(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.
Explanation
(a)
(b)(i) Hydrogen is lighter or-4ess dense than air. The hydrogen particles therefore diffuse faster than air.
(ii) The MnO\(_2\) catalyses the decomposition of the hydrogen peroxide into-water and oxygen
(iii) HCl is covalent, it produces no ions in a non-polar solvent like methylbenzene. In water it ionizes to give H\(^+\) or H\(_{3}\)O\)(^+\) as the only positive charge which turns blue litmus paper red.
(c)(i) MnO\(^{-4}\). Since oxidation number of Mn decreases from +7 to +2.
(ii) C\(_2\)O\(_4\)\(^{-2}\). Since the oxidation number of carbon increases from +3 to +4
(iii) MnO\(^{-4}\) + 8H\(^+\)+ 5e\(^-\) \(\to\) Mn\(^{2+}\) + 4H\(_2\)O
(d)(i) An electrochemical cell is a device which converts chemical energy into electrical energy.
ii)
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Electrochemical cell |
Electrolytic cell |
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1. Chemical reaction produces electricity |
Electric current is required for the reaction to occur |
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2. Electrons are produced as a result of oxidation at an electrode |
Electrons are pushed by an outside source such as a battery. |
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3: Cathode is the positive electrode |
Cathode is the negative electrode |
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4. Anode is negative electrode |
Anode is positive electrode |