Section 2: Understanding Chemical Reactions

Word Equations  Remember: The 2 parts in a chemical reaction are the reactants (things reacting) and the products (things produced).  The left side of the equation lists all the reactants while the right side lists all the products.  Word equations are ways of representing chemical reactions  Tells what reacts and what is produced  An arrow points between the reactants and products Reactants  Products Eg. Iron + Oxygen  Iron(III) oxide

Try it!  Questions 1 – 4 on page 219

Conserving Mass  The Law of Conservation of Mass states that, in a chemical reaction, the total mass of the reactants is always equal to the total mass of the products.  This means that the number of atoms of each element in the reactants (left side) will always have to be the same in the products (right side).  Ex: 2H 2 + O 2  2H 2 O

Balancing Chemical Equations  We can describe chemical reactions in two ways  As word equations  Eg. methane + oxygen  carbon dioxide + water  As skeleton equations  Eg. CH 4 + 2O 2  CO 2 + 2H 2 0  By the Law of Conservation of Mass the mass of the reactants must equal the mass of the products.  There must be a balance between the number of atoms of each type in the reactants and the number of atoms of each type in the products

How to Balance an Equation  Iron reacts with oxygen to form magnetic iron oxide (Fe 3 O 4 ). What is the balanced chemical equation for this reaction?  Steps: 1. Write the word equation for the reaction. iron + oxygen  magnetic iron oxide 2. Write the skeleton equation by replacing each name with a correct chemical formula (*don’t forget about diatomic molecules like O 2 and H 2 ). Fe + O 2  Fe 3 O 4 3. Count the numbers of atoms of each type in reactants and products. Type of atomReactantsProducts Fe13 O24O24 4. Multiply each of the formulas by the appropriate coefficients (#’s in front) to balance the number of atoms. 3Fe + 2O 2  Fe 3 O 4

YOU TRY IT 1  Balance the following equation.  Mg + HNO 3(aq)  H 2 + Mg(NO 3 ) 2 ReactantsProducts Mg11 H12H12 N12N12 O36O36

ANSWER  Mg + 2HNO 3(aq)  H 2 + Mg(NO 3 ) 2

Practice  (a) ___H 2 + ____N 2 ____NH 3  (b) ___Fe + ____O 2 ____Fe 2 O 3  (c) ___Li + ___H 3 PO 4 ___H 2 + ___Li 3 PO 4

Answers  a) 3H 2 + N 2  2NH 3  b) 4Fe + 3O 2  2Fe 2 O 3  c) 6Li + 2H 3 PO 4  3H 2 + 2Li 3 PO 4

Practice  Questions 1 – 3 on page 229