Consider the following reactions: (i) H _ (a q) ⁺+ OH _ (a q) ⁻ H ₂ O _ ( i ) , H =- X ₁ ~kJ ~mol ⁻¹ (ii) H _ 2(a q) + 1 2 O _ 2(a q) H ₂ O _ ( I ) H =- X ₂ ~kJ ~mol ⁻¹ (iii) CO _ 2( ~g ) + H _ 2(g) CO _ (g) + H ₂ O , H =- X ₃ ~kJ ~mol ⁻¹ (iv) C ₂ H _ 2(g) + 5 2 O _ 2(g) 2 CO _ 2(g) + H ₂ O _ (i) , H =+ X ₄ ~kJ ~mol ⁻¹ Enthalpy of formation of H ₂ O _ (l) is:

Step-by-step solution

The given reaction represents the formation of H ₂ O , and the X ₂ represents the enthalpy of formation of H ₂ O . The enthalpy of formation is the heat evolved or absorbed when one mole of substance is formed from its constituent atoms. Related Theory A negative enthalpy change represents an exothermic change where energy is released from the reaction, a positive enthalpy change represents an endothermic reaction where energy is taken in from the surroundings.