Law of Conservation of Mass: BALANCING EQUATIONS NOTES A way to represent chemical reactions on paper DO NOT WRITE ON THIS PAPER. COPY THE NOTES IN YOUR NOTEBOOK Chemical Equations: 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, so there must be the same number of atoms on both sides of the equation

Numbers found in a chemical reaction Subscripts- The small numbers to the lower right of chemical symbols. Subscripts 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 ? Ca= 1 N= 2 O= 6 Example 2: How many atoms of each element are in one formula unit of Magnesium Oxide: MgO ? 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 ? Ca= 3 N= 6 O= 18 Example 2: How many atoms of each element are there in 2 formula units of Magnesium Oxide: 2MgO 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. Chemical Equations give information in 2 major areas: They tell us what substances are reacting and what substances are being produced. Reactants  Products Example: 2 H2+ O2  2H2O, What are the reactants and what are the products? Reactants: H2 and O2 Products: H2O

ASSIGNMENT 2. ____H2 + _____Cl2 → _____HCl 1. Aluminum metal reacts with oxygen to form solid aluminum oxide. 2. ____H2 + _____Cl2 → _____HCl 3. _____HCl + _____Fe2O3 → _____FeCl3 + _____H2O 4. _____MgCl2 → ______Mg + ____Cl2