Question

For a reaction: $C_{\left(s\right)}+O_{2\left(g\right)}\to C{O}_{2\left(g\right)}$
what is the relation between enthalpy of reaction ($\Delta H_r$), enthalpy of formation of $C{O}_2(\Delta H_f$) and enthalpy of combustion of carbon ($\Delta H_{comb}$)?

• A. $\Delta H_r>\Delta H_f=\Delta H_{comb}$
• B. $\Delta H_r=\Delta H_f=\Delta H_{comb}$
• C. $\Delta H_r=\Delta H_f+\Delta H_{comb}$
• D. $\Delta H_r=2\times \Delta H_f$

Answer 1

Answer: (B)

$\Delta H_r=\Delta H_f=\Delta H_{comb}$

Since the reaction shows combustion of one mole of carbon, $\Delta H_{reaction}$ is enthalpy of combustion since 1 mole of $C{O}_2$ is formed. $\Delta H_{reaction}$ is also the enthalpy of formation of $C{O}_2$.