(a) (i) Define heat of combustion.
(ii) What name is given to the container used for determining tne reaction?
(b) The heat of combustion of carbon in excess air is – 3935 kJ.
(i) Sketch an energy profile diagram for the reaction.
(ii) Calculate the heat change when 60 g of carbon undergoes complete combustion to produce carbon (IV) oxide. (C = 12)
(iii) Explain why the value of the heat of neutralization of strong acids by strong bases is constant.
(c) Give reason for the following:
(i) rusting of iron is regarded as a slow combustion process;
(ii) iron filings rust much more faster than iron nails when exposed to the same atmospheric condition;
(iii) iron is better protected from corrosion by plating it with.zinc than with tin
Explanation
(a)(i) Heat of combustion is the heat evolved when one mole of a substance is completely burnt in oxygen
(ii) The name given is calorimeter.
(b)(i)
(ii) C + O(_2) -> CO(_2); H = —3935KJ(^{-1}).
Mole of carbon =12g of carbon will evolve — 3935 KJ
:. 60g of C will evolve — (frac{3935 times 60}{12})
Heat change = -19675 KJ
(iii) Neutralization reaction can be represented generally as H(_2)O(^+) + OH(^-) —> 2H(_2)O. Strong acids and strong bases lame completely in water to furnish H(_3)O(^{+}) and OH(^{-}) respective Thus in all cases, one mole of H(_3)O(^{+}) of a strong acid completely es one mole of OH(^-) of a strong base. Therefore the enthalpy of neutralization is constant.
(c)(i) Both rusting and combustion involve combination with oxygen to produce the oxide. Both reactants are exothermic. Rusting takes a longer period of time than combustion.
(ii) iron filings possess greater net surface area and reaction rate increases with increased surface area.
(iii) Zinc is more reactive which is more reactive than tin. Consequently, when tin – plated iron is exposed, the iron goes into solution and rusts but in the case of galvanized iron, it is the zinc that dissolves thus protecting the iron better.