Chemistry Video 13

Question 1

Kinetics

Answer 1

Study of rates of reaction

Question 2

Reaction rate

Answer 2

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.

Question 3

Instantaneous rate

Answer 3

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

Question 4

Rate constant

Answer 4

k, used in rate law. Independent of reactant concentrations. Depends on temperature, surface area, etc.

Question 5

Rate law

Answer 5

rate = k [A]^m [B]^n [C]^p. The exponents are usually positive integers. The square brackets are reactant concentrations.

Question 6

Reaction order

Answer 6

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

Question 7

Initial rates

Answer 7

compare 2 sets of rate data that differ in the initial concentration of one reactant. Determines the exponent in rate law

Question 8

Reaction mechanism

Answer 8

The pathway by which a reaction occurs.

Question 9

Elementary reaction

Answer 9

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

Question 10

Intermediate

Answer 10

Molecules present in elementary reaction but not in overall reaction

Question 11

Molecularity of the reaction

Answer 11

The number of reactant species in an elementary reaction

Question 12

Unimolecular reaction

Answer 12

involve one reactant molecule. Reaction driven by kinetic energy or collision with container. Rate law is k[A]. Rate is proportional to concentration.

Question 13

Bimolecular reaction

Answer 13

involve 2 reactant molecules. Rate law is k[A][B] or rate law is k[A]^2

Question 14

Termolecular reaction

Answer 14

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]

Question 15

Rate-determining step

Answer 15

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.

Question 16

Collision theory

Answer 16

1. The rate of reaction is proportional to the rate at which collisions occur.
2. Molecules must collide with a particular orientation in order to successfully react
3. Collisions must occur with sufficient energy to allow new chemical bonds to form

Question 17

Collisions and activation energy

Answer 17

Collisions must overcome the activation energy, which is the energy needed to reach the transition state of the reaction.

Question 18

Energy diagram

Answer 18

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.

Question 19

Catalyst

Answer 19

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

Question 20

Integrated rate laws

Answer 20

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

Question 21

Integrated rate law for reaction order of 0

Answer 21

Rate does not depend on the concentration of the reactant. [A] = -kt + [A]subscript 0

Question 22

Integrated rate law for reaction order of 1

Answer 22

ln[A] = -kt + ln[A]subscript 0

Question 23

Integrated rate law for reaction order of 2

Answer 23

1/[A] = kt + 1/[A]subscript 0

Question 24

Units of k for zero order

Answer 24

k units are M/s

Question 25

Units of k for first order

Answer 25

k units are 1/s

Question 26

Units of k for second order

Answer 26

k units are 1/(M*s)

Question 27

Units of k for third order

Answer 27

k units are 1/((M^2)*s)

Question 28

Arrhenius Equation

Answer 28

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.

Question 29

Rearrange Arrhenius Equation

Answer 29

Rearranged to follow “y = mx + b” format. ln(k) = [(-activation energy / R) * (1/T)] + ln(A).

Question 30

Rearrange Arrhenius Equation further to compare rate constants at 2 temperatures

Answer 30

ln (k2/k1) = (activation energy / R) * [(1/T1) – (1/T2)]. There is no frequency factor.

Question 31

Half-life equation for zeroth order reaction

Answer 31

t = [A]subscript0 / (2K)

Question 32

Half-life equation for first order reaction

Answer 32

t = ln2 / k

Question 33

Half-life equation for second order reaction

Answer 33

t = 1 / ((k)*([A]subscript0))