Writing and Balancing Chemical Equations A Review by Mrs. Volpe

What is an equation? An equation is a sentence, with either words or formulas, that describes what happens in a given chemical reaction. Parts of an equation: –Subscripts and Coefficients Subscript tells how many of one particular element in the molecule. Coefficient tells how many of the molecule. –Reactants and Products Reactants are starting materials Products are ending materials –Symbols:  yields (s) solid (g) gas  fire (l) liquid (aq) aqueous

What does a balanced equation look like? 2 H 2(g) + 2O 2(g)  2H 2 O (l) Reactants Products Law of Conservation of Matter: Matter can be neither created nor Destroyed, only changed in form. “If you start with it, you must end with it.” subscript coefficient

How to Balance An Equation Make a list of the elements present on both sides of the reaction. Count the number of atoms on both sides of the reaction. Look for the ones that do not match, and change COEFFICIENTS ONLY to multiply the number of atoms present. Start with metal atoms End with Hydrogen Keep polyatomic ions grouped together, unless they split up in the products.

Balancing in Action Al + Fe 2 O 3  Al 2 O 3 + Fe Al1Al2 Fe2Fe1 O3O

Dealing with word equations When going from words to formulas: –Check subscripts in formulas –Remember naming rules –Remember to balance! Ex: Hydrogen chloride and silver (II) oxide are combined to form water and silver (II) chloride. 2 HCl + AgO  H 2 O + AgCl 2

When going from formulas to words: 2 Na + CaCl 2  2 NaCl + Ca Sodium + calcium chloride  sodium chloride + calcium Sodium is combined with calcium chloride to form sodium chloride and calcium.