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Reaction Kinetics

Key Concepts:

Terms in this set (26)

Protective reaction of Ozone
O3 + photon --> O2 + O

-- O3 degradation absorbs photon and prevents it from reaching the ground and harming us
Elementary Step
Tells about collisions of molecules (typical reactions do not show the exact molecular collisions that occur)

-- What molecules collide to cause bond formation and breakage?
Molecularity
# of molecules involved in a collision

- 2 molecular collisions are most common, 3 molecules are less likely . (anything about 3 molecules in a collision does not occur)
Reaction Mechanism
Series of elementary steps

-- addition of elementary steps results in the net equation of the reaction
What is "M" in the atmosphere
Any combination of molecules that are present in the atmosphere

Nitrogen is the most abundant (generally [N2] = M)
Chapman Mechanism of Ozone formation
O2 + light --> O + O (produces O for the second reaction)

O + O2 + M --> O3 + M (exothermic reaction)

M acts as energy cushion to absorb the energy that is released from the collision of O and O2
Chapman Mechanism of Ozone degradation
O3 + light --> O + O2 (released O is used in next step)

O3 + O --> 2O2
Limitation of the Chapman Mechanisms
Ozone must be produced faster than it is degraded ( not true because we do not account for other possible O3 loss mechanisms in this model)
Reaction Rates
# of products produced (or lost) / unit time
Rate Law equation
rate = concentrations of reactants * k (rate constant)

m and n = reaction orders
** not related to the stoichiometric constants in the overall reaction equation
Arrhenius Equation
A reaction for the temperature dependence of reaction rates
Arrhenius equation written as the line formula
lnk = (-Ea/R)(1/T) + lnA

lnk = y
-Ea/R = m
1/T = x
lnA = b
Steady-State kinetics
A system where substrates are continually added and products are continually removed

** think of a bathtub -- imagine filling a bath tub with the faucet running, but the drain is open

-- differs from equilibrium - @ equilibrium net reaction rate is zero (this may not be true of a system at steady-state)
CFC
Chloro-fluoro-carbons- nonpolar, very stable in the troposphere (do not degrade easily and remain in the atmosphere for decades)
- accelerate the rate at which O3 is degraded due to Cl-
Cl- mechanism for Ozone loss
Cl- + O3 --> ClO + O2
ClO + O --> Cl- + O2
----------------------------------
O + O3 --> O2 + O2
What is the purpose of Cl- in the ozone loss mechanism?
Cl- acts as a catalyst (does not change throughout the reaction and is not present in the net reaction)- acts only to progress reaction forward
What is the importance of ClO in the ozone loss mechanism?
ClO is a temporary intermediate formed in the mechanism
-- proof that this mechanism occurs is the presence of ClO in our atmosphere which has actually been measured
Activation energy
Energy barrier a reaction must overcome to produce the products of the reaction

-- catalysts are able to lower this barrier and allow a reaction to occur faster (ie. Cl- is a catalyst in the ozone loss mechanism which makes the loss of O3 occur faster)
What is the importance of collisions in reactions?
Collisions are responsible for providing energy for bond breaking and formation in a reaction - geometry and probability of these collisions occurring is very low
-- only collisions that can make it over the activation barrier are able to react to form products because they have enough energy for the bond breaking and forming to occur
Time vs. Concentration graphs
responsible for showing the reaction rate (order)
Slope shows the rate of the reaction (this can be a constant rate or changing)
Rate at t = 0 sec
Initial rate (gives most accurate rate without working about the products or intermediates interfering with the reading)
Determining the rate
Take 2 points on the time vs concentration graph and perform following equation (Y2-Y1)/(X2-X1)
Zero order reaction
=k[A]^0 = k

rate is constant -- decreases at a constant rate until reactants are gone
First order reaction
= k[A]^1 = k[A]
Second order reaction
= k[A]^2
slope is very steep (rate of reaction occurs quickly)
Heterogeneous catalyst
provides a surface for the reactants to rest on -- this allows for the molecules to be in a 2D plane with less molecular movement and a higher probability that the molecules will meet at the correct orientation for the reaction to occur

(ie. enzymes in biological systems, car catalytic converters)
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