Question

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

Answer 1

The heat released when ethene reacts with HBr to form CH3CH2Br is estimated by subtracting the total bond formation energy from the total bond dissociation energy, resulting in an exothermic reaction with -1099 kJ/mol of heat released.

Step-by-step explanation:

Calculating Heat Released in a Chemical Reaction

To estimate the heat released when ethene (CH2=CH2) reacts with HBr to form CH3CH2Br, we need to consider the bond enthalpies for bonds broken and formed during the reaction:

Bonds formed: one C-C, two C-H (from ethene) and two H-Br bonds

Bonds broken: C=C (612 kJ/mol) + Br-Br (193 kJ/mol) = 805 kJ/mol

Bonds formed: C-C (348 kJ/mol) + 2 x C-H (2 x 412 kJ/mol) + 2 x H-Br (2 x 366 kJ/mol) = 1904 kJ/mol

The total heat released (exothermic reaction) is the energy required to break bonds minus the energy released forming new bonds.

Heat released = Energy to break bonds - Energy to form bonds

Heat released = 805 kJ/mol - 1904 kJ/mol = -1099 kJ/mol.

This negative value indicates an exothermic reaction, meaning heat is released.

Answer 2

c)

Step-by-step 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.