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.

pre-sliced bread is a relatively recent development in baking technology. previous to that, it was always sold in a bakery in daily-made contiguous loaves with no wrapping, and was moderately perishable, usually being used with a day or two (which is why every bakery had a discounted “day-old” bread table.) therefore, pre-sliced bread would grow stale quickly. the hard crust provided a protective layer, and once the exposed dry slice left from the previous cut was removed, the interior was still soft.

what keeps bread soft is the moisture retained in the crumb (the light spongy part that’s not crust). the reason the crust develops is the surface of the dough gets exposed to hot dry air during baking which browns the outer layer and seals it, preventing all the moisture from escaping the dough. longer exposure to air will also eventually cause your bread to lose flavor and become stale.

people were used to cutting slices as they needed them and storing bread in a specially-made wooden or metal breadbox or pail with a lid to preserve it longer, but the first automatically sliced commercial loaves were produced on july 6, 1928, in chillicothe, missouri, using a machine invented by otto rohwedder, an iowa-born, missouri-based jeweler.[1] pre-sliced bread would indeed dry out, were it not for another 20th century technology: the plastic bag. bread could now be pre-sliced, and if the enclosing bag remains sealed as much as possible, the bread will remain soft and fresh for longer than if sitting unwrapped on a shelf or enclosed in a bread box that is not air-tight. sealed packaging meant bread could also be sold on store shelves and eliminate frequent trips to a bakery.

sealing bread in a plastic bag had another effect: the moisture in the loaf will re-distribute into the crust and soften it, which is sometimes more pleasing in a sandwich - especially for children whose teeth might not yet be strong enough to tear through a hard crust. a homemade loaf of bread can be sealed in a bag overnight and the crust will become soft like store-bought sandwich bread.

these were novel changes in how bread was produced and sold, which led to the coining of the phrase “the greatest thing since sliced bread.”

a 20-cm-long, cylindrical graphite (pure carbon) rod is inserted into an oxidizing atmosphere at 1145 k and 1.013 x 105 pa pressure. the oxidizing process is limited by the diffu- sion of oxygen counterflow to the carbon monoxide that is formed on the cylindrical surface.