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  4. What is the enthalpy of formation of water(l) in terms of X1,X2,X3,X4 is: (i) textH( textaq)+ + textOH( textaq)- arrow ( textH)2 textO (ℓ ​) ; Δ textH = - textX1 text k J mol-1 (ii) ( textH)2 ( textg) + (1/2) ( textO)2 ( textg) arrow ( textH)2 textO (ℓ ) ; Δ textH = - textX2 text k J mol-1 (iii) ( textCO)2 ( textg) + ( textH)2 ( textg) arrow textCO ( textg) + ( textH)2 textO ; Δ textH = - textX3 text k J mol-1 (iv) ( textC)2 ( textH)2 ( textg) + (5/2) ( textO)2 ( textg) arrow 2 ( textCO)2 ( textg) + ( textH)2 textO (ℓ ) ; Δ textH = + 4 textX4 text k J mol-1

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Q. What is the enthalpy of formation of water(l) in terms of $X1,X2,X3,X4$ is:
(i) $\text{H}_{\left(\text{aq}\right)}^{+} + \text{OH}_{\left(\text{aq}\right)}^{-} \rightarrow \left(\text{H}\right)_{2} \text{O} \left(\ell ​\right)$ ; $\Delta \text{H} = - \text{X}_{1} \text{ k J mol}^{-1}$
(ii) $\left(\text{H}\right)_{2} \left(\text{g}\right) + \frac{1}{2} \left(\text{O}\right)_{2} \left(\text{g}\right) \rightarrow \left(\text{H}\right)_{2} \text{O} \left(\ell \right)$ ; $\Delta \text{H} = - \text{X}_{2} \text{ k J mol}^{-1}$
(iii) $\left(\text{CO}\right)_{2} \left(\text{g}\right) + \left(\text{H}\right)_{2} \left(\text{g}\right) \rightarrow \text{CO} \left(\text{g}\right) + \left(\text{H}\right)_{2} \text{O}$ ; $\Delta \text{H} = - \text{X}_{3} \text{ k J mol}^{-1}$
(iv) $\left(\text{C}\right)_{2} \left(\text{H}\right)_{2} \left(\text{g}\right) + \frac{5}{2} \left(\text{O}\right)_{2} \left(\text{g}\right) \rightarrow 2 \left(\text{CO}\right)_{2} \left(\text{g}\right) + \left(\text{H}\right)_{2} \text{O} \left(\ell \right)$ ; $\Delta \text{H} = + 4 \text{X}_{4} \text{ k J mol}^{-1}$

NTA AbhyasNTA Abhyas 2020

Solution:

This reactions shows the formation of $H_{2} O$ , and the $- \text{X}_{2}$ represents the enthalpy of formation of $H_{2} O$ because as the definition suggests that the enthalpy of formation is the heat evolved or absorbed when one mole of substance is formed from its constituent elements in their standard form.