$C_{(graphite)} + O_{2(g)} \to CO_{2(g)}; \Delta H = - 94.05 \ k \ cal \ mol^{-1}$
$C_{(diamond)} + O_{2(g)} \to CO_{2(g)}; \Delta H = - 94.50 \ k \ cal \ mol^{-1}$
Therefore:

  • A
    $C_{(graphite)} \to C_{(diamond)}; \Delta H_{298 \ K} = - 450 \ cal \ mol^{-1}$
  • B
    $C_{(diamond)} \to C_{(graphite)}; \Delta H_{298 \ K} = + 450 \ cal \ mol^{-1}$
  • C
    Graphite is the stabler allotrope
  • D
    Diamond is harder than graphite

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