In the following questions, a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the choices given below each question.
Assertion (A): Combustion of all organic compounds is an exothermic reaction.
Reason (R): The enthalpies of all elements in their standard state are zero.
• Both A and R are true and R is the correct explanation of A.
• Both A and R are true but R is not the correct explanation of A.
• A is true but R is false.
• A is false but R is true.
(ii) Both A and R are true but R is not the correct explanation of A.
Explanation: Combustion reaction are exothermic and are important in industries, rocketry etc. Standard enthalpy of combustion is defined as enthalpy change per mole of substance when it undergoes combustion and all reactant and product are in standard state at specific temperature.
Cooking gas in cylinders contain BUTANE (C4H10).
C4H10(g)+ 
O2(g) → 4CO2(g) + 5H2O
Combustion of glucose
C6H12O6(g) + 6O2(g) → 6CO2(g) + 6H2O(l)
The change in enthalpy is independent of initial and final state and enthalpy change of a reaction is same whether it occurs in 1 step or series of steps.