ThermochemistryHard

Question

Using the data provided, calculate the multiple bond energy (kJ mol-1) of a C ≡ C bond in C2H2. That energy is (take the bond energy of a C - H bond as 350 kJ mol-1)
2C(s) → 2C(g) ; ᐃH = 1410 kJmol-1
2C(s) → 2C(g) ; ᐃH = 1410 kJmol-1
H2(g) → 2H(g) ; ᐃH = 330 kJmol-1

Options

A.1165
B.837
C.865
D.815

Solution

(i) 2C(s) + H2(g) → H - C ≡ C - H(g); ᐃH = 225 kJmol-1
(ii) 2C(s) → 2C(g) ; ᐃH = 1410kJmol-1
(iii) H2(g) → 2H(g) ; ᐃH = 330kJmol-1
From equation (i):
225 = [2  áƒHC(s)→C(g) + BEH-H] - [2 BEC - H +  BEC ≡ C]
225 = [1410 + 330] - [2 ( 350) +  BEC ≡ C ]
225 = [1410 + 330] - [700 + BEC ≡ C]
225 = 1740 - 700 - BEC ≡ C
225 = 1040 - BEC ≡ C
BEC ≡ C = 1040 - 225 = 815 kJ mol-1

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