Chapter 9 Flashcards
Chemical Kinetics
The study of how reactions take place and how fast they occur. (they tell us nothing about the spontaneity of the reactions)
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Intermediate
something that appears in a series of elementary steps as a substance that is produced in one step and consumed in the subsequent step.
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Rate determining step
The slowest step in a process that determines the overall reaction rate
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Rate
Change in something over change in time.
rate = ΔA / Δt
Reaction rate
indicates How fast reactants are being consumed or how fast products are being formed. Depends on several factors which are normally anything that affects the collisions and interactions that are needed to break old bonds and form new bonds:
1) How frequently the reactant molecules collide
2) The orientation of the colliding molecules
3) Their energy
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Activation energy (E_a)
Every chemical reaction has one. It is the minimum energy required for reactant molecules during a molecular collision in order for the reaction to proceed to products. If the reactants don’t possess this energy their collisions wont be able to produce the products and the reaction will not occur. If they do they can reach a high-energy (and short lived) transition state
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Transition state.
also known as the activated complex. Reached when the reactants have the necessary activation energy. It’s a very high energy but very short lived. This is always an energy maximum, and is therefore distinct from an intermediate.
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Common statements made about the reaction rate (3)
1) The lower the activation energy, the faster the reaction rate. The reaction coordinate above suggests that the second step of the mechanism will therefore be the slow step, or the rate determining step, since the second “hill” of the diagram is higher
2) The greater the concentrations of the reactants, the faster the reaction rate. Favourable collisions are more likely as the concentrations of reactant molecules increase.
3) The higher the temp of the reaction mixture, the faster the reaction rate. At higher temp more reactant molecules have a sufficient energy to overcome the activation-energy barrier, and molecules collide at a higher frequency, so the reaction can proceed at a faster rate
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Kinetic quantities and thermodynamic quantities in the reaction coordinate program
Thermodynamic: Reactants, ΔGº, intermediate. Products
Kinetic: activation energy, transition states (activated complexes)
Thermodynamic factors and kinetic factors do not effect each other (very important!!!)
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Catalysts
Provide reactants with a different route, usually a shortcut, to get to products. It almost always makes a reaction go faster by either speeding up the rate determining step of providing an optimized route to products. A catalyst that accelerates a reaction does so by lowering the activation energy of the rate-determining step, and therefore the energy of the highest-energy transition state. The addition of a catalyst will affect the rate of reaction but not the equilibrium or the thermodynamics of the reaction.
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Reactant vs catalyst
The main difference is that the reactants are converted to products, but a catalyst remains unchanged at the end of a reaction. A catalyst can undergo a temporary change during the reaction, but it is always converted back to its original state. They are included in the overall reaction like intermediates.
Rate Law
The date for rate law is determined by the initial rates of reaction and typically are given as the rate at which the reactants disappear. Usually only reactants are added in the rate law. only the reactants that affect the rate are in the rate law which are the ones involved in the rate determining step. This can only be determined experimentally.
generic:
rate = k [A]^x [B]^y
– x = the order of the reaction with respect to A
– Y = the order of the reaction with respect to B
– (x+y) = the overall order of the reaction
– k = the rate constant
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Exception to the fact that the rate law can only be determined experimentally
The exception is for an elementary step in a reaction mechanism. The rate law is first order for a unimolecular and second order for a bimolecular elementary step.
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How to calculate the rate constant
k = rate / rate law
after you figure out the rate law. Each reaction has its own rate constants. The unit for the rate constant isn’t always the same it depends on the rate law. The general unit is M/s but it changes with the order, for example if the rate law was k = [A] [B] the unit would be M-1/s-1
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Arrhenius Equations
another way to calculate Rate constant (k)
k = Ae^-(E_a/RT)
or k = ln A - (E_a / RT)
The rate will increase by a factor of 2-4 everY 10 ºC inc in temp
