Organic Chemistry Homework 3- Alcohols and Organic Acids


Name: …………………………………..                                          ...
2 A list of formulas of several organic compounds is given below.


     C2H5OH              C5H10       C3H7COOH         ...
water in a conical flask. The resultant solution required 22.4 cm3 of 0.18 mol/dm3
     sodium hydroxide for complete neut...
H    O                         H       H           H    H   H           H

     H    C    C   O    H          H     C     ...
(b) Name the reagent used and give the observation in a simple chemical test which
     would confirm that
     (i) linole...
works best.

         Fermentation can only takes place in the absence of oxygen. Hence the
  apparatus    needs to be air...
2 Vegetable fats and oils are organic compounds formed from the reaction of alcohols
  with carboxylic acids. Saturated ca...
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Alcohols and Organic Acid

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Alcohols and Organic Acid

  1. 1. Organic Chemistry Homework 3- Alcohols and Organic Acids Name: ………………………………….. Marks: / 53 Class: …………………………………… Date: ………….. Paper 2 [Structured and Free Response Questions] Section A [Structured Questions] 1 Some information about some alcohols is given in the table. name relative molecular boiling point, oC heat produced on mass, Mr completely burning one mole, kJ methanol 32 65 720 ethanol 78 1370 60 97 2010 butanol 74 2670 pentanol 88 138 (a) Complete the table by (i) adding the name of the other alcohol: propanol (ii) calculating the missing relative molecular mass: 46 (iii) estimating the boiling point of butanol: 117ºC (iv) estimating the heat produced on completely burning one mole of pentanol. 3330 kJ [4] (b) Complete the equation for the complete combustion of butanol. ……..C4H9OH + 6O2  4CO2 + 5H2O ; ∆H = - 2670 kJ/mol [2] (c) Explain why the data for the heat produced in burning one mole increases down the table. More strong bonds are formed in the products, so more heat energy is produced. [1] 1
  2. 2. 2 A list of formulas of several organic compounds is given below. C2H5OH C5H10 C3H7COOH C8H18 C3H8 Choose from these formulas, the ones that fit the descriptions below. (a) a compound that undergoes an addition reaction with steam C5H10 [1] (b) a compound that has a pH less than 7 in water C3H7COOH [1] (c) a compound that is oxidised by air to produce ethanoic acid C2H5OH [1] (d) one compound that is not a hydrocarbon C2H5OH / C3H7COOH [1] (e) two compounds which are in the same homologous series C3H8 and C8H18 [1] (f) two compounds that react together to produce an ester C2H5OH and C3H7COOH [1] 3 (a) The formula of a weak carboxylic acid can be represented by CnH2n+1CO2H (where n is an integer). (i) Explain the meaning of the term ‘weak acid’. A weak acid is one that is only partially ionised when dissolved in water to produce hydrogen ions. [1] (ii) Suggest a test to distinguish between 2 mol/dm3 CH3COOH and 2 mol/dm3 HCl. State the observable change in this test and explain your answer. Place a few drops of Universal indicator into equal volume of both acids in separate test tubes. The Universal indicator will gives a red colour if the acid is 2 mol/dm3 HCl, since dilute hydrochloric is a strong acid. The Universal indicator will gives a orange/yellow colour if the acid is 2 mol/dm3 CH3COOH, since dilute ethanoic acid is a weak acid. [2] (b) A carboxylic acid weighing 0.355 g was dissolved in about 20 cm3 of distilled 2
  3. 3. water in a conical flask. The resultant solution required 22.4 cm3 of 0.18 mol/dm3 sodium hydroxide for complete neutralization. Calculate (i) the number of moles of sodium hydroxide used for the titration; number of moles of sodium hydroxide = 22.4/1000 × 0.18 = 0.004032 mol [2] (ii) the relative molecular mass of the carboxylic acid; number of moles of carboxylic acid = 0.004032 mol thus, relative molecular mass, Mr, of the carboxylic acid = 0.355/0.004032 = 88.0 [2] (iii) deduce the structural formula of the carboxylic acid. The general formula of carboxylic acid is CnH2n+1COOH n(12) + (2n+1) + 12 + 2(16) +1 = 88 n =3 the structural formula of the acid is [2] 4 The structures of six compounds, A - F, are shown below. A B C 3
  4. 4. H O H H H H H H H C C O H H C C H C C C C H H H H H H H H D E F H H H H H H H C O C H H C C O H H C O C C H O H H H H H H (a) Use the letters A to F, to answer the following questions. (i) Which compounds are isomers : A and D (ii) Which compound reacts with sodium carbonate? A (iii) Which compound is an unsaturated hydrocarbon? C (iv) Which compound can be made by reacting an organic acid with an alcohol? D (v) Which compound is formed by the oxidation of compound E? A (b) Compound E is the second member of a homologous series. Write the formula of the first member of this series. CH3OH [1] (c) Compound F has molecular formula C3H8O. Draw the structure of another compound with this molecular formula. [1] 4 Some vegetable oils contain an organic acid called linoleic acid, which is unsaturated. (a) What is meant by the term unsaturated? An unsaturated compound is one which contains multiple bonds between carbon atoms (e.g. C=C bond or C≡C bond), in which other atoms can be added to the molecule by addition reaction. 4
  5. 5. (b) Name the reagent used and give the observation in a simple chemical test which would confirm that (i) linoleic acid is unsaturated. reagent: Add linoleic acid into aqueous bromine in a test tube and shake. observation: If linoleic acid is unsaturated, the reddish-brown aqueous bromine will decolourise rapidly. (ii) linoleic acid is an acid: reagent: Add a small piece of magnesium ribbon into a portion of linoleic acid in a test tube. observation: If linoleic acid is an acid, effervescence will be observed, and the gas evolved will extinguish a lighted splint with a ‘pop’ sound. [4] (c) Linoleic acid has molecular formula C17 H29COOH. How many C=C bonds are present in one molecule of linoleic acid? Explain your answer. number of C=C bonds:3 C=C bonds explanation : The molecular formula of linoleic acid has 6 hydrogen atoms less than a saturated carboxylic acid of formula C17H36COOH. [2] Section B [Free Response Questions] 1 Ethanol is used in some countries as a liquid fuel instead of petrol. The ethanol is manufactured by fermentation of cane sugar. (a) (i) Describe how can sugar can be converted into ethanol, suitable for use as a fuel. Sugar can be converted into ethanol by fermentation. Dissolve the sugar in water in a flask, to which yeast is then added. The flask is then immersed in a beaker of warm water at 37ºC. This is the temperature in which yeast 5
  6. 6. works best. Fermentation can only takes place in the absence of oxygen. Hence the apparatus needs to be air –tight as shown in the diagram below. After about one week, the fermented mixture is filtered. The ethanol formed can then be separated from the mixture by fractional distillation of the filtrate. (ii) Name the gas produced in the fermentation reaction and describe how you would test for it. Carbon dioxide. Bubble the gas through lime water. White precipitate will be formed in lime water indicating that carbon dioxide is produced. (b) Suggest two advantages of using ethanol in place of petrol as a fuel. [2] Two advantages of using ethanol rather than petrol as a fuel for cars: (i) Ethanol burns in air with a clean flame to produce carbon dioxide and steam without soot. Unlike ethanol, petrol may produce soot on burning. (Ethanol has a higher percentage of oxygen and lower percentage of carbon by mass than that of petrol. Hence, it burns more completely and pollutes less.) (ii) Ethanol is a renewable fuel that can be produced by fermentation of sugar solution, whereas petrol is not a renewable fuel. (c) (i)When ethanol is completely burnt in air, 1370 kJ of heat is evolved from one mole of ethanol. Construct the equation for the burning of ethanol, including ∆H in the equation. C2H5OH + 3O2  2CO2 + 3H2O ; ΔH = -1370 kJ (ii) State one other reaction of ethanol with air and write the equation for the reaction. [3] Ethanol can also be oxidised by bacteria in air to form ethanoic acid. C2H5OH + O2  CH3COOH + H2O 6
  7. 7. 2 Vegetable fats and oils are organic compounds formed from the reaction of alcohols with carboxylic acids. Saturated carboxylic acids have the general formula CnH(2n + 1)COOH. (a) What type of organic compound are fats and oils? Esters (b) Unsaturated fats and oils are healthier in peoples’ diets than saturated ones. (i) Explain the meaning of the term unsaturated. [See answer to question 4(a) in sectionA] (ii) Suggest a chemical test that might be used to find out if a fat or oil is saturated or unsaturated, by naming the reagent used and stating the observations expected for both saturated and unsaturated compounds. Add the fat or oil into aqueous bromine in a test tube and shake. If the fat or oil is unsaturated, the reddish-brown aqueous bromine will decolourise rapidly. If the fat or oil is saturated, no visible change will be observed and the reddish- brown aqueous bromine is not decolourised. (iii) Palmitic acid, C15H31COOH, and oleic acid, C17H33COOH, are two common carboxylic acids found combined with alcohols in fats and oils. Which one of these acids is healthier for a person’s diet? [6] Palmitic acid is saturated since it conforms to the general formula of saturated carboxylic acids. Oleic acid has 2 hydrogen atoms less than a saturated carboxylic acid of formula C17H35COOH. Thus, oleic acid is unsaturated as it has one C=C bonds in its molecule. So, oleic acid is healthier for a person’s diet. 7

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