Reaction Rates

Reaction Rates

Reaction Rates Reaction Kinetics, Factors that Influence Rates, and Activation Energy 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. Example Reaction #2 H2 (g) + I2 (g) hydrogen

gas + iodine gas 2 HI (g) hydrogen iodide 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. 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. 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. Factors that Affect Reaction Rates Temperature More reactant molecules are moving fast enough Warm glowsticks 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 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. Number of Molecules

Molecular Speed vs. Temperature 300K 400K Molecule Energy EA Looking at Reaction Energy Energy

exothermic reaction Reactants EA Products Reaction Proceeds Looking at Reaction Energy

Energy endothermic reaction EA Reactants Reaction Proceeds Products

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

Catalyst 2 H2O2 (l) I2 2 H2O (g) + O2 (g) 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. Transition State transition state aka activated complex I + CH3Cl CH3I + Cl Transition State

I + CH3Cl Cl + CH3I Transition-State I + CH3Cl Cl + CH3I Transition State

I + CH3Cl Cl + CH3I HCl(aq) + H2O(l) H3O+(aq) + Cl(aq) hydrochloric acid hydronium ion

+ water ion chloride + N2 (g) + 3 H2 (g) 2 NH3 (g) + 92kJ nitrogen

gas hydrogen gas + ammonia heat gas +

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

Overall Reaction Reaction Steps transition-state 1 transition-state reactants + intermediate 2

1 N2O (g) 2 N2O (g) + O (g) 2 N2O (g) N2 (g) + O (g) (slow) products

N2 (g) + O2 (g) (fast) 2 N2 (g) + O2 (g) mol/L s 1st Order Rate Law A B rate = k[A]1

mol/L 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. 1st Order Rate Law Example SO2Cl2 (g) SO2 (g) + Cl2 (g) 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? rate = k[SO2]1 rate = (4.5 10-2)[500,000] rate = 22,500 ppm/s Other Order Rate Laws A +B C rate = k[A]x[B]y The reaction rate may increase with increasing concentrations of both reactants, [A] and [B].

The powers, x and y, may or may not relate to the coefficients on the balanced chemical equation. 2 N2O5 (g) 2 NO2 (g) + O2 (g) H He

Li Be B C N O F Na Mg Al

Si P S Cl Ar K Ca

Ne Br Kr I Xe H He Li Be B

C N O F Na Mg Al Si

P S Cl Ar K Ca Ne

Br Kr I Xe 4 e in valence shell

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