1. Describing & Balancing Chemical Reactions

2. Describing a Chemical Reaction
A chemical reaction = the process by which one or more substances is changed into one or more different substances.
Reactants = the original substance(s) that react
Products = the substances that form from a reaction
In a chemical reaction, symbols and formulas represent the identities and relative amounts of reactants and products in a chemical reaction.

3. Indications of a Chemical Reaction
There are 4 observable changes that indicate that a chemical reaction is taking place:
Energy released as light or heat
Production of a gas
Formation of a precipitate
Precipitate = a solid produced in a chemical reaction
Color change

4. Precipitate Video

5. Characteristics of Chemical Equations
They must represent known facts.
All reactants and products must be identified.
They must contain the correct formulas for the reactants and products.
Use charges and oxidation states to determine the chemical formulas of the products from the reactants
Recall that charges must cancel—must make sense

6. Characteristics of Chemical Equations
The law of conservation of mass must be satisfied.
The same number of each type of atom must appear on each side of the chemical equation
Coefficients = whole numbers that are placed in front of a formula to balance the chemical equation.
Ex: 2H2O

7. Symbols Used in Chemical Equations
+ if more than one reactant or product is present  “yields,” “reacts to produce,” “produce,” or “form” ( ) Heat is added (s) Reactant or product is a solid or a precipitate (l) Reactant or product is a liquid (g) Reactant or product is a gas (aq) Reactant or product is aqueous – dissolved in water

8. Writing Chemical Formulas
Chemical reactions can be written as:
Word equations:
methane + oxygen  carbon dioxide + water
Formula equations:
CH4 (g) + O2 (g)  CO2 (g) + H2O (g) (not balanced)
CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g)

9. Sample Problem #1 Word Equation:
Write the word and formula equations for the reaction that occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide (dissolved in water).
Word Equation:
Formula Equation:
Balanced Formula Equation:
Sodium oxide + water  sodium hydroxide
Na2O (s) + H2O (l)  NaOH (aq) (unbalanced)
Na2O (s) + H2O (l)  2NaOH (aq)

10. Significance of a Chemical Equation
Chemical reactions contain quantitative information:
Coefficients indicate relative amounts of reactants and products. (moles)
The relative masses of the reactants and products can be determined.

11. Significance of a Chemical Equation
The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.
3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)

12. Balancing Chemical Equations
Steps for balancing chemical equations:
Balance the different types of atoms one at a time.
Balance polyatomic ions that appear on both sides of the equation as single units.
Balance any metals next.
Balance H then O atoms after all other elements have been balanced.

13. Sample Problem #2 __Zn (s) + HCl (aq)  ZnCl2 (aq) + ___ H2 (g)
Zinc reacts with aqueous hydrochloric acid to produce a solution of zinc chloride and hydrogen gas. Write and balance the equation.
__Zn (s) + HCl (aq)  ZnCl2 (aq) + ___ H2 (g)

14. Practice
___C2H4(g) +___ H2(g)  ____ C2H5

15. Practice Balance and Translate to a chemical sentence:
___BaCl2(aq) + ___Na2(CrO4)(aq).  ___Ba(CrO4)(s) + ___NaCl(aq)

16. Practice
Hydrazine, N2H4, is used as rocket fuel. Hydrazine reacts violently with oxygen to produce gaseous nitrogen and water. Write the balanced chemical equation.

17. Practice
Solid aluminum metal reacts with aqueous zinc chloride to produce solid zinc metal and aqueous aluminum chloride. Write the balanced chemical equation.