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Key Definitions, Graphs, formulae.
Key Definitions
Term |
Definition |
Rate of Reaction |
Change in concentration of a reactant or product per unit time. |
Rate Equation |
Rate = k [A]m [B]n where m, n are the orders of the reaction. |
Order of Reaction |
The power to which the concentration of a reactant is raised in the rate law |
Overall Order |
The sum of the powers (orders) of all reactants in the rate equation. Order = m+n. |
Half-life (t₁⁄₂) |
Time taken for the concentration of a reactant to fall to half its value. |
Rate Constant (k) |
Proportionality constant in the rate equation; depends on temperature. |
Activation Energy (Eₐ) |
Minimum energy needed for a reaction to occur. |
Adsorption |
The binding of molecules to a surface (such as a catalyst), which increases reaction rate. |
Desorption |
The release of products from a catalyst surface after the reaction. |
Arrhenius Equation
k=AeEa/RT
Where
k = Chemical Reaction Rate
A = Pre-exponential factor
Ea = Activation Energy
R = Gas Constant
T = Temperature in Kelvin
Mechanisms: SN1 vs SN2
Feature |
SN1 |
SN2 |
Steps |
2 steps (carbocation formed) |
1 step (simultaneous attack/leave) |
Reactivity |
Tertiary haloalkanes |
Primary haloalkanes |
Rate Law |
rate = k[haloalkane] |
rate = k[haloalkane][Nu⁻] |
Steric Hindrance |
Low (carbocation intermediate) |
High (due to nucleophile approach) |
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Experimental Techniques
Method |
Measurement |
Used For |
Mass loss |
Decreasing mass (gas released) |
Rate of gas-producing reactions |
Volume of gas |
Gas syringe / upside-down cylinder |
Gas evolution |
Titration |
Concentration vs time |
Accurate kinetic data |
Colorimetry |
Absorbance vs concentration |
Colored species (e.g. I₂ reaction) |
Rate Laws & Integrated Rate Equations
Order |
Rate Law |
Half-life |
Zero |
rate = k |
t₁⁄₂ = [A]₀ / 2k |
First |
rate = k[A] |
t₁⁄₂ = ln2 / k |
Second |
rate = k[A]² |
t₁⁄₂ = 1 / (k[A]₀) |
Rate Determining Step (RDS)
1. The slowest step in a mechanism controls the overall rate.
2. Only species involved up to RDS appear in the rate equation.
3. Powers in rate law = number of molecules in RDS. |
Catalysis
Type |
Phase |
Example |
Notes |
Homogeneous |
Same as reactants |
Fe²⁺/Fe³⁺ in S₂O₈²⁻ + I⁻ |
Catalyst regenerated via redox steps |
Heterogeneous |
Different from reactants |
Fe in Haber process |
Provides surface, weakens bonds, speeds reaction |
Catalysts lower the activation energy of a reaction by providing an alternate reaction route.
Adsorption (in Heterogeneous Catalysis)
Adsorption is when reactant molecules bind to active sites on the surface of a solid catalyst.
This:
1. Brings molecules close together, increasing collision frequency
2. Weakens bonds in the reactants, lowering activation energy
3. Allows for a faster reaction
After the reaction, products desorb (leave) from the surface, freeing the site for reuse.
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