9 Reaction Kinetics Flashcards
Rate of reaction
Change in concentration of reactant or product per unit time
Rate equation
Experimental relationship between the reaction rate and the concentration of the reactants.
Rate=k[A]^m [B]^n
Order of reaction (with respect to the reactant)
The sum of the powers of the reactant’s concentration term in the experimentally-determined rate equation
Rate constant, k
The proportionality constant in the experimentally determined rate equation.
Half-life of a first order reaction
ln(2)/k
First order reaction
Rate is directly proportional to [A]
Second order reaction
Rate is directly proportional to [A]^2
Zero order reaction
Rate is independent of [A]
Pseudo-order reaction
All reagents except one to be in great excess that their concentrations can be regarded as constant
Rate-determining step
The slowest step in a reaction mechanism
Collision Theory
A reaction will occur only if particles collide with:
1. minimum amount of kinetic energy (activation energy)
2. appropriate collision geometry/orientation
Transition State Theory
Reacting particles with the minimum activation energy will come together in the appropriate orientation to form a transition state/activated complex, which decomposes back into the reactants or rearranges to form products.
Thermodynamic stability
Related to ΔG (For enthalpy driven reactions, ΔG≈ΔH)
ΔH<0 (exothermic), reaction is thermodynamically stable
ΔH>0 (endothermic), reaction is thermodynamically unstable
Kinetic stability
Related to activation energy
Large activation energy, reaction is slow
Low activation energy, reaction is fast
Factors affecting rate of reaction
- Concentration of reactant particles
- Surface area (solid reactants)
- Temperature
- Catalyst
Characteristics of a catalyst
- Provides alternative reaction pathway of lower activation energy
- Usually specific
- Catalyst is regenerated at the end of the reaction
- Does not affect the overall stoichiometry of the reaction
- Does not affect the position of equilibrium
Homogenous catalyst
Catalyst and reactants are in the same phase
Mechanism of a homogenous catalyst
Catalyst forms an intermediate with one of the reactants.
Intermediate undergoes further reaction to form the product and regenerate the catalyst
(Multi-step reaction expected)
Heterogenous catalyst
Catalyst and reactants are in different phases
Adsorption Theory
- Adsorption: Reactant particles form weak bonds to active sites of the catalyst surface. Weakens/breaks the bonds within the reactant molecules, making them more reactive
- Reaction: The reactant particles are held on the catalyst surface in close proximity and in the correct orientation so they can readily react
- Desorption: Weak bonds between products particles and the catalyst surface are broken and products formed diffuse away
Autocatalyst
When one of the products catalyzes the reaction
Enzymes
Proteins that catalyze the chemical reactions in living systems
Factors affecting enzymes
- Substrate concentration (at low [S], reaction is first order, at high [S], reaction is zero order)
- pH of medium
- Temperature
Sampling method
- Reactions carried out at constant temperature
- Known concentration and volumes of reactants are mixed and stopwatch is started
- Samples of reaction mixture are withdrawn at regular time intervals
- Sample is quenched
- Concentration of product/reactant is analyzed via titration
