Class 11th

10.6. Compare the alkali metals and alkaline earth metals with respect to (i) ionisation enthalpy (ii) basicity of oxides and (iii) solubility of hydroxides.

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(i) Ionization enthalpy. Because of high nuclear charge the ionization enthalpy of alkaline earth metals are higher than those of the corresponding alkali metals.

(ii) Basicity of oxides. Basicity of oxides of alkali metals are higher than that of alkaline earth metals.

(iii) Solubility of hydroxides of alkali metals is higher than that of alkaline earth metals. Alkali metals due to lower ionization enthalpy are more electropositive than the corresponding group 2 elements.

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10.5. Explain why sodium is less reactive than potassium.

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It is because ionization enthalpy? H_i of potassium = 419 kJ mol^-1. Ionization enthalpy of sodium = 496 KJ mol^-1. Since Ionization enthalpy of potassium is less than that of sodium, potassium is more reactive than sodium.

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10.4. Find out the oxidation state of sodium in Na₂O₂.

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Let x be the oxidation state of Na in Na₂O₂

Then, 2x + 2 (-1) = 0

=>2x – 2 = 0

=> x = +1.

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10.3. Why are alkali metals not found in nature?

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All the alkali metals have one valence electron, ns1 outside the noble gas core. The loosely held s-electron in the outermost valence shell of these elements makes them the mostelectropositive metals, i.e. they readily lose electron to give monovalent M+ ions. Hence, they are never found in free state in nature.

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10.2. Discuss the general characteristics and gradation in properties of alkaline earth metals.

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The general characteristics and gradation in properties of alkaline earth metals are:

Atomic size goes on increasing down the group.
Ionisation energy goes on decreasing down the group.
They are harder than alkali metals.
They are less electropositive than alkali metals.

Electropositive character increases on going down the group.

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10.1. What are the common physical and chemical features of alkali metals?

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Physical appearance: All the alkali metals are silvery white, soft and light metals.
Density: Because of the large size, these elements have low density which increases down the group except for potassium which is lighter than sodium (most likely due to an unexpected increase in the atomic size.).
The melting and boiling points of the alkali metals are low indicating weak metallic bonding due to the presence of only a single valence electron in them.
Atomic volume:The atomic volume, atomic and ionic radii rise as the group number reduces from Li to Cs.
Melting and boiling points:The weak crystal lattice bonding causes low melting and boili

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Physical appearance: All the alkali metals are silvery white, soft and light metals.
Density: Because of the large size, these elements have low density which increases down the group except for potassium which is lighter than sodium (most likely due to an unexpected increase in the atomic size.).
The melting and boiling points of the alkali metals are low indicating weak metallic bonding due to the presence of only a single valence electron in them.
Atomic volume:The atomic volume, atomic and ionic radii rise as the group number reduces from Li to Cs.
Melting and boiling points:The weak crystal lattice bonding causes low melting and boiling points of all alkali metals.

The alkali metals and their salts impart characteristiccolour to an oxidizing flame and thus can be detected bythe respective flame tests and can bedetermined by flame photometry or atomic absorption spectroscopy.
These elements whenirradiated with light, the light energy absorbedmay be sufficient to make a can atom lose electron.This property makes caesium and potassiumuseful as electrodes in photoelectric cells.
Ionisation power:Alkali metals have the lowest initial ionisation energy among their periods.The ionisation energy drops as one moves down the group.
Electropositivity character:Alkali metals are highly electropositive or metallic in nature, and this feature grows in strength as the alkali metals progress from Li to Cs.
Electrode potentials:Alkali metals are strongly electropositive and oxidise easily in water, releasing hydrogen gas.
Reducing nature:Due to their low ionisation energies, alkali metals are good reductants. Their lowering character is Na, Cs, Rb, K, Li (in aqueous solution).
Electronegativity:The electronegativity of alkali metals decreases as one moves down the periodic table.
Oxidation states:The outermost valence shell of all alkali metals contains only one electron. After the penultimate shell is completed, these atoms lose one electron to the nearest inert gas. As a result, the monovalent elements have an oxidation state +1.
Hydration of ions:All alkali metal salts (excluding lithium) are ionic and soluble in water due to a strong hydration tendency of the alkali metal cations;
Colour of flame:All alkali metals and associated salts give a non-luminous flame a distinct colour.Because of the absorption of energy, the electrons of an alkali metal and its salt are stimulated to higher energy levels when placed in a burning flame. This is a volatile state. Thus, a specific light colour is released as the electrons return towards the ground state. As one moves from Li to Cs, the ionisation energy decreases, increasing the frequency of the light emitted.
The photoelectric effect:Alkali metals, in particular K and Cs, have a photoelectric effect because of their low ionisation activity and work function.
The electrical conductivity:Since the valence electrons of all alkali metals may freely travel through the crystal structure, they are excellent heat and electric conductors. Electrical conductivity increases as one moves down the group.

Common chemical features of alkali metals are:

The alkali metals are highly reactive due totheir large size and low ionization enthalpy. Thereactivity of these metals increases down thegroup.

Reactivity towards air: The alkali metalstarnish in dry air due to the formation oftheir oxides which in turn react withmoisture to form hydroxides. They burnvigorously in oxygen forming oxides.Because oftheir high reactivity towards air and water, alkali metals are normally kept in kerosene oil.
Reactivity towards water: The alkalimetals react with water to form hydroxideand dihydrogen. Reaction of lithium with water is less vigorous thanthat of sodium due to itssmall size and very high hydration energy.Other metals of the group react explosivelywith water.They also react with proton donors suchas alcohol, gaseous ammonia and alkynes.
Reactivity towards dihydrogen: Thealkali metals react with dihydrogen atabout 673K to formhydrides. All the alkali metal hydrides areionic solids with high melting points.
Reactivity towards halogens: The alkalimetals readily react vigorously withhalogens to form ionic halides, butlithium halides are somewhatcovalent due to highpolarisation capability of lithium ion.
Reducing nature: The alkali metals arestrong reducing agents, lithium being themost and sodium the least powerful. With the small size of its ion, lithium hasthe the highest hydration enthalpy whichaccounts for its high negative E0 value andits high reducing power.
Solutions in liquid ammonia: The alkalimetals dissolve in liquid ammonia givingdeep blue solutions which are conductingin nature.

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6.30 Consider the decay of a free neutron at rest : n $\to$ p + e .Show that the two-body decay of this type must necessarily give an electron of fixed energy and, therefore, cannot account for the observed continuous energy distribution in the $β$ -decay of a neutron or a nucleus (Fig. 6.19).

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6.30 The decay process of free neutron at rest is given as:

n $? p + \overset{?}{e}$

From Einstein's mass-energy relation, we have the energy of electron as? $m c^{2}$ where,

? m = Mass defect = Mass of neutron – ( Mass of proton + mass of electron)

c = speed of light

? m and c are constant. Hence, the given two-body decay is unable to explain the continuous energy distribution in the $?$ -decay of a neutron or a nucleus. The presence of neutrino von the LHS of the decay correctly explain the continuous energy distribution.

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6.29 Which of the following potential energy curves in Fig. 6.18 cannot possibly describe the elastic collision of two billiard balls ? Here r is the distance between centers of the balls.

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6.29 The potential energy of two masses in a system is inversely proportional to the distance between them. The potential energy of the system of two balls will decrease as they get closer to each other. When the balls touch each other, the potential energy becomes zero, I.e. at r = 2R. The potential energy curve in (i), (ii), (iii), (iv) and (vi) do not satisfy these conditions. So there is no elastic collision.

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6.28 A trolley of mass 200 kg moves with a uniform speed of 36 km/h on a frictionless track. A child of mass 20 kg runs on the trolley from one end to the other (10 m away) with a speed of 4 m s^–1 relative to the trolley in a direction opposite to the its motion, and jumps out of the trolley. What is the final speed of the trolley? How much has the trolley moved from the time the child begins to run?

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6.28 Mass, m = 200 kg

Speed, v = 36 km/h = 10 m/s

Mass of the boy, M = 20 kg

Initial momentum = (M + m)v = (20 + 200) x 10 kg-m/s = 2200 kg-m/s

If v' is the final velocity of the trolley, then

The final momentum = (M+m) x v' – M x 4= 220v'-80

According to the law of conservation of energy,

Initial momentum = final momentum

2200 = 220v'-80

V' = 10.36 m/s

Time required by the boy to travel 10m = 10/4 = 2.5 s

Distance travel by trolley in 2.5 s = 10.36 x 2.5 m = 25.9 m

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6.27 A bolt of mass 0.3 kg falls from the ceiling of an elevator moving down with an uniform speed of 7 m s^–1. It hits the floor of the elevator (length of the elevator = 3 m) and does not rebound. What is the heat produced by the impact ? Would your answer be different if the elevator were stationary ?

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