general chemistry 1

Question

Use the molar bond enthalpy data in the table to estimate the value of Δ𝐻∘rxn for the equation.

C2H4(g)+HBr(g)⟶C2H5Br(g)

Bond:                    𝐤𝐉⋅𝐦𝐨𝐥−1

C−H                       414

C=C                       615

H−Br                      368

C−C                       347

C−Br                      276

Answer: Δ𝐻∘rxn=_________ 𝐤𝐉⋅𝐦𝐨𝐥−1

Answer 1

It requires energy to break bonds, and then energy is released when bonds are made. So, we will find the energy needed to break the bonds in the reactants, and then the energy released when the products are made. That way we can determine the CHANGE in energy (∆H) of the reaction.

Reactants:

615 + 4x414 = 615 + 1656 = 2271 kJ/mol for C2H4 which is H2-C=C-H2

368 for H-Br

TOTAL REACTANTS = 2271 + 368 = 2639 kJ/mol (positive)

Products:

5x414 + 347 + 276 = -2693 kJ/mol = TOTAL FOR PRODUCTS (negative since heat is released)

The ∆Hrxn = -422 kJ/mol (the difference between bond breaking and bond making)