Energy & Chemical Reactions

Energy & Chemical Reactions

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 (H)H) 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 (H)H) 1. Also known as Enthalpy, the heat given off or absorbed by reactants during a reaction 2. (H)H) = P.E.(products) P.E.(reactants) 3. Negative (-H)H) = exothermic reaction Can be written as a product

4. Positive (H)H) = 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 H)G = H)H - T H)S - H)G for a reaction = spontaneous + H)G 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

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