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Presentation On Stoichiometry: Chemical Formulae

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Published in: Chemistry | Science
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This PPT is suitable for the IGCSE and O Level Chemistry students. It only provides theoretical background of the topic. The practice questions, worksheets and topical questions will be discussed during the tutoring sessions

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  1. Chemical Formulae Chemistry (IGCSE / O-Level)
  2. What will you learn in this lesson? • Chemical symbol • Chemical formulae • Rules for naming chemical compounds • Chemical formulae of ionic and covalent compounds
  3. Chemical Symbol • A letter or group of letters representing an element in a chemical formula • All elements are either a single capital letter, such as hydrogen (H), oxygen (O), nitrogen (N) etc., or made up of two letters, aluminium (Al), iron (Fe), magnesium (Mg) etc. • The second letter in the symbol is always in lowercase ' Iver odium lead argentum cuprum natrium ka!ium ferrum I umbum pb
  4. Chemical Formulae • Formula represents an element and its structure • For noble gases, their structures are made up of single atom. So, their symbol is the formula • For giant ionic and covalent lattice structures, their formulae is actually the symbols of elements ca Sc Zh G? Ge AS se Br2 Formulae of the elements reflect their structure and position in the Periodic Table.
  5. Naming Chemical Compounds • Simple and Compounds Ions: Ionics compounds consist of two simple ions (Nal+ Kl+ Cll- Mg2+ 02-) or a group of atoms(C032- , NOg- , S042- , NH') forming a compound ion These compounds ions or groups are made up of covalently bonded atoms These groups have negative charge, but there is an important compound ion ammonium NH4+ that is positively charged Wth With With
  6. Naming Chemical Compounds Rules for Naming Compounds: Ifthere is a metal in the compound, it is named first. Where the metal can form more than one ion, then the name indicates which ion is present. For example, iron(ll) chloride contains the Fe2+ ion, while iron(lll) chloride contains the Fez + ion. Compounds containing only two elements have names ending in —ide . For example, sodium chloride (NaCI), calcium bromide (CaBr2) and magnesium nitride (Mg3N2). The important exception to this is the hydroxides, Which contain the (OHO ion. Compounds containing a compound ion (usually containing oxygen) have names that end with —ate . For example, calcium carbonate (CaC03), potassium nitrate (KN03), magnesium sulfate (MgS04) and sodium ethanoate (CH3COONa). The names of some compounds use prefixes to tell you the number of that particular atom in the molecule. This is useful if two elements form more than one compound. For example, carbon monoxide (CO) and carbon dioxide (C02), nitrogen dioxide (N02) and dinitrogen tetmxide ("1204), and sulfur dioxide (S02) and sulfur trioxide (S03). The names for the important mineral acids follow a logical system but are best simply learnt at this stage, e.g. sulfuric acid (H2S04). Some common and important compounds have historical names that do not fit with the above rules. Examples of these include water 0420), ammonia (NH,) and methane (CHA).
  7. Formulae of Covalent Compounds The combining power of an element in the covalent molecule is the number of bonds that atom makes (referred to as valency) The valency pf an element in the main groups of the periodic table can be deduced from the group number of the element The 'cross-over' method for working out chemical formulae can be applied to simple covalent compounds with single central atom Valency = Group Number 11 Valency = 8 - Group Number Valency = O roup alency com bin ing ower) cal NaC1 mgC12 AIC13 SiC14 pH3
  8. Formulae of Covalent Compounds (contd.) 'Cross-over' Method for Carbon dioxide (C02): A method involving the symbols and valencies of the elements can be used: 1 Write down the symbols of the el ements. 2 Write the valencies of the elements beneath their symbols (Figure AA). 3 If necessary, simpUW the ratio. Formula of carbon dioxide c Wite down fie symbols down the valencies Can simplify: Figure 4.4: A method for finding the formula of carbon dioxide. 2
  9. Formulae of Covalent Compounds (contd.) 'Cross-over' Method: Now apVly this 'cross-over' method to finding the formula of: a methane (the simplest compound of carbon and hydrogen) b nitrogen chloride (a Simple molecular compound)
  10. Formulae of Covalent Compounds (contd.) Molecular Formula: A formula that show the actual number of atoms of element present in the molecule of a compound We do this simply by counting the number of each type of atoms present in a molecule The compounds consist of multiple central atom do not follow cross- over method. For e.g. C3H6 !H Diagrams showing the structures of BrF3, NHB and C3H6e
  11. Formulae of Covalent Compounds (contd.) • Molecular formula is used in chemical equations • These structures fulfill the valency condition but their formulae can't be found using 'cross-over' method hß•ogen Each oxygen atom makes two bonds; each hydrogen makes one bond. Each carbon atom makes four bonds; each hydrogen makes one bond. Structures of hydrogen peroxide (8202) and ethane (C2H6), showing the bonds made.
  12. Formulae of Covalent Compounds (contd.) Empirical Formula: It is the simplest whole number ratio of different atoms in that compound The empirical formula of propene, C3H6 is CH2 What will be the molecular and empirical formulae of ethane? The formula for any giant covalent structure can be worked out by the cross-over method using valencies of elements. For e.g., silicon (IV) oxide
  13. Formulae of Ionic Compounds • They are giant ionic structures, ionic compounds are always represented by their empirical formulae and the cross-over method can be used to work these formulae out • In ionic compounds, the valency of each ion is equal to its charge Sinwle . Cue al u minivn, iron 4.3: simple and i I eve h • Wide, chlMide, sulfide, nitride, NH. hvdroö&, OW nitrate, NO 2 - wtfate, SO. ate, C02>
  14. Formulae of Ionic Compounds (contd.) 'Cross-over' method for aluminium oxide and calcium oxide: For,muta foe aluminium oxide Formota for calcium charges • Simplify the FermuJa Cab
  15. Formulae of Ionic Compounds (contd.) 'Cross-over' method: Now find the formulae of: a magnesium iodide b StumTnium bromide
  16. Formulae of Ionic Compounds (contd.) 'Cross-over' method for Compound Ions: The same rule applies to compound ions. It is useful to put the formula of compound ion in brackets Formula down symbols N a,C06 the brackets are notneeded if +ere only one ion present Now find te 6mulae of: a m phosphate b ammonium nltnte
  17. Formulae of Ionic Compounds (contd.) Ionic compounds can be represented by diagrams of their structures Cl-cr Cl- Cl- Cl- Cl-c:r The diagram represents 4 Ca2+ ions and 8 Cll- ions. If we divide by 4, then there will be 1 Ca2+ ion and 2 Cll- ions. So the formula will be CaC12
  18. Formulae of Ionic Compounds (contd.) The formulae of ionic compounds is always the simplest whole number ratio of the ions present in the compound (empirical formula) Ions present sodium chloride ammonium nitrate potassium sulfate calcium hydrogen carbonate copper(ll) sulfate magnesium nitrate aluminium chloride Naa NH4N02 K2S04 cuso„ AIC13 NH4+ Cu2+ "102+ A13+ HCO, SOA 2—

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