Energy & Chemical
Reactions
I. Kinetics – area of chemistry concerned
with rates of chemical reactions
A. Reaction Rates
1. The change of concentration of reactants in a
unit of time
2. 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 Reactants
2. Temperature
• As temp. ↑, reactant particles speed up =
more collisions
• Increasing the temp. 10oC doubles reaction
rate
3. Surface area - More surface area allows more
contact between reactants = faster reaction rate
4. Concentration - ↑ concentration of 1 or more
reactants ↑ # of collision = faster reaction rate
5. Catalyts – speed up reaction rates without being
permanently changed
II. Heat of Reactions
A. 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 Energy
1. Energy is needed to move reactants into a
temporary group of atoms = activated complex
2. As an activated complex, reactant particles
rearrange to from products
3. 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 reaction
2. (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 Catalyst
1. Lowers amount of activation energy needed to
form an activation complex
2. Only changes the potential energy of the
activation complex. Everything else remains
the same!
E. Entropy & Spontaneous Reactions
1. Reactions tend to move from a point of high
energy to low energy with an increase in Entropy
2. 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 Reactions
1. Every reaction is exothermic in one direction
and endothermic in the other.
Labelling Potential Energy Diagrams