Question

$O_{\left(g\right)}+e^{-}\to O_{\left(g\right)}^{-}\Delta H_1$
$O_{\left(g\right)}^{-}+e^{-}\to O_{\left(g\right)}^{-2}\Delta H_2$
$O_{2\left(g\right)}\to O_{2\left(g\right)}^{+}+e^{-}\Delta H_3$
$O_{\left(g\right)}\to O_{\left(g\right)}^{+}+e^{-}\Delta H_4$
$H_{2\left(g\right)}\to H_{2\left(g\right)}^{+}+e^{-}\Delta H_5$
$H_{\left(g\right)}\to H_{\left(g\right)}^{+}+e^{-}\Delta H_6$
$O_{\left(g\right)}+2e^{-}\to O^{-2}\Delta H_7$
Which of the following option(s) is/are correct?

• A. $|△H_1|>|△H_2|$
• B. $|△H_4|>|△H_3|$
• C. $|△H_5|>|△H_6|$
• D. $|△H_7|$ is +ve

Answer 1

Answer: (B), (C), (D)

$|△H_7|$ is +ve

A) $O_{\left(g\right)}+e^{-}\to O_{\left(g\right)}^{-}△H_1=-142kJ$
$O_{\left(g\right)}^{-}+e^{-}\to O_{\left(g\right)}^{-2}△H_2=+844kJ$
since electron is added to negatively charged species, it will experience repulsion from incoming electron, hence the process is very endothermic.

B) $O_{2\left(g\right)}\to O_{2\left(g\right)}^{+}+e^{-}$
$\left(\text{Electron removed from antibonding molecular orbitals and hence it requires less energy than an electron from atomic orbital}\pi 2P_{y}or\pi 2P_x\right)$
Electronic configuration of $O_2$ is:
$\sigma \left(1s{)}^2{\sigma }^{\ast }\right(1s{)}^2\sigma \left(2s{)}^2{\sigma }^{\ast }\right(2s{)}^2\sigma \left(2p_z{)}^2\pi \right(2p_x{)}^2\pi \left(2p_y{)}^2{\pi }^{\ast }\right(2p_x{)}^1{\pi }^{\ast }(2p_y{)}^1$
$O_{\left(g\right)}\to O_{\left(g\right)}^{+}+e^{-}\left(\text{Electron removed from 2P}\right)$

C) $H_{2\left(g\right)}\to H_{2\left(g\right)}^{+}+e^{-}$
$(\text{Electron removed from bonding molecular orbital and it requires more energy than the electron from an atomic orbital}1s$
Electronic configuration of $H_2$ is:
$\sigma (1s{)}^2$
$H_{\left(g\right)}\to H_{\left(g\right)}^{+}+e^{-}(\text{Electron removed from}1s$
D) $O_{\left(g\right)}+2e^{-}\to O_{\left(g\right)}^{-2}△H_7=702kJ$