BALANCING EQUATIONS Day 73

 Subscript  Coefficient  Law of Conservation of Mass  Molecule  Atom  Skeleton Equation  Balanced Equation  Word Equation

 Chemical reactions result in chemical changes.  Chemical changes occur when new substances are created.  The original substance(s), called reactants, change into new substance(s) called products. (c) McGraw Hill Ryerson 2007 See pages

(c) McGraw Hill Ryerson 2007 See pages

 When a chemical reaction occurs, new compounds are created, BUT…  No new matter is created or destroyed rearrangedchange partners  No new matter is created or destroyed; atoms are just rearranged as the atoms change partners to form new compounds. 3  If there are 3 atoms of oxygen in the reactants, there MUST be 3 atoms of oxygen in the products.  Number of each atom in reactants = number of each atom in products.  The law of conservation of mass:  Mass of reactants = mass of products (c) McGraw Hill Ryerson 2007 If you could collect and measure all of the exhaust from this car, you would find that mass of reactants (gas + O 2 ) = mass of products (exhaust).

The simplest form of chemical equation is a word equation. –Potassium metal + oxygen gas  potassium oxide A skeleton equation shows the formulas of the elements/compounds. –A skeleton equation shows which atoms are involved, but not how many molecules are involved. K + O 2  K 2 O

balanced chemical equation coefficientsA balanced chemical equation shows all atoms and the coefficients tells us how many molecules (and atoms) there are. ensures number of each atom same on both sides –Balancing ensures that the number of each atom is the same on both sides of the reaction arrow. 42 4K + O 2  2K 2 O K K K K K K K K K K K K K K K K OO O O

count atoms to balance  Using the law of conservation of mass, we can count atoms to balance the number of atoms in chemical equations.  Word equation: methane + oxygen  water + carbon dioxide  Skeleton equation:CH 4 + O 2  H 2 O + CO 2  To balance the compounds, take note of how many atoms of each element occur on each side of the reaction arrow. (c) McGraw Hill Ryerson 2007

Skeleton equation:CH 4 + O 2  H 2 O + CO Carbon = 1 Carbon = Hydrogen = 4 Hydrogen = Oxygen = 2 Oxygen = Balanced equation:CH 4 + 2O 2  2H 2 O + CO 2 Carbon = 1 Carbon = Hydrogen = 4 Hydrogen = Oxygen = 4 Oxygen = 4 (c) McGraw Hill Ryerson 2007 The same number of atoms must be on each side.

BALANCING EQUATIONS O 2 O 2  ReactantsProducts H22 hydrogen + oxygen  water H2OH2OH2OH2O H 2 H 2 +O21

BALANCING EQUATIONS H 2 + O 2  H 2 O 2 ReactantsProducts H22 O22 hydrogen + oxygen  hydrogen peroxide X YOU CANNOT CHANGE THE SUBSCRIPTS

BALANCING EQUATIONS H 2 + O 2  H 2 O ReactantsProducts H22 O21 hydrogen + oxygen  water 2

BALANCING EQUATIONS H 2 + O 2  H 2 O ReactantsProducts H24 O22 hydrogen + oxygen  water 2

BALANCING EQUATIONS 2 2 H 2 + O 2  H 2 O ReactantsProducts H44 O22 hydrogen + oxygen  water 2 (g) (l)

 Balance chemical equations by following these steps:  Trial and error will work but can be very inefficient.  USE A TABLE  USE A TABLE (write atoms underneath reactants and products) treat polyatomic ions as a group  If they look the same on both sides of the equation, treat polyatomic ions (such as SO 4 2– ) as a group & balance them as such. write water as HOH.  If ‘OH’ and H 2 O are in the equation, write water as HOH.  Balance one compound at a time rewrite the # of atoms as things change  Balance one compound at a time & rewrite the # of atoms in the chart as things change.  Only add coefficients  Only add coefficients; NEVER change subscripts!!! H and O LAST  If H and O appear in more than one place, attempt to balance them LAST.  Balance everything that isn’t ‘H’ or ‘O’ 1 st.  Balance the ‘H’s 2 nd to last.  Balance the ‘O’s last.  Always double-check  Always double-check after you think you are finished.  CHECK YOUR ANSWERS!!! (c) McGraw Hill Ryerson 2007 See pages

 Balance the following:  Fe + Br 2  FeBr 3  Sn(NO 2 ) 4 + K 3 PO 4  KNO 2 + Sn 3 (PO 4 ) 4  C 2 H6 + O 2  CO 2 + H 2 O

____Ba + ____H 2 O  ____Ba(OH) 2 + ____H 2 ____CO 2 + ____H 2 O  ____H 2 CO 3 ____Fe 2 O 3 + ____C  ____Fe + ____CO ____Fe + ____H 2 O  ____H 2 + ____Fe 2 O 3