Writing And Balancing Equations Chemistry Dr. May

The Road To Writing Sentences  Know the alphabet  Know vowels and consonants  Know how to spell words  Know the meaning of words  Know sentence structure  Know capitalization and punctuation

The Road To Writing Equations  Know the atomic symbols  Know oxidation numbers of elements  Know the charge on polyatomic ions  Be able to create the correct formulas  Identify the reactants and products  Balance the equation

The Language of Equations Ca(OH) 2 + H 2 SO 4  CaSO H 2 O  The equation above is like a sentence  The compounds Ca(OH) 2, H 2 SO 4, CaSO 4, and H 2 O are like correctly spelled and chosen words  The elements Ca, O, H, and S are like the letters of the alphabet

Equations in English Ca(OH) 2 + H 2 SO 4  CaSO H 2 O  One molecule of calcium hydroxide plus one molecule of sulfuric acid yields one molecule of calcium sulfate plus two molecules of water.

Equations Zn + 2 HCl   ZnCl 2 + H 2   On the left, Zn and HCl are reactants  On the right, ZnCl 2 and H 2 are products  The arrow (  ) means “yields”  The up arrow (  ) indicated a gas product  The 2 in front of HCl is a coefficient  The small 2 to the right of H is a subscript

“Skeleton” Equation  Identify the reaction  Zinc reacts with hydrochloric acid to give zinc chloride and hydrogen gas  Determine the correct formulas and write the “skeleton” (unbalanced) equation  Zn + HCl   ZnCl 2 + H 2 

Coefficients  A coefficient placed before a formula multiplies every atom in that formula by that number 2 H 2 SO 4  2 H 2 SO 4 means 2 molecules of sulfuric acid but also  4 H atoms, 2 S atoms, and 8 O atoms

Conservation Of Mass  A balanced equation contains the same number of each kind of atom on each side of the equation  The balanced equation obeys the Law of Conservation of Mass

Balance The Equation Zn + HCl   ZnCl 2 + H 2   Count atoms of each element on each side  Balance each element by placing whole numbers (coefficients) in front of each formula as needed Zn + HCl   ZnCl 2 + H 2  2

Write Equation  Calcium hydroxide reacts with sulfuric acid to give calcium sulfate and water Calcium = Ca +2 Sulfate = SO 4  2 Hydrogen = H +1 Hydroxide = OH  1 Ca(OH) 2 + H 2 SO 4  CaSO 4 + H 2 O

Balance Equation 1Ca, 2OH, 2H, 1SO 4 1Ca, 1OH, 1H, 1SO 4 Ca(OH) 2 + H 2 SO 4  CaSO 4 + HOH Count atoms on both sides Balance the water Ca(OH) 2 + H 2 SO 4  CaSO 4 + HOH 2

Write Equation  Aluminum hydroxide plus sulfuric acid gives aluminum sulfate plus water Aluminum = Al +3 Hydroxide = OH  1 Hydrogen = H +1 Sulfate = SO 4  2 Al(OH) 3 + H 2 SO 4  Al 2 (SO 4 ) 3 + H 2 O

Balance The Equation Al(OH) 3 + H 2 SO 4  Al 2 (SO 4 ) 3 + HOH 1 Al, 3 OH, 2 H, 1 SO 4 2 Al, 3 SO 4, 1 H, 1 OH 2 Al(OH) 3 + H 2 SO 4  Al 2 (SO 4 ) 3 + HOH 2 Al(OH) H 2 SO 4  Al 2 (SO 4 ) 3 + HOH 2 Al(OH) H 2 SO 4  Al 2 (SO 4 ) HOH

Write The Equation  Propane burns to produce carbon dioxide and water C 3 H 8 + O 2  CO 2 + H 2 O The above skeleton equation must be balanced

Balance The Equation C 3 H 8 + O 2  CO 2 + H 2 O  First balance the carbons C 3 H 8 + O 2  3 CO 2 + H 2 O  Next balance the hydrogens C 3 H 8 + O 2  3 CO H 2 O  Finally balance the oxygens C 3 H O 2  3 CO H 2 O = 10 2 x 5 = 10

Chemical Equations  Chemical equations are the short hand description of a chemical reaction  It tells us:  The reactants and products  The formulas of the reactants and products  The number of molecules of reactants and products  The number of atoms of each element  The number of moles of each substance

The End  This presentation was created for the benefit of our students by the Science Department at Howard High School of Technology  Please send suggestions and comments to