reaction rates reaction kinetics, factors that influence rates, and activation energy
TRANSCRIPT
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