What are metallic/interstitial hydrides? How do they differ from molecular hydrides?
Metallic/interstitial hydrides are formed by many $d$-block and $f$-block elements. These hydrides conduct heat and electricity.
Unlike saline hydride, they are almost always non-stoichiometric, being deficient in hydrogen. e.g., $\mathrm{LaH}_{2.87}, \mathrm{YbH}_{2.55}, \mathrm{TiH}_{1.5-1.8}, \mathrm{ZrH}_{1.3-1.75}, \mathrm{VH}_{0.56}, \mathrm{NiH}_{0.6-0.7}, \mathrm{PdH}_{0.6-0.8}$ etc. In such hydrides, the law of constant composition does not hold good.
Comparision between molecular and metallic hydrides
| Molecular hydrides | Metallic hydrides |
|---|---|
| These are mainly formed by $p$-block elements and some $s$-block elements (Be and Mg ). | These are formed by group 3, 4, 5 (Sc, Ti, V, Y, Zr, $\mathrm{Nb}, \mathrm{La}, \mathrm{Hf}, \mathrm{Ta}, \mathrm{Ac}$ etc..) 10, 11, 12 (Pd, Cu, Zn etc..) and f-block elements (Ce, Eu, Yb, Th, U etc.) |
| Those are usually volatile compounds having low melting and boiling point. | These are hard, have a metallic lustre. |
| It conduct electricity. | These do not conduct electricity. |
Name the classes of hydrides to which $\mathrm{H}_2 \mathrm{O}, \mathrm{B}_2 \mathrm{H}_6$ and NaH belong.
$\mathrm{H}_2 \mathrm{O}-$ Covalent or molecular hydride (electron rich hydride).
$\mathrm{B}_2 \mathrm{H}_6-$ Covalent or molecular hydride (electron deficient hydride).
NaH - lonic or saline hydride.
If same mass of liquid water and a piece of ice is taken, then why is the density of ice less than that of liquid water?
In ice, molecules of $\mathrm{H}_2 \mathrm{O}$ are not packed so closely as in liquid water. There exists vacant spaces in the crystal lattice. This results in larger volume and lower density (density = mass/volume).
In other words, density of ice is lower than liquid water and hence ice floats on water.
Complete the following equations
(i) $\mathrm{PbS}(s)+\mathrm{H}_2 \mathrm{O}_2(a q) \longrightarrow$
(ii) $\mathrm{CO}(\mathrm{g})+2 \mathrm{H}_2(\mathrm{g}) \xrightarrow[\text { catalyst }]{\text { Cobalt }}$
(i) When PbS react with hydrogen peroxide, then $\mathrm{PbSO}_4$ and water are formed.
$$\mathrm{PbS}(\mathrm{s})+4 \mathrm{H}_2 \mathrm{O}_2(a q) \longrightarrow \mathrm{PbSO}_4+4 \mathrm{H}_2 \mathrm{O}$$
(ii) When carbon mono-oxide reacts with hydrogen in the presence of cobalt catalyst, then methanol is formed.
$$\mathrm{CO}(g)+2 \mathrm{H}_2(g) \xrightarrow[\text { catalyst }]{\text { Cobalt }} \mathrm{CH}_3 \mathrm{OH}(l)$$
Give reasons
(i) Lakes freeze from top towards bottom.
(ii) Ice floats on water.
(i) Density of ice is less than that of liquid water. During severe winter, the temperature of lake water keeps on decreasing. Since, cold water is heavier, therefore, it moves towards bottom of the lake and warm water from the bottom moves towards surface. This process continues. The density of water is maximum at 277 K .
Therefore, any further decrease in temperature of the surface water will decrease in density. The temperature of surface water keeps on decreasing and ultimately it freezes. Thus, the ice layer at lower temperature floats over the water below it. Due to this, freezing of water into ice takes place continuously from top towards bottom.
(ii) Density of ice is less than that of liquid water, so it floats over water.