CHAPTER 8 CHEMICAL EQUATIONS The Chemical Equation is a shorthand expression for a chemical change or reaction

The Arrow The divides the reactants from the products Ex. Reactant(s) Product(s)

The Plus The + sign is placed between reactants or products showing that they are separate until the reaction occurs Reactant + Reactant Product + Product

Heat The sign indicates that HEAT was added to the reaction This is usually placed over or under the arrow.

States of matter in chemical equations The physical state of the reactants and products are given by: (l) for liquid; (s) solid; (g) gas; (aq) substances in aqueous solution. You must be told this to know which state to write.*

BALANCING REACTIONS Law of Conservation of Matter

Coefficients Coefficients are placed in front of a substance to balance the equation and to indicate the number of units of each substance reacting or being produced

8.2 Writing Balanced Equations According to the Law of Conservation of Matter you must have the same number of each kind of atom before and after the reaction occurs; You can use COEFFICIENTS to achieve this you cannot change the SUBSCRIPTS 1. Write the word equation for what is happening 2. Write the charge balanced compounds; molecular formulas for each substance in the rxn 3. Write the unbalanced reaction 4. Use COEFFICIENTS to balance the reaction 5. Check to see that there are the same number of each kind of atom on the reactant and product side.

Types of Reactions Most reactions fit into some general form or type of reaction. There are 100s of types; we’ll look at 5!

Composition = Combination = Synthesis Reactions two or more reactants combine to form one product A + B  AB H 2 + Cl 2  2HCl

ZINC COMBINES WITH SULFUR (:31) Zn + S  ZnS

Decomposition Reactions One reactant decomposes to two or more substances AB  A + B 2HCl  H 2 + Cl 2

ELEPHANT TOOTHPASTE H 2 O 2  H 2 O + O 2 (:35)

Single Replacement Reactions occur when one element reacts with a compound to take the place of one elements of the compound. A + BC  AC + B (provided A is more reactive than B)

Activity Series

LINK TO SINGLE REPLACEMENT REACTION (1:55) Zn + 2HCl  ZnCl2 + H2

Double Replacement Reactions Two compounds exchange partners with each other to produce two different compounds. AB + CD  AD + CB NaCl + KOH  NaOH + KCl

DOUBLE REPLACEMENT Video Review (2:42)

Combustion Reactions Occur when a hydrocarbon (substance with the formula C x H y ) is burned in the presence of oxygen (O 2 ) to produce water (H 2 O) and carbon dioxide (CO 2 ) Example: C x H y + O 2  CO 2 + H 2 O Incomplete combustion produces CO (carbon monoxide)

DON’T TRY THIS AT HOME (1:31) Combustion and the Hindenburg

CHEMICAL REACTION TYPES A video review (1:36)

8.4 Types of Chemical Reactions Combination, Composition, or Synthesis Reaction --two or more reactants combine to form one product A + B  AB Decomposition Reaction -- One reactant decomposes to two or more substances AB  A + B Single Replacement Reactions occur when one element reacts with a compound to take the place of one elements of the compound. Usually a metal replaces another according to the activity series on p. 161 A + BC  AC + B (provided A is more reactive than B) but if not, then X + YZ  no rxn (if X is not more reactive than Y) Double Replacement Rxn. Two compounds exchange partners with each other to produce two different cds. AB + CD  AD + CB (does not rely on the activity series) Not all reactions take place here either but you are not responsible to know which will and will not.

8.5 Heat in Chemical Reactions As discussed before: some rxns produce heat (feel hot) and others require heat (feel cold) Exothermic reactions are ones that give off energy in the form of heat and the amount of energy is written as a product Example of Exothermic: H 2 + Cl 2 2HCl kJ An Endothermic reaction requires energy and feels cold. Here, heat is written as a reactant since it is required to make the reaction occur Example of Endothermic: N 2 + O kJ  2NO