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”.
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
In an Exothermic reaction, energy is released, products have a lower P.E. than the reactants, and the sign of Δ H is negative. Exothermic graph. Δ H = 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 Δ H is positive. Endothermic graph. Δ H = (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: 2SO 2( g ) + O 2( g ) → 2SO 3( g ) Reverse reaction: 2SO 2( g ) + O 2( g ) ← 2SO 3( g )
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
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.
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 Equilibrium only write down if you’re taking AP next year:
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 ( - Δ H ) – an increase in Entropy ( + Δ S ) 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: Δ G = Δ H - T Δ S Δ G = Free energy change Δ H = Total Heat T = Temp in Kelvin Δ S = Entropy
According to Gibb Δ G = Δ H - T Δ S If Δ G = negative value, then reaction is spontaneous If Δ G = positive value, then reaction is Non-spontaneous Zero Δ G means reactions are at equilibrium.