The Law of Conservation of Matter

2 Conservation of Matter l The law states –During a chemical reaction, matter cannot be created or destroyed.

All chemical reactions have two parts l Reactants - the substances you start with l Products- the substances you end up with l The reactants turn into the products. –Reactants  Products

In a chemical reaction l Mass of reactant = mass of product l Can be described several ways –In a word equation –Aluminum + Oxygen Aluminum Oxide –In a chemical equation –Al + O 2 Al 2 O 3

Balancing Chemical Equations

Balanced Equation l Atoms can’t be created or destroyed l All the atoms we start with we must end up with l A balanced equation has the same number of atoms of each element on both sides of the equation.

C + O 2  CO 2 l This equation is already balanced l What if it isn’t already? C + O O  C O O

C + O 2  CO l We need one more oxygen in the products. l Can’t change the formula, because it describes what actually happens +  O C O O C O CC

l Must have started with two C 2 C + O 2  2 CO +  O CC O CC l Must be used to make another CO l But where did the other C come from? O O

Rules for balancing  Write the correct formulas for all the reactants and products  Count the number of atoms of each type appearing on both sides  Balance the elements one at a time by adding coefficients (the numbers in front)  Check to make sure it is balanced.

11 Coefficients l A number written before a chemical formula l It shows how many molecules of a compound are present. 2 NaCl coefficient

Never l Change a subscript to balance an equation. –If you change the formula you are describing a different reaction. –H 2 O is a different compound than H 2 O 2 l Never put a coefficient in the middle of a formula –2 NaCl is okay, Na2Cl is not.

Example H 2 +H2OH2OO2O2  Make a table to keep track of where you are at RP H O Need twice as much O in the product H 2 +H2OH2OO2O2  2 2 Changes the OAlso changes the H 4 Need twice as much H in the reactant 2 Recount 4 The equation is balanced, has the same number of each kind of atom on both sides

Example H 2 +H2OH2OO2O2  RP H O This is the answer Not this

An equation l Describes a reaction l Must be balanced to follow the Law of Conservation of Mass l Can only be balanced by changing the coefficients.

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