Chemical Rxn Rates

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

Chemical KineticsThe 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 fastermore collisionsfaster rxn rate

A 10°C increase in Tempdouble the rate of rxn

Temp Collisions Rxn rate

Concentration

Increasing concentration increases chance of effective collisions faster rxn rate

Conc Rxn Rate

Catalysts

Provides a shorter pathway for the reactants to follow as they change into products (like a tunnel cut through a mountain)

Lowers Activation EnergySpeeds up rxn rate without being consumed in

the rxn

+ Catalysts Activation E Rxn Rate

ENZYMES

Activation Energy (Ea)

depends on reactantsminimum energy required for a reaction to

occurlow Ea = fast rxn rate

Ea

Endothermic Reaction witha Catalyst

Exothermic Reaction with a Catalyst

high SA = fast rxn ratemore opportunities for collisionsIncrease surface area by…

using smaller particlesdissolving in water

SA Collisions Rxn Rate

Surface Area

Chemical Equilibrium

Reversible Reactions:

A chemical reaction in which the products

can react to re-form the reactantsChemical 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) + O2(g)

Arrows going both directions ( ) indicates equilibrium in a chemical equation

LeChatelier’s PrincipleWhen 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

N2 + 3H2 2NH3

What is the effect of increasing the amount of ammonia? (which way would the equilibrium shift?)

Equilibrium shift

Consider the following reaction

H2O2 H2O + O2

What is the effect of decreasing the amount of H2O2 ? (which way would the equilibrium shift?)

An increase in temp. speeds the rxn rate because theA) distance between molecules increasesB) distance between molecules decreasesC) # of collisions increasesD) # of collisions decreases

As the # of collisions between reactants increases, the

A) Reaction stopsB) rate of rxn decreasesC) rate of rxn increasesD) rate of rxn remains the same