Answer to Question #137963 in Organic Chemistry for Cameron Rennie

Question

The standard enthalpy of formation of H2O (l) is -286 kJ/mol and the standard enthalpy of combustion of ethane is -1560 kJ/mol. Given that the enthalpy change when one mole of ethene is redcued by hydrogen gas to ethane is - 138kJ/mol, calculate the standard enthalpy of combustion of ethene.

Answer 1

$1) H_2(g) + \frac{1}{2}O_2(g) \rightarrow H_2O(l)$ $-286 \frac{kJ}{mol}$

2) $C_2H_6(g) + \frac{7}{2} O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g)$ $-1560 \frac{kJ}{mol}$

3) $C_2H_4(g)+ H_2(g) \rightarrow C_2H_6(g)$ $-138 \frac{kJ}{mol}$

To get the equation of etene combustion

$C_2H_4 (g) + 3O_2(g) \rightarrow 2CO_2 (g) + 2H_2O (g)$

we should "read" the first equation form right to left and then add all three euqations together

1) $H_2O(l) \rightarrow H_2(g) + \frac{1}{2}O_2(g)$ $+286 \frac{kJ}{mol}$

2) $C_2H_6(g) + \frac{7}{2} O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g)$ $-1560 \frac{kJ}{mol}$

3) $C_2H_4(g)+ H_2(g)\rightarrow C_2H_6(g)$ $-138 \frac{kJ}{mol}$

$H_2O(l) +C_2H_6(g) + \frac{7}{2} O_2(g) +C_2H_4(g)+ H_2(g) \rightarrow H_2(g) + \frac{1}{2}O_2(g)+2CO_2(g) + 3H_2O(g)+C_2H_6(g)$

$[phone] = -1412 \frac{kJ}{mol}$

cancel the same species:

$C_2H_4(g) + 3O_2(g) \rightarrow 2CO_2(g) +2H_2O(g)$ $-1412 \frac{kJ}{mol}$