C(graphite) +O2( g) ⟶CO2( g) ; ΔH=94.05 kcal mol^-1 .C(dianwed )+𝐎2 𝐭 g) ⟶CO2( g) ; ΔH=94.50 kca

step 1 Carbon in graphite form plus oxygen gas forms carbon dioxide. Change of enthalpy is 94 .05 kiloc

C(graphite) +O2( g) ⟶CO2( g) ; ΔH=94.05 kcal mol^-1...

$C_{\text {(graphite) }}+\mathrm{O}_{2}(\mathrm{~g}) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g}) ; \Delta H=94.05 \mathrm{kcal}$ $\mathrm{mol}^{-1}$ $\left.C_{\text {(dianwed })}+\mathbf{O}_{2}{ }_{\mathbf{t}} \mathrm{g}\right) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g}) ; \Delta H=94.50 \mathrm{kcal}$ Therefore, (1) $\mathbf{C}_{\text {(diamond) }} \rightarrow C_{\text {tgraphite }} ; \Delta I I_{298 \mathrm{~K}}^{\gamma}=-450$ cal mol 1 (2) $\mathbf{C}_{(\text {graptile) }} \rightarrow \mathrm{C}_{\text {idiannond }} ; \Delta / I_{2 v \mathrm{~K}}^{\theta}=-450 \mathrm{cal} \mathrm{mol}$ (3) Diamond is harder than graphitc. (4) Graphite is stable isotope.

$C_{\text {(graphite) }}+\mathrm{O}_{2}(\mathrm{~g}) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g}) ; \Delta H=94.05 \mathrm{kcal}$
$\mathrm{mol}^{-1}$
$\left.C_{\text {(dianwed })}+\mathbf{O}_{2}{ }_{\mathbf{t}} \mathrm{g}\right) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g}) ; \Delta H=94.50 \mathrm{kcal}$
Therefore,
(1) $\mathbf{C}_{\text {(diamond) }} \rightarrow C_{\text {tgraphite }} ; \Delta I I_{298 \mathrm{~K}}^{\gamma}=-450$ cal mol 1
(2) $\mathbf{C}_{(\text {graptile) }} \rightarrow \mathrm{C}_{\text {idiannond }} ; \Delta / I_{2 v \mathrm{~K}}^{\theta}=-450 \mathrm{cal} \mathrm{mol}$
(3) Diamond is harder than graphitc.
(4) Graphite is stable isotope.

Key Concepts

Carbon Allotropes

Carbon exists in different structural forms known as allotropes, with diamond and graphite being two prominent examples. These allotropes exhibit distinct bonding patterns and crystal structures, which lead to vastly different physical and chemical properties even though they consist solely of carbon atoms.

Standard Enthalpy of Formation and Reaction Energies

The standard enthalpy of formation is a measure of the energy change when a compound is formed from its elements under standard conditions. In combustion reactions, comparing the enthalpy changes for different allotropes provides insight into their relative internal energies and thermodynamic stabilities.

Thermodynamic Stability

Thermodynamic stability refers to the intrinsic energy state of substances, where a lower energy state corresponds to a more stable form. In the context of carbon allotropes, graphite is generally more stable than diamond at standard conditions, as evidenced by the energy differences calculated from their combustion enthalpies.

Physical Properties Versus Chemical Stability

It is important to distinguish between chemical stability, which relates to the energy content and reactivity of a substance, and physical properties such as hardness. While diamond is physically harder due to its strong covalent bonding network, its higher energy state compared to graphite makes it less thermodynamically stable under standard conditions.

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