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CHEMISTRY, M5 2025 HSC 13 MC

Which row of the table correctly shows the expected signs of the enthalpy \((\Delta H)\) and entropy \((\Delta S)\) changes for the complete combustion of octane above 100°C?

\begin{align*}
\begin{array}{l}
\rule{0pt}{2.5ex} \ \rule[-1ex]{0pt}{0pt}& \\
\rule{0pt}{2.5ex}\textbf{A.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{B.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{C.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{D.}\rule[-1ex]{0pt}{0pt}\\
\end{array}
\begin{array}{|c|c|}
\hline
\rule{0pt}{2.5ex}\quad \quad \Delta H \quad \quad\rule[-1ex]{0pt}{0pt}& \quad \quad\Delta S \quad \quad\\
\hline
\rule{0pt}{2.5ex}>0\rule[-1ex]{0pt}{0pt}&<0\\
\hline
\rule{0pt}{2.5ex}<0\rule[-1ex]{0pt}{0pt}& <0\\
\hline
\rule{0pt}{2.5ex}<0\rule[-1ex]{0pt}{0pt}& >0 \\
\hline
\rule{0pt}{2.5ex}>0\rule[-1ex]{0pt}{0pt}& >0 \\
\hline
\end{array}
\end{align*}

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\(C\)

Show Worked Solution
  • When the complete combustion of octane occurs at above 100°C, all \(\ce{H2O}\) will be gas. Therefore the chemical equation for the reaction is:
    • \(\ce{2C8H18(l) + 25O2(g) -> 16CO2(g) + 18H2O(g)}\)
  • A combustion reaction will always be an exothermic reaction \((\Delta H < 0)\).
  • As there are 34 gas molecules produced from 25 gas molecules and 2 liquid molecules, the entropy (disorder of the system) increases, hence  \(\Delta S > 0\).

\(\Rightarrow C\)

CHEMISTRY, M5 2018 HSC 30

Over the last 50 years, scientists have recorded increases in the following:

  • the amount of fossil fuels burnt
  • atmospheric carbon dioxide levels
  • average global air temperature and ocean temperature
  • the volume of carbon dioxide dissolved in the oceans.

Analyse the factors that affect the equilibrium between carbon dioxide in the air and carbon dioxide in the oceans. In your answer, make reference to the scientists' observations and include relevant equations.  (7 marks)

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Fossil fuel combustion:

  • Combustion of fossil fuels releases \(\ce{CO2}\) and heat energy, both of which are released into the atmosphere.
  • Increased burning of fossil fuels will contribute to further rises in atmospheric \(\ce{CO2}\), as described in the equation for the combustion of octane
  •    \(\ce{2C8H18 + 25O2 -> 16CO2 + 18H2O\qquad \triangle H –ve}\)

Carbon dioxide and other climate interactions:

  • \(\ce{CO2}\) combines with water according to the following equilibrium in an exothermic reaction.
  •    \(\ce{CO2(g) + H2O(l) \rightleftharpoons H2CO3(aq) \rightleftharpoons H+(aq) + HCO^3-(aq)\qquad   \triangle H –ve}\)
  • This is an equilibrium and by Le Chatelier’s principle when a system is changed, the system will adjust to oppose the change.
  • Factors that affect equilibrium in this system are temperature, pressure and concentration of reactants and products.
  • The increase of \(\ce{CO2}\) in the air due to the combustion of fossil fuels described above, increases the pressure due to \(\ce{CO2}\) in the system.  By Le Chatelier’s principle, the system will oppose this by absorbing more \(\ce{CO2}\) into the oceans.
  • Scientists have been measuring the level of \(\ce{CO2}\) in oceans due to this effect and confirmed the increase in \(\ce{CO2}\).
  • However, this equilibrium is exothermic and as it causes temperature rises, by Le Chatelier’s principle, the reverse reaction may be subsequently favoured. This would have the effect of decreasing the amount of \(\ce{CO2}\) dissolving in the oceans.
  • In summary, if global temperatures continue to rise and \(\ce{CO2}\) in the atmosphere becomes stable or reduces, the system may adjust so that oceans may release \(\ce{CO2}\) rather than absorbing it.
Show Worked Solution

Fossil fuel combustion:

  • Combustion of fossil fuels releases \(\ce{CO2}\) and heat energy, both of which are released into the atmosphere.
  • Increased burning of fossil fuels will contribute to further rises in atmospheric \(\ce{CO2}\), as described in the equation for the combustion of octane
  •    \(\ce{2C8H18 + 25O2 -> 16CO2 + 18H2O\qquad \triangle H –ve}\)

Carbon dioxide and other climate interactions:

  • \(\ce{CO2}\) combines with water according to the following equilibrium in an exothermic reaction.
  •    \(\ce{CO2(g) + H2O(l) \rightleftharpoons H2CO3(aq) \rightleftharpoons H+(aq) + HCO^3-(aq)\qquad   \triangle H –ve}\)
  • This is an equilibrium and by Le Chatelier’s principle when a system is changed, the system will adjust to oppose the change.
  • Factors that affect equilibrium in this system are temperature, pressure and concentration of reactants and products.
  • The increase of \(\ce{CO2}\) in the air due to the combustion of fossil fuels described above, increases the pressure due to \(\ce{CO2}\) in the system.  By Le Chatelier’s principle, the system will oppose this by absorbing more \(\ce{CO2}\) into the oceans.
  • Scientists have been measuring the level of \(\ce{CO2}\) in oceans due to this effect and confirmed the increase in \(\ce{CO2}\).
  • However, this equilibrium is exothermic and as it causes temperature rises, by Le Chatelier’s principle, the reverse reaction may be subsequently favoured. This would have the effect of decreasing the amount of \(\ce{CO2}\) dissolving in the oceans.
  • In summary, if global temperatures continue to rise and \(\ce{CO2}\) in the atmosphere becomes stable or reduces, the system may adjust so that oceans may release \(\ce{CO2}\) rather than absorbing it.

♦♦ Mean mark 41%.