At 1200 K the standard Gibbs energies (per mole of $O X_{2}$ , in $kJ$ ) are: $2 C + O X_{2} 2 CO$ , $- 440$ ; $2 CO + O X_{2} 2 CO X_{2}$ , $- 360$ ; and for the oxides of four metals formed as $2 M + O X_{2} 2 MO$ : A, $- 300$ ; B, $- 380$ ; C, $- 420$ ; D, $- 520$ . How many of these oxides can be reduced by carbon but not by $CO$ ?

Answer: A

An oxide is reduced if its formation value is less negative than that of the reductant couple. Carbon (

- 440

) reduces A, B and C but not D.

CO

(

- 360

) reduces only A. So B and C are reduced by carbon but not by

CO

: 2 oxides.