Question

Heat of which of the following reaction gives enthalpy of formation directly ?
1)H2(g) + I2 (g)=2HI(g)
2)N2(g) +3H2(g)=2NH3(g)
3)C(graphite) +2H2(g) = CH4(g)
4) All of these

Answer 1

third option is correct because in heat of formation we take the change in energy which is required to form one mole of the compound

Answer 2

Answer: The correct option is 3.

Explanation: The enthalpy of formation of a compound is defined as the change in enthalpy when 1 mole of a substance is formed from its constituent elements in their standard states. This is written as $\Delta H_f$

From the given option:

Option 1 & Option 2: In these, 2 moles of HI and 2 moles of $NH_3$ are getting formed and hence these reaction will not give the enthalpy of formation.

Option 3: The moles of $CH_4$ is 1 in this reaction from Carbon and hydrogen in their respective standard states. Hence, this reaction will give the enthalpy of formation directly.