Elemental carbon usually exists in one of two forms: graphite or diamond. It is

Question

Elemental carbon usually exists in one of two forms: graphite or diamond. It is generally believed that diamonds last forever. The table shows the standard enthalpy of formation (?H?f) and the standard molar entropy (S?) values for diamond and graphite.

What is the standard Gibbs free energy for the transformation of diamond to graphite at 298 K?

Cdiamond?Cgraphite

Express your answer to three significant figures and include the appropriate units.

Explanation / Answer

C(d) -> C(g)

Where d is for diamond and g is for graphite

Calculate first

G(g)

G(d)

Then ?G = G(g) - G(d)

For this you need

?G = ?H - T?S

?H = Hproduct - Hreactant = Hgraphite - Hdiamond = 0-1.897 kJ/mol = -1.897 kJ/mol

?S = Sproduct - Sreactant = Sgraphite - Sdiamond = 5.74- 2.38 J/molK = 3.36 J/molK

?G = ?H - T?S = -1.897 kJ/mol -(298K)(3.36J/molK) = -1.897 kJ/mol - 1001.28 J/mol = -1.897 kJ/mol - 1.00 kJ/mol = -2.897 kJ/mol

?G = -2.90 kJ/mol

Since the symbol is negative, this is the favoured condition. Therefore Diamonds are actually "not forever" and will form the more stablea allotrope graphite

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