MCAT Chemistry

Kinetics & Equillibrium

MCAT Chemistry > Kinetics & Equilibrium

Background

Reaction Mechanisms offer a step-by-step view of the reaction. Overall reaction is a sum

A2 + 2B → 2AB

A2 + B → A2B (slow) = intermediate, rate determining step

A2 + B → 2AB (fast)

Rate Determining Step is the slowest step.

Collision Theory of Chemical Kinetics states that the rate of a reaction is proportional to the number of collisions per second between reacting molecules. For a reaction to occur, molecules must collide with each other at the proper angle and with a minimum amount of kinetic energy at or greater than the energy maximum or activation energy (Ea)

Rate = Fz

Reaction Rates (mol/L/S or M/s)

aA + bB → cC + dD

rate = -1Δ[A]/aΔT = -1Δ[B]/bΔT = -1Δ[C]/cΔT = -1Δ[D]/dΔT

Example:

2A + B → C

rate = -1Δ[A]/2Δt = -Δ[B]/Δt = Δ[C]/Δt

Determination of Rate Law only uses reactant concentration and the exponent values of X, Y, Z are determined experimentally

Rate = k[A]x[B]y[C]z

Zero Order Reactions occur when C formation is independent of A/B

Rate = k[A]0[B]0 = k

First Order Reactions occur when the rate is proportional to one reactant

Rate = k[A]1[B]0 or Rate = k[A]0[B]1

Second Order Reactions are when the rate is proportion to either both of the reactants or the square of one

Rate = k[A]1[B]1 = k[A]0[B]2 = k[A]2[B]0

Transition State Theory states that old bonds are weakened and new ones form an activation complex, which has a greater ΔE than the products or reactants. Transitional molecules exist at energy maxima and can dissociate into reactants or form products

Factors Affecting Reaction Rate

Reactant Concentrations will directly increase rate as concentrations increase for all but zero order reactions

Temperature will directly increase the rate as temperature increases. However, if the temperature get too high, catalysts may denature and become useless

Mediums depends on the solvent (polar vs. organic), and usually polar is preferred because the dipole destabilizes and weakens bonds

Catalysts are substances that increase the reaction rate without being consumed (enzymes are essential to biological processes). They increase the frequency of reaction, often changing the position of molecules to a more favorable one and decreases activation energy (Ea)

Homogeneous catalysts are in the same phase as reactants

Heterogeneous catalysts are in a distinct phase from reactants

Irreversible Reactions only proceed one way

Reversible Reactions can proceed forwards and backwards

Dynamic Equilibrium occurs when the forward/reverse reaction rates are still occurring but at the same rate and there is no net gain or loss

A ←→ B

Law of Mass Action states that if a system is at equilibrium (forward rate = reverse rate) at a specific temperature it will follow a specific ratio. Keq is dependent on temperature and will vary if temperature does. The larger the value of Keq, the further right and more complete the reaction will be.

Concentrations of pure substances (solids & fluids) are assigned a value of 1

Kc = Keq = [products] / [reactants] = ([C]c[D]d) / ([A]a[B]b) = kf/kr

Reaction Quotient states where the process is in relation to equilibrium

Qc = ([C]c[D]d) / ([A]a[B]b)

Le Chatelier's Principle

Le Chatelier's principle states that when a system is given a certain stressor, the reaction will react in an equal an opposite manner to compensate for the change

Adding or removing reactants or products by varying the concentration will alter the equilibrium state in such a way that the reaction will always move in the direction away from the added species and towards the removed species.

If Qc < Keq then the reaction will move in a forward direction until Qc = keq. This occurs when the reactant is added, or the product is removed. If Qc > keq then the reaction will act in reverse. This occurs when the reactants are removed or the product is added.

When a system is compressed by increasing the pressure, the overall volume decreases. The system will move towards the side with the lower number of moles. When a system expands, the volume increases and the pressure decreases and the reactants move towards the greater mole of gas present

Reaction shifts to the right occur when:

Reaction Shifts to the left occur when:

Change What Occurs Effect on Equilibrium Effect on K
Addition of reactant Added reactant consumed Shift to right No change
Addition of product Added product consumed Shift to left No change
Decrease V, Increase P Pressure decreases Shift to fewer gas molecules No change
Increase V, Decrease P Pressure decreases Shift to more gas molecules No change
Increase T Heat is consumed Shift in endo direction Change
Decrease T Heat is generated Shift in exo direction Change

MCAT Chemistry

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