Ch 5 - Chemical Kinetics Flashcards
What does the change in Gibbs free energy determine?What does a negative/positive Gibbs free energy correlate with?
- whether or not a reaction is spontaneous
- +G = endergonic = energy absorbed
- -G = exergonic = energy given off
What do chemical mechanisms propose?
a series of steps that make up the overall reaction
What are intermediates?
molecules that exist within the course of a reaction but are neither reactants nor products overall
What is the rate-determining step?
- slowest step
- determines the overall rate of the reaction because the reaction can only proceed as fast as this step
- rate related to the concentration of the reactants in the rate determining step
What is the collision theory?
states that a reaction rate is proportional to the number of effective collisions between the reacting molecules
- focuses on energy and orientation of reactants, and considers each potential reaction to be all or nothing (either enough energy to form products or not)
rate = Z x f
What must happen in order for a collision theory to be effective?
molecules must be in the proper orientation and have sufficient kinetic energy to exceed the activation energy
What is the Arrhenius equation?
a mathematical way of representing collision theory
k = Ae^(-Ea/RT)
- low Ea and high temperature make the negative exponent of the equation smaller in magnitude and increase the rate constant k
What is the transition state theory?
forms high energy activated complex that can proceed forward or backward, forming products or reverting to reactants
Where does the reaction proceed from the transition state?
toward products or revert back to reactants
Where is the transition state on a free energy reaction diagram?
the highest point
How does the concentration of the reactant affect the reaction rate?
increasing the concentration of reactant will increase reaction rate (except for zero-order reactions) because there are more effective collisions per time
How does the temperature affect the reaction rate?
increasing the temperature will increase reaction rate because the particles’ kinetic energy is increased
How can changing the medium affect the reaction rate?
changing the medium can increase or decrease the reaction rate, depending on how the reactant interact with the medium
How does a catalyst affect the reaction rate?
adding a catalyst increases reaction rate because it lowers the activation energy
What is the difference between a homogenous and heterogeneous catalyst”
- homo: same phase as the reactants
- hetero: different phase
How are reactions rates measured?
in terms of the rate of disappearance of a reactant or appearance of a product
What is the form for rate laws?
rate = k[A]^x[B]^y[C]^z
determine how rate is affected during trials
change in rate = change in […]^*
How are rate laws determined?
must be from experimental data
What is the rate order?
- the sum of all individual rate orders in the rate law
- usually do not match the stoichiometric coefficient
What are zero-order reactions and how are they graphed?
- have a constant rate that does not depend on the concentration of reactant
rate = k[A]^0[B]^0 - concentration v time curve is a straight line; the slope is equal to -k
What affects the rate of a zero-order reaction?
only changing the temperature or adding a catalyst
- lower temperature = decreased rate
- added catalyst = increased rate
What are first order reactions and how are they graphed?
- has a rate that is directly proportional to only one reactant such that doubling the concentration of that reactant results in a doubling of the rate of formation of product
rate = k[A]^1 or rate = [B]^1 - concentration v time curve is non linear
- the slope of a ln[A] v time plot is -k
What is a second order reaction and how are they graphed?
- has a rate that is proportional to either the concentrations of 2 reactants or to the square of the concentrations of a single reactant
rate = k[A]^1[B]^1 or rate = k[A]^2 or rate = k[B]^2 - concentration v time curve is non linear
- slot of a 1/[A] v time plot is k
What are broken order reactions?
those with non integer orders
What are mixed order reactions?
those that have a rate order that changes over time
What is activation energy?
the minimum energy required for a chemical reaction to occur
- on the graph of reaction coordinate v free energy, Ea represented by the distance on the y axis from the energy of the reactants to the peak energy prior to formation of products
What affects the rate of a first order reactions?
- lower temperature = decreased rate
- all reactants’ concentrations double = rate doubled
- catalyst added = rate increased
What affects the rate of a second order reactions?
- lower temperature = decreased rate
- all reactants’ concentrations double = rate x 4
- catalyst added = rate increased
What type of reaction is a certain equilibrium process if the activation energy of the forward reaction (Gf) is greater than the Ea of the reverse reaction (Gr) and why?
nonspontaneous reaction
- if the Ea of the forward reaction is greater than the Ea of the reverse reaction, then the products must have a higher free energy and the overall energy of the system is higher at the end that it was at the beginning, meaning the net free energy is positive (nonspontaneous)
How do saturated catalysts affect reactions?
saturated solutions containing a catalyst have a maximum turnover rate and cannot increase the rate constant or the reaction rate any higher by adding more reactant molecules
What are exergonic reactions?
- if energy is released by the reaction
- the net energy change is negative, and the free energy of the final products is lower than the free energy of the initial reactants
