Question

Calculate the $\Delta H$in joules for: C (graphite) $\to$C (diamond) from the data:
C (graphite) + ${O_2}$(g) $\to$ $C{O_2}$(g); $\Delta {H^o}$ = -393.5kJ
C (graphite) + ${O_2}$(g) $\to$ $C{O_2}$(g); $\Delta {H^o}$ = -395.4kJ
A.1.9 kJ
B.$- 7889 \times {10^3}$
C.190000
D.$+ 7889 \times {10^3}$

Answer 1

Hint: In this question, we have to find the transfer of heat from graphite to Diamond. Use the values given in the question to solve and find the heat energy. Use Hess's law for chemical equations to find your answer.

Complete answer:
According to the given information
C (graphite) + ${O_2}$(g) $\to$ $C{O_2}$(g); $\Delta {H_1}$ = -393.5kJ (equation 1)
C (graphite) + ${O_2}$(g) $\to$ $C{O_2}$(g); $\Delta {H_2}$ = -395.4kJ (equation 2)
On reversing equation 2
$C{O_2} \to C + {O_2};\vartriangle H_2^1 = + 395.4kJ$ (Equation 3)
Adding equation 1 and 3
$C + {O_2} + C{O_2} = C{O_2} + {O_2}$
Now $\Delta H = \Delta {H_1} + \Delta {H_2}$
$\Delta H = - 393.5 + 395.4$
$\Delta H = 1.9kJ$
$\Delta H = 1900Joules$
Thus option A”1.9kJ” is the correct answer to the problem.

Note: According to Hess’ Law, if a chemical equation can be written as the sum of many other chemical equations, the enthalpy change of the first chemical equation is equivalent to the sum of the enthalpy changes of the other chemical equations.