Presentation on theme: "Reaction Rates and Chemical Equilibrium"— Presentation transcript:
1 Reaction Rates and Chemical Equilibrium Chapter 7Reaction Rates and Chemical Equilibrium
2 Chemical Kinetics Thermodynamics – does a reaction take place? Kinetics – how fast does a reaction proceed?Reaction rate is the change in the concentration of a reactant or a product with time (M/s).A Brate = -D[A]DtD[A] = change in concentration of A overtime period Dtrate =D[B]DtD[B] = change in concentration of B overtime period DtBecause [A] decreases with time,D[A] is negative.
3 A BEvery reaction has its own raterate = -D[A]Dtrate =D[B]Dt
4 Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) red-brownBr2 (aq) + HCOOH (aq) Br- (aq) + 2H+ (aq) + CO2 (g)time393 nmlightDetectort1< t2 < t3D[Br2] a D Absorption
5 Collision TheoryCollision theory: In order for a reaction to take place, the following conditions must be met:1. Two molecules must collide2. The molecules must have the proper orientation during collision3. The molecules must collide with enough energyEffective Collision: a collision that results in chemical reaction
6 Importance of Molecular Orientation effective collisionineffective collision
7 A + B AB C + D+Exothermic ReactionEndothermic ReactionThe activation energy (Ea ) is the minimum amount of energy required to initiate a chemical reaction.
8 Transition State (AB )++One or more original bonds are partially broken and one or more new bonds may be in the process of formation
9 Rate of Reaction The rate of reaction depends on several conditions: 1. Nature of Reactants2. Concentrations3. Temperature3. Catalysts-speeds up a reaction by providing an alternate pathway that has a lower activation energy
10 Chemical ReactionA reversible reaction is one which moves in both the forward and backward direction.Both forward and backward reactions have different initial rates.Eventually the forward reaction and backward reactions have the same rate. When this happens, there is no more change in concentration. This is called equilibrium.
11 Chemical equilibrium is achieved when: Equilibrium is a state in which there are no observable changes as time goes by.Chemical equilibrium is achieved when:the rates of the forward and reverse reactions are equal andthe concentrations of the reactants and products remain constantPhysical equilibriumH2O (l)H2O (g)NO2Chemical equilibriumN2O4 (g)2NO2 (g)
13 Equilibrium Expressions aA + bB cC + dDK =[C]c[D]d[A]a[B]bK = Equilibrium ConstantN2O4 (g) NO2 (g)K =[NO2]2[N2O4]= 4.63 x 10-3Problems 7.3, 7.4, 7.5 pages 224 and 225
14 Equilibrium Will K = [C]c[D]d [A]a[B]b aA + bB cC + dD K >> 1 Lie to the rightFavor productsK << 1Lie to the leftFavor reactants
15 Le Châtelier’s Principle LeChatelier’s Principle: When a stress is placed on a system in equilibrium, the system responds to minimize the stress.What’s a stress?Changes in ConcentrationChange in PressureChange in TemperatureUse of a catalyst
16 Le Châtelier’s Principle Changes in Concentration continuedRemoveAddAddRemoveaA + bB cC + dDN2 (g) + 3H2 (g) NH3 (g)ChangeShifts the EquilibriumIncrease concentration of product(s)Left (Reactants)Decrease concentration of product(s)Right (Products)Increase concentration of reactant(s)Right (Products)Decrease concentration of reactant(s)Left (Reactants)
17 Le Châtelier’s Principle Changes in Volume and PressureA (g) + B (g) C (g)ChangeShifts the EquilibriumIncrease pressureSide with fewest moles of gasDecrease pressureSide with most moles of gasIncrease volumeSide with most moles of gasDecrease volumeSide with fewest moles of gas
18 Le Châtelier’s Principle Changes in TemperatureChangeExothermic RxEndothermic RxIncrease temperatureK decreasesK increasesDecrease temperatureK increasesK decreasesN2O4 (g) NO2 (g)colderhotter
19 Le Châtelier’s Principle Adding a Catalystdoes not change Kdoes not shift the position of an equilibrium systemsystem will reach equilibrium soonerCatalyst lowers Ea for both forward and reverse reactions.Catalyst does not change equilibrium constant or shift equilibrium.
20 Le Châtelier’s Principle - Summary ChangeShift EquilibriumChange EquilibriumConstantConcentrationyesnoPressureyes*noVolumeyes*noTemperatureyesyesCatalystnono*Dependent on relative moles of gaseous reactants and products