(a) Draw an energy profile diagram to illustrate a catalysed exothermic reaction and label parts of the curves representing the following: (i) activated com...
Question 1 Report
(a) Draw an energy profile diagram to illustrate a catalysed exothermic reaction and label parts of the curves representing the following:
(i) activated complex (without catalyst);
(ii) activated energy (with catalyst)
(iii) enthalpy change
(b) Give the reasons for the following observations:
(i) A balloon filled with liyilrogen becomes deflated faster than a balloon filled with air under the same conditions.
(ii) Hydrogen peroxide decomposes slowly at room temperature but when a pinch of MnO, is added, bubbles form rapidly.
(iii) A solution of hydrogen chloride as in methylbenzene has no effect on `litmus but a solution of the gas in water turns blue litmus paper red.
(c) Consider the reaction represented by the following equation: 2MnO\(^-_{4(aq)}\) + 5C\(_2\)O\(^{2-}_4\) + 16H\(^+\) \(\to\) 2Mn\(^{2+}_{(aq)}\) + 8H\(_2\)O\(_{(l)}\) + 10C\(_{2(g)}\) .
Write down: (i) the species undergoing reduction giving reasons;
(ii) the reducing agent giving reasons;
(iii) the reduction half equation;
(iv) one observation made during the reaction.
(d)(i) What is an electrochemical cell?
(ii) State three differences between an electrochemical cell and an electrolytic cell.
Answer Details
(a) Energy profile diagram for a catalysed exothermic reaction
The products are at a lower potential energy than the reactants, so energy is released and \(\Delta H\) is negative. A catalyst provides an alternative pathway with a lower activation energy. It does not change the enthalpy change of the reaction.
(b)(i) Hydrogen is much less dense than air because its particles have a much smaller molar mass. Hydrogen particles diffuse through the balloon material faster than air particles, so a hydrogen-filled balloon deflates more quickly.
(b)(ii) Manganese(IV) oxide, \(\mathrm{MnO_2}\), is a catalyst for the decomposition of hydrogen peroxide. It lowers the activation energy, so many more successful collisions occur each second. Hydrogen peroxide therefore decomposes rapidly to form water and oxygen gas, seen as bubbles:
\[2H_2O_2 \rightarrow 2H_2O + O_2\]
(b)(iii) Hydrogen chloride is covalent. In methylbenzene, a non-polar solvent, it remains as HCl molecules and does not form hydrogen ions, so it does not affect litmus. In water, HCl ionises:
\[\mathrm{HCl + H_2O \rightarrow H_3O^+ + Cl^-}\]
The \(\mathrm{H_3O^+}\) ions make the solution acidic, so blue litmus paper turns red.
(c)
- Species undergoing reduction: \(\mathrm{MnO_4^-}\). The oxidation number of manganese decreases from \(+7\) in \(\mathrm{MnO_4^-}\) to \(+2\) in \(\mathrm{Mn^{2+}}\). A decrease in oxidation number means gain of electrons, so reduction has occurred.
- Reducing agent: \(\mathrm{C_2O_4^{2-}}\), the oxalate (ethanedioate) ion. Carbon increases in oxidation number from \(+3\) in \(\mathrm{C_2O_4^{2-}}\) to \(+4\) in \(\mathrm{CO_2}\). It is oxidised and supplies electrons to reduce the permanganate ion; therefore it is the reducing agent.
- Reduction half-equation: \[\mathrm{MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O}\]
- Observation: The purple colour of the permanganate ion decolourises. Bubbles of carbon dioxide may also be observed.
(d)(i) An electrochemical cell is a device in which a spontaneous redox reaction converts chemical energy into electrical energy.
(d)(ii)
| Electrochemical (galvanic) cell | Electrolytic cell |
|---|---|
| A spontaneous chemical reaction produces electrical energy. | An external electrical supply drives a non-spontaneous chemical reaction. |
| The anode is the negative electrode. | The anode is the positive electrode. |
| The cathode is the positive electrode. | The cathode is the negative electrode. |
Examination reminder: Oxidation is loss of electrons and an increase in oxidation number; reduction is gain of electrons and a decrease in oxidation number.
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