Question

Calculate the heat of combustion of glucose from the following data -
(i) $C _{\text {(graphite) }}+ O _{2} (g) \longrightarrow CO _{2}(g), \Delta H =-395.0\, kJ$
(ii) $H _{2} (g) +1 / 2 O _{2} \longrightarrow H _{2} O (l), \Delta H =-269.5\, kJ$
(iii) $6 C _{\text {(graphite) }}+6 H _{2} (g) +3 O _{2} (g) \longrightarrow C _{6} H _{12} O _{6} (s), \Delta H =-1170\, kJ$

Answer 1

From given,
(i) $6 C +6 O _{2} \longrightarrow 6 CO _{2} ; \Delta H =6 \times(-395.0)\, kJ$
(ii) $6 H _{2}$ (g) $+\frac{6}{2} O _{2} \longrightarrow 6 H _{2} O$;
$\Delta H =6 \times(-269.5)\, kJ$
$\frac{(iii) C _{6} H _{12} O _{6} \longrightarrow 6 C +6 H _{2}+3 O _{2} ; \Delta H =1170\, kJ}{(i) + (ii) + (iii) C _{6} H _{12} O _{6}+6 O _{2} \longrightarrow 6 CO _{2}+6 H _{2} O}$;
$\Delta H ; 6 \times(-395.0)+6 \times(-269.5)+1170$
$=-2370-1617+1170$
$=-2817\, kJ$