chem 4 (kinetics/equilib)

what is a reaction rate equation

Δ[ ] / Δtime

[ ] is molarity of whatever’s inside

4 factors that increase the rate of a chemical reaction

1) Increasing temp ( more KE, faster, more collisions, reacts faster)

2) Add a catalyst (new pathway that is a faster reaction)

3) Increasing molarity of reactants (more reactants means more collisions and reactions, so faster)

4) decreasing the particle size of reactant (smaller particle size is more surface area, so more collisions) (the powder form of a metal will be much faster than a solid chunk) this is for heterogeneous mixtures (diff phases)

5) stirring a mixture of different phases (heterogeneous) will increase interaction and access

Differentiate law eqn

rate = k[ X ]^m

k is rate constant

[ X ] is reactant concentration

n, m, etc is the e order of reactants (how rate is affected by the reactant)

when do you use differential rate laws

when you have concentrations and rate, and only works for forward rxns

How do you solve for order of reactants (m, n, etc)

1. Find rate law for different experiments where one of the reactants is the same for both experiments

2. Set up a ratio of the different equations and cancel out what’s the same

3. You’ll have something like: 2 = 2^n

4. and then figure out n based on this

Repeat for order of each reactant

how do you find overall reaction order

sum of the reactant orders (sum of exponents m, n, etc)

How do you determine k (and units) for differential rate laws

choose any experiment’s rate law and plug in numbers!

to find units include units for the rate and the concentrations (exponents apply to labels too!!) and then solve to find what the units are (may need to cancel stuff from fraction, or multiply reciprocal from bottom, etc)

1) 2 = 2^n ; n=?

2) 8 = 2^m ; m=?

3) 4=2^o ; o=?

n=1

m=3

o=2

What happens with a zero order reaction

The rate doesn’t change, thus the reactant doesn’t appear in the rate law (if [reactant] changes and rate doesn’t, it’s zero order

When are integrated rate laws used?

When given concentration and time (when conc. depends on time)

We will only be dealing with zero, first, and second order reactions

integrated rate law for 0 order reaction

[A]t - [A]0 = -kt

integrated rate law for 1st order reaction

ln[A]t - ln[A]0 = -kt

integrated rate law for 2nd order reaction

1/[A]t - 1/[A]0 = -kt

how do you know which order it is based on integrated rate law graph?

if the equation for the rate law produces a linear graph (NOT curved)

what is a half life (for a 1st order reaction(only one you need))

time required for a reactant to reach HALF its original concentration

formula for half life

t ½ = 0.693 / k IN AP PACKET

While you don’t need to know the equation for a 0 or 2nd order half life, how can you determine if it is NOT first order

If the half lives vary (different time for it to go from 100→50 than 50→25) for second and 0 order, so if it is not half for SAME amount of time it is not first order

how do you solve for k using integrated rate law when given time vs concentration?

plug in the times and concentrations into specific equation for order of reaction and solve for k

What are units for k in an integrated rate law?

1/units of time (1/sec, 1/hr, etc)

how do you rearrange half life to solve for k?

k = 0.693/t ½

0 order differential rate law eqn

Rate = k

1st order differential rate law eqn?

rate = k[A]

what is the eqn for second order differential rate law?

rate = k[A]²

how do you determine the rate law for a reaction if you know the elementary steps and slow step?

use the coefficients of reactants in the slow step to determine the exponent of said chemical in the rate law. reactants to power of coefficients of SLOW STEP

what is the rate determining step of elementary steps in a chemical reaction?

the slow step (the reaction rate can only go as fast as the slowest step)

For a reaction mechanism to be “correct” what two criteria need to be met?

1. sum of elementary steps must add up to overall reaction (hess law kind of canceling if something is on both sides of diff elem. steps)

2. the rate law for the overall r n must agree with the rate determining (slow) step

rates are ALWAYS

positive!

to determine the rate of consumption of a particular chemical

find the concentration of a certain chemical and then use the coefficients to determine if the concentration of OTHER chemicals are same, half, double, etc. based on coefficients, and compare products and reactants to determine how fast a certain chemical disappears

Zero order reactions have a (pos/neg) slope

negative (-k)

First order reactions have a (pos/neg) slope

negative (-k)

second order reactions have a (pos/neg) slope

positive (+k)

What is the molecularity and rate law for a mechanism with an elementary step: A → products

unimolecular, Rate = k[A]

What is the molecularity and rate law for a mechanism with an elementary step: A + A → products or 2A → products

bimolecular, Rate = k[A]²

What is the molecularity and rate law for a mechanism with an elementary step: A + B → products

bimolecular, Rate = k[A][B]

What is the molecularity and rate law for a mechanism with an elementary step: A + A + B → products (2A + B → products)

termolecular, Rate = k[A]²[B]

What is the molecularity and rate law for a mechanism with an elementary step: A + B + C → products

termolecular, Rate = k[A][B][C]

If the fast step comes before a rate determining (slow) step

it will start to reverse while the slow step occurs, and if the product of this step is a reactant in the next (it’s an intermediate) sub the reactants in the fast step for the intermediate reactant in the slow step

Chemical reactions _____ when temp is increased

speed up

what is activation energy

the minimum amount of energy that must be overcome to produce a chemical reaction

What two conditions must be met for an effective collision to occur

1. collisions must have enough activation energy

2. reactants must be oriented correctly

More activation energy means a _____ rxn rate

slower

less activation energy means a ____ rxn rate

higher

Why does a higher temp increase the rxn rate

more molecules have enough energy to overcome activation energy threshold and will therefore have more successful

collisions faster

what does each letter represent in the following graph:

A: reactants

B: products

C: Activation energy

D: Overall ΔE (positive, but can be negative)

what is an activated complex, where is it on a reaction progress graph

an aggregate of all of the reactant atoms in one big clump, located at the top of the hump of activation energy

What is an intermediate?

something that appears in elementary steps of a mechanism first as a product and then again as a reactant

what is a catalyst?

Substance that speeds up a reaction without being consumed itself, gives another pathway for a reaction that has a lower activation energy

how do you determine a catalyst in a mechanism

it appears over multiple elementary steps, first as a reactant, then as a product

what does a reaction progress and energy graph look like for a multi step mechanism

it will have two humps compared to one big one with two different steps

What is chemical equilibrium

The state where concentrations of reactants and products are constant with time (rate of forward reaction proceeeds at same rate as reverse reaction), a system is at equilibrium when it can no longer do work ΔG = 0

What is the equilibrium constant?

CAPITAL K, tells you the rate at which a reaction will proceed to completion

what is the equilibrium expression formula if

jA + kB ⇌ lC + mD

K = [C]^l [D]^m / [A]^j [B]^k

PRODUCTS OVER REACTANTS (concentrations to power of coefficients)

if K > 1 then ____ are favored

products are favored (forward reaction is favored)

if K < 1 then _____ are favored

reactants are favored (reverse rxn is favored)

K_c is

Equilibrium constant for molar concentrations

K_p is

Equilibrium constant for partial pressures

K_sp is

solubility product (no denominator because the reactants are solid)

K_a is

acid dissociation constant for weak acids

K_b is

base dissociation constant for weak bases

K_w describes

ionization of water (K_w = 1E^-14)

K modifications:

if the rxn is reversed, K will be

1/K (reciprocal)

K modifications:

if coefficients are changed by n, K is

raised to the power of n

halved: K^1/2

doubled: K²

K modifications:

when reactions are ADDED together then K overall will be…

the product of K for each individual step (K overall = K1 * K2)

for the rxn jA + kB ⇌ lC + mD, Kp = ?

(Pc)^l (PD)^m / (PA)^j (PB)^k

if pure solid or pure liquids are in a chemical equation, for equilibrium expressions they are…

NOT USED (concentrations stay the same)

What is Q

the constant that is used for initial concentrations (K but when the system is NOT in equilibrium)

if something says [H2]₀ that means

initial concentration

determining which way a reaction will shift:

Q = K

no shift (already at equilibrium)

determining which way a reaction will shift:

Q > K

Left shift, initial has too much product and a larger numerator

determining which way a reaction will shift:

Q < K

right shift, because there is too much initial reactant and a larger denominator

In a chemical reaction, equilibrium occurs when (using G):

Gproduct = Greactant

Gproduct - Greactant = 0

or 🔺G = 0

(when it no longer does work because nature is lazy and wants the lowest possible energy state which is 🔺G = 0)

not every reaction has to reach completion (all product) in order to be at equilibrium

formula for ΔG involving K

ΔG = -RTln(K)

if ΔG = 0 (at equilibrium), K=?

K=1 (equal concentrations of reactants and products, equilibrium)

if ΔG < 0 (-ΔG or SPONTANEOUS (reacts readily)) then K is

K > 1, there is more product

if ΔG > 0 (POSITIVE ΔG or non spontaneous) then K is

K < 1, more reactant

What are the steps to this problem?

1. solve for overall ΔH (sum ΔH products times coefficients) minus (sum of ΔH reactants times coefficients

2. Solve for overall ΔS (same as ΔH, sum of products - reactants)

3. Solve for ΔG (ΔG = ΔH-TΔS)

4. Solve for K (ΔG = -RTlnk, plug in G, R and T)

Steps to solve equilibrium problems (ICE problems)

1. if you have nonzero initial concentrations for everything, find Q and compare to K to determine left or right shift

2. Complete ICE table (initial concentrations, change (-x, 2x, x, -2x, etc), equilibrium (take the initial plus or minus the change)

3. solve for x (based on K or based on info given for equilibrium concentrations)

4. Substitute in X into final row of ice table

Three types of ice problems

1. Solve for K (find equilibrium concentrations with ice table and plug into equilibrium expression)

2. If you are given a larg K value and initial concentrations, and asks to calculate equilibrium concentrations

3. if you are given a small K value and asks to calculate equilibrium concentrations

What is a TINY K value?

K< x10^-2

what does a tiny K value mean

when you see +x, - x, +2x, -2x, etc, cross them out when solving for x they won’t impact answer

What is Le Chateliers principle

If a system AT equilibrium is subject to stress, it will shift to reduce that stress

Le chateliers:

Add a substance, how will it shift

it will shift AWAY from added substance to “use it up”

substances can include chemicals or energy (temp)

Le chateliers:

Remove a substance and it will shift

TOWARD that substance to replace what was removed

substances can include chemicals or energy (temp)

Le chateliers:

if a pure solid or liquid is affected, there will be ____ shift

NO SHIFT (concentrations are constant)

Le chateliers (pressure):

Add or remove a gaseous substance, shift or no

YES SHIFT, away/toward affected substance

Le chateliers (pressure):

Change in volume of container, yes or no shift

YEA SHIFT, will shift to undo this

Le chateliers (pressure):

Add an INERT (last column) gas, yes or no shift?

NO SHIFT

Le chateliers (pressure):

Volume of container decreases, how will it shift

it will shift to DECREASE PRESSURE (less space=more pressure on gasses, so it wants to undo that) it will go towards side with LESS mols of gas

Le chateliers (pressure):

Volume of container increases, how will it shift?

it will shift to INCREASE pressure, more space means less pressure in gasses so it will want to undo this, thus shifting toward the side with MORE mols of gas

Le chateliers (pressure):

How do you determine HOW to increase or decrease pressure

Look at the gaseous species in the reaction and their coefficients, find out which side has more coefficients (mols), and to inc pressure it will shift toward larger mol side and to dec pressure it will shift toward smaller mol side

If there is an endothermic process (+ΔH) energy is a…

Reactant

If there is an exothermic process (-ΔH) energy is a…

product

ONLY a ______ causes the value of K to change

CHANGE IN TEMPERATURE, because Energy (temp) is not a Chemical that is inducing a shift, therefore when the reaction shifts to change the energy, the amount of chemicals are the same, and it will affect the overall ratio of reactants and products when it shifts which will affect K

When a stress is placed on a system, even when equilibrium is re established, the stress is..

never FULLY eliminated

Pressure changes only affect…

gas

good luckkkk

😝

what would a graph look like for the effect of a catalyst on a reaction with activation energy

one higher hump and one smaller hump, representing with and without a catalyst

K (equilibrium constant) will ONLY change from a…

CHANGE IN TEMPERATURE, there is one value for K for a system at a certain temp, change the temp and the K will change, otherwise the equilibrium will be maintained at the same constant