Reaction Rates and Chemical Equilibrium Chapter 7 Reaction Rates and Chemical Equilibrium

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 B rate = - D[A] Dt D[A] = change in concentration of A over time period Dt rate = D[B] Dt D[B] = change in concentration of B over time period Dt Because [A] decreases with time, D[A] is negative.

A B Every reaction has its own rate rate = - D[A] Dt rate = D[B] Dt

Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) red-brown Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) time 393 nm light Detector t1< t2 < t3 D[Br2] a D Absorption

Collision Theory Collision theory: In order for a reaction to take place, the following conditions must be met: 1. Two molecules must collide 2. The molecules must have the proper orientation during collision 3. The molecules must collide with enough energy Effective Collision: a collision that results in chemical reaction

Importance of Molecular Orientation effective collision ineffective collision

A + B AB C + D + Exothermic Reaction Endothermic Reaction The activation energy (Ea ) is the minimum amount of energy required to initiate a chemical reaction.

Transition State (AB ) + + One or more original bonds are partially broken and one or more new bonds may be in the process of formation

Rate of Reaction The rate of reaction depends on several conditions: 1. Nature of Reactants 2. Concentrations 3. Temperature 3. Catalysts-speeds up a reaction by providing an alternate pathway that has a lower activation energy

Chemical Reaction A 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.

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 and the concentrations of the reactants and products remain constant Physical equilibrium H2O (l) H2O (g) NO2 Chemical equilibrium N2O4 (g) 2NO2 (g)

N2O4 (g) 2NO2 (g) equilibrium Start with NO2

Equilibrium Expressions aA + bB cC + dD K = [C]c[D]d [A]a[B]b K = Equilibrium Constant N2O4 (g) 2NO2 (g) K = [NO2]2 [N2O4] = 4.63 x 10-3 Problems 7.3, 7.4, 7.5 pages 224 and 225

Equilibrium Will K = [C]c[D]d [A]a[B]b aA + bB cC + dD K >> 1 Lie to the right Favor products K << 1 Lie to the left Favor reactants

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 Concentration Change in Pressure Change in Temperature Use of a catalyst

Le Châtelier’s Principle Changes in Concentration continued Remove Add Add Remove aA + bB cC + dD N2 (g) + 3H2 (g) 2NH3 (g) Change Shifts the Equilibrium Increase 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)

Le Châtelier’s Principle Changes in Volume and Pressure A (g) + B (g) C (g) Change Shifts the Equilibrium Increase pressure Side with fewest moles of gas Decrease pressure Side with most moles of gas Increase volume Side with most moles of gas Decrease volume Side with fewest moles of gas

Le Châtelier’s Principle Changes in Temperature Change Exothermic Rx Endothermic Rx Increase temperature K decreases K increases Decrease temperature K increases K decreases N2O4 (g) 2NO2 (g) colder hotter

Le Châtelier’s Principle Adding a Catalyst does not change K does not shift the position of an equilibrium system system will reach equilibrium sooner Catalyst lowers Ea for both forward and reverse reactions. Catalyst does not change equilibrium constant or shift equilibrium.

Le Châtelier’s Principle - Summary Change Shift Equilibrium Change Equilibrium Constant Concentration yes no Pressure yes* no Volume yes* no Temperature yes yes Catalyst no no *Dependent on relative moles of gaseous reactants and products