Calculate the enthalpy of the reaction
4b(s)+3o2(g)→2b2o3(s)
given the following pertinent information:
b2o3(s)+3h2o(g)→3o2(g)+b2h6(g), δh∘a=+2035 kj
2b(s)+3h2(g)→b2h6(g), δh∘b=+36 kj
h2(g)+12o2(g)→h2o(l), δh∘c=−285 kj
h2o(l)→h2o(g), δh∘d=+44 kj

Answer : The enthalpy of the reaction is, -2552 kJ/mole

Explanation :

According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.

According to this law, the chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.

The given enthalpy of reaction is,

The intermediate balanced chemical reactions are:

(1)    

(2)    

(3)    

(4)    

Now we have to revere the reactions 1 and multiple by 2, revere the reactions 3, 4 and multiple by 2 and multiply the reaction 2 by 2 and then adding all the equations, we get :

(when we are reversing the reaction then the sign of the enthalpy change will be change.)

The expression for enthalpy of the reaction will be,

Therefore, the enthalpy of the reaction is, -2552 kJ/mole

"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."