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. 10 o C 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