MCQ
Enthalpy for the reaction $C + {O_2} \to C{O_2}$ is
- APositive
- ✓Negative
- CZero
- DNone
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(Given: Atomic no. $Eu, 63; Sm , 62 ; Tm , 69 ; Tb$, $65 ; Yb , 70 ; Dy , 66]$

$\left[\mathrm{CoCl}\left(\mathrm{NH}_3\right)_5\right]^{2+}, \quad\left[\mathrm{Co}(\mathrm{CN})_6\right]^{3-} \text {, }$
$(A)$ $(B)$
$\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5\left(\mathrm{H}_2 \mathrm{O}\right)\right]^{3+}, \quad\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_4\right]^{2+}$
$(C)$ $(D)$
The correct order of $A$, $B$, $C$ and $D$ in terms of wavenumber of light absorbed is :