Chemistry Unit 5 Flashcards

Kinetics

• the area of chemistry that is concerned with reaction rates

Spontaneous reactions

• reactions that occurs under a specific set of conditions without interference (not necessarily quickly)

Rate unit

M/s or mol/ (L^-1*s^-1)

4 Factors that Affect Reaction Rates

• concentration of reactants

• Temperature

• Physical state of reactants

• Presence of catalyst

Concentration of reactants as factor

• as concentration increases, so does the likelihood that reactant particles will collide. (Bc particles closer together)

• If there is more contact between the reactant particles, the rate will be faster

Temperature as a factor

• as higher temperatures, reactant particles have more kinetic energy and move faster, colliding more. If there is more contact between the reactant particles, the rate will be faster

Physical state of the reactants as a factor

• in order to react, particles must come i contact with each other. If there is more contact (gas>liquids>solids) between reactant particles, then the rate will be faster

Presence of Catalyst

• speed up reactions by changing the mechanism of the reaction (how the particles come tg)

• Not consumed and neither a reactant or product

• Ex. Enzymes, potassium iodide (H2O2 decomposition)

When particles collide, they must…

1. Have enough energy for bonds to break and reform

2. Be oriented correctly for the new bonds to form

Kinetic Molecular Theory/Collision Theory

• molecules collide with each others

• These collisions cases bonds to break and new bonds to form

• Orientation of bonds is important to form new bonds

Average rate of change for A offer the first t units

• change in concentration of a reactant or product per unit time

• Rate = change of [A]/change of t

Average rate of appearance and disappearance

• average rate of appearance = change of [A]/change of time

• Average rate of disappearance = - change of [A]/change of time

Average Rate Stoichiometry

Rate = -1/a change in [A]/change in [t] = -1/b change in [B]/change in [t] = 1/c change in [C]/change in [t] = 1/d change in [D]/change in [t]

Average rate

Slope of secant line

• rise/run

Instantaneous rate

• slope of a line tangent to the cure at given point

Initial rate

• instantaneous rate calculated infinitely close to the beginning of a reaction

• Can be compared to other reactions

• If negative, demonstrates disappearance

Differential rate law

• expresses how rate depends on concentrations

• K[A]^n

• K = rate constant and n = rate order

• n must be determined thru experimentation

Integrated rate law

• expresses how concentration depends on time

Zero order

When rate does not change when the concentration of a reactant changes

First order

• the rate is based on the factor that the concentration of a given reactant changes by (exponent = 1)

Second order

• rate is based on the factor that the concentration of a reactant changes by squared (exponent = 2)

Overall reaction order

Sum of the order of each component/reactant in the rate law

How to find the unit of the rate constant?

Set M/s (rate) = k * M^n. Plugging in n for the overall order of the reaction, you can find the unit of k.

First order differential rate law

rate = k[A]

First order integrated rate law

Ln[A] - ln[A0] = -k*t

Zero order differential rate law

Rate = k

Zero order integrated rate law

[A0] - [A] = - k*t

If plot of ln[A] vs. t is a straight line

• reaction is first order

If plot of [A] vs. time is a straight line

Reaction is zero order

Half life of First-order reactions

• time required for a reactant to reach half its original concentration

Half life of first order reactions

T1/2 = 0.693/k

Second order differential rate law

Rate = k[A]²

2nd order Integrated rate law

1/[A] - 1/[A0] = kt

If the plot of 1/[A] vs. time is a straight line

The reaction is second order

Half life of a second order reaction

T1/2 = 1/(k*[A0])

• half life gets longer as the reaction progresses and the concentration of the reactants dec

• Each successive half-life is double the preceding one

Half life of the zero order reaction

T1/2 = [A]0/2k

Relationship of rate constant to the slope of the straight line produced by first order

slope = -k

Relationship of a rate constant to the slope of straight line in first order reaction

Slope = -k

Relationship of rate constant to the slope of straight line in second order reaction

Slope = k

Rate Law Proportion

Rate1/rate2 = k[A1/A2]^m * [B1/B2]^n

Reaction mechanism

• the series of steps by which a chemical reaction occurs

• Bc a chemical equation does not tell us how the reactants become roducts - its simply a summary of the overall process

Elementary steps

• individual steps that make up the overall reaction Mechanism (add to find overall reaction)

Intermediates in elementary steps

• species formed as a product in one step then used up as a reactant in another step

Catalyst in elementary steps

• a species that is a reactant in one step then a product in another

Molecularity in Elementary steps

• refers to how rate laws are written for individual elementary steps

• Does not refer to the overall reaction order

Unimolecular

• elementary step reaction involving one molecular, first order

Bimolecular

• reaction of elementary steps involving the collision of two species; second order

Termolecular

• elementary step Reaction involving the collision of three species; third order

• Very rare

Rate-determining step

• slowest step of the reaction

• Determines the rate law and molecularity of the overall reaction

Mechanism Evaluation to reach overall reaction equation

• elementary steps can be flipped (rate constant becomes 1/k)

• Elementary steps may be multiplied by a whole number (k doesn’t change)

Order of cancellation

• cancel out catalyst first diagonally from left to right

• Cancel our intermediates next diagonally from right to left

Elementary steps rate laws

• can use stoichiometry to determine order (if 2 before then second order, etc.)

• If the rate law of the RDS includes a catalyst or intermediate then substitute it with a solved equilibrium step

How are the factors that affect rate shown in rate law.

1. Concentration seen in main part with orders

2. Temperature, physical state, and catalyst reflected in k

Activation energy (Ea)

• energy that must be overcome to produce a chemical reaction

• Distance between reactants and top of peak

• Transition state is at these peaks

Exothermic reaction

Products have less energy than reactants b/x energy as heat was lost

Endothermic reaction

• products have more energy than reactants b/c energy as heat gained

Transition state

• a species that exists momentarily as reactants become products

• When a molecule is in its transition state, it will form an “activation complex”

• If A + BC → AB + C

  • Transition state would be ABC, an activation complex for the briefest moment

A in the Arrhenius Equation

• represents the frequency of molecular collisions with the correct orientation to lead to a reaction

How many elementary steps in the mechanism model?

Count number of peaks

Where intermediates on model for chemical kinetics?

• product in on step then a reactant in next step

• Located in valleys of the graph

RDS based on model for chemical kinetics

• highest peak

Catalyst def

• a substance that speeds up a reaction w/o being consumed

Enzyme def

• a large molecular (usually a protein) that catalyzed biological reactions

Homogenous catalyst

• present in the same phase as the reacting molecules

Heterogenous catalyst

• present in a different phase than the reacting molecules

Catalyst does what for pathway of reaction

• allow reactions to proceeds by a different mechanism - a new pathway

• New pathway has a lower activation energy

Caveat to catalysis

• will speed up a reaction but only to a certain point.

• Past a certain point making more reactants won’t change the rate b/c the active sites are all full (zero order at this point)

What specifically does concentration do for the collision model?

• allows for more collisions due to closer proximity

What specifically does temperature for the collision model?

• high temperatures make the average kinetic energy > Ea)

What specifically does the physical state (surface area) do for the collision model?

• looser/more free flowing states provide more surface area and therefore more collisions are possible

Collision theory

• molecules must collide to react

• Collisions must have the correct orientation

• Collisions must have enough energy

Delta E

• change in energy (triangle E)

• Difference of Ep -Er

• Can be negative if exothermic reaction