Unit 5: Kinetics

5.1 Reaction Rates, 5.2 Introduction to Rate Law, 5.3 Concentration Changes Over Time, 5.4 Elementary Reactions, 5.5 Collision Model, 5.6 Reaction Energy Profile, 5.7 Introduction to Reaction Mechanisms, 5.8 Reaction Mechanism and Rate Law, 5.9 Pre-Equilibrium Approximation, 5.10 Multistep Reaction Energy Profile, 5.11 Catalysis

kinetics

rate at which reactions occur and the factors that affect them

what happens to concentration of reactants and products as reaction progresses

[reactant] decreases, [product] increases

rate unit

usually M/s

rate with respect to reactant A

-\frac{\Delta\left\lbrack A\right\rbrack}{\Delta t}

rate with respect to product B

\frac{\Delta\left\lbrack B\right\rbrack}{\Delta t}

why is the reactant’s rate negative

it is being consumed, so the concentration at the end is lower than the beginning

what must occur in order for a reaction to occur (collision theory)

particles must collide with correct orientation and sufficient energy

how does increased SA (breaking up into pieces) affect reaction rate

increase frequency of collisions, increase rate

how does increased concentration of reactants affect reaction rate

more molecules, increase rate

how does increased pressure (in gas) affect reaction rate

forces particles closer together, increase rate

how does decreasing the volume (of gas) affect reaction rate

forces particles closer together, increase rate

how does increased temperature affect reaction rate

molecules move faster & have more kinetic energy, increase rate

how does using a catalyst affect reaction rate

lowers activation energy, increases rate

rate law

rate = k [reactant₁]ⁿ[reactant₂]^m

what does k represent in rate law

rate constant

initial rate

reaction rate as close to t = 0 as possible

reaction order

how impactful reactant is on the rate of reaction

what happens if you double concentration of a 0 order reactant

rate stays the same

what happens if you double concentration of a 1st order reactant

rate doubles

what happens if you double concentration of a 2nd order reactant

rate quadruples

overall order of a reaction

add up the individual orders of the reactants

how to determine rate law experimentally

find trials where only one reactant changes concentration, then calculate its order based on how the rate of reaction changes

how to find units for rate constant

1/M^xs, where x = overall order - 1

what does 0 order graph look like

linear when y-axis is [A], slope is negative

what does 1st order graph look like

linear when y-axis is ln[A], slope is negative

what does 2nd order graph look like

linear when y-axis is 1/[A], slope is positive

what is half-life associated with

1st order reactions (change exponentially)

elementary reactions

series of simple reactions

reaction mechanism

sequence of steps in a reaction

rate-determining step in elementary reactions

slow step

when can we directly derive rate law from the equation

if the slow step is the first step, coefficient of reactants is their order

which temperature is greater?

T₂ because there are more collisions/molecules at the activation energy

label

A - reactants

B - activation energy

C - intermediate

D - transition state/activated complex
E - products

F - ΔH

G - slow step

H - fast step

how to tell if reaction is exo or endothermic

positive ΔH is endothermic, negative ΔH is exothermic

relationship between temperature and rate constant

direct according to the Arrhenius equation

intermediate

produced during the reaction, then immediately used up; concentration stays relatively low; does not appear in overall equation

catalyst

speeds up reaction without chemically changing, present in beginning and and ending steps

pre-equilibrium approximation

used to determine rate law if the first step is fast and reversible and the second step is slow; use substitution

how do you deal with a -[product] when performing pre-equilibrium approximation

put it in the denominator of the rate law

how does catalyst speed up reaction

lowers activation energy, can provide alternate mechanism, can assist in orientation of reactants

types of catalysts

heterogeneous, homogeneous, enzymatic

heterogeneous catalyst

usually metals, interacts with reactants on the surface

homogeneous catalyst

catalyst fully mixes with reactants, same phase

enzymatic catalyst

contains an active site that facilitates the breaking/formation of bonds, drastically lowers activation energy