Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 13: Reaction Rate • Rate = ∆ [A]/ ∆t • • • • Related to (1) Concentration: Rate Laws (2) Temperature (3) Catalyst Chapter 13: Reaction Rate: Effect of Temperature • CONSIDERATIONS – As temperature increases, the rate increases for most reactions – The rate constant is temperature related. • Reaction Rate is Proportional to Temperature. • NOT ALWAYS A LINEAR RELATIONSHIP Temperature and Rate Reaction Rate: Effect of Temperature The Collision Model • The collision model: in order for molecules to react they must collide. • The greater the number of collisions the faster the rate. Reaction Rate: Effect of Temperature The Collision Model Reaction Rate: Effect of Temperature The Collision Model • The more molecules present, the greater the probability of collision and the faster the rate. • The higher the temperature, the more energy available to the molecules and the faster the rate. • In order for reaction to occur the reactant molecules must collide in the correct orientation and with enough energy to form products. Reaction Rate: Effect of Temperature Activation Energy • Arrhenius: molecules must posses a minimum amount of energy to react. • In order to form products, bonds must be broken in the reactants. – Bond breakage requires energy. • Activation energy, Ea, is the minimum energy required to initiate a chemical reaction. Temperature and Rate Activation Energy Reaction Rate: Effect of Temperature Activation Energy • Consider the rearrangement of acetonitrile: H3C N C H3C N C H3C C N – In H3C-NC, the C-NC bond bends until the C-N bond breaks and the NC portion is perpendicular to the H3C portion. This structure is called the activated complex or transition state. – The energy required for the above twist and break is the activation energy, Ea. – Once the C-N bond is broken, the NC portion can continue to rotate forming a C-CN bond. Temperature and Rate Activation Energy Reaction Rate: Effect of Temperature The Arrhenius Equation • Arrhenius discovered most reaction-rate data obeyed the Arrhenius equation: k Ae Ea RT – k is the rate constant, Ea is the activation energy, R is the gas constant (8.314 J/K-mol) and T is the temperature in K. – A is called the frequency factor. – A is a measure of the probability of a favorable collision. – Both A and Ea are specific to a given reaction. Reaction Rate: Effect of Temperature The Arrhenius Equation • If we have a lot of data, we can determine Ea and A graphically by rearranging the Arrhenius Ea equation: ln k ln A RT • If we do not have a lot of data, then we can use k1 Ea 1 1 ln k2 R T2 T1 Problem • A certain first order reaction has a rate constant of 2.75E-2s-1 at 20 ºC. What is the value of “k” at 60 º C if (a) Ea = 75.5 kJ/mole Problem: Solution • • • • • • • VARIABLES Ln (k1/k2) =Ea/R (1/T2 – 1/T1) k1 = 2.75E-2s-1 T1 = 20 º C + 273 K/ º C = 293 K T2 = 60 º C + 273 K/ º C = 333 K k2 = x R = 8.314 Joules/(k) (mole) Problem: Solution • Ln (k1/k2) =Ea/R (1/T2 – 1/T1) • LN (2.75E-2/x) = 75.5 kJ/0.008314 kJ/Kmole (1/333 K –1/293 K) • Ln (2.75E-2/x) = 75.5kJ/0.008314 kJ/Kmole (-4.10E-4) • Ln (2.75E-2/x) = -3.723 • take ex or antilog/inverse of both sides • 2.75E-2/x = 2.416E-2 • x = k2 = 1.1s-1 Reaction Mechanism • Few reactions go to products in one step. • Determination of number of steps and formulas of compounds constitutes a reaction mechanism. Reaction Mechanism STEP ONE: NO (g) + O3 (g) NO2 (g) + O2 (g) STEP TWO: NO2 (g) + O (g) NO (g) + O2 (g) SUMMARY: O3(g) + O(g) 2O2(g) Chapter 13: Reaction Rate-Catalyst • A catalyst changes the rate of a chemical reaction. • There are two types of catalyst: – homogeneous, and – heterogeneous. Reaction Rate: Catalyst Homogeneous Catalysis O3(g) + O(g) 2O2(g) – STEP ONE: – STEP TWO: NO (g) + O3 (g) NO2 (g) + O2 (g) NO2 (g) + O (g) NO (g) + O2 (g) – SUMMARY: O3(g) + O(g) 2O2(g) • Catalysts usually increase rate by lowering the activation energy for a reaction. Reaction Rate: Homogenous Catalyst • Catalysts usually increase rate by lowering the activation energy for a reaction. • Increases the number of effective collisions. Catalysis Homogeneous Catalysis Chapter 15: Reaction Rate & Heterogeneous Catalyst • The catalyst is in a different phase than the reactants and products. • Typical example: solid catalyst, gaseous reactants and products (catalytic converters in cars). • First step is adsorption (the binding of reactant molecules to the catalyst surface). • Adsorbed species (atoms or ions) are very reactive. • Molecules are adsorbed onto active sites (red spheres) on the catalyst surface. Reaction Rate: Heterogeneous Catalyst Heterogeneous Catalysis Reaction Rate: Heterogeneous Catalyst Enzymes • Enzymes are biological catalysts. • Enzymes have very specific shapes. • Substrates undergo reaction at the active site of an enzyme. • A specific reactant/substrate fits a specific active site Reaction Rate: Heterogeneous Catalyst Enzymes