Reaction Rate How Fast Does the Reaction Go

Collision Theory l In order to react molecules and atoms must touch each other. l They must hit each other hard enough to react. l Anything that increase these things will make the reaction faster.

Energy Reaction coordinate Reactants Products

Energy Reaction coordinate Reactants Products Activation Energy - Minimum energy to make the reaction happen

Energy Reaction coordinate Reactants Products Activated Complex or Transition State

Energy Reaction coordinate Reactants Products Overall energy change

Things that Effect Rate l Temperature l Higher temperature faster particles. l More and harder collisions. l More concentrated closer together the molecules. l Collide more often

Things that Effect Rate l Particle size l Molecules can only collide at the surface. l Smaller particles bigger surface area. l Smallest possible is molecules or ions. l Dissolving speeds up reactions.

Things that Effect Rate l Catalysts- substances that speed up a reaction without being used up.(enzyme). l Speeds up reaction by giving the reaction a new path. l The new path has a lower activation energy. l More molecules have this energy. l The reaction goes faster. l Inhibitor- a substance that blocks a catalyst.

Energy Reaction coordinate Reactants Products

Pt surface HHHH HHHH l Hydrogen bonds to surface of metal. l Break H-H bonds Catalysts

Pt surface HHHH Catalysts C HH C HH

Pt surface HHHH Catalysts C HH C HH l The double bond breaks and bonds to the catalyst.

Pt surface HHHH Catalysts C HH C HH l The hydrogen atoms bond with the carbon

Pt surface H Catalysts C HH C HH HHH

Reaction Mechanism l Reaction mechanism is a description of how the reaction really happens. l An intermediate is a product that immediately gets used in the next reaction.

+ This reaction takes place in three steps

+ EaEa First step is fast Low activation energy

Second step is slow High activation energy + EaEa

Second step is rate determining

Energy l Substances tend react to achieve the lowest energy state. l Most chemical reactions are exothermic. l Doesn’t work for things like ice melting. l An ice cube must absorb heat to melt, but it melts anyway. Why?

Entropy l The degree of randomness or disorder. l The first law of thermodynamics. The energy of the universe is constant. l The second law of thermodynamics. The entropy of the universe increases in any change. l Drop a box of marbles. l Watch your room for a week.

Entropy Entropy of a solid Entropy of a liquid Entropy of a gas l A solid has an orderly arrangement. l A liquid has the molecules next to each other. l A gas has molecules moving all over the place.

Entropy increases when... l Reactions of solids produce gases or liquids, or liquids produce gases. l A substance is divided into parts -so reactions with more reactants than products have an increase in entropy. l the temperature is raised -because the random motion of the molecules is increased. l a substance is dissolved.

Spontaneous reaction l Reactions that will happen. l Nonspontaneous reactions don’t. l Two factors influence. l Enthalpy (heat) and entropy(disorder).

Two Factors l Exothermic reactions tend to be spontaneous. Reactions where the entropy of the products is greater than reactants tend to be spontaneous..

Equilibrium l Eventually you reach a point where the reverse reaction is going as fast as the forward reaction. l This is dynamic equilibrium. l The rate of the forward reaction is equal to the rate of the reverse reaction. l The concentration of products and reactants stays the same, but the reactions are still running.

Equilibrium l Equilibrium position- how much product and reactant there are at equilibrium. l Shown with the double arrow. l Reactants are favored l Products are favored l Catalysts speed up both the forward and reverse reactions so don’t affect equilibrium position.

Measuring equilibrium l At equilibrium the concentrations of products and reactants are constant. l We can write a constant that will tell us where the equilibrium position is. l K eq equilibrium constant l K eq = [Products] coefficients [Reactants] coefficients l Square brackets [ ] means concentration in molarity (moles/liter)

Writing Equilibrium Expressions l General equation aA + bB cC + dD l K eq = [C] c [D] d [A] a [B] b l Write the equilibrium expressions for the following reactions. l 3H 2 (g) + N 2 (g) 2NH 3 (g) l 2H 2 O(g) 2H 2 (g) + O 2 (g)

What it tells us l If K eq > 1 Products are favored l If K eq < 1 Reactants are favored

LeChâtelier’s Principle l If something is changed in a system at equilibrium, the system will respond to relieve the stress. l Three types of stress are applied.

Changing Concentration l If you add reactants (or increase their concentration). l The forward reaction will speed up. l More product will form. l Equilibrium “Shifts to the right” Reactants  products

Changing Concentration l If you add products (or increase their concentration). l The reverse reaction will speed up. l More reactant will form. l Equilibrium “Shifts to the left” Reactants  products

Changing Concentration l If you remove products (or decrease their concentration). l The forward reaction will speed up. l More product will form. l Equilibrium “Shifts to the right” Reactants  products

Changing Concentration l If you remove reactants (or decrease their concentration). l The reverse reaction will speed up. l More reactant will form. l Equilibrium “Shifts to the left”. Reactants  products l Used to control how much yield you get from a chemical reaction.

Changing Temperature l Reactions either require or release heat. l Endothermic reactions go faster at higher temperature. l Exothermic go faster at lower temperatures. l All reversible reactions will be exothermic one way and endothermic the other.

Changing Temperature l As you raise the temperature the reaction proceeds in the endothermic direction. l As you lower the temperature the reaction proceeds in the exothermic direction. Reactants + heat  Products at high T Reactants + heat  Products at low T

Changes in Pressure l As the pressure increases the reaction will shift in the direction of the least gases. At high pressure 2H 2 (g) + O 2 (g)  2 H 2 O(g) At low pressure 2H 2 (g) + O 2 (g)  2 H 2 O(g)