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Elements rarely occur alone Found as compounds Molecule- a group of atoms that are held together by chemical forces Diatomic molecules- molecules that are made up of only two atoms › Oxygen– O2 2 atoms of oxygen bonded together Chemical formula- a combination of letters and numbers that shows which elements make up a compound › H2O 2 atoms of hydrogen 1 atom of oxygen Elements and compounds combine through chemical reactions to form new compounds › Described in a chemical equation › Reactants on the left side of the arrow › Products on the right side › Arrow means gives or yields CH4 + 2O2 CO2 + H2O Balancing a chemical equation is much like the work of an accountant who has to show every penny that comes in and where it has gone to. The Law of Conservation of Mass states: that mass is neither created nor destroyed in any chemical reaction. Therefore balancing of equations requires the same number of atoms on both sides of a chemical reaction. The number of atoms in the Reactants must equal the Number of atoms in the Products The mass of reactants = mass of products Reactant + Reactant = Product 2 Al + ___ 3 Br2 ---> ___ Al2Br6 ___ First you need an equation with the correct “formulae” ………. You’ll probably be given this in the question Just like this one Mg + O2 MgO Then all you do is list the atoms that are involved on each side of the arrow Mg + O2 MgO Mg O Mg O Then start balancing: [1] Just count up the atoms on each side Mg + O2 MgO 1 Mg 1 2 O 1 [2] The numbers aren’t balanced so then add “BIG” numbers to make up for any shortages Mg + O2 2 MgO And adjust totals 1 Mg 1 2 2 O 1 2 But the numbers still aren’t equal, so add another “BIG” number 2 Mg + O2 2 MgO 2 1 Mg 2 2 O 2 And adjust totals again NOW BOTH SIDES HAVE EQUAL NUMBERS OF ATOMS WE SAY THAT THE EQUATION IS BALANCED!! Try to balance these equations using the same method: [1] Na + Cl2 NaCl [2] CH4 + O2 CO2 + H2O [3] Li + HNO3 LiNO3 + H2 [4] Al + O2 Al2O3 How did you get on?? Here are the answers: [1] 2 Na + Cl2 2 NaCl [2] CH4 + 2 O2 CO2 + 2 H2O [3] 2 Li + 2 HNO3 2 LiNO3 + H2 [4] 4 Al + 3 O2 2 Al2O3 NH3 + O2 Reactants NO + H2O Products N appears once on both sides in equal numbers, so the coefficient for NH3 is the same as for NO. Next look at H which appears only once on each side but has different numbers of atoms, 3 on the left and 2 on the right. The least common multiple of 3 and 2 is 6, so rewrite the equation to get 6 atoms of H on both sides: 2NH3 + O2 NO + 3H2O There are 2 oxygen atoms on the left and 5 on the right — the least common multiple of 2 and 5 is 10, so rewrite the equation as: 2NH3 + 5O2 4NO + 6H2O 1. Write all reactants on the left and all products on the right side of the equation arrow. Make sure you write the correct formula for each element 2. Use coefficients in front of each formula to balance the number of atoms on each side. 3. Multiply the coefficient of each element by the subscript of the element to count the atoms. Then list the number of atoms of each element on each side. 2NH3 + 5O2 4NO + 6H2O Write them out keeping them on the appropriate side of the chemical equation 2 N (nitrogen atoms) 4 N (nitrogen atoms) 6 H (hydrogen atoms) 12 H (hydrogen atoms) 10 O (oxygen atoms) atoms) 10 O (oxygen This shows the equation not to be balanced “YET” If you double the N and H on the left the equation will be balanced: 4NH3 + 5O2 4NO + 6H2O 4NH3 + 5O2 4 N (nitrogen atoms) 12 H (hydrogen atoms) 10 O (oxygen atoms) The 4NO + 6H2O 4 N (nitrogen atoms) 12 H (hydrogen atoms) 10 O (oxygen atoms) equation is Balanced