McAuley Catholic College

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1 McAuley Catholic College JUNIOR SCIENCE ANSWERS Name: Worksheet 1 Unit 1 Semester 1 Chemical Reactions Instructions: Work though the exercise over the page. Marks achieved Marks possible 25 R.N.Waters (Xavier Catholic College) Page 1 of 6

2 WRITING CHEMICAL FORMULAE 1 Name: Chemical formulae are very useful to chemists as they show the numbers and types of atoms that make up chemical compounds. Many people know that the chemical formula for water is H 2 O, but how many know that this tells us that a water particle is made up of two hydrogen atoms and one oxygen atom, which are chemically combined. Some compounds are very simple and are made up of just two atoms. Others, like the proteins that are found in all living organisms can be made up of tens of thousands of atoms. By following some simple rules we can write the formulae of most of the chemical compounds that we are likely to encounter in this course of study. Chemical names. A chemical compound will always have two, or more, parts to its name. The name of a chemical compound tells us about the numbers and types of atoms that have combined to form it. The name can also be used to help us write the chemical formula of the compound. Sometimes one of the parts of the compound s name will have a prefix, which indicates how many atoms of that element are present in the formula. The following prefixes are often encountered. Prefix. Number of atoms. Example. Formula Mon(o)- One Carbon monoxide CO Di- Two Carbon dioxide CO 2 Tri- Three Sulphur trioxide SO 3 Tetr(a)- Four Carbon tetrachloride CCl 4 Pent(a)- Five Di-phosphorus pentoxide P 2 O 5 Hex(a)- Six Uranium hexafluoride UF 6 Hept(a)- Seven Iodine heptafluoride IF 7 Oct(a)- Eight Octane C 8 H 18 Non(a)- Nine Nonane C 9 H 20 Dec(a)- Ten Decane C 10 H 22 I. In the first example, carbon monoxide is made up of one atom of carbon and one atom of oxygen, chemically combined. (When writing chemical formulae we do not write the number 1. This is because the presence of a chemical symbol in the formula tells us that there is at least one atom of that element present in the compound.) In the final example, decane is composed of ten carbon atoms and twenty- two hydrogen atoms chemically combined; a total of thirty-two atoms. Name endings also tell us about the chemical make-up of a compound. Compounds whose name ends in IDE are composed of two elements only. Compounds whose name ends in ATE contain the element oxygen. Compounds whose name contains the prefix PER contain extra atoms of one of the elements. eg Hydrogen oxide H 2 O, hydrogen peroxide H 2 O 2. R.N.Waters (Xavier Catholic College) Page 2 of 6

3 Valency Number. Each element is assigned a valency number. A valency number is assigned to each group in the periodic table. All the elements in a particular group have that assigned number. An element s valency number is used to calculate the ratios in which it combines with other elements. Group Valency number (The noble, or inert, gases are assigned a valency number of zero because they do not react with any other elements.) You will see valency number being used to write chemical formulae in the following sections. Writing formulae for simple, two element, compounds Chemical compounds that are composed of two elements only, can be recognised by their IDE name ending. eg Magnesium oxide is composed of the elements magnesium and oxygen, chemically combined. The chemical formulae of two element compounds can be determined by using the Valency number method. The Valency numbers of the elements in a compound can be used to determine the numbers of atoms of each element in the compound. The chemical formula of carbon dioxide will be determined to illustrate the method used to write chemical formulae. Method 1. Write the symbols of the elements that make up the compound. C O 2. Write the valency number of each element at the bottom, right hand side of each symbol. C 4 O 2 3. Swap the valency numbers of the elements. C 2 O 4 4. Divide the valency numbers by any common denominator that will give whole number answers. (In this case divide by 2.) C 1 O 2 5. Re-write the formula omitting the number 1. Formula = CO 2 R.N.Waters (Xavier Catholic College) Page 3 of 6

4 Use the valency swapping method to determine the formulae of the following two - element compounds. 1. Magnesium oxide MgO 2. Lithium fluoride LiF 3. Phosphorus hydride PH3 4. Calcium chloride CaCl2 5. Aluminium iodide AlI3 6. Magnesium nitride Mg3N2 7. Sodium oxide Na2O 8. Silicon chloride SiCl4 9. Potassium sulphide K2S 10. Hydrogen chloride HCl R.N.Waters (Xavier Catholic College) Page 4 of 6

5 Writing formulae for two element compounds which contain a variable valency element. If an element can have more than one valency number, then the number is written after the name of the element in the compound. The valency number is written in Roman numerals and is in brackets. eg Iron (II) oxide FeO Iron (III) oxide Fe 2 O 3 The method of writing the formulae for these compounds is exactly the same as shown in the previous section. eg Write the formula of lead (IV) oxide. Write the symbols. PbO Write in the valency numbers. Pb 4 O 2 Swap the valency numbers. Pb 2 O 4 Cancel down. In this case using a common denominator of 2. Pb 1 O 2 Formula = PbO 2 Use the valency swapping method to determine the formulae of the following two - element compounds. 1. Copper (II) oxide CuO 2. Iron (III) chloride FeCl3 3. Tin (IV) iodide SnI4 4. Chromium (III) oxide Cr2O3 5. Cobalt (II) chloride CoCl2 R.N.Waters (Xavier Catholic College) Page 5 of 6

6 What are the names of the following compounds? 1. MnO 2 Manganese Oxide 2. PbO Lead Oxide 3. CrCl 3 Chromium Chloride 4. SnO 2 Tin Oxide 5. FeS Iron Sulfide 6. NaCl Sodium Chloride 7. CuF Copper Fluoride 8. CaI 2 Calcium Iodide 9. AlBr 3 Aluminium Bromide 10. Li 2 O Lithium Oxide R.N.Waters (Xavier Catholic College) Page 6 of 6