Question

A.What is the valency of Nitrogen in the following compounds?
i) $N_2{O}_3$ ii) $N_2{O}_5$ iii) $N{O}_2$ [3 marks]

B. (i) Why metallic character decreases across a period and increases down a group? [1 mark]

(ii) Considering the atomic number and position in the periodic table, arrange the following elements in the increasing order of metallic character: $Si,Be,Mg,Na,P.$ [2 marks]

C. (i) Why are decomposition reactions called the opposite of combination reactions? Write equations for decomposition reactions. [3 marks]

(ii) Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity. [1 mark]

Answer 1

A. (i) In $N_2{O}_5$, there are two nitrogen atoms and five oxygen atoms. As a result, it has a valency of 5. [1 mark]
(ii) In $N_2{O}_3$, there are two nitrogen atoms and three oxygen atoms. As a result, it has a valency of 3. [1 mark]
(iii) In $N{O}_2$, the charge of oxygen is -2 and the charge on the nitrogen is: x + 2(-2) = 0 x = +4 [1 mark]

B. (i) As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease.
Down the group, the effective nuclear charge experienced by valence electrons decreases because the outermost electrons move farther away from the nucleus. Therefore, these can be lost easily. [1 Mark]
Hence metallic character decreases across a period and increases down a group.

(ii) Arranging the elements into different groups and periods in order of their increasing atomic numbers.

$1$ $2$ $13$ $14$ $15$

----- $Be$ ---- ----- -----

$Na$ $Mg$ ---- $Si$ $P$

We know, metallic character increases down a group and decreases along a period as we move from left to right. [1 Marks]

Therefore, $Na$ is the most metallic element, followed by $Mg$ and $Si$ while $P$ is the least metallic element. Comparing $Be$ and $Mg,Mg$ is more metallic than $Be.$ [1 Marks]

So, the overall order of increasing metallic character is:
$P<Si<Be<Mg<Na$ [1 Marks]

C.(i) Combination reaction is said to be the reaction between two or more molecules to form a larger molecule; whereas the decomposition reaction is defined as the splitting of larger molecules into two or more smaller molecules. This essentially explains that the decomposition reaction is the opposite of the combination reaction. [2 Marks]

Few examples of decomposition reactions are:
$ZnC{O}_3\left(s\right)\to ZnO\left(s\right)+C{O}_2\left(g\right)$
$CaC{O}_3\left(s\right)\to CaO\left(s\right)+C{O}_2\left(g\right)$
$2HgO\left(s\right)\to 2Hg\left(s\right)+O_2\left(g\right)$ [1 Mark]

(ii) In the presence of heat (Thermal decomposition reaction):
${\text{CaCO}}_3\text{(s)}\to \text{CaO(s)}+{\text{CO}}_2\text{(g)}$


In the presence of sunlight (Photolytic decomposition reaction):
$2\text{AgCl(s)}\to 2\text{Ag(s)}+{\text{Cl}}_2\text{(g)}$
[0.5 mark]
In the presence of electricity (Electrolytic decomposition reaction):
$2{\text{H}}_2\text{O(l)}\to 2{\text{H}}_2\text{(g)}+{\text{O}}_2\text{(g)}$
[0.5 mark]