Estimate the heat released when ethene (CH2=CH2) reacts with HBr to give CH3CH2Br.

Bond enthalpies are:

C--H : 412 kJ/mol; C--C : 348 kJ/mol

C=C : 612 kJ/mol; C--Br : 276 kJ/mol;

Br--Br : 193 kJ/mol; H--Br : 366 kJ/mol.

a) 470 kJ/mol b) 200 kJ/mol c) 58 kJ/mol d) 1036 kJ/mol e) 424 kJ/mol

c)

Explanation:

The reaction between ethene and HBr is:

CH2=CH2 + HBr → CH3CH2Br

Thus, the double bond C=C and the bond H-Br were broken, and the bonds C-C, C-H and C-Br were formed. The enthalpy of the reaction (the energy change), is the sum of the enthalpies of the reactions that were broken with the bonds that were created.

To break a bond, it's necessary to the molecule to gain energy, thus the process is endothermic and H>0, and to form a bond, it's necessary to release energy, and so H<0, so:

ΔH = ∑n*Hbroken - ∑n*Hformed, where n is the coefficient of the substance, so:

ΔH = (612 + 366) - (348+412+276)

ΔH = - 58 kJ/mol

The minus signal indicates that the energy was released.