Comparing strength of the C-H, N-H, and O-H bonds


> 
> $$
> \begin{align}
> \ce{CH4(g) + Cl(g) &-> CH3(g) + HCl(g)} &\quad ΔH^\circ &= \pu{-14 kJ mol-1}\_\mathrm{rxn}\\
> \ce{NH4(g) + Cl(g) &-> NH2(g) + HCl(g)} &\quad ΔH^\circ &= \pu{-36 kJ mol-1}\_\mathrm{rxn}\\
> \ce{H2O(g) + Cl(g) &-> OH(g) + HCl(g)} &\quad ΔH^\circ &= \pu{+40 kJ mol-1}\_\mathrm{rxn}
> \end{align}
> $$
> 
> 
> **41**. Based on the data above, what can be concluded regarding the strength of the $\ce{C-H}$, $\ce{N-H}$, and $\ce{O-H}$ bonds in the molecules shown?
> 
> 
> (**A**) The $\ce{C-H}$ bond is the strongest.  
> 
>  (**B**) The $\ce{N-H}$ bond is the strongest.  
> 
>  (**C**) The $\ce{O-H}$ bond is the strongest.  
> 
>  (**D**) Nothing can be concluded without knowing the strength of the $\ce{H-Cl}$ bond. 
> 
> 
> 


I am not sure how to approach the problem **41**. In exothermic reactions, more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants. So would the answer be **B** because the $ΔH$ is exothermic and is the greater than $ΔH$ of -14 for **A**?




The answer should be **C**.
One way would be to write the thermochemical equations, however since we just want to compare, we can do it without going into that. 
The formation of the H-Cl bond, (like all bonds) is exothermic. And the **breaking** of the C-H, N-H and O-H bonds is endothermic (since their formation is exothermic too).
Since the contribution of the H-Cl bond to the enthalpy is the same for all three, we can focus on the C-H, N-H and O-H bonds alone.


Now, the stronger the bond, the more energy it would **need**, and hence make a positive contribution to the enthalpy change. Hence we can see that since the total enthalpy of the reaction is highest(**most positive**) for the reaction involving O-H bond, the O-H bond is the strongest. (Infact, it is stronger than the H-Cl bond too as the enthalpy change is positive).


Note that this question also demonstrates the order of **electronegativity**, which is 
O>Cl >~ N> C>H. Since the bond with most electronegativity difference would be the strongest, it would be the **O-H** bond. 


 3