Section 7.2 Chemical Equations 1.understand the information given by a chemical equation 2.balance an equation so that the numbers of each atom are the same on both sides of the arrow Learning Goals – I will be able to… Section 7.1 AB + CD  AC + DB

Section 7.2 Chemical Equations Recall from previous units  Section 7.1 A NEW SUBSTANCE IS FORMED

Section 7.2 Chemical Equations Chemical reactions involve a rearrangement of the ways atoms are grouped together. A chemical equation represents a chemical reaction. –Reactants are shown to the left of an arrow. –Products are shown to the right of the arrow. Section 7.1

Section 7.2 Chemical Equations Physical states of compounds are often given in a chemical equation. Section 7.1

Section 7.2 Chemical Equations Atoms as Building Blocks ++ 

Section 7.2 Chemical Equations What’s wrong with this picture? + +  The numbers of reactanct and product blocks do not match.

Section 7.2 Chemical Equations How can we “balance” this equation using numbers? + +  2 Coefficients are numbers used to balance an equation so we have the same number of each building block on each side of the equation. 2

Section 7.2 Chemical Equations In a chemical reaction atoms are not created or destroyed. All atoms present in the reactants must be accounted for in the products. –Same number of each type atom on both sides of arrow Chemical Reactions and Conservation of Matter

Section 7.2 Chemical Equations The reaction between methane and oxygen

Section 7.2 Chemical Equations Balancing a Chemical Equation Unbalanced equation

Section 7.2 Chemical Equations Balancing the equation The balanced equation CH 4 + 2O 2  CO 2 + 2H 2 O ** Use coefficients, do NOT change subscripts! **

Section 7.2 Chemical Equations Steps for Balancing a Chemical Equation (Section 7.3) Ex.: Hydrogen gas and oxygen gas combine to form liquid water. 1.Write the unbalanced chemical equation showing the formulas for reactants and products H 2 (g) + O 2 (g)  H 2 O(l) 2.Start balancing with the most complicated molecule (H 2 O in this case)

Section 7.2 Chemical Equations 3.Check to be sure the equation is balanced 2H 2 (g) + O 2 (g)  2H 2 O(l) Reactant side: Product side: 4 H, 2 O4 H, 2 O