The four enthalpy-change equations are listed below. $ \begin{aligned} \text{Na(g)} &\rightarrow \text{Na}^+(\text{g}) + e^- && \Delta H = W \\ \text{Na(g)} &\rightarrow \text{Na}^{2+}(\text{g}) + 2e^- && \Delta H = X \\ \text{Na(s)} &\rightarrow \text{Na(g)} && \Delta H = Y \\ \text{Na(s)} &\rightarrow \text{Na}^{2+}(\text{g}) + 2e^- && \Delta H = Z \end{aligned} $ Which equation represents the second ionisation energy of sodium?
- A$X$
- B$X + Y - W$
- C$X - W$
- D$Z - W$