Question

Hexane is a saturated hydrocarbon that exists in liquid form. It burns in air to give CO2 and water
C6H6 (mc003-1.jpg) + mc003-2.jpgO2 ® 6 CO2 + 3H2O (l)
The potential energy diagram for this reaction is:

mc003-3.jpg

The activation energy for the reverse reaction is
Question 3 options:

a)

25 kJ

b)

65 kJ

c)

–80 kJ

d)

–25 kJ

e)

–65 kJ

Answer 1

Answer: (b) 65 kJ.

Reason: From the diagram E_a(forward) = 25 kJ and ΔH = E_products − E_reactants = −40 kJ (products are 40 kJ lower). The reverse activation energy is
E_a(reverse) = E_a(forward) − ΔH = 25 − (−40) = 65 kJ. Activation energies are positive, so negative choices are not physically meaningful.