Unit 6 – Chemical Reactions and Equations Evidence of a Chemical Reaction Chemical Equations Balancing Chemical Equations 1

Unit 6 – Chemical Reactions and Equations Upon completion of this unit, you should be able to do the following: 1. Define a chemical reaction and describe the evidence that indicates a chemical change has occurred. 2. List the various parts of a chemical equation including the symbols representing the physical state of the substances in the reaction. 3. Balance equations by inspection, given the names or formulas for reactants and products. 2

Unit 6 – Chemical Reactions Evidence of a Chemical Reaction pgs Color change Formation of a precipitate (solid) Formation of a gas (bubbles) Heat is produced (exothermic) OR Heat is absorbed (endothermic) 3

Unit 6 – Chemical Reactions pgs When the methane in natural gas combines with the oxygen in air and burns, carbon dioxide and water vapor are formed. This chemical change is called a chemical reaction. We can represent the reaction by writing a chemical equation where the reactants are on the left and the products are on the right. 4

Chemical Equations The chemist’s method of summarizing all information about a chemical reaction. General form Reactants → ProductsExample 2H 2 + O 2 → 2H 2 O The coefficients in front of each formula tell how many molecules or units are involved. 5

Unit 6 – Chemical Equations methane + oxygen carbon dioxide + water CH 4 + O 2 CO 2 + H 2 O reactantsproducts 6

Unit 6 – Balancing Chemical Equations pages In a chemical reaction, atoms are neither created nor destroyed. There must be the same number of atoms on the reactant side of the equation as there are on the product side of the equation. This process is called balancing the equation. 7

Unit 6 – Chemical Equations CH 4 + O 2 CO 2 + H 2 O CH O 2 CO H 2 O ReactantsProducts 1C 4H2H 2O3O ReactantsProducts 1C 4H4H4H 4O4O4O4O Unbalanced Balanced 8

Unit 6 – Chemical Equations Combustion of ethanol C 2 H 5 OH (l) + O 2 (g) CO 2 (g) + H 2 O (g) Physical states g – gas l – liquid s – solid aq - aqueous 9

Unit 6 – Chemical Equations Combustion of ethanol C 2 H 5 OH (l) + O 2 (g) CO 2 (g) + H 2 O (g) ReactantsProducts 2C1C 6H2H 3O ReactantsProducts 2C 6H 7O Unbalanced Balanced

Unit 6 – Chemical Equations Solid potassium reacts with liquid water to form gaseous hydrogen and potassium hydroxide that is soluble in water. Start by writing an unbalanced equation. 11

Unit 6 – Chemical Equations Solid potassium reacts with liquid water to form gaseous hydrogen and potassium hydroxide that is soluble in water. Start by writing an unbalanced equation. K (s) + H 2 O (l) H 2 (g) + KOH (aq) Note that H is unbalanced, 2 on reactant side, 3 on product side. ReactantsProducts 1K 2H3H 1O 12

Unit 6 – Chemical Equations 2K (s) + 2H 2 O (l) H 2 (g) + 2KOH (aq) Add coefficient of 2 to KOH and 2 to H 2 O to balance O. Add coefficient of 2 to K to balance K. ReactantsProducts 2K 4H 2O 13

Unit 6 – Chemical Equations Page 163 – example 6.3 Under appropriate conditions at 1000 o C, ammonia (NH 3 ) gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water. Write the unbalanced and balanced equations for this reaction. 14

Unit 6 – Chemical Equations Pages 167 and 168; Problems 16, 20, 24, 30, 38 and 40 15