Question

Which reaction is exothermic?

CH4 + 2O2 -->CO2 + 2H2O + 891 kJ
NH4NO3 + H2O + 25 kJ --> NH4+(aq) + NO3-(aq)
2H2O + 286 kJ --> 2H2 + O2
N2 + O2 + 181 kJ-->2NO

Answer 1

Ans: a) CH4 + 2O2 -->CO2 + 2H2O + 891 kJ

Chemical reactions can be broadly classified as: Exothermic and endothermic reactions.

Exothermic reactions proceed with the release of heat. Here the potential energy of the reactants is greater than that of the products and the reaction enthalpy is negative.

In contrast, Endothermic reactions proceed with the absorption of heat. Here the potential energy of the reactants is lower than that of the products and the reaction enthalpy is positive.

In the given examples only

CH4 + 2O2 -->CO2 + 2H2O + 891 kJ

is an exothermic reaction as it involves production or release of 891 kJ of heat.

The other reactions are endothermic since they involve absorption of heat.

Answer 2

Answer: Correct option is
`CH_4+2O_2\rightarrow CO_2+2H_2O+891kJ`

Explanation: Exothermic reactions are the reactions in which energy is released to the surroundings.

For these reactions, the energy term should be present on the product side and in the options given the energy term is present in the reaction,
`CH_4+2O_2\rightarrow CO_2+2H_2O+891kJ`

All other reactions have energy term on the reactant side and hence, they are considered as endothermic reactions.