Chemical Equations and Reactions

1. Chemical Equations and Reactions

2. What is a Chem. Rxn.? • Chemical Reaction: • Process of one or more substances converting to form new substances with different properties. • Some examples: • Iron rusting • Baking soda and vinegar mixed • A campfire • Plants converting CO2 and H2O to sugars

3. Indications of Chemical Rxns. • What are the five signs of a chemical reaction? • Change in heat or light (energy) • Production of a gas • Production of a precipitate • Precipitate – an ionic solid formed as the product of a chemical rxn. • Change in odor • Change in color

4. Traits of Chem. Rxns. • Reactants: • Substances that enter a chemical reaction. • Ex.: Vinegar (acetic acid) and baking soda (sodium bicarbonate) • Products: • Substances that are produced by a chemical reaction. • Ex.: Carbon dioxide, sodium acetate, and water • Do all combinations result in a chemical reaction? Yes or No? Example?

5. Chemical Equations (Eqns.) • Chemical Equation: • Expression using chemical symbols to represent a chemical reaction • A chemical equation represents the identities and relative amounts of reactants and products in a chemical reaction • Word Equations: • Ex. Acetic acid + Sodium Bicarbonate  Carbon Dioxide + Sodium Acetate + Water • Formula Equations:

6. Symbols for use in Chemical Equations

7. Do you remember the diatomic elements? HIFNOBrCl BrO FINClH Hockey Stick & Puck Writing Equations • Example: • Sodium metal is added to water. Hydrogen gas bubbles off, and sodium hydroxide is left behind, in solution. • Step 1: • Put text into word equation: • Sodium + water  hydrogen + sodium hydroxide • Step 2: • Translate words in word equation to formulas • Na + H2O  H2 + NaOH • Step 3: • Add symbols to tell states of reactants and products. • Na(s) + H2O(l)  H2(g) + NaOH(aq)

8. Law of Conservation of Mass • Law of Conservation of Mass: • Mass cannot be lost or gained in a chemical reaction • ALSO, the total mass of the reactants must equal total mass of the products • Therefore, the NUMBER of each type of atom on each side of the equation MUST be the same!

9. Types of Chemical Rxns. • Synthesis (Direct Combination) Reaction: • Occurs when two or more simpler substances combine to form a more complex substance. • General Form: • A + B  AB • Eqn.: • Fe(s) + O2(g)  FeO2(s)

10. Synthesis Reactions • Signs of a synthesis reaction: • Start with elements and end with a single compound • OR start with simple compounds and end with a single compound. • What is always true of a synthesis reaction? • At the end, there is a SINGLE product, a compound. • Examples: • Iron metal and oxygen gas combine to form rust • Sodium metal and chlorine gas combine to form salt

11. Decomposition Reaction • Occurs when a substance breaks down into two or more simpler substances. • Opposite of a synthesis reaction. • General Form: • AB  A + B • Eqn.: • 2H2O(l)  2H2(g) + O2(g)

12. Decomposition Reaction • Signs of a decomposition reaction • A single, complex substance breaks into the elements that make it up • OR A single, complex substance breaks into simpler substances • What is always true of a decomposition reaction? • The reactant is ALWAYS a single, complex compound. • Example • Liquid hydrogen peroxide, in the presence of light, breaks down to form water and oxygen gas.

13. Single Replacement Rxn. • Atoms of one element replace atoms of another element in a compound. • General Form: • A + BC  AC + B • Eqn.: • Cu(s) + AgNO2(aq)  CuNO2(aq) + Ag(s)

14. Single Replacement Rxn. • Signs of a Single Replacement Rxn. • One element reacts with a compound, and this produces another free element and a new compound. • What is always true of a single replacement reaction? • The reactants include an element and the products include a different element. • Example • Zinc is put into acid, and “eaten away”, producing hydrogen bubbles.

15. Double Replacement Rxn. • Atoms in two different compounds trade places with each other. • General Form: • AB + CD  AD + CB • Eqn.: • AgNO3(aq) + KCl(aq)  AgCl(↓) + KNO3(aq)

16. Double Replacement Rxn. • What are the signs of a Double Replacement Reaction? • Two compounds react and product two NEW compounds. • There is USUALLY a gas or precipitate formed • What is always true of a DR Reaction? • There are two compounds at the beginning, and two compounds at the end of the reaction. • Example • When vinegar and baking soda are combined, carbon dioxide gas bubbles off, vigorously!

17. Combustion Reaction • Any hydrocarbon (compound made of only C, H, and occasionally O) reacts with oxygen to produce carbon dioxide and water. • A large amount of energy is released as light and heat. • General Form: • CxHx + O2 CO2 + H2O • Eqn. • CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)

18. Combustion Reaction • What are the signs of a combustion reaction? • There is always light and heat, as well as the production of gases (smoke). Something is BURNING! • What is always true of combustion reactions? • They always take place in oxygen (in air), with a source of heat (like a match), and start with a hydrocarbon (like oil, paper, or anything organic) • Example • When paper burns, it produces light, heat, and gases. • Anything else flammable: ethanol, gasoline, marshmallows

19. Exothermic and Endothermic • Exothermic reaction: • A reaction that releases energy and gives off heat • An exothermic reaction may also release energy in the form of light or sound. • Endothermic reaction: • A reaction that absorbs energy resulting in lowering of temperature. • Endothermic reactions also include any reactions that need light to proceed, such as photosynthesis.