Energy & Chemical Reactions

I. Kinetics – area of chemistry concerned with rates of chemical reactions

A. Reaction Rates1. The change of concentration of reactants in a

unit of time2. Rate or speed of a reaction depends upon 2 or

more molecules colliding so that reactant bonds break and new product bonds form

B. Reaction rates are affected by:1. Nature of Reactants2. Temperature

• As temp. ↑, reactant particles speed up = more collisions• Increasing the temp. 10oC doubles reaction

rate3. Surface area - More surface area allows more

contact between reactants = faster reaction rate4. Concentration - ↑ concentration of 1 or more

reactants ↑ # of collision = faster reaction rate5. Catalyts – speed up reaction rates without being

permanently changed

II. Heat of ReactionsA. Review of Terms

1. Exothermic – heat is released by reactants, or given off

2. Endothermic - heat is absorbed by reactants from surroundings

3. Enthalpy (H) – total energy content of a substance• Also called Heat of Reaction, the heat given off

or absorbed during a chemical reaction (ΔH)4. Entropy (S) – measure of randomness or disorder

in a system

B. Activation Energy1. Energy is needed to move reactants into a

temporary group of atoms = activated complex2. As an activated complex, reactant particles

rearrange to from products3. Activation Energy – amount of energy needed

to move reactants into the activated complex

C. Heat of Reaction (ΔH)1. Also known as Enthalpy, the heat given off or

absorbed by reactants during a reaction2. (ΔH) = P.E.(products) – P.E.(reactants)3. Negative (-ΔH) = exothermic reaction• Can be written as a product

4. Positive (ΔH) = endothermic reaction• Can be written as a reactant

D. Effect of a Catalyst1. Lowers amount of activation energy needed to

form an activation complex2. Only changes the potential energy of the

activation complex. Everything else remains the same!

E. Entropy & Spontaneous Reactions1. Reactions tend to move from a point of high

energy to low energy with an increase in Entropy2. Spontaneous Reactions = reactions that move

from high energy to low energy with an increase in entropy

3. Gibbs Free Energy (G) – indicates how spontaneous a reaction is• Is determined by the effects of heat, temp., and

entropy• ΔG = ΔH - T ΔS• - ΔG for a reaction = spontaneous• + ΔG for a reaction = not spontaneous

Table Summarizing the parts of Gibbs equation to predict if a reaction is spontaneous or not

F. Reversible Reactions1. Every reaction is exothermic in one direction

and endothermic in the other.

Labelling Potential Energy Diagrams