Chemistry - Arrhenius Made Easy

"At least as easy it's ever going to get"

The Collision Model

• rate of chemical reaction —> (dependent) on concentration of reactants

• order of each reactant depends on the detailed reaction mechanism

• temperature of the reaction (affects) —> the value of the rate constant

• 10C increase doubles the reaction rate

• MOLECULES MUST COLLIDE in order to react (+ COLLIDE W SUFFICIENT ENERGY + CORRECT ORIENTATION) —> WHY? SO COLLISIONS ARE EFFECTIVE

Thermodynamically reactions

• Don’t occur spontaneously

• (i.e.) combustion reactions

• Relationship between increase in temp. + increase in rate isn’t in a direct relationship

Relationship Between Energy & Speed

Activation Energy

• Must be overcome to produce a chemical reaction

Taller the Hump (on graph)

• Higher the Activation energy (Ea)

• Smaller the value of the rate constant, k, and the slower the rate

Exothermic

• Energy is lost

• Lower energy state therefore energy is lost (mainly lost in heat)

Endothermic

Energy is gained

Temperature Affects Ea (concentration)…

• temperature is defined as average kinetic energy of the molecules

• higher the temperature the higher the average kinetic energy

• the more energetic molecules, the more energetic collisions

Arrhenius Equation

y=mx+b

lnK = (-Ea/R) (1/T) + lnA

K = Rate Constant

(-Ea/R) = activation energy + gas constant

R=8.31J/K x Mole —> Because it’s measuring SPEED

(1/T) = reciprocal kelvin

• you’ll be asked to solve for K, Ea, or T

Factors That Affect Reaction

Nature of Reactants - Some reactant molecules react slowly, some react more quickly.

Concentration of Reactants - More molecules, more collisions.

Temperature - The faster the kinetic energy the more collisions will occur.

Catalysts - Accelerate chemical reactions.

Surface Area of Reactants - Exposed surfaces affect speed.

Average Rate of Change

Molarity / Time

Orders of Reaction

Zero Order

Half Order

First Order

Second Order

Third Order

Mechanism

A mechanism is a sequence of events at the molecular level that controls the speed and outcome of a reaction.

The Collision Theory of Reaction Rates

• Particles must collide

• Only 2 particles may collide at one time

• Proper orientation, so they can come in contact.

• Collision must occur with enough energy to overcome electron/electron repulsion of valence shell (+ must have enough energy to transform translational energy —> vibrational energy.. WHY? Penetrate each other so electron can rearrange and form new bonds.

• New collision product is at the peak of the activation energy hump. Known as the activated complex. —> A.C. can fall to either reactants or to product

• All these criteria met, reaction may proceed

Rate

• Isn’t constant, it changes with time

Asked to write the expression (rate of law)

• Rate=k[A]^order you determined from analyzing graphs