You Should Be Able To… 1. Define and explain the law of conservation of mass 2. Write and balance chemical equations from formula, word equations, or descriptions of experiments

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

(c) McGraw Hill Ryerson 2007 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. See pages

(c) McGraw Hill Ryerson 2007 See pages

(c) McGraw Hill Ryerson 2007 Chemical reactions can be written in different ways. –A word equation: Nitrogen monoxide + oxygen  nitrogen dioxide –A symbolic equation: 2 gg 2 g2NO (g) + O 2(g)  2NO 2(g) STATE OF MATTER - Letters indicate the state of each compound. (aq) = aqueous/dissolved in water (s) = solid ( ) = liquid (g) = gas COEFFICIENTS - Indicates how many of each molecule there is. -Ie: there are 2 molecules of NO.

(c) McGraw Hill Ryerson 2007 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 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, a DESCRIPTION –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

(c) McGraw Hill Ryerson 2007 count atoms to balanceUsing 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. See Page 207

(c) McGraw Hill Ryerson 2007 Skeleton equation:CH 4 + O 2  H 2 O + CO 2 11 Carbon = 1Carbon = 1 42 Hydrogen = 4Hydrogen = 2 23 Oxygen = 2Oxygen = Balanced equation:CH 4 + 2O 2  2H 2 O + CO 2 Carbon = 1Carbon = 1 44 Hydrogen = 4Hydrogen = 4 44 Oxygen = 4Oxygen = 4 The same number of atoms must be on each side. See Page 207

Balancing Equations ReactantsProducts H22 hydrogen + oxygen  water O 2 O 2  H2OH2OH2OH2O H 2 H 2 +O21

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)

(c) McGraw Hill Ryerson 2007 Balance chemical equations by following these steps: –Trial and error will work but can be very inefficient. USE A TABLEUSE A TABLE (write atoms underneath reactants and products) Balance one compound at a time rewrite the # of atoms as things changeBalance one compound at a time & rewrite the # of atoms in the chart as things change. Only add coefficientsOnly add coefficients; NEVER change subscripts!!! H and O LASTIf 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!!! 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