WJEC Chemistry for AS: Study and Rev Guide

How to use this book As experienced senior examiners for the Specification we have written this study guide to help you be aware of what is required, and structured the content to guide you through to success in the AS Chemistry examination. Knowledge and Understanding The first section of the book covers the key knowledge and understanding required for the examination and provides notes for each of the two examination theory papers: Unit 1 The Language of Chemistry, Structure of Matter and Simple Reactions Unit 2 Energy, Rate and the Chemistry of Carbon Compounds In addition, we have tried to give you additional pointers so that you can develop your work: ■ Questions may be based on any term in the Specification so these terms are defined and highlighted. ■ There are ‘Quickfire’ questions designed to test your knowledge and understanding of the material. ■ ‘Pointers’ pick out things that may be useful in answering questions. ■ ‘Grade Boost’ inserts point out key ways in which candidates can impress the examiners by their knowledge and understanding. ■ ‘Extra’ comprises a type of Quickfire question that increases in difficulty. ■ There is a comprehensive set of candidate answers to questions in all sections along with marking, analysis and explanation by the examiners of these answers. Thehydrogen spectrum Grade boost The IE of a hydrogen atom can be shown on its electron energy level diagram by drawing an arrow upwards from the n = 1 to the n =∞ level. Pointer The convergence limit iswhen the spectral lines become so close together they have a continuous band of radiation and separate lines cannot be distinguished. QUICKFIRE QUICKFIRE QUICKFIRE  Briefly outlinehow the atomic spectrumof hydrogen canbeused tomeasure themolar first ionisation energy of hydrogen. QUICKFIRE QUICKFIRE QUICKFIRE  Thevalue of the frequency at the start of the continuum in the lithium emission spectrum is 1.30 × 10 15 Hz.Calculate the first ionisation energy of lithium. Pointer Electronic transition iswhen an electronmoves from one energy level to another. 20 ASChemistry: Study and RevisionGuide Anatomofhydrogenhasonlyoneelectron so itgives the simplest emission spectrum.Theatomic spectrumofhydrogen consistsof separate seriesof linesmainly in theultraviolet,visibleand infrared regionsof the electromagnetic spectrum. 2500– 2000– 1500– 1000– 500– H He Li Be B C N O F Ne Element 1st IE (kJmol -1 ) Paschen Infrared 700 ∞ /nm 400 200 100 Visible Ultraviolet Balmer Lyman 500 Wavelength /nm 400 n=5 n=4 n=3 n=2 600 700 Balmer series (visible) Whenanatom isexcitedbyabsorbingenergy,anelectron jumpsup toahigher energy level.As theelectron fallsbackdown toa lowerenergy level, itemits energy in the formofelectromagnetic radiation.Theemittedenergycanbe seen asa line in the spectrumbecause this isequal to thedifferencebetween the two energy levels,∆E, in thiselectronic transition, i.e. it isafixedquantityorquantum. Since∆E=hf,electronic transitionsbetweendifferentenergy levels result in emissionof radiationofdifferent frequenciesand thereforeproducedifferent lines in the spectrum. As the frequency increases, the linesgetcloser togetherbecause theenergy differencebetween the shellsdecreases.Each line in theLyman series (ultraviolet region) isdue toelectrons returning to thefirst shellor n =1energy level,while theBalmer series (visible region) isdue toelectrons returning to the n =2energy level. 2500– 2000– 1500– 1000– 500– H He Li Be B C N O F Ne Element 1st IE (kJmol -1 ) Paschen Infrared 700 ∞ /nm 400 200 100 Visible Ultraviolet Balmer Lyman 500 Wavelength /nm 400 n=5 n=4 n=3 n=2 600 700 Balmer series (visible) Ionisationof thehydrogen atom The spectral linesbecome closerand closer togetheras the frequencyof the radiation increasesuntil they converge toa limit.The convergence limit corresponds to thepointatwhich theenergyofanelectron isno longer quantised.At thatpoint thenucleushas lostall influenceover theelectron; theatomhasbecome ionised. Measuring the convergent frequencyof theLyman series (difference from n =1 to n =∞)andusing∆E=hfallows the ionisationenergy tobe calculated. Thevalueof∆E ismultipliedbyAvogadro’s constant togive thefirst ionisationenergy foramoleofatoms. AS Chemistry: Study and Revision Guide 4