Select all of the following reactions that are endothermic.
-h2(g) + ½o2(g) → h2o(g), δh = -57.83 kcal/mole
-½n2(g) + ½o2(g) → no(g), δh = +21.6 kcal/mole
-½n2(g) + o2(g) + 8.1 kcal → no2(g)
-½n2(g) + 3/2h2(g) → nh3(g) + 11.0 kcal/mole
-nh3(g) → ½n2(g) + 3/2h2(g), δh = +11.0 kcal/mole

Answer : The correct options that show endothermic reactions are,

Explanation :

Endothermic reaction : It is a type of chemical reaction in which the energy is absorbed by the surroundings. In this reaction, the energy of reactants are less than the energy of products.

Exothermic reaction : It is a type of chemical reaction in which the energy is released into the surroundings. In this reaction, the energy of reactants are more than the energy of products.

Enthalpy of a reaction : It is defined as the energy change that takes place when the reactants go to the products. It is represented as,

If the heat is absorbed during the reaction then the enthalpy of reaction is positive.

If the heat is released during the reaction then the enthalpy of reaction is negative.

That means,

when the value of enthalpy of reaction is negative then the energy is released and the reaction will be exothermic reaction.

when the value of enthalpy of reaction is positive then the energy is absorbed and the reaction will be endothermic reaction.

From the given options we conclude that, the reaction which has positive value of enthalpy of reaction then those reaction are endothermic reaction and the reaction which has negative value of enthalpy of reaction then those reaction are exothermic reaction.

The endothermic reactions are those in which heat is needed (or added) into the system for the reactants to react. the value of ΔH is positive. From the reactions in the choices presented, the endothermic reactions are the second, third, and fifth. All the rest of the choices are exothermic reactions. 

Explanation:

CH₃C(CH₃)=CH-CH₂-CH₃ ( 2 - methylpent-2-ene)

Cl₂ ⇒ Cl⁺ + Cl⁻

Cl⁺( electrophile)

CH₃C(CH₃)=CH-CH₂-CH₃  + Cl⁺ ( electrophile)  ⇒ CH₃C⁺(CH₃)-C(Cl)H-CH₂-CH₃ (  intermediate )

CH₃C⁺(CH₃)-C(Cl)H-CH₂-CH₃ + Cl⁻  ⇒ CH₃CCl(CH₃)-C(Cl)H-CH₂-CH₃ ( final product )

It is electrophilic addition reaction.