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Miracle Learning Centre Chemistry Tuition Question on Energy Changes. (O Level / IP Chemistry)

1. Hydrazine, N2H4 is commonly used as a rocket fuel. Ethanol is also a fuel which can be in cars. (a) (i) Draw a ‘dot and cross’ diagram to show the bonding in hydrazine. Your diagram should show all the electrons present.

(ii) The combustion of one mole of hydrazine produces nitrogen and water vapour as the only products. The overall energy change is 662 kJ/mol. Construct a labelled energy level diagram for the combustion of hydrazine.

(b) (i) Write down a balanced chemical equation (with state symbols) showing the complete combustion of ethanol.

(ii) Calculate the overall energy involved in the combustion of 1 mole of ethanol using the table of bond energies as shown.

(iii) Discuss if hydrazine or ethanol is a better fuel.

Miracle Learning Centre Answer to Chemistry Tuition Question on Energy Changes. (O Level / IP Chemistry)

(b) (i) C2H5OH (l) + 3O2 (g)  2CO2(g) + 3H¬2O(l)

(ii) Energy absorbed during bond breaking = 5(412) + 348 + 360 + 464 + 3(496) = 4720 kJ/mol Energy released during bond forming = 2(2)(774) + 3(2)(464) = 5880 kJ/mol Enthalpy change = 4720 – 5880 = - 1160 kJ/mol

(iii) Ethanol is a better fuel as it releases more energy than hydrazine when burnt.

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