Method because their sizes are so small and they have very low polarizability. Due to these factors when high pressure is applied they experienced very less intermolecular forces. When pressure is released suddenly, they don’t liquefy due to the absence of sufficient intermolecular forces. So in order to liquefy them, their temperature has to bring nearer to absolute zero.
 15. CO2 gas deviates largely from the general gas equation at 0°C, but to less extent at 100°C.
 The size of CO2 is large so that at 0°C the forces of attraction in CO2 become dominant so it deviates largely from the general gas equation. But less extent at 100°C because at high temperature, kinetic energy will also become high due to which forces of attraction in CO2 molecule become less dominant.
 16. Why is a critical temperature for CO2(31. 1°C) less than NH3 (132. 4°C)?
 The critical temperature for CO2 is less than NH3 because in NH3 hydrogen bonding experience stronger intermolecular forces while in CO2 there is no hydrogen bonding. So stronger the intermolecular forces then higher the critical temperature. Moreover, the size of the CO2 is bigger than NH3. But the absence of hydrogen bonding in CO2 makes its critical temperature less than NH3.
 17. Why do gases show non-ideal behavior at low temperatures and high pressure?
 Gases are ideal only at low pressure and high temperature and become non-ideal at low temperature and high pressure. Actually, low temperature and high pressure become responsible for the creation of forces of attraction and moreover, the actual volume does not remain negligible. And only non-ideal gases show forces of attraction between them.
 18. The pressure of NH3 gas at given conditions (say 20 atm pressure and room temperature) is less as calculated by Van der Waals equation than that calculated by the general gas equation.
 NH3 is a polar or non-ideal gas. It has attractive forces among its molecules which give less value of pressure calculated by Van der Waal equation than the general gas equation.

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