Reaction Rates

1. Reaction Rates Reaction Kinetics, Factors that Influence Rates, and Activation Energy

2. Standards • 8. Chemical reaction rates depend on factors that influence the frequency of collision of reactant molecules. As a basis for understanding this concept: • a. Students know the rate of reaction is the decrease in concentration of reactants or the increase in concentration of products with time. • 8. b. Students know how reaction rates depend on such factors as concentration, temperature, and pressure. • 8. c. Students know the role a catalyst plays in increasing the reaction rate. • 8. d.* Students know the definition and role of activation energy in a chemical reaction.

5. Example Reaction #2 H2 (g) + I2 (g)2 HI (g) hydrogen iodine hydrogen gas gas iodide +

6. Definitions • 2A AA • r (reaction rate) – • how fast we use up a reactant • how fast we make a product • chemical kinetics – the study of chemical reaction rates.

7. Definitions • 2A AA • r (reaction rate) – • how fast we use up a reactant • how fast we make a product • chemical kinetics – the study of chemical reaction rates.

8. Definitions • 2A AA • r (reaction rate) – • how fast we use up a reactant • how fast we make a product • chemical kinetics – the study of chemical reaction rates.

9. Factors that Affect Reaction Rates • Temperature • More reactant molecules are moving fast enough • Warmglowsticks glow brighter • Chilled food spoils slower • Concentration (pressure for gases) • Reactants are hitting each other more often • Greater concentration = Faster reaction • Lower concentration = Slower reaction • Surface Area • Reactants are hitting each other more often • Start bonfires with crumpled paper or small twigs • Catalysts • They hold reactant molecules in an optimal orientation, which speeds up the reaction

10. Activation Energy • Activation Energy (EA) – The amount of energy reactants need to form the transition-state (the in-between) and the reaction to proceed. • aka “the hurdle” the reactants need to get over.

11. Molecular Speed vs. Temperature 300K Number of Molecules 400K EA Molecule Energy

12. Looking at Reaction Energy exothermic reaction Energy EA Reactants Products Reaction Proceeds

13. Looking at Reaction Energy endothermic reaction Energy EA Products Reactants Reaction Proceeds

14. Looking at Reaction Energy EA Energy With catalyst Reactants Products Reaction Proceeds

15. Catalyst • Catalyst – a substance not used up in a reaction that speeds up the reaction by lowering the activation energy, EA. • A catalyst also holds onto one of the reactants and positions the reactant so that it is easier to hit in the critical spot. I2 2 H2O2 (l) 2 H2O (g) + O2(g)

16. Transition State I– + CH3Cl CH3I + Cl– transition state aka activated complex

17. Transition State I– + CH3Cl Cl– + CH3I

18. Transition-State I– + CH3Cl Cl– + CH3I

19. Transition State I– + CH3Cl Cl– + CH3I

20. HCl(aq) + H2O(l) H3O+(aq)+ Cl–(aq) hydro- water hydronium chloride chloric acid ion ion + +

21. N2(g) + 3 H2(g) 2 NH3(g) + 92kJ nitrogen hydrogen ammonia heat gas gas gas + +

22. Reaction Steps rate-determining step 2 N2O (g) 2 N2(g) + O2(g) N2O (g) N2(g) + O (g) (slow) N2O (g) + O (g) N2(g) + O2 (g) (fast) + Overall Reaction

23. Reaction Steps transition-state 1 1 2 2 2 N2O (g) 2 N2(g) + O2(g) N2O (g) N2(g) + O (g) (slow) N2O (g) + O (g) N2(g) + O2 (g) (fast) transition-state intermediate reactants products +

24. 1st Order Rate Law mol/L s • The greater the concentration, [A], the greater the reaction rate. • This is a first-order rate law because it is concentration, [A], to the power of 1. mol/L A B rate = k·[A] 1

25. 1st Order Rate Law Example At 660 Kelvin, the rate constant, k, equals 4.5 × 10-2 s-1. What is the reaction rate when there is a concentration of 500,000 ppm? SO2Cl2 (g) SO2 (g) + Cl2 (g) rate = k·[SO2]1 rate = (4.5 × 10-2)·[500,000] rate = 22,500 ppm/s

26. Other Order Rate Laws • The reaction rate may increase with increasing concentrations of both reactants, [A] and [B]. • The powers, x and y, mayormay not relate to the coefficients on the balanced chemical equation. A + B C rate = k·[A]x·[B]y

27. 2 N2O5 (g) 2 NO2 (g) + O2 (g)

29. K H Na Li Mg Ca Be He O S Cl Ar F P N Br Kr C Si Al Ne B I Xe

30. K H Na Li Mg Ca Be He O S Cl Ar F P N Br Kr C Si Al Ne B I Xe

32. 4 e– in valence shell