Law of Conservation of Mass Balancing Equations: Law of Conservation of Mass

Law of Conservation of Mass: Mass is neither created nor destroyed during a chemical reaction- it is conserved Mass reactants = Mass products

Chemical Equations/ Law of Conservation of Mass For an equation to follow the Law of Conservation of Mass, the equation must be balanced. Atoms can neither be created nor destroyed in an ordinary chemical reaction There must be the same number of atoms on both sides of the equation

Parts of a Chemical Equation: Chemical equations tell us what substances are reacting and what substances are being produced. Reactants  Products Reactants- starting substances H2 + O2 → Products- substances formed during the reaction. → H2O

Numbers found in a chemical reaction Subscripts- The small numbers to the lower right of chemical symbols Represent the number of atoms of each element in the molecule Example 1: How many atoms of each element are in one formula unit of Calcium Nitrate: Ca(NO3)2 ? Example 2: How many atoms of each element are in one formula unit of Magnesium Oxide: MgO ? Ca= 1 N= 2 O = 6 Mg= 1 O= 1

Numbers found in a chemical reaction Coefficients The large numbers in front of chemical formulas Coefficients represent the number of molecules of the substance in the reaction. Example 1: How many atoms of each element are there in 3 formula units of Calcium Nitrate: 3Ca(NO3)2 ? Example 2: How many atoms of each element are there in 2 formula units of Magnesium Oxide: 2MgO Ca= 3 N= 6 O= 18 Mg = 2 O = 2

Writing Chemical Equations Check for diatomic molecules: H2 , N2 , O2 , Cl2 , F2 , Br2 , I2 If these elements appear by themselves in an equation, they must be with the subscript 2.

How do we balance the chemical equation: Balance equations by changing coefficients Never by changing subscripts

Practice Problems: Circle and label the reactants and the product(s) _____Zn + ______HCl  _____ZnCl2 + _____H2 _____KClO3  _____KCl + _____O2 _____S8 + _____F2  _____SF6 _____Fe + _____O2  _____Fe2O3