Question

What is the order of heat of combustion of the following ?
(i) Heptane
(ii) Pentane
(iii) 2-Methyl pentane
(iv) 2, 2-Dimethyl butane

• A. (i) > (ii) > (iii) > (iv)
• B. (iv) > (iii) > (ii) > (i)
• C. (i) > (iii) > (iv) > (ii)
• D. (i) > (iii) > (ii) > (iv)

Answer 1

The correct option is C (i) > (iii) > (iv) > (ii)

More the number of carbon atoms, more will be the heat of combustion. For compounds of same number of carbon atoms heat of combustion decreases with increase in branching.
Heptane - 7 carbon atoms
Pentane - 5 carbon atoms
2-Methyl pentane - 6 carbon atoms and 1 branching
2, 2-Dimethyl butane - 6 carbon atoms and 2 branching
Hence, the order of heat of combustion is
(i) > (iii) > (iv) > (ii)