WJEC Chemistry for A2: Study and Rev Guide

How to use this book As experienced senior examiners for the WJEC Chemistry 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 GCE Chemistry examination. The book is divided into two main sections, for Unit 3 and for Unit 4. Knowledge and understanding The first section of the book covers the key knowledge and understanding that is required for the examination and provides notes for each of the two examination theory papers, as well as some comments about the practical examination, Unit 5. In addition we have tried to give you pointers that will help you with your learning. ■ Questions may be based on certain terms in the specification, so these 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 candidates’ answers to questions in all sections, along with marking, analysis and explanation by the examiners, of these answers. 3.4 Chemistry of the d -block transition elements d -block transition elements Theelementsof the d -blockare classedas transition elements if theyhave partiallyfilled d -orbitals in theiratomsor their ions.They share somekey properties thatyouneed to recallandexplain: ■ Theyhave severaldifferentoxidation states. ■ They form complexesby co-ordinatebonding,whichareusually coloured. ■ Both themetals themselvesand their compounds canactas catalysts. Electronic con gurations Electronic configurations showhow theelectronsarearranged in s , p and d -orbitals.The inner shellsare fulland contain18electrons,and theseare arrangedas1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 .Theouterelectronsare in the3 d and4 s orbitals – remember that the4 s orbital isfilledbefore the3 d orbitals. 3 d 3 4 s 2 3 d 6 4 s 2 Electronic con guration of vanadium Electronic con guration of iron Transitionmetal ions Whenyouwrite theelectronic configuration for transitionmetals ions, remember that theatoms lose4 s electronsbefore the3 d electrons.This means the4 s orbital isempty inall the transitionmetal ions. 3 d 2 4 s 0 3 d 5 4 s 0 Electronic con guration of V 3+ Electronic con guration of Fe 3+ Oxidation states The ionisationenergies forelectrons in d -orbitalsareall similar,which causes each transitionmetal tohaveavarietyofdifferentoxidation states.Youneed to remember the commonoxidation statesof some commonmetals: ■ Chromium:+3and+6 ■ Manganese:+2,+4and+7 ■ Iron:+2and+3 ■ Cobalt:+2and+3 ■ Copper:+1and+2 Pointer Transition elements are titanium to copper. Copper is a transition element because the Cu 2+ ion has a partially lled d -orbital but zinc isn’t because the atom and ions have full d -orbitals. Grade boost If you are asked towrite electron arrangements for chromium or copper atoms, remember they are exceptions as they contain only one electron in their 4 s orbitals, which gives a half-full or full set of d -orbitals. Pointer You need to recall the colours ofmany transitionmetal ions and oxoions that you have seen throughout the course. These include Cr 3+ (dark green), CrO 4 2– (yellow), Cr 2 O 7 2– (orange),MnO 4 – (purple), Co 2+ (pink), Fe 2+ (pale green), Fe 3+ (red-brown), Cu 2+ (pale blue). Transition elements = elementswithpartially lled d -orbitals in their atomsor ions. KeyTerm QUICKFIRE QUICKFIRE QUICKFIRE  Work out the electronic con gurationsof the atomsTiandCo, and the ionsCr 3+ andNi 2+ . 24 A2Chemistry: Study and RevisionGuide A2 Chemistry: Study and Revision Guide 4