What are the common physical and chemical features of alkali metals?
Physical properties of alkali metalsare as follows.
(1) They are quite soft and can be cut easily. Sodium metal can be easily cut using a knife.
(2) They are light coloured and are mostly silvery white in appearance.
(3) They have low density because of the large atomic sizes. The density increases down the group from Li to Cs. The only exceptionto this is K, which has lower density than Na.
(4) The metallic bonding present in alkali metals is quite weak. Therefore, they have low melting and boiling points.
(5) Alkali metals and their salts impart a characteristic colour to flames. This is because the heat from the flame excites the electron present in the outermost orbital to a high energy level. When this excited electron reverts back to the ground state, it emits excess energy as radiation that falls in the visible region.
(6) They also display photoelectric effect. When metals such as Cs and K are irradiated with light, they lose electrons.
Chemical properties of alkali metals
Alkali metals are highly reactive due to their low ionization enthalpy. As we move down the group, the reactivity increases.
(1) They react with water to form respective oxides or hydroxides. As we move down the group, the reaction becomes more and more spontaneous.
(2) They react with water to form their respective hydroxides and dihydrogens.
(3) They react with dihydrogen to form metal hydrides. These hydrides are ionic solids and have high melting points.
2M + H2 → 2M+H-
(4) Almost all alkali metals, except Li, react directly with halogens to form ionic halides.
2M + Cl2 → 2MCl
(M = Li,K,Rb,Cs)
Since Li+ion is very small in size, it can easily distort the electron cloud around the negative halide ion. Therefore, lithium halides are covalent in nature.
(5) They are strong reducing agents. The reducing power of alkali metals increases on moving down the group. However, lithium is an exception. It is the strongest reducing agent among the alkali metals. It is because of its high hydration energy.
(6) They dissolve in liquid ammonia to form deep blue coloured solutions. These solutions are conducting in nature.
M + (x+y)NH3 → [M(NH3)x]+ + [M(NH3)y]-
The ammoniated electrons cause the blue colour of the solution. These solutions are paramagnetic and if allowed to stand for some time, then they liberate hydrogen. This results in the formation of amides.
M+(am) + e- + NH3(l) → MNH(am) + 1/2 H2(g)
In a highly concentrated solution, the blue colour changes to bronze and the solution becomes diamagnetic.
The mass of an electron is 9.1 × 10–31 kg. If its K.E. is 3.0 × 10–25 J, calculate its wavelength.
Calculate the amount of carbon dioxide that could be produced when
(i) 1 mole of carbon is burnt in air.
(ii) 1 mole of carbon is burnt in 16 g of dioxygen.
(iii) 2 moles of carbon are burnt in 16 g of dioxygen.
Calculate the wavelength of an electron moving with a velocity of 2.05 × 107 ms–1.
Balance the following redox reactions by ion – electron method :
(a) MnO4 – (aq) + I – (aq) → MnO2 (s) + I2(s) (in basic medium)
(b) MnO4 – (aq) + SO2 (g) → Mn2+ (aq) + HSO4– (aq) (in acidic solution)
(c) H2O2 (aq) + Fe 2+ (aq) → Fe3+ (aq) + H2O (l) (in acidic solution)
(d) Cr2O7 2– + SO2(g) → Cr3+ (aq) + SO42– (aq) (in acidic solution)
In a process, 701 J of heat is absorbed by a system and 394 J ofwork is done by the system. What is the change in internal energy for the process?
What will be the minimum pressure required to compress 500 dm3 of air at 1 bar to 200 dm3 at 30°C?
In a reaction A + B2 → AB2 Identify the limiting reagent, if any, in the following reaction mixtures.
(i) 300 atoms of A + 200 molecules of B
(ii) 2 mol A + 3 mol B
(iii) 100 atoms of A + 100 molecules of B
(iv) 5 mol A + 2.5 mol B
(v) 2.5 mol A + 5 mol B
At 0°C, the density of a certain oxide of a gas at 2 bar is same as that of dinitrogen at 5 bar. What is the molecular mass of the oxide?
Which one of the following will have largest number of atoms?
(i) 1 g Au (s)
(ii) 1 g Na (s)
(iii) 1 g Li (s)
(iv) 1 g of Cl2(g)
Density of a gas is found to be 5.46 g/dm3 at 27 °C at 2 bar pressure. What will be its density at STP?
Consider the reactions:
(a) 6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)
(b) O3(g) + H2O2(l) → H2O(l) + 2O2(g)
Why it is more appropriate to write these reactions as:
(a) 6CO2(g) + 12H2O(l) → C6H12O6(aq) + 6H2O(l) + 6O2(g)
(b) O3(g) + H2O2(l) → H2O(l) + O2(g) + O2(g)
Also suggest a technique to investigate the path of the above (a) and (b) redox reactions.
An aqueous solution of borax is
(a) neutral (b) amphoteric (c) basic (d) acidic
Enthalpies of formation of CO(g), CO2(g), N2O(g) and N2O4(g) are –110, – 393, 81 and 9.7 kJ mol–1 respectively. Find the value of ΔrH for the reaction:
N2O4(g) + 3CO(g) → N2O(g) + 3CO2(g)
Use the periodic table to answer the following questions.
(a) Identify an element with five electrons in the outer subshell.
(b) Identify an element that would tend to lose two electrons.
(c) Identify an element that would tend to gain two electrons.
(d) Identify the group having metal, non-metal, liquid as well as gas at the room temperature.
If B-Cl bond has a dipole moment, explain why BCl3 molecule has zero dipole moment.
In Milikan’s experiment, static electric charge on the oil drops has been obtained by shining X-rays.
If the static electric charge on the oil drop is –1.282 × 10–18C, calculate the number of electrons present on it.
Give a brief description of the principles of the following techniques taking an example in each case.
(a) Crystallisation
(b) Distillation
(c) Chromatography
The compound AgF2 is an unstable compound. However, if formed, the compound acts as a very strong oxidizing agent. Why?
The species: H2O, HCO–3, HSO-4 and NH3 can act both as Brönsted acids and bases. For each case give the corresponding conjugate acid and base.
For the following equilibrium, Kc = 6.3 × 1014 at 1000 K
NO (g) + O3 (g) ↔ NO2 (g) + O2 (g)
Both the forward and reverse reactions in the equilibrium are elementary bimolecular reactions. What is Kc, for the reverse reaction?