# What is the heat of hydrogenation in a hydrogenation reaction?

Sep 3, 2015

Hydrogenation reactions consist of the addition of (guess what?) hydrogen to a molecule. For example...

$\text{Ethene + "H_2 "" stackrel("Pd/C")(->) "Ethane}$

The heat of whatever event at constant pressure, ${q}_{p}$, is simply the enthalpy of such an event, $\Delta H$. For a hydrogenation reaction, the enthalpy of hydrogenation is simply the enthalpy of reaction, or $\Delta {H}_{\text{rxn}}$.

This enthalpy could be broken down into which bonds were broken or made. One might call those $\Delta {H}_{\text{broken}}$ and $\Delta {H}_{\text{made}}$.

Whatever the case, the heat of hydrogenation is fundamentally based on which bonds were broken, which were made, and the overall differences in them throughout a hydrogenation reaction typically on a per-$\text{mol}$ basis, commonly of alkenes, sometimes of alkynes.

In the example I listed above, you break:

• $1$ $C = C$ bond

and make:

• $1$ $C - C$ bond

since you had a double bond and then you just have a single bond. Then you break:

• $1$ $H - H$ bond

before you allow ${H}_{2}$ to make:

• $2$ $C - H$ bonds

These enthalpies are:

$C = C$ bond: $\text{~602 kJ/mol}$
$C - C$ bond: $\text{~346 kJ/mol}$
$H - H$ bond: $\text{~436 kJ/mol}$
$C - H$ bond: $\text{~413 kJ/mol}$

Breaking a bond takes outside energy and puts it into the bond, and is thus positive. Making a bond releases energy into the atmosphere and is thus reported as negative. Overall, you get about:

$\Delta {H}_{\text{rxn" = sum_i DeltaH_"broken,i" - sum_i DeltaH_"made,i}}$

$= {\overbrace{\left[\left({\underbrace{602}}_{b r o k e n} - {\underbrace{346}}_{m a \mathrm{de}}\right) + {\underbrace{436}}_{b r o k e n}\right]}}^{\text{Step 1") - overbrace([underbrace(413 + 413)_(made)])^("Step 2}}$

$= {\overbrace{\left(602 + 436\right)}}^{b r o k e n} - {\overbrace{\left(346 + 413 + 413\right)}}^{m a \mathrm{de}}$

$\approx \textcolor{b l u e}{\text{-134 kJ/mol}}$

The enthalpy or heat of hydrogenation of ethene into ethane is exothermic.