The heat of combustion of acetylene is 312 kcal- If heat of formation of CO2 and H2O are 94-38 and 68-38 kcal respectively- C - C bond energy is x kcal- Given that heat of atomisation of C and H are 150-0 and 51-5 kcal respectively and C- -H bond energy is 93-64 kcal-Value of x nearest integer value is -

Given, #160; C_2H_2 + 52O_2⟶ 2CO_2 + H_2O; #160; Δ H = 312 kcal #160; #160; #160; ... (i) #160; #160; #160; #160; #160; #160; ...

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Standard XII

Chemistry

Geometrical Isomerism

The heat of c...

Question

The heat of combustion of acetylene is 312 kcal. If heat of formation of $C O_{2}$ and $H_{2} O$ are 94.38 and 68.38 kcal respectively, $C \equiv \equiv C$ bond energy is $x k c a l$ . Given that heat of atomisation of C and H are 150.0 and 51.5 kcal respectively and $C - - H$ bond energy is 93.64 kcal.

Value of $x$ (nearest integer value) is _______.

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Solution

Given, $C_{2} H_{2} + \frac{5}{2} O_{2} ⟶ 2 C O_{2} + H_{2} O; Δ H = - 312 k c a l$ ... (i)
( $∵$ Heat of combustion are exothermic)

$C + O_{2} ⟶ C O_{2}; Δ H = - 94.38 k c a l$ ... (ii)

$H_{2} + \frac{1}{2} O_{2} ⟶ H_{2} O; Δ H = - 68.38 k c a l$ ... (iii)

Multiply eq. (ii) by 2 and add in eq. (iii)

$2 C + H_{2} + (5 / 2) O_{2} ⟶ 2 C O_{2} + H_{2} O Δ H = - 257.14 k c a l$ ...( iv)
Subtracting eq. (i) from eq. (iv)
$C_{2} H_{2} + (5 / 2) O_{2} ⟶ 2 C O_{2} + H_{2} O Δ H = - 312 k c a l$ ...( i)
- - - - +
........................................................................................................................
$2 C + H_{2} ⟶ C_{2} H_{2}; Δ H = + 54.86 k c a l$ ... (v)

Also, $Δ H$ for $C + H_{2} ⟶ C_{2} H_{2}$

$Δ H =$ Bond energy data of formation of bond + Bond energy data of dissociation of bond

$Δ H = - [e_{(C \equiv \equiv C)} + 2 \times e_{(C - - H)}] + [2 C_{s \to g} + e_{(H - - H)}]$

Also given, $C_{s \to g} = 150 k c a l$

$\frac{1}{2} H_{2} (g) ⟶ H (g) - 51.5 k c a l$

$∴ e_{H - - H} = 51.5 \times 2 k c a l = 103.0 k c a l$
$e_{C - - H} = 93.64 k c a l$

$∴ 54.86 = - [e_{(C \equiv \equiv C)} + 2 \times 93.64] + [2 \times 150 + 1 \times 103.0]$

$∴ e_{C \equiv \equiv C} = 160.86 k c a l ≃ 161$

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