WJEC Chemistry for AS Level Student Book: 2nd Edition

Stretch and challenge boxes may provide extra information not in the main text, but relevant. They may provide more examples, or questions, but do not contain information that will be tested in an examination. Exam tips provide general or speci c advice to help you prepare for an examination. Read these very carefully. Links to other sections of the course are highlighted in the margin, near the relevant text. They are accompanied by a reference to any areas where sections relate to one another. It may be useful for you to use these Links to recap a topic, before beginning to study the current topic. Occasionally a topic covers an experiment or a practical that is a speci ed practical task. This feature appears alongside in the margin to highlight its importance and to give you some extra information and hints on understanding it fully. Maths tips provide further explanation about the mathematics described in the text. Mathematical skills Maths crops up quite a lot in AS Level Chemistry (a minimum of 20% of the mark in the exams will depend on your maths skills) so it’s really important that you’ve mastered all the maths skills you’ll need before sitting your exams. Don’t panic, there is nothing dif cult here, you’re preparing for a chemistry exam not a maths one! Mathematical requirements are given in Appendix B at the end of the speci cation course content. The level of understanding is equivalent to Level 2 or GCSE. You’ll be expected to apply a range of maths skills in your exams. These skills are highlighted at the beginning of each chapter and relevant examples to your AS Level Chemistry course are included throughout the book. In addition, further explanations are provided in Maths tips boxes and there is a maths skills chapter on page 191 which gives more explanation and examples of the key mathematical concepts that you need to understand. These features should help you to successfully apply the maths skills that you need. Practical work and use of your lab book This is an important and intrinsic part of the speci cation and is covered in two ways: rstly as a part of the written papers in which its weighting is at least 15% and, secondly, through direct practical work in the laboratory that will prepare you to deal with the written work. Throughout the AS Level Chemistry course, you will do a lot of practical work and use it to develop many skills. You will record your practical work in a lab book, which is an essential record of what you do. Your lab book is a working document. It is designed to be an ongoing record of the practical tasks you undertake. It is a record of your progress in developing practical techniques, in recording, in making drawings and measurements, plotting graphs, mathematical analysis, evaluating and drawing conclusions. Practical check Preparing a soluble salt by titration is a speci ed practical task. When the titration is repeated without an indicator, make sure that when nearing the exact amount needed, the acid is added a drop at a time in order to avoid overshooting. If you evaporate all of the water then a powder will form instead of crystals. Stretch & challenge The mechanism for the hydration of ethene involves electrophilic attack on the π bond by the b + H on the water. Try and draw this mechanism using curly arrows. Exam tip Detailed knowledge of the mass spectrometer is not required, however an understanding of how to interpret a mass spectrum is vital. Link Mass spectrometry page 161 Maths tip When you divide by the smallest to give a whole number ratio, do not cheat! If the ratio is, for example, 1 : 1.5 this is 2 : 3. If the ratio is 1 : 1.67 this is 3 : 5. If your ratio does not simplify to give whole numbers, go back and check – you have made a mistake! One common mistake is to round numbers too soon. Don’t do this until the end. 5 About this book