Reaction Rates Reaction Kinetics, Factors that Influence Rates, and Activation Energy.
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Transcript of 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.
Definitions
• r (reaction rate) – how fast we use up a reactanthow fast we make a product
2A AA
• chemical kinetics – the study of chemical reaction rates.
Definitions
• r (reaction rate) – how fast we use up a reactanthow fast we make a product
2A AA
• chemical kinetics – the study of chemical reaction rates.
Definitions
• r (reaction rate) – how fast we use up a reactanthow fast we make a product
2A AA
• 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.
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.
2 H2O2 (l) 2 H2O (g) + O2 (g)I2
Reaction Steps
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
rate-determining step
Reaction Steps
2 N2O (g) 2 N2 (g) + O2 (g)
N2O (g) N2 (g) + O (g) (slow)
N2O (g) + O (g) N2 (g) + O2 (g) (fast)+
1
2
1
2
transition-state
transition-state
intermediatereactants
products
1st Order Rate Law
• 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.
A B
rate = k·[A] mol/L
mol/L s
1
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
Other Order Rate Laws
• 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.
A + B C
rate = k·[A]x·[B]y