Consider the reaction.

2 upper c (s, graphite) plus upper h subscript 2 right arrow upper c subscript 2 upper h subscript 2 (g).

standard enthalpies of formation
substance
dhf (kj/mol)
c2 h2 (g)
226.73
caco3 (s)
–1206.92
cao (s)
–635.09
co (g)
–110.525
co2 (g)
–393.509
h 2 o (i)
–285.8
h 2 o (g)
–241.818
c (s), diamond
1.895
c (s), graphite
0.0

based on the equation and the information in the table, what is the enthalpy of the reaction?
use delta h r x n equals the sum of delta h f of all the products minus the sum of delta h f of all the reactants..

–453.46 kj
–226.73 kj
226.73 kj
453.46 kj

we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.