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Chemical kinetics
The study of factors that affect the rates of chemical reactions
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Reaction rate
How fast a chemical reaction proceeds
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Rate
How much a quantity changes in a given time
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Rate for the concentration of a reactant
Rate= - ⋀[A]/⋀t
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Rate for the concentration of a product
Rate= ⋀[A]/⋀t
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Average rate of reaction equation
Rate = - ⋀[A]/⋀t
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Instantaneous reaction rate equation and definition
The change in concentration at one particular time
On graph:
Rate = ⋀[A]/⋀t
Slope= - ⋀[A]/⋀t
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Average rate
Change in concentrations over a period of time
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What happens between the instantaneous and average rate as time interval increases?
As the time interval increases, the more the average rate begins to deviate from the instantaneous rate
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What is the rate law?
The mathematical relationship between the rate of reaction and concentrations of the reactants and homogenous catalysts
Rate= k[A]^m[B]^n
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Integrated rate law (concentration-time)
Allows you to estimate the amount of reactants left in the reaction at a given time
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Integrated rate law for a zero order reaction
[A]t= -kt + [A]o
Slope= -k, y- intercept is [A] initial
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Integrated rate law for first order reaction
ln[A]t= -kt + [A]o
Slope= -k, y-intercept = ln[A]
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Integrated rate law for second order reaction
1/[A]t= kt + 1/[A]o
Slope = k, y-intercept = 1[A]o
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Half-life
The amount of time it takes for the initial concentration of a reactant to decrease by half
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Half life for zero order reaction
t1/2= [A]o/2k
Proportional: as initial concentration increases, half life increases
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Half-life for first order reaction
t1/2 = .693/k
Concentration is independent of half-life for first order reactions
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Half-life for second order reaction
t1/2= 1/k[A]o
Half-life is not proportional: as concentration increases, rate decreases
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What are the factors that affect reaction rate?
- Temperature
- Catalysts
- The nature of the reaction
- Reactant concentration
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The nature of reaction:
The kind of molecules reacting and their physical condition
- 1) Small molecules react faster than large ones
- 2) Gases react faster than liquids, react faster than solids
- 3) Powdered solids are more reactive than "cube" solids
- 4) Ions react faster than molecules
- 5) Certain chemicals are more reactive than others
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Reactant concentration
- 1) For 1st and 2nd order: higher concentration = faster reaction
- 2) The concentration of gas depends on their partial pressure; higher pressure = higher concentration
- 3) No temperature change = no rate constant change
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Temperature
Higher temperature = higher reaction rate through increase of reaction constant
ex. for each 10 celsius rise in temp, reaction speed increases by 2
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Catalysts
- Used in one step, remade in another. Not consumed, but still speed up reaction
- Alter activation energy and rate constant
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Activation energy
The difference in energy between reactants and activated complex
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Activated complex
Chemical species with partially broken/ partially formed bonds and is highly unstable
(this is where intermediates are created, and is the hill on a graph)
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What happens to the amount of molecules able to pass the activation energy barrier as temperature increases?
The amount of molecules able to reach the Ea increases with increasing temperature
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How to calculate Ea from a graph
Ea= -m * R
R= 8.314
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The collision theory of kinetics and it's two factors
Molecules must collide for a reaction to take place.
1) The molecules must have enough energy when they collide that the activated complex can form
2) They must be oriented in the proper way
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Reaction mechanisms
Most reactions occur in a series of small reactions that involve 1 or 2 molecules (sometimes 3)
This makes the elementary steps, the intermediates, and the fast/slow steps
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Molecularity
The # of reactant particles in an elementary step
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Intermediate
Something that is produced in one elementary step and then used in another
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How to determine the rate law for an elementary step
The order of the reactants is equal to the coefficients
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Catalysts
- Substances that speed up the rate of a reaction but are not consumed
- Create a lower activation energy
- Consumed in one step, but then remade in another
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Mass action equation
Kc= Products/reactants
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When Kc > 1
There are more products than reactants
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When Kc < 1
There are more reactants than products
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Calculating Kp
Kp= Kc (RT) ^products-reactants
R= 0.0821
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Reaction quotient
- Found in mass action equation
- Compares concentration ratios, in Qc, to Kc
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If Qc > Kc
Too much product, equation will shift left (reverse reaction)
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If Qc < Kc
Too much reactant, equation will shift right (forward reaction)
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If Qc = Kc
Equation is at equilibrium
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Le Chatliers Principle
When something disturbs equilibrium, the equation will shift itself to minimize the disturbance
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Factors that can disturb equilibrium
- Change in concentration
- Change in volume (can change concentrations of reactants/products)
- Change in pressure (for gases) (changes concentrations)
- Change in temperature
Whenever there is a change in pressure for gases, there will be a change in concentration
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How do catalysts affect the rate of a reaction at equilibrium
- Affects both the forward and reverse reactions the same
- Catalysts do not effect the position of the equilibrium
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Rules of shifting for Le Chatliers Principle
- Add to left= shift right
- Add to right = shift left
- Remove right = shift right
- Remove left= shift left
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