22 May Vinegar – An Overview
9) Low grade wine containing ethanol can be turned into vinegar as shown in the diagram below:
The wine is sprayed downward and air passes upwards over a packing material.
Bacteria, growing on the packing material, converts the ethanol in the wine into ethanoic acid, the main component in vinegar.
a) (i) Give evidence from the diagram which demonstrates that the above reaction is exothermic.
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(ii) Construct the balanced chemical equation for the above reaction.
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b) Ethanol can be manufactured by the catalytic addition of steam to ethene. Calculate the percentage yield of the reaction if 3.496 g of ethanol is produced from 1920 cm3 of ethene at room temperature and pressure.
10. Ammonium dichromate, (NH4)2Cr2O7, is an orange crystalline substance. When heated, it starts decomposing with sparks of evolution of heat. The reaction then proceeds with its own heat, and a green fluffy powder called chromic oxide (chromium(III) oxide) is thrown out of the test tube. The other products are water vapour and nitrogen.
- a) (i) Construct the balanced chemical equation, including state symbols, for this reaction.
(ii) State if chromium has been oxidised or reduced in this reaction. Explain your answer in terms of oxidation state.
(iii) Explain, using the information given, if the reaction is an exothermic or endothermic reaction.
(iv) From your answer in a)(iii), sketch the energy profile diagram for this reaction.
- b) (i) Chromic oxide is said to exhibit amphoteric properties. Explain what this means.
(ii) Chromic oxide is used as an abrasive compound to polish knives and razors. State the physical property chromic oxide possesses that allows it to be used as an abrasive and relate this physical property to the structure of chromic oxide
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9.a.i. The cold air entering the tank became warm air when it is released.
- 2 C2H5OH + O2 → 2 CH3COOH
- No. of moles of ethene = 1920/24 000 = 0.0800 mol
No. of moles of ethanol = 0.0800 mol
Theoretical mass of ethanol = 0.0800 x [2(12) + 5(1) + 16 + 1] = 3.68 g
Percentage yield = 3.496/3.68 x 100% = 95.0 %
10.a.i. (NH4)2Cr2O7(s) à Cr2O3(s) + 4H2O(g) + N2(g)
- Chromium has been reduced. Its oxidation state has decreased from +7 in (NH4)2Cr2O7 to +3 in chromic oxide Cr2O3.
iii. Reaction is exothermic, as it is described that there is evolution of heat. The heat provided is to provide the activation energy just to start off the reaction.
b.i. Chromic oxide can react with both acids and bases
to produce salt and water.
ii. It is hard. This is because it has very strong electrostatic forces of attraction between chromium and oxide ions, which require a lot of energy to overcome.
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