Writing and Balancing Chemical Reactions

Chemical Equation A description of a chemical reaction, using symbols and formulas to represent the elements and compounds involved Like an equation in Math ::: 2+3=5

A chemical equation includes all substances involved in the reaction and all products that are formed. A + X  AX + AX+ : indicates that A and X react together  yields  : read as “yields” points to the result of the reaction

A + X  AX The starting substances in the reaction are called the reactants and are left of the . A and X are the reactants The substances that are formed are called the products. AX is the product The products are to the right of the .

When writing a chemical equation it is important to know the physical form of the products and reactants. The symbols are: (g) = gas (l) = liquid (s) = solid (aq) = aqueous solution (water)

The symbols are written directly after the substance you are describing. A(s) + X(g)  AX(g) If heat is needed to move a reaction, the Greek letter delta (Δ) is used above the yield arrow. Δ A(s) + X(g)  AX(g)

We can interpret a chemical equation in several ways EX. C (s) + O 2(g)  CO 2(g) Qualitatively Solid C and gaseous O 2 react to form gaseous CO 2 Microscopic Representation One C atom reacts with one O molecule to form one molecule of CO 2 Macroscopic Representation One mole of C reacts with 1 mole of O gas to produce one mole of CO 2

Not all reactions are this simple. H 2(g) + O 2(g)  H 2 O (l) This equation as it is written does not conform to the law of conservation of mass. There 2 atoms of oxygen in the reactants ; but only one atom in the products!! atoms cannot be created or destroyed We need to balance this equation so it will agree with the law of conservation of mass

STOICHIOMETRIC COEFFICIENTS ratioproducts and reactantsWe can use STOICHIOMETRIC COEFFICIENTS to adjust the ratio of the products and reactants. H 2(g) + O 2(g)  H 2 O (l)H 2(g) + O 2(g)  H 2 O (l) 2H 2(g) + O 2(g)  2H 2 O (l)2H 2(g) + O 2(g)  2H 2 O (l) 4 H atoms + 2 O atoms  4 H atoms and 2 O atoms4 H atoms + 2 O atoms  4 H atoms and 2 O atoms

We can only balance equations by adjusting coefficients We cannot change subscripts H 2 O to H 2 O 2 X We cannot add a reactant or product H 2 + O 2  H 2 O + O X Yes, the reaction looks balanced, but it no longer describes the desired reaction.

Balancing Chemical Equations 1. LEARN THESE DIATOMIC MOLECULES: O 2 H 2 N 2 F 2 Cl 2 Br 2 I 2 2. Do NOT change the chemical formulas of the reactants or products (subscripts). 3. Start with the most complicated molecule and proceed element by element to determine coefficients. Save simplest molecule for last, particularly single element molecules. 4. Check equation by counting the number of each type of atom in reactants and products, making sure there is the same number on each side of the arrow.

MINOH Metals, Ions, nonmetals, oxygen, hydrogen

Practice

Balancing Equations

What is a chemical equation?

A description of a chemical reaction, using symbols and formulas to represent the elements and compounds involved

What does the symbol Delta (Δ) mean when it is above the yield arrow?

Heat is needed

It is important to know the ________ form of the products and reactants in a chemical equation. It is important to know the ________ form of the products and reactants in a chemical equation.

Physical

Balance the following equation: H 2(g) + O 2(g)  H 2 O (l)

2H 2(g) + O 2(g)  2H 2 O (l)

We can only balance equations by...

adjusting coefficients

4Fe (s) + 3O 2(g) ---> 2Fe 2 O 3(g) Name the Reactants and the Products

4Fe (s) and 3O 2(g) = reactants 2Fe 2 O 3(g) = products 2Fe 2 O 3(g) = products

Chemical equations must conform to the...

Law of Conservation of Mass

Balance the following equation: Zn (s) + AgNO 3 (aq)  Zn(NO 3 ) 2 (aq) + Ag (s)

Zn (s) + 2AgNO 3(aq)  Zn(NO 3 ) 2(aq) + 2Ag (s)

To balance an equation you cannot add...

Reactants, Products or subscripts

Balance the following equation: N 2 (g) + H 2 (g)  NH 3 (g)

N 2 (g) + 3H 2 (g)  2NH 3 (g)

Balance the following equation: N 2 (g) + O 2 (g)  N 2 O 5 (g)

2N 2 (g) + 5O 2 (g)  2N 2 O 5 (g)

Balance the following equation: Ca (s) + H 2 O (l)  Ca(OH) 2 (aq) + H 2 (g)

Ca (s) + 2H 2 O (l)  Ca(OH) 2 (aq) + H 2 (g)

Balance the following equation: K (s) + H 2 O (l)  KOH (aq) + H 2 (g)

2K (s) + 2H 2 O (l)  2KOH (aq) + H 2 (g)

Balance the following equation: Zn (s) + S 8 (s)  ZnS (s)

8Zn (s) + S 8 (s)  8ZnS (s)

Balance the following equation: C 5 H 12 (l) + O 2 (g)  H 2 O (g) + CO 2 (g)

C 5 H 12 (l) + 8O 2 (g)  6H 2 O (g) + 5CO 2 (g)