Which balanced equation represents an endothermic reaction?
(1) c(s) + o2(g) → co2(g)
(2) ch4(g) + 2o2(g) → co2(g) + 2h2o(l)
(3) n2(g) + 3h2(g) → 2nh3(g)
(4) n2(g) + o2(g) → 2no(g)

4)  N2(g) + O2(g) → 2NO(g)

Explanation:

In a chemical equation, the location of the word "heat" can be used to quickly determine whether the reaction is endothermic or exothermic. If heat is released as a product of the reaction, the reaction is exothermic. If heat is listed on the side of the reactants, the reaction is endothermic.

However in the options given, heat was not mentioned. this means we would have to undergo elimination process.

Option 2 represents combustion of methane, heat is a natural by product of any combustion reaction. hence, the correct answer cannot be 2.

Option 1 - The C=O bonds in CO2 have a high bond energy (the energy you would need to break them) which is more than either of the C-C or O=O bonds in the elements and their sum. Thus to create C and O2 from CO2 you would have to put in energy to break the two C=O bonds. You would get some back from the C-C and O=O bonds but less. Thus you need to add a lot of energy to convert CO2 into its elements. (You can decompose it at high temperatures.)

The reverse process gives off heat. So again, the correct answer cannot be 1.

Option 3 - The heat of formation of H2 ( g ) and N2( gas) under the standard condition is zero ( 0), and heat of formation of NH3 ( g ) is -46.11 kj / mol, so for the formation of 2 moles of NH3( g ) it would be - 92.22 kj,

The -ve symbol is indicative of the above reaction is exothermic in nature. So again, the correct answer cannot be 3.

Our only option left is 4 - The standard enthalpy of formation of nitric oxide (NO) in the gas phase is about +90 kJ/mol, which is endothermic. The primary reason for this is that the nitrogen-nitrogen triple bond is very strong (941 kJ/mol). The energy cost to break that bond as well as the oxygen-oxygen double bond (495 kJ/mol) is not compensated by the forming of the nitrogen-oxygen double bond (607 kJ/mol). Endothermic reactions are the result of breaking strong bonds and forming weaker ones.

"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."