Chemical Kinetics Flashcards
Transition state
- (Also called an activated complex)
- unstable, high-energy state
- briefly exists between a reactant and intermediate, or between a reactant and product.
- At the peak of the hill in a reaction coordinate diagram
- state of the reactant(s) in which old bonds are breaking and new bonds are forming
- both the incoming group and the leaving group are present transiently on the molecule
- cannot be isolated from a reaction mixture
What does the collision theory state?
for a chemical reaction to occur:
1. molecules must collide
2. Both molecules must collide with enough energy: molecules must have sufficient energy to overcome the activation energy
3. Both molecules must collide in the propert 3D orientation
What increases the collision frequency?
Increasing temperature increases the collision frequency and percentage of high energy collisions
What will affect the rate of an irreversible chemical reaction?
Increasing the concentration of the reactants will affect the rate
A chemical reaction occurs when two or more reactant molecules hit each other with enough fore and in the proper orientation. The more molecules hitting each other, the higher the chance of a chemical reaction and the faster the reaction proceeds. Therefore, the higher the concentration of the reactants, the higher the chance for the molecules to strike (collide) each other, causing a reaction and an increase in the rate
According to the Arrhenius equation, an increase in temperature of a chemical reaction results in:
Increase in reaction rate
Increase in collision frequency
Increase in rate constant
what does chemical kinetics explain?
- the speed of a reaction
- answers the question: “how fast?”
- spontaneity does not affect speed of a reaction
energy diagram for an exothermic reaction:
- products are lower in energy than reactants
activation energy
- refers to the minimum amount of energy the reactants must possess in order for the reaction to occur
- energy where reactants start to top of the “hill”
- even if a reaction is spontaneous, if the minimum amount of energy is not provided, the reaction will not occur
what is a catalyst?
- a substance that lowers the activation energy
- speeds up the rate of the reaction
- provides an alternate pathway for the reaction
- they are not consumed by the reaction
- they are not altered in the process, so they are present at the end of the reaction
what are intermediates in energy diagrams?
- species that are formed during the course of the reaction
- they are not part of the reactants or the products
- their energy is less than the transition states energy
- possible to isolate them from a mixture
- there can be more than one
stopovers on a road trip
zero-order reaction rate law expression
rate = k[A]0 = k(1) = k
* anything raised to the power of 0 = 1
zero-order reactions:
- rate = k
- the rate of the reaction does NOT depend on the concentration of the reactants
- increasing/decreasing concentration of reactants does NOT change the rate of the reaction
first order reaction rate law expression:
rate = k[A]1 = k[A]
first order reactions
- rate = k[A]
- rate of the reaction is proportional to the concentration of the reactant
- if the concentration of A were to double, the rate would also double
second order rate law expression
- rate = k[A]2 or
- rate = k[A][B]
second order rate law
rate = k[A]2
* the rate of the reaction is proportional to [A]2
* if we double [A], the rate of the reaction would increase by 22
rate = k[A][B]
* the rate of the reaction is directly proportional to both [A] and [B]
* halving [A] would also halve the rate
distinguishing between catalysts and intermediates:
- catalyst: a reactant first and is then regenerated as a product
- intermediate: is a product first and then a reactant
arrhenius equation
- describes the temperature dependent nature of reaction rates
- k = Ae-Ea/RT
- k = rate constant
- A = arrhenius constant
- e = mathematical constant (~2.71)
- Ea = activation energy
- R = universal gas constant
- T = temperature
temp and activation energy are the only ones that can be manipulated
what would an increase in T (in the arrhenius equation) result in?
- increase in temperature = molecules move faster and collide more often = increase in collision frequency = increased percentage of high energy collisions - sufficient energy to overcome the activation energy barrier
- increase in temperature = increase in the rate of the reaction = increase in k
k and rate of reaction are DIRECTLY related
what would an increase in Ea (in the arrhenius equation) result in?
- increase in activation energy = rate of the reaction slows down = decrease in k
how does increasing or decreasing T affect Ea?
- it doesn’t
- temperature and activation energy are independent of each other
- the only thing that can alter the activation energy is a catalyst
According to the Arrhenius equation, a decrease in Ea of a chemical reaction results in:
- increase rate constant (k)
- increase in rate of the reaction
