Question

When $O_2$ is absorbed on a metallic surface, electron transfer occurs from the metal to $O_2$. The true statement(s) regarding this adsorption is(are):

• A. $O_2$ is physisorbed
• B. heat is released
• C. occupancy of ${\pi }_{2p}^{\ast }$ of $O_2$ is increased
• D. bond length of $O_2$ is increased

Answer 1

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

The correct options are
B heat is released
C occupancy of ${\pi }_{2p}^{\ast }$ of $O_2$ is increased
D bond length of $O_2$ is increased

When $O_2$ is adsorbed on a metallic surface, electron transfer occurs from the metal to $O_2$.
The MO configuration of $O_2$ is $KK\left(\sigma 2s{)}^2\right({\sigma }^{\ast }2s{)}^2\left(\sigma 2p_x{)}^2\right(\pi 2p_y{)}^2\left(\pi 2p_z{)}^2\right({\pi }^{\ast }2p_y{)}^1({\pi }^{\ast }2p_z{)}^1$
The MO configuration of $O_2^{-}$ is $KK\left(\sigma 2s{)}^2\right({\sigma }^{\ast }2s{)}^2\left(\sigma 2p_x{)}^2\right(\pi 2p_y{)}^2\left(\pi 2p_z{)}^2\right({\pi }^{\ast }2p_y{)}^2({\pi }^{\ast }2p_z{)}^1$
(A) Thus, oxygen is chemisorbed. Hence, the statement (A) is false.
(B) Chemisorption is an exothermic process. Heat is released. Hence, the statement (B) is true.
(C) The occupancy of ${\pi }_{2p}^{\ast }$ of $O_2$ is increased. The added electron enters ${\pi }_{2p_y}^{\ast }$ MO. Hence, the statement (C) is true.
(D) The bond order is decreased (as the electron enters anti bonding MO) and the bond length of $O_2$ is increased. Hence, the statement (D) is true.