(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...
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.
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.
