Chemical Rxn Rates

Chemical Kinetics The area of chemistry that concerns reaction rates. However, only a small fraction of collisions produces a reaction. Why? Key Idea: Molecules must collide to react.

Collision Model Orientation of reactants must allow formation of new bonds. Collisions must have enough energy to produce the reaction (must equal or exceed the activation energy).

Reaction Rate Speed at which a chemical reaction takes place Reaction rate depends on the collisions between reacting particles. Determined by measuring the change in concentration of a reactant or product per unit of time

Factors Affecting Reaction Rates Temperature: Measure of average kinetic energy of the molecules in a substance Concentration: amount of molecules present in a unit volume Catalyst: an agent that speeds up the rate of chemical rxns w/out being permanently changed or used up Surface area: area of the surface

Temperature Molecules at higher temp. have higher KE and move faster  more collisions  faster rxn rate A 10°C increase in Temp  double the rate of rxn Temp Collisions Rxn rate

high SA = fast rxn rate  more opportunities for collisions  Increase surface area by… using smaller particles dissolving in water SA Collisions Rxn Rate Surface Area

Concentration Increasing concentration  increases chance of effective collisions  faster rxn rate ConcCollisionsRxn Rate

Heat of Reaction The amount of heat released or absorbed during a chemical reaction. Endothermic: Exothermic: Reactions in which energy is absorbed as the reaction proceeds. Reactions in which energy is released as the reaction proceeds.

Endothermic Reactions

Exothermic Reactions

Catalysts Provides a shorter pathway for the reactants to follow as they change into products (like a tunnel cut through a mountain) Lowers Activation Energy Speeds up rxn rate without being consumed in the rxn + CatalystsActivation E Rxn Rate ENZYMES

Activation Energy (E a ) depends on reactants mminimum energy required for a reaction to occur llow E a = fast rxn rate EaEa

Endothermic Reaction with a Catalyst

Exothermic Reaction with a Catalyst

Chemical Equilibrium Reversible Reactions: A chemical reaction in which the products can react to re-form the reactants Chemical Equilibrium: When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and reactants remains unchanged 2HgO(s)  2Hg(l) + O 2 (g) Arrows going both directions (  ) indicates equilibrium in a chemical equation

LeChatelier’s Principle When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress.

When you take something away from a system at equilibrium, the system shifts in such a way as to replace what you’ve taken away. Le Chatelier Translated: When you add something to a system at equilibrium, the system shifts in such a way as to use up what you’ve added.

Equilibrium Shift Consider the following reaction N 2 + 3H 2  2NH 3 What is the effect of increasing the amount of ammonia? (which way would the equilibrium shift?)

Equilibrium shift Consider the following reaction H 2 O 2  H 2 O + O 2 What is the effect of decreasing the amount of H 2 O 2 ? (which way would the equilibrium shift?)

An increase in temp. speeds the rxn rate because the A) distance between molecules increases B) distance between molecules decreases C) # of collisions increases D) # of collisions decreases As the # of collisions between reactants increases, the A) Reaction stops B) rate of rxn decreases C) rate of rxn increases D) rate of rxn remains the same