The average bond enthalpy for a C=C double bond is 614kj/mol and that of a C-C single bond is 348kj/mol . Estimate the energy needed to break the pi bond in 2-butene.

CH3 CH3 CH3 H
\ / \ /
C=C ------> C=C
/ \ / \
H H H CH3
Cis-2-butene Trans-2-butene

Think about this. A single bond C-C is a sigma bond. The double bond is a sigma bond + a pi bond. So would the difference between energy of double bond and single bond be the pi bond?

To estimate the energy needed to break the pi bond in 2-butene, we need to calculate the difference between the bond enthalpies of the reactants and the products.

Trans-2-butene has a single C-C bond and a pi bond.
Cis-2-butene also has a single C-C bond and a pi bond.

First, let's calculate the bond enthalpy for the reactant, which is the sum of the bond enthalpies in the reactant molecule.
Reactant bond enthalpy = 348 kJ/mol (C-C single bond) + 614 kJ/mol (C=C double bond)

Now, let's calculate the bond enthalpy for the products, which is the sum of the bond enthalpies in the product molecules.
Products bond enthalpy = 2 * 348 kJ/mol (C-C single bonds)

To break the pi bond, we need to subtract the bond enthalpies of the products from the bond enthalpy of the reactant.

Energy needed to break the pi bond = Reactant bond enthalpy - Products bond enthalpy
Energy needed to break the pi bond = (348 kJ/mol + 614 kJ/mol) - (2 * 348 kJ/mol)

Simplifying the equation, we get:

Energy needed to break the pi bond = 614 kJ/mol - 696 kJ/mol
Energy needed to break the pi bond = -82 kJ/mol

Therefore, an estimated energy of -82 kJ/mol is needed to break the pi bond in 2-butene. The negative sign indicates that this process releases energy.

To estimate the energy needed to break the pi bond in 2-butene, you need to calculate the total bond enthalpy for the pi bond.

First, let's determine the number of pi bonds in 2-butene. In both cis-2-butene and trans-2-butene, there is only one pi bond between the two carbon atoms in the middle.

Next, we need to calculate the total bond enthalpy for the pi bond. Since each C=C double bond has an average bond enthalpy of 614 kJ/mol, the total bond enthalpy for one pi bond is 614 kJ/mol.

Finally, to calculate the energy needed to break the pi bond in 2-butene, you multiply the total bond enthalpy for the pi bond by the number of pi bonds in the molecule. Since there is only one pi bond in 2-butene, the energy needed to break the pi bond in 2-butene is 614 kJ/mol.

Therefore, the estimated energy needed to break the pi bond in 2-butene is 614 kJ/mol.