Gaseous State And Periodic Table

Question 1

Define Dalton’s Law of Partial pressure

Answer 1

Dalton’s law of partial pressure states that the total pressure of a mixture of gases is the sum of the partial pressures of the constituent gases

Question 2

What is the partial pressure of a gas in an inert gas mixture?

Answer 2

Partial pressure of a gas in an inert gas mixture is the pressure that it would exert if it occupied the container alone

Question 3

State the 4 basic assumptions of ideal gases

Answer 3

1. Gas PARTICLES have negligible volume compared to the volume of the gas container
2. There are no intermolecular forces of attraction between gas particles
3. The collisions between gas particles and the walls of the gas container are perfectly elastic
4. Gas particles are in constant and random motion

Question 4

Describe the behaviour of real gases

Answer 4

1. The volume of gas particles is significant
2. IMFA between gas particles are significant/not negligible
3. Collisions between gas particles are not elastic (ie will result in the loss of kinetic energy)
4. Gas particles are not in constant, random motion.

Question 5

Why do gases deviate from ideal gas behaviours at (a) High pressures and (b) low temperatures??

Answer 5

At high pressures:
Volume of gas is relatively small. Vol of gas particles take up a large portion of the gas container. Volume of gas particles become significant

At low temperatures:
There exist significant IMFA between gas particles. Collisions between gas particles and walls of the container are inelastic, resulting in loss of average KE

Question 6

Describe how volume of gas deviates from the vol of an ideal gas as pressure of real gases increase

Answer 6

See graph in notes for ref

At v low pressures, real gases exhibit ideal gas behaviour as IMFA between particles are negligible

At low pressures, IMFA between particles are significant, decreasing molar volume of real gas relative to ideal gases

At high pressures, gas particles packed densely tgt, avg separation between gas particles decrease to the extent where repulsive forces dominate, driving gas particles apart. Molar volume of gas particles is larger than than of an ideal gas

Question 7

Compare and account for the different deviations from ideal gas behaviour for NH3, O2 and H2 at low pressures

Answer 7

NH3 exhibits intermolecular hydrogen bonds between gas particles which are STRONGER than the idid interactions between O2 molecules. NH3 thus has MORE significant IMFA and displays a larger deviation from ideal gas behaviour at low pressures. Hence, NH3 curve has a large dip

idid interactions in H2 are the WEAKEST due to its smallest electron cloud. Its behaviour is close to that of an ideal gas and does not display much deviation from ideal gas behaviour. Hence, curve does not have a dip

Since its a comparative qn, use comparatives

Question 8

Explain why real gases show more deviation from ideal gas behaviour as temp decreases

Answer 8

As temp decreases, average KE of molecules decrease, IMFA between particles become significant as particles with lower energy become closer together. Hence, lower temps show a larger dip on the graph

Question 9

explain why ionic radius of a cation is smaller than its corresponding atomic radius

Answer 9

Electrons are removed from the valence shell to form the cation. The cation has one less shell of electrons, thus valence electrons are less shielded and closer to the nucleus. Hence, ionic radius of a cation is smaller than its corresponding atomic radius

Question 10

Explain why anionic radius is larger than the corresponding atomic radius

Answer 10

Electrons are added to the valence shell to form the anion.
Greater repulsion occurs between the electrons, while nuclear charge remains constant (number of protons does not change). Hence, valence electrons are further away from the nucleus, and ionic radius of anion is larger

Question 11

Trend across period for atomic radius of iso electronic ions (What are iso electronic ions?)

Answer 11

Isoelectronic ions are ions which have the same number of electrons. Examples are: Na+, Mg2+, Al3+, Si4+, P3-, S2-, Cl-

Across period, number of protons increase, hence nuclear charge increases. Shielding effect remains unchanged as the number of electrons remain the same. Effective nuclear charge increases, and there is stronger electrostatic attraction between VE and nucleus, valence electrons pulled progressively closer to the nucleus. Hence, ionic radius decreases

Question 12

WITHIN THE SAME PERIOD, ionic radii of anions are larger than cations. Explain why

Answer 12

Anions have an additional electron shell compared to the cations. Despite larger NC of anions (due to more protons), valence electrons are more shielded and are further away from the nucleus than the VE of cations. (I guess screening effect stronger), hence ionic radii are larger than cations in the same period.

Question 13

The maximum attainable oxidation state generally increases across a period until a maximum is reached. Explain why max OS of S with Cl is only +4 and not +6 like expected

Answer 13

Bulky Cl atoms result in steric hindrance about the central S atom. There will be great repulsion between large electron clouds of Cl atoms

Question 14

Why is Al2O3 ionic but AlCl3 covalent?

Answer 14

Al2O3 is ionic with a considerable degree of covalent character. High charge density of Al3+ polarises the electron cloud of O2- ions, resulting in some form of “electron sharing”

AlCl3 is covalent as high charge density of Al3+ polarises the LARGE electron cloud of CL- ion, making AlCl3 predominantly covalent

Extent of covalent character depends on: 1. Polarising power of cation and 2. Polarisability of anion (size of electron cloud)

Question 15

State the pH of the period 3 chlorides from NaCl to PCl5, and the reactions that occur

Answer 15

NaCl: 7 (dissolves readily)
MgCl2: hydrolysis to form a weakly acidic solution: 6.5
- (Mg(H2O)6)2+ + H2O to form H3O+ ions

AlCl3: hydrolyses in water to form acidic solution: 3

When limited cold water added, AlCl3 forms Al(OH)3 and HCl
When more water added, (Al(H2O)6)3+ and H2O forms (Al(H2O)5(OH-))2+ + H3O+

SiCl4 and PCl5: COMPLETELY hydrolyses in water to form strongly acidic solutions : pH 2
SiCl4 and PCl5 undergo reaction with water to form aqueous HCl and H3PO4 or SiO2 (a solid) .

When limited water added, white fumes of HCl GAS is formed along with POCL3. When more water added, POCL3 further reacts to give H3PO4

Question 16

What are the reactions of oxides with water? From NaO to SO3

Answer 16

NaO dissolves completely to form NaOH

MgO is only sparingly soluble to form Mg(OH)2 (reversible arrow for sparingly soluble salts)
More exothermic lattice energy, harder to break the ionic lattice structure
- lattice energy refers to the energy evolved in forming 1 mole of ionic compound from its constituent ions.

Al2O3, SiO2 -> insoluble in water
Al2O3 has very exothermic lattice energy
SiO2 has strong and extensive covalent bonds between Si and O atoms in lattice structure, and thus SiO2 is insoluble in water.

P4O10 and SO3
P4O10 reacts with water to form H3PO4 as the only product
SO3 reacts with water to form H2SO4 as the only product

Question 17

What is the pH of the resulting solutions when period 3 oxides REACT with water 13

Answer 17

NaOH: 13
Mg(OH)2: 9 (MgO sparingly soluble)

Al2O3, SiO2: 7 (insoluble in water)

P4O10, SO3 —> 2 (forms acidic solution)

Question 18

Write the equation for reaction of Al2O3 with base (functions as an acid). Write equation for reaction of BeO OR Be(OH)2 with NaOH (diagonal relationships—Be similar to Al)

Answer 18

Al2O3 + 2NaOH + 3H2O —> 2NaAl(OH)4

BeO + 2NaOH + H2O—> Na2Be(OH)4 (note that positive charge always adds up to 4)

Question 19

What are the products formed when SiO2, P4O10 and SO3 react with a base like NaOH? (Note: they are acidic oxides).

Bonus: are there any special conditions for reaction?

Answer 19

SiO2 —> Na2SiO3 (pretend Si has +4 charge)

P4O10 —> Na3PO4 (form PO4 ion)

SO3 —> form Na2SO4

SiO2 only reacts with HOT, CONCENTRATED NaOH