Unit 12: Kinetics and Equilibrium Tyna L. Meeks

Unit 12: Kinetics and Equilibrium Kinetics - Speed with which reactions occur b Rate of reactions Collision TheoryCollision Theory –molecules must collide in order to react –it’s not just the collision however, they must collide effectively –Effective collisions depend on orientation of molecules and energy of molecule at time of collision

Unit 12: Kinetics and Equilibrium Kinetics - Speed with which reactions occur b Rate of reactions Affected byAffected by –nature of reactants –concentration –surface area –catalyst –temperature

Unit 12: Kinetics and Equilibrium Kinetics - Reaction rates b Nature of the reactants involve the breaking of old bonds and creating of new bondsinvolve the breaking of old bonds and creating of new bonds covalently bonded molecules are slower to react than ionic substances due to number of molecules that require breaking - breaking more bonds requires more energy and causes reaction to go slowercovalently bonded molecules are slower to react than ionic substances due to number of molecules that require breaking - breaking more bonds requires more energy and causes reaction to go slower

Unit 12: Kinetics and Equilibrium Kinetics - Reaction rates b Concentration Reactions go faster if more particles are present to reactReactions go faster if more particles are present to react more molecules present per unit volume equals a higher concentrationmore molecules present per unit volume equals a higher concentration

Unit 12: Kinetics and Equilibrium Kinetics - Reaction rates b Surface Area Reactions go faster if more surface is available for collisionReactions go faster if more surface is available for collision finely divided powders always react faster than lumps of the same massfinely divided powders always react faster than lumps of the same mass

Unit 12: Kinetics and Equilibrium Kinetics - Reaction rates b Pressure Only important when gases are presentOnly important when gases are present Changing pressures will alter the number of collisions between gas moleculesChanging pressures will alter the number of collisions between gas molecules –increasing pressure causes more collisions –decreasing pressure causes fewer collisions

Unit 12: Kinetics and Equilibrium Kinetics - Reaction rates b Catalysts Substances that lower the energy required for an effective collision to occurSubstances that lower the energy required for an effective collision to occur These molecules are not actually part of the reaction, they enter and leave the reaction as exactly the same moleculeThese molecules are not actually part of the reaction, they enter and leave the reaction as exactly the same molecule

Unit 12: Kinetics and Equilibrium Kinetics - Reaction rates b Temperature remember this is defined as the average kinetic energy a molecule hasremember this is defined as the average kinetic energy a molecule has The more energy a substance has, the more collisions are effective collisionsThe more energy a substance has, the more collisions are effective collisions

Unit 12: Kinetics and Equilibrium Kinetics - Potential Energy Diagrams b When discussing the rate of a reaction, following the energy is important graphing the potential energy, or energy contained in the bonds, is very usefulgraphing the potential energy, or energy contained in the bonds, is very useful

Unit 12: Kinetics and Equilibrium Kinetics - Potential Energy Diagrams

Unit 12: Kinetics and Equilibrium Equilibrium b Reactions can proceed in the forward direction and in the reverse direction indicated by a double arrow ( )indicated by a double arrow ( ) if the reverse is happening just as fast as the forward, the reaction is said to be in equilibriumif the reverse is happening just as fast as the forward, the reaction is said to be in equilibrium

Unit 12: Kinetics and Equilibrium Equilibrium b Important word associations Rates are EQUALRates are EQUAL Concentrations are CONSTANTConcentrations are CONSTANT

Unit 12: Kinetics and Equilibrium Equilibrium b Unique situations always cause equilibriums: Physical equilibriumPhysical equilibrium –Phase –Solutions Chemical equilibriumChemical equilibrium –Reversible reactions

Unit 12: Kinetics and Equilibrium Equilibrium b Phase Equilibrium exists between two phases of matter during a phase changeexists between two phases of matter during a phase change Not the same amount of each phase, but their amounts stay constant and in proportionNot the same amount of each phase, but their amounts stay constant and in proportion Forward phase change occurs at same rate as reverse phase changeForward phase change occurs at same rate as reverse phase change

Unit 12: Kinetics and Equilibrium Equilibrium b Solution Equilibrium occurs in a saturated solutionoccurs in a saturated solution Not exactly the same crystals stay undissolved all the time, but the number of crystals undissolved stays constantNot exactly the same crystals stay undissolved all the time, but the number of crystals undissolved stays constant Rate of dissolving is exactly the same as the rate of crystallizationRate of dissolving is exactly the same as the rate of crystallization

Unit 12: Kinetics and Equilibrium Equilibrium b Chemical Equilibrium in the very beginning, when only reactants exist, the forward reaction is the only thing that occursin the very beginning, when only reactants exist, the forward reaction is the only thing that occurs as time goes on, and products have been created, the forward reaction slows down, and the reverse reaction begins to startas time goes on, and products have been created, the forward reaction slows down, and the reverse reaction begins to start

Unit 12: Kinetics and Equilibrium Equilibrium b Chemical Equilibrium eventually, the reverse reaction and forward reaction occur at exactly the same rateeventually, the reverse reaction and forward reaction occur at exactly the same rate If a precipitate is formed, or a gas is formed that leaves the system, equilibrium can never be reached, this reaction must occur in a closed environmentIf a precipitate is formed, or a gas is formed that leaves the system, equilibrium can never be reached, this reaction must occur in a closed environment

Unit 12: Kinetics and Equilibrium Equilibrium b Le Châtelier’s Principle once a system has reached equilibrium, any changes to the system cause stress, and the system will respond to that stressonce a system has reached equilibrium, any changes to the system cause stress, and the system will respond to that stress –change in temperature  –change in pressure  –change in concentration  –actual molecule –common ion

Unit 12: Kinetics and Equilibrium Equilibrium b Le Châtelier’s Principle CH 4 (g) + H 2 O (g) 3H 2 (g) + CO (g) + heat

Unit 12: Kinetics and Equilibrium Equilibrium b Le Châtelier’s Principle N 2 (g) + 3H 2(g) 2NH 3 (g) + heat

Unit 12: Kinetics and Equilibrium Consider the following equation: X + Y  2Z + heat The concentration of the reactants could be increased by (1) adding a catalyst (2) adding more heat to the system (3) increasing the concentration of Y (4) decreasing the concentration of X

Unit 12: Kinetics and Equilibrium Consider the following equation: Cu(s) + HCl(aq)  CuCl 2 (aq) + H 2 (g) As the concentration of the HCl(aq) decreases at constant temperature, the rate of the forward reaction (1) decreases (2) increases (3) remains the same (4) equals the rate of the reverse reaction

Unit 12: Kinetics and Equilibrium Le Chat Lab: observations CoCl 4 +2 H2OH2OH2OH2O Co(H 2 O) Cl - Shift Add H 2 O Increase [H 2 O] Add CaCl 2 Increase [Cl - ] Add AgNO 3 Decrease [Cl - ] Add acid Add base Heating Bath Increase temp Cooling Batch Decrease temp

Unit 12: Kinetics and Equilibrium Spontaneity b How do you determine if an overall reaction is truly spontaneous, not just an atom? EnthalpyEnthalpy EntropyEntropy Gibbs Free EnergyGibbs Free Energy

Unit 12: Kinetics and Equilibrium Spontaneity b Enthalpy - you want this to be negative the amount of heat or energy involved in a reactionthe amount of heat or energy involved in a reaction –if released it is NEGATIVE –EXOTHERMIC –if absorbed it is POSITIVE –ENDOTHERMIC

Unit 12: Kinetics and Equilibrium Spontaneity b Entropy - you want this to be positive the amount of chaos involved in the systemthe amount of chaos involved in the system nature wants things to become more chaotic or disorderednature wants things to become more chaotic or disordered –if something is more disordered it is POSITIVE –if something is less disordered it is NEGATIVE