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# Chapter 3 part 2 (3.5 3.6)

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### Chapter 3 part 2 (3.5 3.6)

1. 1. 3.5 Chemical Formulae  Used to represent a chemical compound  It shows:- the elements (denoted by symbols) - the relative numbers (indicated by subscript after the symbol)  Example:- H2 O
2. 2. Chemical formulae of some covalent compounds Name of compound Chemical formula Number of each element in the compound Oxygen O2 2 oxygen atoms Water H2O 2 hydrogen atoms 1 oxygen atom
3. 3. Chemical formulae of some ions (cations) Charge Cation Symbol +1 Sodium ion Na+ +2 Magnesium ion Mg 2+ +3 Iron(III) ion Fe3+
4. 4. Chemical formulae of some ions (anions) Charge Anion Symbol -1 Fluoride ion F- -2 Oxide ion O2- -3 Nitride ion N3-
5. 5.  To write the chemical formula of an ionic compounds:- write the formula of the ions involved in forming the compound - balance the positive and negative charge (use subscript) - Write the chemical formula of the ionic compound without the charges.
6. 6. Example:-
7. 7. Formulae of some ionic compounds Cation Anion Chemical formula Na+ Cl- NaCl Ca2+ Cl- CaCl2 Al3+ N3- AlN
8. 8.  Empirical formulae of a compound:- shows the simplest ratio of the atoms of the elements that combine to form a compound - steps to determine the empirical formula of a compound:1) write the mass / percentage of each element in the compound 2) calculate the number of moles for each element 3) Divide each number by the smallest number to obtain simplest ratio 4) Write the empirical formula of the compound
9. 9. Example:-
10. 10.  Molecular formulae of the compound:- shows the actual numbers of the atoms of the elements that combine to form the compound Compound Molecular formula Simplest ratio of the elements Empirical formula Water H2O H:O = 2:1 H2O Ethene C2H4 C:H=1:2 CH2 Glucose C6H12O6 C:H:O=1:2:1 CH2O
11. 11. 3.6 Chemical Equations  Chemical reaction can be represented by a chemical equation  Reactants – chemicals that are reacting. Written on LHS.  Products – chemicals formed in the reaction. Written on the RHS
12. 12.  Writing a chemical equation:1) write the correct formulae of all reactants on the LHS of the equation 2) write the correct formulae of all products on the RHS of the equation 3) the equation is then balanced. 4) make sure the number of atoms before and after reaction are the same 5) Write the physical state of each reactants and products
13. 13. Example:-