Chemistry Video 13 Flashcards
Kinetics
Study of rates of reaction
Reaction rate
rate = change in concentration over change in time (M/s). rate = delta [molecule] / delta t. Rate is negative for a reactant. Rate is positive for a product. The rate of a particular molecule is also multiplied by the inverse of the coefficient in front of the molecule in an equation.
Instantaneous rate
the rate at any particular instant. Determined by taking the average rate over a short period of time OR by finding the slope of the tangent line in graphical analysis
Rate constant
k, used in rate law. Independent of reactant concentrations. Depends on temperature, surface area, etc.
Rate law
rate = k [A]^m [B]^n [C]^p. The exponents are usually positive integers. The square brackets are reactant concentrations.
Reaction order
refers to exponents in the rate law. If the exponent in rate law is 1, then the reaction is first order with respect to the specific molecule A. If the exponent is 2, the reaction is second order with respect to B. If the exponent is 0, the reaction is zero order with respect to C. The overall reaction order is the sum of all exponents in the rate law
Initial rates
compare 2 sets of rate data that differ in the initial concentration of one reactant. Determines the exponent in rate law
Reaction mechanism
The pathway by which a reaction occurs.
Elementary reaction
Part of the pathway by which the overall reaction occurs. All add up to give the overall reaction. Tells us the mechanism. Happen at different rates
Intermediate
Molecules present in elementary reaction but not in overall reaction
Molecularity of the reaction
The number of reactant species in an elementary reaction
Unimolecular reaction
involve one reactant molecule. Reaction driven by kinetic energy or collision with container. Rate law is k[A]. Rate is proportional to concentration.
Bimolecular reaction
involve 2 reactant molecules. Rate law is k[A][B] or rate law is k[A]^2
Termolecular reaction
involve 3 reactant molecules. Uncommon; difficult for 3 molecules to collide simultaneously. Rate law is k[A][B]^2 or k[A]^2[B] or k[A][B][C]
Rate-determining step
The elementary reaction that is the slowest. Limits the rate at which the overall reaction can occur. The exponents in the rate law of the overall reaction does correlate with the stoichiometric coefficients of the rate-determining step. The rate law for the rate-determining step is the overall rate law, without needing experimental data.
Collision theory
- The rate of reaction is proportional to the rate at which collisions occur.
- Molecules must collide with a particular orientation in order to successfully react
- Collisions must occur with sufficient energy to allow new chemical bonds to form
Collisions and activation energy
Collisions must overcome the activation energy, which is the energy needed to reach the transition state of the reaction.
Energy diagram
x-axis is reaction coordinate (time), y-axis is potential energy. The peak is transition state. Distance from reactants to transition state is activation energy. Distance from reactants to products is delta H, which is the change in enthalpy. If products is lower than reactants in potential energy, then it is exothermic. If products is higher than reactants in potential energy, then it is endothermic. Some reactions are reversible and the activation energy of the reverse reaction is the distance from products to transition state.
Catalyst
Substance that lowers the activation energy of a reaction. Causes a faster reaction rate. A catalyst is not used up stoichiometrically in a reaction; Catalyst is not consumed or it is regenerated as the reaction proceeds. Provides alternative pathway that can cause ‘reactions that have too high of a pathway to proceed’ to finally cause a reaction. The alternative pathway can be a one-step or multi-step pathway that may differ from the uncatalyzed pathway
Integrated rate laws
Used to determine rate laws by graphical analysis. Can calculate the concentration of a reactant at any time. Follows “y = mx + b” format. The slope “m” is the rate constant “k”. If we choose the correct integrated rate law to do the graphical analysis, we will get a linear relation between the 2 axis; it will be a straight line
Integrated rate law for reaction order of 0
Rate does not depend on the concentration of the reactant. [A] = -kt + [A]subscript 0
Integrated rate law for reaction order of 1
ln[A] = -kt + ln[A]subscript 0
Integrated rate law for reaction order of 2
1/[A] = kt + 1/[A]subscript 0
Units of k for zero order
k units are M/s
Units of k for first order
k units are 1/s
Units of k for second order
k units are 1/(M*s)
Units of k for third order
k units are 1/((M^2)*s)
Arrhenius Equation
Relates activation energy and rate constant. K = Ae^[-activation energy/(R*T)]. R = gas constant of 8.314 J/mol K. T = temperature in Kelvins. e = Euler’s number of 2.71828. A = frequency factor, which is related to frequency of collisions and orientation of molecules; it is the faction of molecules that are able to get over the activation barrier.
Rearrange Arrhenius Equation
Rearranged to follow “y = mx + b” format. ln(k) = [(-activation energy / R) * (1/T)] + ln(A).
Rearrange Arrhenius Equation further to compare rate constants at 2 temperatures
ln (k2/k1) = (activation energy / R) * [(1/T1) – (1/T2)]. There is no frequency factor.
Half-life equation for zeroth order reaction
t = [A]subscript0 / (2K)
Half-life equation for first order reaction
t = ln2 / k
Half-life equation for second order reaction
t = 1 / ((k)*([A]subscript0))
