CHEMISTRY. Time Allowed 2 hours. Candidates should answer all five questions. All questions, including the essay question (Q1) carry equal marks.

Transcription

1 EXAMINATION FOR ENTRANCE SCHOLARSHIPS AND EXHIBITIONS FEBRUARY 0 CHEMISTRY Time Allowed hours Candidates should answer all five questions. All questions, including the essay question (Q) carry equal marks.. Write an essay on one of the topics below. The essay needs to be detailed and relevant to the topic. In all cases marks will be awarded for examples used to illustrate the essay and equations where appropriate. a. Mechanisms in organic reactions b. Polyesters and polyamides c. Variation in st Ionisation Energy across a period d. How le Chatelier s principle can help industrial chemists []. a. A radioactive isotope of potassium, 0 K, decays by β emission. Its half-life is. x 0 9 years. i. Give the mass number and symbol for the product of this emission. ii. Calculate the length of time it would take for the radioactivity of a sample of 0 K to fall to /8 th of its original activity. iii. This potassium isotope does contribute to background radiation. Explain why it does not pose a significant risk to health in spite of being radioactive. [] b. Molar first and second ionisation energies of potassium and calcium are listed in the table below. Molar first ionisation energy (kj mol - ) Molar second ionisation energy (kj mol - ) Potassium 9 0 Calcium 89

2 i. Give the equation for the process associated with the second ionisation energy of potassium. ii. Explain why the value for nd IE the value is greater for potassium than calcium. [] c. Potassium reacts with chlorine in a similar way to sodium. i. Give a balanced equation for the reaction. Explain which element is oxidised and which is reduced ii. Explain the high melting point of potassium chloride (770 C) in terms of the forces within its crystal structure. [6]. a. X is a metallic element in Group of the Periodic Table. A solution of its nitrate ( X(NO ) (aq) ) undergoes the following tests. No observation, (solution remains clear). Dil H SO added. Na CO (aq ) added White precipitate P X(NO ) (aq). Reagent Y added White precipitate Q i. What conclusion can you draw from test. about X sulphate? ii. Write a balanced equation for the test. reaction between X(NO ) (aq) and sodium carbonate solution (Na CO (aq) ). Show clearly which product is Precipitate P. iii. Give possible identities for Reagent Y and precipitate Q. iv. Which Group metal is X likely to be? [6] b. The table below gives data about three elements in Group 7 of the Periodic Table.

3 Appearance at room Electronegativity temperature Chlorine Green gas.8 Bromine Dark red liquid.7 Iodine Purple/black solid.0 i. Describe the intermolecular forces in the purple/black solid, iodine, and use them to explain the ease with which iodine changes to vapour (sublimes) when gently warmed. ii. Explain the trend in melting point for the three elements iii. Explain the trend in electronegativity down the group. [6]. a. Below are listed five organic compounds that contain oxygen. A CH CHOHCH CH CH B CH CH COCH CH C CH CH COOCH CH D CH OHCH CHCHCH OH E CH CH CHOHCH CH From the compounds above i. Give the letter and the displayed formula for a compound that would show optical isomerism. Indicate the chiral centre in the molecule with a *. ii.give the letter for a compound that would show E-Z isomerism. iii. Give the letter of a compound that would give a positive Brady s test (, dinitrophenyl hydrazine) but not a silver mirror with Tollens reagent. iv. Give the letter of a compound that would give a positive iodoform test. n.b. Compounds may appear once, more than once or not at all in your answers b. I A straight chain amine, compound Z has an empirical formula C H 6 N and an Mr of 88. [] II III Compound Z turns red litmus blue. One mole of compound Z reacts with two moles of ethanoyl chloride in a condensation reaction. i. Give the molecular formula of Z and a possible displayed formula.

4 ii. Explain why compound Z turns red litmus blue. iii. Draw the displayed formula for the organic product of the reaction in III, using your chosen structure in i. [] c. Two amino acids molecules can condense together to make a dipeptide. Give the structures of a possible dipeptides that can be made from aminoethanoic acid (glycine) and amino propanoic acid (alanine) when they react together. []. a. A student uses a titration procedure to find the concentration of a solution of hydrochloric acid, HCl, using a standard 0.00 mol dm - solution of sodium carbonate. i. Why is the burette rinsed twice with the hydrochloric acid before it is filled? ii. After filling the burette, using a funnel, why is it recommended that the funnel is removed from the burette before the titration proceeds? iii. cm of the standard sodium carbonate solution is then measured carefully and placed in a conical flask and a few drops of indicator added. What piece of apparatus is used to measure the cm of solution accurately? iv. The acid is then added, a little at a time, until the endpoint approaches. The acid is then added drop by drop until the indicator changes colour. Give one further suggestion about the addition of acid to the conical flask, to improve the reliability of the procedure. [] b. The following titration readings were obtained..60 cm.70 cm.9 cm.6 cm i. Use appropriate readings to calculate the mean titre for the experiment. ii. The equation for the reaction is as follows: Na CO + HCl NaCl + H O CO Use the information above to calculate the concentration of the hydrochloric acid solution. [] c. Molar enthalpy of a reaction can be calculated using the equation H = mc T n (m = total mass of solution and c = specific heat capacity of the solutions)

5 Describe a method to find the T for a number of moles (n) and hence the molar enthalpy of neutralisation of hydrochloric acid by sodium hydroxide. Assume that you are provided with hydrochloric acid and sodium hydroxide solutions, both of.00 mol dm - concentration. You would also have a thermometer and usual lab equipment. Make sure to include the following in your answer: The container(s) used for the neutralisation. The method for measuring the volumes of solutions used. Precautions to ensure accuracy in measuring the temperature change. [] d. In a laboratory two solutions have lost their labels. They are known to be calcium nitrate and sodium nitrate. Give a test that could be carried out on both solutions to confirm which is which. Make sure to describe the observations expected for each compound. []

6 0.8 B Boron.0 C Carbon 6.0 N Nitrogen O Oxygen F Fluorine 9 0. Ne Neon Al Aluminium 8. Si Silicon.0 P Phosphorus. S Sulfur 6. Cl Chlorine Ar Argon Ga Gallium 7.6 Ge Germanium 7.9 As Arsenic 79.0 Se Selenium 79.9 Br Bromine 8.8 Kr Krypton 6 In Indium 9 9 Sn Tin 0 Sb Antimony 8 Te Tellurium 7 I Iodine Xe Xenon 0 Tl Thallium 8 07 Pb Lead 8 09 Bi Bismuth 8 (0) Po Polonium 8 (0) At Astatine 8 () Rn Radon 86.9 Mn Manganese.8 Fe Iron Co Cobalt Ni Nickel 8 6. Cu Copper 9 6. Zn Zinc 0 9. K Potassium 9 0. Ca Calcium 0.0 Na Sodium. Mg Magnesium 6.9 Li Lithium.0 H Hydrogen.00 He Helium 9.0 Be Beryllium.0 Sc Scandium 7.9 Ti Titanium 0.9 V Vanadium.0 Cr Chromium 98.9 Tc Technetium 0 Ru Ruthenium 0 Rh Rhodium 06 Pd Palladium 6 08 Ag Silver 7 Cd Cadmium 8 8. Rb Rubidium Sr Strontium Y Yttrium 9 9. Zr Zirconium Nb Niobium 9.9 Mo Molybdenum 7 Gd Gadolinium 6 9 Tb Terbium 6 6 Ho Holmium Er Erbium Tm Thulium 69 7 Yb Ytterbium 70 7 Lu Lutetium 7 6 Dy Dysprosium 66 0 Ce Cerium 8 Pr Praseodymium 9 Nd Neodymium 60 (7) Pm Promethium 6 0 Sm Samarium 6 () Eu Europium 6 (7) Cm Curium 96 () Bk Berkelium 97 () Es Einsteinium 99 () Fm Fermium 00 (6) Md Mendelevium 0 () No Nobelium 0 (7) Lr Lawrencium 0 () Cf Californium 98 Th Thorium 90 () Pa Protactinium 9 8 U Uranium 9 (7) Np Neptunium 9 () Pu Plutonium 9 () Am Americium 9 86 Re Rhenium 7 90 Os Osmium 76 9 Ir Iridium 77 9 Pt Platinum Au Gold 79 0 Hg Mercury 80 Cs Caesium 7 Ba Barium 6 9 La Lanthanum 7 79 Hf Hafnium 7 8 Ta Tantalum 7 8 W Tungsten 7 () Fr Francium 87 (6) Ra Radium 88 (7) Ac Actinium 89 Lanthanoid elements Actinoid elements Key Group Period f Block 6 7 THE PERIODIC TABLE A r Symbol Name Z relative atomic mass atomic number d Block p Block s Block