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Physical Chemistry Understanding our Chemical World Physical Chemistry Understanding our Chemical World Paul Monk Manchester Metropolitan University, UK Copyright  2004 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England Telephone (+44) 1243 779777 Email (for orders and customer service enquiries): cs-books@wiley.co.uk Visit our Home Page on www.wileyeurope.com or www.wiley.com All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London W1T 4LP, UK, without the permission in writing of the Publisher. 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Other Wiley Editorial Offices John Wiley & Sons Inc., 111 River Street, Hoboken, NJ 07030, USA Jossey-Bass, 989 Market Street, San Francisco, CA 94103-1741, USA Wiley-VCH Verlag GmbH, Boschstr. 12, D-69469 Weinheim, Germany John Wiley & Sons Australia Ltd, 33 Park Road, Milton, Queensland 4064, Australia John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark, Singapore 129809 John Wiley & Sons Canada Ltd, 22 Worcester Road, Etobicoke, Ontario, Canada M9W 1L1 Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Library of Congress Cataloging-in-Publication Data Monk, Paul M. S. Physical chemistry : understanding our chemical world / Paul Monk. p. cm. Includes bibliographical references and index. ISBN 0-471-49180-2 (acid-free paper) – ISBN 0-471-49181-0 (pbk. : acid-free paper) 1. Chemistry, Physical and theoretical. I. Title. QD453.3.M66 2004 541 – dc22 2004004224 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0-471-49180-2 hardback 0-471-49181-0 paperback Typeset in 10.5/12.5pt Times by Laserwords Private Limited, Chennai, India Printed and bound in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire This book is printed on acid-free paper responsibly manufactured from sustainable forestry in which at least two trees are planted for each one used for paper production. Contents Preface xv Etymological introduction xix List of symbols xxiii Powers of ten: standard prefixes 1 Introduction to physical chemistry 1.1 What is physical chemistry: variables, relationships and laws Why do we warm ourselves by a radiator? Why does water get hot in a kettle? Are these two colours complementary? Does my radio get louder if I vary the volume control? Why does the mercury in a barometer go up when the air pressure increases? Why does a radiator feel hot to the touch when ‘on’, and cold when ‘off’? xxviii 1 1 1 2 2 3 5 7 1.2 The practice of thermodynamic measurement 9 What is temperature? How long is a piece of string? How fast is ‘greased lightning’? Why is the SI unit of mass the kilogram? Why is ‘the material of action so variable’? 9 14 15 17 18 1.3 Properties of gases and the gas laws Why Why How Why Why How do we see eddy patterns above a radiator? does a hot-air balloon float? was the absolute zero of temperature determined? pressurize the contents of a gas canister? does thunder accompany lightning? does a bubble-jet printer work? 20 20 20 21 23 25 26 vi CONTENTS What causes pressure? Why is it unwise to incinerate an empty can of air freshener? 1.4 Further thoughts on energy Why is the room warm? What do we mean by ‘room temperature’? Why do we get warmed-through in front of a fire, rather than just our skins? 2 Introducing interactions and bonds 2.1 Physical and molecular interactions What is ‘dry ice’? How is ammonia liquefied? Why does steam condense in a cold bathroom? How does a liquid-crystal display work? Why does dew form on a cool morning? How is the three-dimensional structure maintained within the DNA double helix? How do we make liquid nitrogen? Why is petrol a liquid at room temperature but butane is a gas? 2.2 Quantifying the interactions and their influence How does mist form? How do we liquefy petroleum gas? Why is the molar volume of a gas not zero at 0 K? 2.3 Creating formal chemical bonds Why Why How Why Why Why Why Why Why Why Why Why Why is chlorine gas lethal yet sodium chloride is vital for life? does a bicycle tyre get hot when inflated? does a fridge cooler work? does steam warm up a cappuccino coffee? does land become more fertile after a thunderstorm? does a satellite need an inert coating? does water have the formula H2 O? is petroleum gel so soft? does salt form when sodium and chlorine react? heat a neon lamp before it will generate light? does lightning conduct through air? is argon gas inert? is silver iodide yellow? 30 32 33 33 34 35 37 37 37 38 39 40 42 44 47 49 50 50 52 54 59 59 59 60 61 63 64 66 67 69 69 72 74 75 3 Energy and the first law of thermodynamics 77 3.1 Introduction to thermodynamics: internal energy 77 Why Why Why Why does the mouth get cold when eating ice cream? is skin scalded by steam? do we sweat? do we still feel hot while sweating on a humid beach? 77 79 81 83 CONTENTS Why is the water at the top of a waterfall cooler than the water at its base? Why is it such hard work pumping up a bicycle tyre? Why does a sausage become warm when placed in an oven? Why, when letting down a bicycle tyre, is the expelled air so cold? Why does a tyre get hot during inflation? Can a tyre be inflated without a rise in temperature? How fast does the air in an oven warm up? Why does water boil more quickly in a kettle than in a pan on a stove? Why does a match emit heat when lit? Why does it always take 4 min to boil an egg properly? Why does a watched pot always take so long to boil? 3.2 Enthalpy How does a whistling kettle work? How much energy do we require during a distillation? Why does the enthalpy of melting ice decrease as the temperature decreases? Why does water take longer to heat in a pressure cooker than in an open pan? Why does the temperature change during a reaction? Are diamonds forever? Why do we burn fuel when cold? Why does butane burn with a hotter flame than methane? 3.3 Indirect measurement of enthalpy How How Why Why How do we make ‘industrial alcohol’? does an ‘anti-smoking pipe’ work? does dissolving a salt in water liberate heat? does our mouth feel cold after eating peppermint? does a camper’s ‘emergency heat stick’ work? 4 Reaction spontaneity and the direction of thermodynamic change 4.1 The direction of physicochemical change: entropy Why does the colour spread when placing a drop of dye in a saucer of clean water? When we spill a bowl of sugar, why do the grains go everywhere and cause such a mess? Why, when one end of the bath is hot and the other cold, do the temperatures equalize? Why does a room containing oranges acquire their aroma? Why do damp clothes become dry when hung outside? Why does crystallization of a solute occur? 4.2 The temperature dependence of entropy Why do dust particles move more quickly by Brownian motion in warm water? Why does the jam of a jam tart burn more than does the pastry? vii 85 86 87 88 89 89 90 91 94 95 98 99 99 102 104 106 107 109 111 114 118 118 120 123 125 127 129 129 129 130 131 133 134 137 139 139 139 viii CONTENTS 4.3 Introducing the Gibbs function Why is burning hydrogen gas in air (to form liquid water) a spontaneous reaction? How does a reflux condenser work? 4.4 The effect of pressure on thermodynamic variables How Why Why How 144 144 144 148 much energy is needed? does a vacuum ‘suck’? do we sneeze? does a laboratory water pump work? 148 151 152 153 4.5 Thermodynamics and the extent of reaction 156 Why Why Why Why Why is a ‘weak’ acid weak? does the pH of the weak acid remain constant? does the voltage of a battery decrease to zero? does the concentration of product stop changing? do chicken eggs have thinner shells in the summer? 4.6 The effect of temperature on thermodynamic variables Why does egg white denature when cooked but remain liquid at room temperature? At what temperature will the egg start to denature? Why does recrystallization work? 5 Phase equilibria 5.1 Energetic introduction to phase equilibria Why does an ice cube melt in the mouth? Why does water placed in a freezer become ice? Why was Napoleon’s Russian campaign such a disaster? 5.2 Pressure and temperature changes with a single-component system: qualitative discussion How How How How is the ‘Smoke’ in horror films made? does freeze-drying work? does a rotary evaporator work? is coffee decaffeinated? 5.3 Quantitative effects of pressure and temperature change for a single-component system Why is ice so slippery? What is ‘black ice’? Why does deflating the tyres on a car improve its road-holding on ice? Why does a pressure cooker work? 5.4 Phase equilibria involving two-component systems: partition Why How Why How Why does a fizzy drink lose its fizz and go flat? does a separating funnel work? is an ice cube only misty at its centre? does recrystallization work? are some eggshells brown and some white? 156 158 159 162 165 166 166 170 171 177 177 177 181 182 184 184 185 188 189 192 192 193 198 199 205 205 207 208 209 211 CONTENTS 5.5 Phase equilibria and colligative properties Why How Why Why does a mixed-melting-point determination work? did the Victorians make ice cream? boil vegetables in salted water? does the ice on a path melt when sprinkled with salt? 5.6 Phase equilibria involving vapour pressure Why does petrol sometimes have a strong smell and sometimes not? How do anaesthetics work? How do carbon monoxide sensors work? Why does green petrol smell different from leaded petrol? Why do some brands of ‘green’ petrol smell different from others? Why does a cup of hot coffee yield more steam than above a cup of boiling water at the same temperature? How are essential oils for aromatherapy extracted from plants? 6 Acids and Bases 6.1 Properties of Lowry–Brønsted acids and bases Why does vinegar taste sour? Why is it dangerous to allow water near an electrical appliance, if water is an insulator? Why is bottled water ‘neutral’? What is ‘acid rain’? Why does cutting an onion make us cry? Why does splashing the hands with sodium hydroxide solution make them feel ‘soapy’? Why is aqueous ammonia alkaline? Why is there no vinegar in crisps of salt and vinegar flavour? How did soldiers avoid chlorine gas poisoning at the Second Battle of Ypres? How is sherbet made? Why do steps made of limestone sometimes feel slippery? Why is the acid in a car battery more corrosive than vinegar? Why do equimolar solutions of sulphuric acid and nitric acid have different pHs? What is the pH of a ‘neutral’ solution? What do we mean when we say blood plasma has a ‘pH of 7.4’? 6.2 ‘Strong’ and ‘weak’ acids and bases Why Why Why Why is a nettle sting more painful than a burn from ethanoic acid? is ‘carbolic acid’ not in fact an acid? does carbonic acid behave as a mono-protic acid? is an organic acid such as trichloroethanoic acid so strong? 6.3 Titration analyses Why does a dock leaf bring relief after a nettle sting? How do indigestion tablets work? ix 212 212 216 217 218 221 221 222 224 224 225 229 229 233 233 233 235 236 237 239 239 240 241 242 244 244 245 250 251 251 253 253 254 259 260 261 261 262