Page 13 - Physical chemistry understanding our chemical world

xii CONTENTS

8.5 Thermodynamic considerations: activation energy, absolute reaction rates
and catalysis 408
Why prepare a cup of tea with boiling water? 408
Why store food in a fridge? 408
Why do the chemical reactions involved in cooking require heating? 409
Why does a reaction speed up at higher temperature? 411
Why does the body become hotter when ill, and get ‘a temperature’ ? 415
Why are the rates of some reactions insensitive to temperature? 416
What are catalytic converters? 420

9 Physical chemistry involving light: spectroscopy and
photochemistry 423
9.1 Introduction to photochemistry 423
Why is ink coloured? 423
Why do neon streetlights glow? 424
Why do we get hot when lying in the sun? 425
Why is red wine so red? 426
Why are some paints red, some blue and others black? 427
Why can’t we see infrared light with our eyes? 429
How does a dimmer switch work? 433
Why does UV-b cause sunburn yet UV-a does not? 434
How does a suntan protect against sunlight? 436
How does sun cream block sunlight? 439
Why does tea have a darker colour if brewed for longer? 442
Why does a glass of apple juice appear darker when viewed against a
white card? 442
Why are some paints darker than others? 444
What is ink? 445
9.2 Photon absorptions and the effect of wavelength 446
Why do radical reactions usually require UV light? 446
Why does photolysis require a powerful lamp? 452
Why are spectroscopic bands not sharp? 453
Why does hydrogen look pink in a glow discharge? 455
Why do surfaces exposed to the sun get so dusty? 457
Why is microwave radiation invisible to the eye? 458
9.3 Photochemical and spectroscopic selection rules 459
Why is the permanganate ion so intensely coloured? 459
Why is chlorophyll green? 461
Why does adding salt remove a blood stain? 462
What is gold-free gold paint made of? 462
What causes the blue colour of sapphire? 463
Why do we get hot while lying in the sun? 464
What is an infrared spectrum? 467
Why does food get hot in a microwave oven? 469
Are mobile phones a risk to health? 471