Important questions in chemistry How far? How fast?
Learning objectives ►
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Describe fundamental principles behind reaction kinetics Describe factors that affect reaction rates Define rate of reaction Describe concept of equilibrium in terms of forward and reverse reactions Write equilibrium constant expressions Predict concentrations of reactants and products using equilibrium constant expressions Describe physical basis of Le Chatelier’s principle Predict responses of systems to changes in conditions using Le Chatelier’s principle
Equilibrium ► Not
all reactions go to completion – equilibrium ► State of equilibrium is when system has reached lowest energy state Not a static event but a dynamic one Rate of formation of products = rate of formation of reactants ► Le
Chatelier’s principle predicts how systems at
equilibrium respond to changes in conditions
Underlying principles behind reactions: echoes of kinetic theory ► Molecules
are in motion ► Molecules undergo collisions ► Only some collisions result in products ► Temperature increases motion of molecules
Reaction pathway in energy ► In
order for a reaction to proceed reacting molecules must leap over energy barrier; molecules with insufficient energy don’t make it
Bond breaking
Bond making Returns 100 kJ
Requires 100 kJ
Kinetics deals with things in the box Reactants at equilibrium
Products at equilibrium Province of kinetics
Exo-thermic and endo-thermic ► H2
+ O2 gives out energy – exothermic
► N2
+ O2 absorbs energy - endothermic
The pathway of a reaction
Chance meetings ► On
the molecular level reactions occur via collisions
Big But: Not all collisions result in reaction ►Two
considerations
► Energy
of the molecules
Overcoming the barrier ► Orientation
of the molecules
Getting them lined up Very significant for larger molecules
Insufficient energy
Wrong orientation
Factors that affect reaction rate ► Concentration
► Temperature ► Presence ► Physical
of REACTANTS
(kinetic energy of molecules)
of a catalyst
state of reactants (surface area)
Higher concentration = more collisions
Ways over (or around) the barrier ► Temperature
increases reaction rate by increasing fraction of molecules with enough energy to jump barrier
►A
catalyst is a way to lower the barrier. A catalyst increases the reaction rate, but is not consumed itself during the reaction
Catalysts modify the pathway ► Addition
of chlorine catalyst increases rate of decomposition of O3 to O2 – reason for ozone hole (CFCs) ► Pathway is modified: two barriers smaller than one without catalyst
Clean air and catalysis ► The
metal surface catalyzes oxidation of unwanted exhaust to CO2, H2O and N2
Without catalysts, there would be no life at all, from microbes to humans ► ENZYMES
are biological catalysts ► Most enzymes are proteins – large molecules ► Have correct shape to bring reactant molecules together Enzyme in correct orientation
Equilibrium: a rate of reaction perspective ► Forward
reaction A+B→C+D
► Backward
reaction A+B←C+D
► Equilibrium
results: Rate of forward reaction = rate of backward reaction A+B↔C+D
Equilibrium constant expression aA + bB ↔ cC + dD Coefficient
c
K eq Reactants
d
[C ] [ D ] a b [ A] [ B ]
Always Products over Reactants
Products
Not all products and reactants are included ► Ignore
all pure solids and liquids – they do not change concentrations during reactions ► Consider MnO2(s) + 4HCl(aq) = MnCl2(aq) + Cl2(g) + 2H2O(l)
Not all products and reactants are included ► Ignore
all pure solids and liquids – they do not change concentrations during reactions MnO2(s) + 4HCl(aq) = MnCl2(aq) + Cl2(g) + 2H2O(l)
K eq
[ MnCl2 ][Cl2 ] 4 [ HCl ]
Significance of large Keq ► The
finishing point is the same coming from either direction
Significance of small Keq
Calculations – putting numbers in ► Consider
the reaction 2HI(g) ↔ H2(g) + I2(g)
What is the value of Keq if [HI] = 0.54 M, [H2] = [I2] = 1.72 M?
K eq K eq
[ H 2 ][ I 2 ] 2 [ HI ]
[1.72 ][1.72 ] 10 .2 2 [0.54 ]
Solving problems with K ► Equilibrium
constant for reaction: N2O4(g) = 2NO2(g) is 4.6 x 10-3 ► If [NO2] = 0.050 M, what is [N2O4]?
Solving problems with K ► Equilibrium
constant for reaction: 2NOBr(g) = 2NO(g) + Br2(g) is 2.0
►
If [NO] = 2.0 M and [Br2] = 1.0 M, what is [NOBr]?
Upsetting the applecart ► What
happens to the equilibrium when changes are made? ► Le Chatelier’s Principle If a stress is placed on a system at equilibrium, the system will respond by changing its position to minimize the stress
Changes in composition ► Consider
the reaction at equilibrium 2HI(g) ↔ H2(g) + I2(g)
► What
happens if additional H2(g) is added?
The system responds by trying to reduce the amount of added material; H2 is converted into HI – the equilibrium shifts away from the point of change 2HI(g) ↔ H2(g) + I2(g)
In general: Add products: products → reactants aA + bB ↔ cC + dD Add reactants: reactants → products aA + bB ↔ cC + dD ► Other
effects;
Temperature Pressure
Specific example
Temperature and equilibrium N2(g) + 3H2(g) = 2NH3(g) + heat Exothermic Reaction: Supply heat: equilibrium adjusts to disperse heat: shifts towards reactants ►Less
NH3 is made
Endothermic reactions show opposite response Heat shifts towards products ►Why
