Съдържание

Условие Chemicals A — Standardization of iodine solution B — Kinetic study B — Calculations Source notes Решение

IChO 1999, Експеримент — Задача 1. A Kinetic Study of the Acid Catalyzed Reaction Between Acetone and Iodine in Aqueous Solution

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Всички материали от IChO 1999

Thirty-first International Chemistry Olympiad — 1999 — Practical problems (Volume 2, 2009)

Внимание: Бележка към темата

Attributed archive/compiler teaching source. Retain the Volume 2 attribution and native copying conditions; the printed teaching grant does not establish a broader commercial licence. This 2009 collection is not an unchanged complete jury packet: the editorial preface describes recreated presentation and schemes, numerical corrections, brief keys and omitted practical material. Printed errors, absent measurements, incomplete methods, missing forms and missing answers remain disclosed.

Compilation edition and teaching attribution

Anton Sirota (editor), The Competition Problems from the International Chemistry Olympiads, Volume 2, IChO International Information Centre, Bratislava, IUVENTA, 2009.

Native copyright and teaching-copy conditions

THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS,

Volume 2
21st – 40th IChO (1989 – 2008)

Editor: Anton Sirota
ISBN 978-80-8072-092-6
Copyright © 2009 by IUVENTA – IChO International Information Centre, Bratislava, Slovakia
You are free to copy, distribute, transmit or adapt this publication or its parts for unlimited teaching purposes, however, you are obliged to attribute your copies, transmissions or adaptations with a reference to "The
Competition Problems from the International Chemistry Olympiads, Volume 2" as it is required in the chemical literature. The above conditions can be waived if you get permission from the copyright holder.

Issued by IUVENTA in 2009
with the financial support of the Ministry of Education of the Slovak Republic

Number of copies: 250
Not for sale.

International Chemistry Olympiad
International Information Centre
IUVENTA
Búdková 2
811 04 Bratislava 1, Slovakia
Phone: +421-907-473367
Fax: +421-2-59296123
E-mail: anton.sirota@stuba.sk
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Native editorial preface

Preface
Preface
Preface
Preface
This publication contains the competition problems (Volume 2) from the 21st – 40th
International Chemistry Olympiads (IChO) organized in the years 1989 – 2008 and is a continuation of the publication that appeared last year as Volume 1 and contained competition problems from the first twenty IChOs. The whole review of the competition tasks set in the IChO in its fourty-year history is a contribution of the IChO International
Information Centre in Bratislava (Slovakia) to the development of this world known international competition. This Volume 2 contains 154 theoretical and 46 practical competition problems from the mentioned years. The review as a whole presents altogether 279 theoretical and 96 practical problems.
In the elaboration of this collection the editor had to face certain difficulties because the aim was not only to make use of past recordings but also to give them such a form that they may be used in practice and further chemical education. Consequently, it was necessary to make some corrections in order to unify the form of the problems. However, they did not concern the contents and language of the problems.
Unfortunately, the authors of the particular competition problems are not known and due to the procedure of the creation of the IChO competition problems, it is impossible to assign any author's name to a particular problem. As the editor I would appreciate many
times some discussion with the authors about any critical places that occurred in the text.
On the other hand, any additional amendments to the text would be not correct from the
historical point of view. Therefore, responsibility for the scientific content and language of the problems lies exclusively with the organizers of the particular International Chemistry
Olympiads.

Some parts of texts, especially those gained as scanned materials, could not be used directly and thus, several texts, schemes and pictures had to be re-written or created again. Some solutions were often available in a brief form and necessary extent only, just for the needs of members of the International Jury.

Recalculations of the solutions were made in some special cases only when the numeric results in the original solutions showed to be obviously not correct. Although the numbers of significant figures in the results of several solutions do not obey the criteria generally accepted, they were left without change.

In this publication SI quantities and units are used and a more modern method of chemical calculations is introduced. Only some exceptions have been made when, in an effort to preserve the original text, the quantities and units have been used that are not SI.
There were some problems with the presentation of the solutions of practical tasks, because most of the relatively simple calculations were based on the experimental results of contestants. Moreover, the practical problems are accompanied with answer sheets in the last years and several additional questions and tasks have appeared in them that were not a part of the text of the original experimental problems. Naturally, answer sheets could not be included in this publication and can only be preserved as archive materials.

When reading the texts of the IChO problems one must admire and appreciate the work of those many known and unknown people – teachers, authors, pupils, and organizers – who contributed so much to development and success of this important international competition.

I am sure about the usefulness of the this review of the IChO problems. It may serve not only as archive material but, in particular, this review should serve to both competitors and their teachers as a source of further inspiration in their preparation for this challenging competition.

Bratislava, July 2009

Anton Sirota, editor

Условие

The reaction between acetone and iodine in aqueous solution is catalyzed by H⁺.

\mathrm{CH_3COCH_3(aq)+I_2(aq)}\xrightarrow{\mathrm{H^+}} \mathrm{CH_3COCH_2I(aq)+H^+(aq)+I^-(aq)}.

The rate law for this reaction is:

-\frac{d[\mathrm{I_2}]}{dt}=k[\mathrm{CH_3COCH_3}]^x[\mathrm{I_2}]^y[\mathrm{H^+}]^z,\qquad v_0=k[\mathrm{CH_3COCH_3}]_0^x[\mathrm{I_2}]_0^y[\mathrm{H^+}]_0^z.

where H⁺ ions are the catalyst. In order to determine the rate constant $k$ and the kinetic orders $x$ , $y$ , $z$ , the initial rate of reaction is measured, where $[\,]_0$ are the initial concentrations of acetone, I₂ and H⁺, respectively.

If the initial rates are measured for various initial concentrations of the reactants then the order with respect to each reactant can be obtained.

The initial rate is obtained by measuring the decrease in the I₂(aq) concentration after a short time interval (7.0 min. in this experiment) after the start of the reaction. Aqueous sodium acetate solution is added to stop the reaction after 7 minutes. The acetate ion reacts immediately with the H⁺ to produce acetic acid and so reducing the concentration of H⁺. The reaction is thus stopped as there is no catalyst present.

Since the reaction does not come to a complete halt, the solution should be titrated immediately after the addition of the sodium acetate solution.

The remaining iodine I₂(aq) is determined by titration with sodium thiosulphate, Na₂S₂O₃. As the end point of the titration is approached, starch indicator is added and the titration is continued until the blue colour disappears.

Chemicals

Supplied chemical	Quantity
Aqueous iodine in 0.4 M KI	80 cm³
0.100 M aqueous HCl	50 cm³
0.50 M aqueous sodium acetate	80 cm³
Standard approximately 0.02 M sodium thiosulphate	200 cm³; exact concentration announced at the practical's start
50% by-volume aqueous acetone	50 cm³; pure acetone density 0.787 g cm⁻³, molecular weight 58.08
Starch indicator	7 cm³

A — Standardization of iodine solution

1
Pipette 5.00 cm³ aqueous iodine into a clean 125 cm³ Erlenmeyer flask.

2
Add 10 cm³ distilled water using a graduated cylinder.

3
Titrate with standard 0.02 M sodium thiosulphate until pale yellow.

4
Add 3–4 drops starch; continue until blue disappears.

5
Record initial/final thiosulphate volumes and volume used on the answer sheet.

6
Repeat steps 1–5 as necessary.

7
Give the titre used for calculation on the answer sheet.

8
Calculate iodine concentration.

B — Kinetic study

1
Label stoppered flasks I–IV.

2
Add these volumes, in cm³, and stopper each flask immediately.

Flask	Water	0.100 M HCl	50% acetone
I	5.00	5.00	5.00
II	0.0	5.00	5.00
III	0.0	5.00	10.00
IV	0.0	10.00	5.00

3
Measure 10 cm³ of 0.50 M sodium acetate in a graduated cylinder.

4
Set the stopwatch to 0.0000.

5
Pipette 5.00 cm³ iodine into flask I. Start timing as its first drop is added.

6
Stopper and swirl continuously.

7
Remove stopper just before 7.0 min. At 7.0 min immediately pour in the measured 10 cm³ sodium acetate; shake well.

8
Titrate remaining iodine with standard thiosulphate.

9
Record thiosulphate volume.

10
Repeat steps 3–9 for II–IV, using iodine volumes 10.00, 5.00 and 5.00 cm³ respectively at step 5.

B — Calculations

1 (B-1)

Calculate initial molar concentrations of iodine, acetone and HCl in I–IV, assuming additive volumes.

2 (B-2)

Calculate iodine concentrations remaining after 7.0 min.

3 (B-3)

Calculate initial reaction rates in M s⁻¹.

4 (B-4)

The rate law is $-d[\mathrm{I_2}]/dt=k[\mathrm{CH_3COCH_3}]^x[\mathrm{I_2}]^y[\mathrm{H^+}]^z$ . Calculate $x,y,z$ from rates and initial concentrations; round to nearest integers and fill the answer sheet. Write the rate equation.

5 (B-5)

Calculate each flask's rate constant with proper units.

6 (B-6)

Give the mean rate constant.

Source notes

Attributed Volume 2, Anton Sirota / IChO International Information Centre / IUVENTA, Bratislava, 2009; physical331–334, printed733–736. Actual titres, iodine concentration, announced exact thiosulphate concentration, numeric rate constants and the separate answer sheet are absent. The brief printed key gives orders and a calculation method, not measured data. It includes the mean under B5 although the question labels it B6. All procedural instructions and native B-1–B-6 identifiers are preserved within their connected practical assignment. Teaching copying, distribution and adaptation require Volume 2 attribution; no blanket commercial permission is asserted.

Решение

Внимание: Непълно решение

Attributed Volume 2, Anton Sirota / IChO International Information Centre / IUVENTA, Bratislava, 2009; Actual titres, iodine concentration, announced exact thiosulphate concentration, numeric rate constants and the separate answer sheet are absent. The brief printed key gives orders and a calculation method, not measured data. It includes the mean under B5 although the question labels it B6. All procedural instructions and native B-1–B-6 identifiers are preserved within their connected practical assignment. Teaching copying, distribution and adaptation require Volume 2 attribution; no blanket commercial permission is asserted.

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