Hydrogenation reactions are used to add hydrogen across double bonds in hydrocarbons and other organic compounds. Use average bond energies to calculate ΔH_rxn for the hydrogenation reaction. H₂C=CH₂(g) + H₂(g) → H₃C–CH₃(g)

Verified step by step guidance

1. Identify the bonds broken and formed in the reaction. In the reactants, you have one C=C double bond and one H-H single bond. In the products, you have two C-H single bonds and one C-C single bond.
2. Use the average bond energies to calculate the total energy required to break the bonds in the reactants. Look up the bond energies for C=C, H-H, C-H, and C-C bonds.
3. Calculate the total energy released when the new bonds are formed in the products. Use the bond energies for C-H and C-C bonds.
4. Determine the change in enthalpy (ΔH_{rxn}) by subtracting the total energy of the bonds formed from the total energy of the bonds broken: ΔH_{rxn} = (Energy of bonds broken) - (Energy of bonds formed).
5. Interpret the sign of ΔH_{rxn}. If it is negative, the reaction is exothermic, meaning it releases energy. If it is positive, the reaction is endothermic, meaning it absorbs energy.

Key Concepts

Hydrogenation Reaction

A hydrogenation reaction involves the addition of hydrogen (H2) to an unsaturated compound, typically an alkene or alkyne, resulting in a saturated compound. This process is commonly used in organic chemistry to convert double or triple bonds into single bonds, thereby increasing the hydrogen content of the molecule.

Average Bond Energies

Average bond energies represent the energy required to break a specific type of bond in a molecule, averaged over various compounds. These values are crucial for calculating the enthalpy change (ΔHrxn) of a reaction, as they allow chemists to estimate the energy absorbed or released during bond formation and breaking in the reactants and products.

Enthalpy Change (ΔHrxn)

The enthalpy change (ΔHrxn) of a reaction quantifies the heat absorbed or released during the reaction at constant pressure. It can be calculated using the formula ΔHrxn = Σ(Bond Energies of Reactants) - Σ(Bond Energies of Products), which helps determine whether a reaction is exothermic (releases heat) or endothermic (absorbs heat).