Balancing Chemical Equations Unit 2 – Lecture 8

Chemical Equations A shorthand way to describe a chemical reaction using chemical symbols and formulae Three Parts to a Chemical Equation Reactants substances present before the reaction Yield Arrow indicates the direction of a reaction some rxns are reversible [double-sided arrow] Products substances present after the reaction

Chem. Equations – cont’d Chemical symbols give a “before-and-after” picture of a chemical reaction Reactants Products MgO + C CO + Mg magnesium oxide to form carbon monoxide reacts with carbon and magnesium or… magnesium oxide reacting with carbon yields carbon monoxide and magnesium

Chemical Equations – cont’d Coefficient – (large) # in front of an element applies to all elements in the group to the right of it ex: 4C 4 carbon atoms ex: 5KI 5 potassium atoms & 5 iodine atoms ex: 4SO4 4 Sulfur atoms & 16 Oxygen atoms (4x4)

Chemical Equations – cont’d Subscript – (small) # to the lower right of an element applies only to the element it is attached to if outside of parentheses [(XY)3] applies to everything inside of parentheses ex: H2 2 hydrogen atoms ex: CO2 1 carbon atom, 2 oxygen atoms ex: (SO4)3 3 Sulfur atoms (1x3), 12 Oxygen atoms (4x3)

Chemical Equations - cont’d Try this: Al2(SO4)3 Aluminum? 2 atoms Sulfur? 3 atoms Oxygen? 12 atoms (4x3)

Balancing Chem Equations Law of Conservation of Matter matter is neither created nor destroyed in a chemical reaction, it is conserved biology: energy is lost, matter is RECYCLED (changed from one thing to another) ie, if you begin a reaction with 5 atoms of Hydrogen, you must end up with 5 atoms of Hydrogen

Balancing – cont’d Al + S Al2S3 2Al + 3S Al2S3 Balanced = same # of each element on each side Al + S Al2S3 2Al + 3S Al2S3 Unbalanced Balanced

Balanced or Unbalanced? C3H8 + 5O2 4H2O + 3CO2 Carbon? = 3 vs 3 Hydrogen? = 8 vs 8 Oxygen = 10 vs 10 (4 + 6) Is it balanced? Yes.

Balanced or Unbalanced? Fe3O4 + H2 Fe + H2O Iron = 3 vs 1 Oxygen = 4 vs 1 Hydrogen = 2 vs 2 Is it balanced? No. So, now what?

Balancing a Chemical Reaction Set up two columns: labeled “reactants” &“products.” Identify & record reacting elements on the left. Record elements in the SAME ORDER on the right. Count & Record # of each reacting element. Count & Record # of each element produced. Unequal? Balance Coefficients. [can’t change subscripts] you can treat Polyatomic Ions as one if they appear on both sides of the equation like (SO4) in parentheses change your greatest inequality first leave solitary elements for last

Practice Fe3O4 + H2 Fe + H2O

Practice On your worksheet, complete the two problems on the front of your sheet. ____H2 + ____O2  ____H2O ____Cl2 + ____NaBr  ____Br2 + ____NaCl 2 1 2 1 2 1 2

Homework Page 6 – Balancing Chemical Equations w/s 1-9 on-level all for honors