Rates of Reaction & Equilibrium

Part 1:Rates of Reaction

Collision Theory

• Atoms/molecules must collide in order to react.

• Increasing the rate of collision will increase how fast a reaction takes place

Effective collisions depend upon:

• Nature of Reactants • Concentration • Temperature • Surface Area • Catalysts

Activation Energy

• The energy put in to a reaction to get it started.

• Small activation energies can be as simple as heat from the classroom, or the spark from a striker.

• Large activation energies can be a lot of heat (like baking a cake).

• Creates an “activated complex”.

Activation Energy is B

Activated Complex

Catalysts

• lower the activation energy, thus change the rate of rxn.

• change the mechanism of a rxn, involving less activation energy

• do not change the overall process • Do not get used up in the process. • Do not start a chemical rxn • ex. Enzymes

Catalysts lower activation energy

Inhibitor

• a substance that interferes with the action of a catalyst.

• reduces the amount of catalyst available and therefore lowers the reaction rate.

Enthalpy

• Heat of reaction• The difference in potential energy

(PE) between products and reactants represents Enthalpy

• ΔH = Hproducts - Hreactants

• table I

• In an Exothermic reaction, energy is released, products have a lower P.E. than the reactants, and the sign of ΔH is negative.

Exothermic graph. ΔH = C = amount of heat given off during rxn

• In an Endothermic reaction, energy is absorbed, products have a higher P.E. than the reactants, and the sign of ΔH is positive.

Endothermic graph. ΔH = (3) = amount of heat absorbed by rxn

Speeding up reactions:

• Most reactions, especially Endothermic reactions, will go faster with higher temperatures

• Exothermic reactions will be inhibited by very high temperatures

• Increasing surface area will increase rate of reaction

• Increasing concentrations will increase rate of reaction

Reversible Reactions

• A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

• Example: • Forward reaction: 2SO2(g) + O2(g) →

2SO3(g)

• Reverse reaction: 2SO2(g) + O2(g) ← 2SO3(g)

Part 2:Equilibrium

Chemical Equilibrium

• When the rates of the forward and reverse reactions are equal, the reaction has reached a state of balance

• no net change occurs in the actual amounts of the components of the system.

• (Concentrations of Products and Reactants do not change)

Types of equilibrium

• Solution Equilibrium– dissolving is occurring at the same rate

at precipitation

• Phase Equilibrium– Vaporization occurring at the same rate

as condensation

• Reaction Equilibrium– Products are forming equal to the rate of

the reverse reaction re-forming reactants

Equilibrium Graphs…

adding catalyst to a system that was already at equilibrium?

• A catalyst will bring a system to equilibrium sooner

• The rates of the forward and reverse reactions would increase but the overall net reactions would not change.

Equilibrium Continued

Le Châtelier’s Principle Concentration

• If additional reactants (or products) are added to a reaction system at equilibrium, the eq point (point of equilibrium) will shift favoring the reaction that would relieve the stress.

Le Châtelier’s Principle Temperature

• If additional heat were added to a reaction system at equilibrium (raise the temperature of the system), the eq point will shift favoring the endothermic reaction to relieve the stress.

• An increase in temperature favors all reactions, but endothermic reactions benefit more.

Le Châtelier’s Principle Pressure

• Changing the pressure of a system only affects reactions that have components in the gaseous phase.

• If additional pressure were added to a system at equilibrium, the eq point will shift favoring the reaction that makes less gas molecules to relieve the stress.

The Haber Process• http://mail.kenton.k12.ny.us/~Bob_Ventola/chemistry/habermovie.swf

Law Of Chemical Equilibriumonly write down if you’re taking AP next year:

• ►When a reversible reaction reaches equilibrium at a given temp. the following mathematical relationship occurs

• Ex. aA + bB cC + dD • ►lower case = coefficient • ►upper case = formula • § Keq = [products] = [C]c[D]d

[reactants] [A]a[B]b

Spontaneous Reactions

• occur naturally and favors the formation of products at the specified conditions.

• produce substantial amounts of products at equilibrium and release free energy.

• fireworks

Entropy

• is a measure of the disorder of a system. (randomness) (S)

• §Recall that heat (Enthalpy) changes accompany most chemical and physical processes.

“E” words

• Enthalpy is a measure of heat energy + value = endothermic

- value = exothermic • Entropy is a measure of the disorder,

randomness, or lack of organization of a system. – ex. solid (less random) - liquid - gas

(more random) – High temp. = High entropy

Forces of the Universe

• Systems move naturally toward – a decrease in Enthalpy ( - ΔH) – an increase in Entropy ( + ΔS)

• The universe naturally makes things go to lower energy and more disorder.

Enthalpy, Entropy and Free Energy

• every chemical reaction, heat is either released or absorbed and entropy either increases or decreases.

• size and direction of enthalpy changes and entropy changes together determine whether a reaction is spontaneous

Gibb’s Free Energy Change

• The difference between energy change (DH ) and entropy change (DS ) was studied by Willard Gibb

• Gibb formula: ΔG = ΔH - TΔS • ΔG = Free energy change • ΔH = Total Heat • T = Temp in Kelvin • ΔS = Entropy

According to Gibb

• ΔG = ΔH - TΔS • If ΔG = negative value, then reaction

is spontaneous • If ΔG = positive value, then reaction

is Non-spontaneous • Zero ΔG means reactions are at

equilibrium.