(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 exces...
Question 1 Report
(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
Answer Details
(a)(i) The heat of combustion is the heat evolved when one mole of a substance is completely burnt in oxygen.
(a)(ii) The container used to determine the heat change is a calorimeter. A bomb calorimeter is a type of calorimeter used for combustion reactions.
(b)(i) Combustion of carbon is exothermic:
\[ C(s) + O_2(g) \rightarrow CO_2(g) \]
The products are at a lower energy level than the reactants because heat is released. Therefore, \( \Delta H \) is negative.
(b)(ii) The stated heat of combustion is understood as \( -3935\ \text{kJ mol}^{-1} \).
\[ \text{Moles of carbon}=\frac{60\ \text{g}}{12\ \text{g mol}^{-1}}=5\ \text{mol} \]
\[ \text{Heat change}=5\ \text{mol}\times(-3935\ \text{kJ mol}^{-1}) \]
\[ \boxed{\text{Heat change}=-19675\ \text{kJ}} \]
The negative sign shows that \(19675\ \text{kJ}\) of heat is released to the surroundings.
(b)(iii) Strong acids and strong bases completely ionise in water. Thus, every neutralisation between them has the same net ionic equation:
\[ H_3O^+(aq)+OH^-(aq)\rightarrow 2H_2O(l) \]
Since the same reaction occurs in each case, the same heat is released per mole of water formed. Therefore, the heat of neutralisation of a strong acid by a strong base is constant.
(c)(i) Rusting is regarded as slow combustion because both processes involve oxidation: iron combines with oxygen to form an oxide, and heat is released. Rusting occurs much more slowly than ordinary burning.
(c)(ii) Iron filings rust faster than iron nails because filings have a much larger surface area exposed to oxygen and water. A larger surface area increases the rate of oxidation.
(c)(iii) Zinc protects iron better than tin because zinc is more reactive than iron. If a zinc coating is scratched, zinc oxidises in preference to iron, giving sacrificial protection. Tin is less reactive than iron; therefore, if a tin coating is broken, the exposed iron can rust.
Examination reminder: For corrosion questions, compare the reactivity of the coating metal with iron. A metal more reactive than iron can protect it even when the coating is damaged.
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