AP Chem Unit 8 Kinetics

Standards: Reaction Rates 8a. rate of reaction is the decrease in concentration of reactants or the increase in concentration of products with time. 8b. reaction rates depend on such factors as concentration, temperature, and pressure. 8c. the role a catalyst plays in increasing the reaction rate.

Chemical Reactions and Kinetics * What is your favorite dish? What are the main ingredients and how do you prepare this dish? Lets connect to Chemistry *What is a chemical reaction? *What does the word “kinetics” remind you of? *What are the parts of a chemical reaction?

REVIEW: Parts of a chemical reaction Reaction #1: A + B  AB Reactants Product Reaction #2: 2A + B  A2B Coefficient Subscript

REVIEW: Types of a chemical reaction Direction: Match the chemical reaction with the name of the type of reaction 1. Double Replacement 2. Single Replacement 3. Combustion 4. Synthesis Reaction D. Na3PO4 + 3 KOH  3 NaOH + K3PO4 5. Acid-Base 6. Decomposition A. P4 + 3 O2  2 P2O3 B. 2 NO2  2 O2 + N2 C. Pb + FeSO4  PbSO4 + Fe E. C6H12 + 9 O2  6 CO2 + 6 H2O +Energy F. HNO3 + NaOH  H2O + NaNO3

NaCl (s)  Na+ (aq) + Cl- (aq) Sketch of what is happening on a molecular model of a chemical reaction (specific vs. general) NaCl (s)  Na+ (aq) + Cl- (aq)  + Cl Na Cl Have some colored salts and solutions to mix Na  + B A B A

Direction: Using the balanced equation below, create a molecular model of this chemical reaction (use 3 different colored pencils) Mg + 2HCl  MgCl2 + H2 +

Collision Theory: reactants forming products using energy and proper orientation

Rate of reaction-- decrease in [reactants] or the increase [products] over time. REAGENTS  PRODUCTS

What are the factors that determines the rate of reaction?

Explain how reaction rates depend on such factors as concentration, temperature, and pressure (use the pictures below to support your argument) Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move. That greater density of molecules increases the number of collisions When you raise the temperature of a system, the molecules bounce around a lot more because they have more energy. When they bounce around more, they are more likely to collide. If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed. Low Pressure High Pressure

-- the frequency of collisions between reactants Catalysts -- the frequency of collisions between reactants -- rearrange the orientation of reactants -- speeds up reaction, not consumed Ex. biological enzymes, catalase etc. --reducing intramolecular bonding within reactant molecules, or donating electron density to the reactants.

Energy Diagram *Reaction needs to surpass activation energy (Ea)

AP Chem Kinetics Activity: Rate Order Activity 1: The Burning Candle Activity 2: The Empty Bucket Activity 3: Penny Decomposition Goals for each activity: *Graph data and determine which one corresponds to zero/1st/2nd rate order *What are the formulas for each rate order

AP Chem Kinetics Activity: Decomposition of Crystal Violet with NaOH (Colorimetry) Goal: Determine the rate order of the decomposition of crystal violet by colorimetry (if possible spectro-vis) Direction: *Groups will collect at least 3 data time points *Record time and height/ calculate concentration *Materials: --Crystal Violet 25µM and NaOH 0.2M CV + NaOH Decomposing -- add 10 mL from class set-up Standard --CV 6mL + 4mL of water (concentration =1.5µM)

Reaction Mechanism *Rate determining step-- gives us the rate law (slow step) hint: look at coefficient (ex. A2 because A + A) *Intermediate-- it is produced then consumed *Catalyst – it is consumed first then produced reaction favors simpler (bi-molecular) 1st rxn: A + A  E If A + B  C 2nd rxn: E + B  G 3rd rxn: G  C + A *To check mechanism, if 1st rxn is slow, what is the rate law, is it similar to the predicted rate law expression? If so, then the mechanism is correct. *Draw molecular model for each rxn step.

Reaction Mechanism Rate = k [H2O2] m

Kinetics Practice Problem The reaction 3 A + B  (products) is known to follow the mechanism: A + B  C + E A + E  G G + A  D + F   If the reaction rate is observed to be the first order in A and first order in B, write the rate law for the reaction.   Which step must be the slow step? If step III were the slow step, what would the rate law become?   Which letters from the reaction mechanism are used to represent the products of this reaction? Which letters from the reaction mechanism are used to represent the unstable intermediate products?

Kinetics Practice Problem For the reaction A + B  C the following rate data were obtained at constant temperature: Exp. Rate (moles/sec) [A]0 [B]0 I 2.0 x 10-4 0.10 0.10 II 8.0 x 10-4 0.20 0.20 III 8.0 x 10-4 0.10 0.20  Determine the rate order and write the rate law for this reaction. b) Calculate the rate constant, k. Include units. c) If each reactant has a molarity of 0.3M, determine the rate d) Propose a reasonable TWO step mechanism for this reaction and designate the slow step in the reaction. _________________________ _______________________