question_answer

Calculate \[\Delta H\](in Joules) for, C (graphite) \[\to C\](diamond), from the following data C(graphite)\[+\,{{O}_{2}}(g)\to C{{O}_{2}}(g);\] \[\Delta H=-393.5\,kJ\] C (diamond)\[+\,{{O}_{2}}(g)\to C{{O}_{2}}(g);\] \[\Delta H=-395.4\,kJ\]

A) \[1,900\]        

B) \[~-788.9\times {{10}^{3}}\]

C) \[1,90,000\]    

D) \[~+\,788.9\times \,{{10}^{3}}\]

Correct Answer: A

Solution :

 \[C(\text{graphite})\,+{{O}_{2}}(g)\xrightarrow{{}}C{{O}_{2}}(g);\] \[\Delta H=-393.5\,kJ\] ?(i) \[\text{C}\,\text{(diamond)}\,\text{+}\,O{{}_{2}}(g)\xrightarrow{{}}C{{O}_{2}}(g);\] \[\Delta H=-395.4\,kJ\] ?(ii) C (graphite)\[\xrightarrow{{}}\]C(diamond); Subtract the Eq (ii) form (i) C (graphite) \[\xrightarrow{{}}\]C(diamond) \[\Delta H=-393.5-(-395.4)\] \[=1.9\,kJ=1900\,J\]