Chemical reactions Unit Learning Goal # 2: Demonstrate how chemical equations are used to describe chemical reactions.

Chemical Equation A representation of a chemical reaction showing the relative numbers of reactants and products. Reactants Products “Yields”

Chemical reactions A chemical reaction involves changing the ways the atoms are grouped. H2 + Cl2   -->   2 HCl 2 H2 + O2   -->   2 H2O

Parts of a chemical Equation

Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

Law of Conservation of Mass In a chemical change, matter is neither created or destroyed. Mass of Reactants = Mass of Products

Conservation of Mass

Counting Atoms

Atoms are Conserved in a Chemical Reaction The identities (formulas) of the compounds must never be changed in balancing chemical equations. DO NOT change the Subscripts. You may only change the coefficients in an equation to balance the equation.

Steps To Balancing an Equation Here are the basic steps to follow when balancing equations. You should always use pencil when balancing equation so that you can erase any mistakes. 1st  Count the atoms of each element on each side of the arrow in an equation. Ex. CH4 + O2  CO2 + H2O C = 1 C = 1 H = 4 H = 2 O = 2 0 = 3

Steps To Balancing an Equation 2nd We need to add compounds to get equal numbers of each element. You can only add whole amounts of a molecule. Ex. CH4 + O2 + O2  CO2 + H2O + H2O C = 1 C = 1 H = 4 H = 4 O = 4 O = 4

Steps To Balancing an Equation The correct way to write this is to put a coefficient to represent Multiple molecules or formula units. - A coefficient is placed before a molecule. - A coefficient pertains to every element in the compound, regardless of parenthesis. Ex. CH4 + 2O2  CO2 + 2H2O C = 1 C = 1 H = 4 H = 4 O = 4 O = 4 The equation is now balanced because there are equal amounts of carbon, hydrogen, and oxygen on the reactant side and on the product side.

Steps to Balancing Equations There are four basic steps to balancing a chemical equation. Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation. Check your answer to see if: The numbers of atoms on both sides of the equation are now balanced. The coefficients are in the lowest possible whole number ratios. (reduced)