Chapter 8

Chemical Reactions

8-1 Nature of Chem Rxn’s

*form a new substance

-Characteristics of chem rxn’s

*reactants: substance that enters into a chem rxn

*products: substance being produced by the rxn

-Characteristics of chem rxn’s

-reactants yields products

-involve a change in nrg as well

-nrg is either absorbed or released

-Capacity to React

-determined by the arrangement of valence e-’s

8-2 Chemical Equations

-involves the rearrangements of atoms

*expression in which symbols and formulas are used to represent a chem rxn

-reactants yields products

Conservation of mass

-atoms can be neither created nor destroyed during a chem rxn

-Conservation of mass

-if the # of atoms remains the same, then the mass must remain the same

*Law of conservation of mass: mass remains constant in a chem rxn

-Balancing Chem

*# of atoms of each element is the same on both sides of the equation

*Coefficients:

#’s that indicate how many atoms or molecules of each substance

-Balancing Chem

Ex:

-remember to balance a chem equation you can change coeff but never symbols or formulas

-Balancing Chem

-steps:

1. Write the chem equat(look for diatomics)

-Balancing Chem

2. Write down the oxidation #’s

3. Make oxidation #’s = 0

4. Balance left and right side

8-3 Types of Chem Rxn’s

-synthesis, decomposition, single replacement, and double replacement…. and Combustion

-Synthesis Rxn

*two simple substances form a more complex substance

EX: corrosion of metals

-Decomposition Rxn

*complex substance breaks down into two or more simpler substances

Ex

-Single-replacement rxn’s

*an uncombined element replaces an element that is part of a cmpnd

EX:

-Double-Replacement Rxn

*different atoms in two different cmpnds replace eachother

-2 cmpnds react to form 2

new cmpnds

EX:

Combustion

_____ + O2 ----> CO2 + H2O

8-4 Energy of Chem Rxn’s

-chem rxn = change in nrg

-nrg is never created or destroyed in chem rxn’s

-Two types of chem rxn’s

1. Exothermic Rxn

*nrg is released

-Ex: combustion

-nrg of product is less than the nrg of the reactant

- Equation: R -----> P + nrg

2. Endothermic Rxn’s

*nrg is absorbed

-nrg of the products is more than the nrg of the reactants

-Equation: R + nrg ----> P

-Activation Energy

*nrg needed to start a rxn

-form short-lived, high nrg, extremely unstable molecule

-all chem rxn’s require activation nrg

-nrg diagrams:

8-5 Rates of Chem Rxn

*Kinetics:

study of rxn rates

*Rxn rates:

how quickly reactants turn into products

-Collision Theory

*relates molecular coll to rxn rate

-reacting molecules must collide with sufficient nrg

-Rates of rxn are determined by the following:

1. Concentration

2. Surface area

3. Temp

4. Catalysts

1.Concentration

*measure of the amount of that subst in a given unit of vol.

-high conc. = many particles per vol

-usually if you increase the conc of reactants you increase the rate of the rxn

2. Surface Area

-increase in surface area increases the collision between reacting molecules

3. Temperature

-increase in temp generally increases the rate of rxn

3. Temperature

*Temp: measure of the nrg of their motion

-at room temp, the rates of many chem rxn’s 2bl or 3pl with a rise in temp of 10 degrees C

4. Catalysts

*substance that increases the rate of rxn but is not itself changed by the rxn

-reduces the activation nrg required to start a chem rxn