chem 14B last weeks

redox couple

a couple formed by the oxidizied and reduced state of a singlular species that results from either a reduction half reaction or a oxidation half reaction; the couple is in the same half reaction

oxidizing agent

the one that is reduced or induces oxidation

reducing agent

the one that is oxidizied or induces reduction/causes

nuances to balancing a redox reaction in a acidic environment

-balance O by using h2o on the otherside

-balance H by using H+ (not h3o)

-INCLUDE PHYSICAL STATES

-DONT FORGET TO CANCEL OUT SPECIES PRESENT ON BOTH SIDES OF THE EQUATION

nuances to balancing a redox reaction in a basic environment

-balance o by using h2o

-balance h by using h2o AGAIN, and then to balance h by using OH

-INCLUDE PHYSICAL STATES

-DONT FORGET TO CANCEL OUT SPECIES THAT ARE PRESENT ON BOTH SIDES OF THE EQUATION

oxidation numbers to remember

• elements alone/in elemental form

• group 1 metals

• group 2 metals

• Aluminum

• hydrogen

• oxygen

• flourine (what is the nuance with this element)

• halogens

• 0

• -1

• -2

• +3

• +1

• -2

• -1

• -1

Electrochemical cell

A device in which an electric current is either produced by a spontaneous chemical reaction or is user to bring about a nonspontaneous reaction

Electric current

Flow of electrons

Galvanic/voltaic cell

an electrochemical cell in which a spontaneous chemical reaction is used to generate a electrical current: uses voltage

Voltage

The potential difference/ ability of a galvanic cell to push electrons through a circut

What are the components of a galvanic cell

1. Electrodes

2. Electrolyte

3. Salt bridge

4. Anode

5. Cathode

6. Wire

Electrode

Metallic conductor that makes contact with an electrolyte: a half cell is also called an electrode

Electrolyte

Ionically conducting medium that is in contact with an electrode: carries the electrical current with the movement of ions within the solution

Anode is the site of …. And the cathode is the site of …..

Anode is oxidation

Cathode is reduction

Half cell

the half of a full cell that includes an electrode and electrolyte

Importance of salt bridges

If charge build up occurs on either side of the cell, then the current would stop moving: usuaally cl ions balance out that charge difference in the electrolyte solution

Cell maximum work potential

electrical work is a type of nonexpansion work that a system can do

At constant TEMPERATURE and PRESSURE delta G-= Wemax

Wemax= -nFEcell

The equation for Wemax is only applicable to a cell that is working

Reversibly

Potential difference

The difference in electric potential energy: defines how STRONG the pull of electrons is

Ecell

Potential difference with the galvanic cell

Ecell= delta G/-nF

What does each part of the equatioj for Ecell represent: Ecell=deltaG/-nF

Ecell (volts)

N= number of electrons per mole REACTION

delta G: kj/mole rxn

F=faraday constant

What is the si unit of electric charge/electron charge

Coloumb=1ampherex1sec

1 amphere means

The electron charge current: how much electric charge (coloumbs) flows through a single point per second

Si unit for potential difference

1 volt: 1J/1 coloumb

Nuance about stoichiometric influence on Ecell

• the sixe of the cell does jot influence the Ecell value, although delta G increases by a factor of whatever stoichiometric coefficient the reaction was multiplied by, e cell does not change

• Gou can only increase the ecell value if you make a battery

Cell diagram

-what is always on the left and right

Electrode|Reducing agent|oxidizied form||reduced form|oxidizing agent|electrode

Left is always the site of oxidation

Right is always the sight of reduction

Nuances when drawing a cell diagram

-include phases

-species with the same phase are separated by a comma

Standard potential (E knaught) measures what

Different from Ecell

It is the standard potential of a half cell that makes a contribution to the standard cell

• measures REDUCTION POWER: electron pulling power

What does a negative and positive standard potential (E) indicate

Positive E indicates that the half reaction is likely to get reduced

Negative means less likely: oxidized

How to calculate Ecell from Eknaught

Ecell = Ereduced-Eoxidizied

What does a positive Ecell and negative Ecell mean

Positive: reaction is spontaneous under standard conditions where delta G is negative and K>1 forward reaction is spontaneous

Negative: reaction is nonspontaneous and delta G is positive, forward reaction is not spontaneous, the reactants are favored: K<1

E knaught is measured by what (equation?)

Its reduction potential relative to the SHE cell, SHE is always 0 at all temps

Eelectrode of interest=Einterest-Eshe

Reduction potential or W knaught increases as you go to the right or left of the periodic table

Right

How do you know that the arrangement of Ered and Eox is correct when you are calculating Ecell

If Ecell isnpositive

How to find standard potential of a half reaction not listed

1.convert e values to delta g and manipulate delta g to get the desired half reaction and convert back to ecell

Pattern on electrochemical series: what is the relationship between the reactions that lie above and below

A oxidizied species in the list which would be on the left has a tendency to get recuced by a reduced species below

A reduced species in the right had a tendency to get oxidizied by a species that lies aboce it

What does it mean for Ecell when K=1

Ecell=0 no desire to favor either side of the reaction, the cell stops working

How to find Ecell for a reaction that is not redox

Form the reaction from known half reactions

The reverse reaction for E knaught of a reduction reaction is -E knaught

What equation should you use for calculating Ecell of a concentration cell

The nerst equation Ecell=-RT/nF lnQ

Concentration cells

The two half cells are jdentical excelt for confentration: the side wjth the lower concentration is the anode and the side with the higher is the carhode: seen in precipitation reactions

at equillbirum, Ecell is equal to Eknaughtcell is equal to

Ecell = 0 (bc delta G is derived from delta G=delta G knaught -rtlnQ

E knaught cell= dependent on K/ delta G knaught

Nerst equation

calculates the reduction potential of an electrochemical cell under non-standard conditions, relating it to standard potential (

E0

𝐸0

), temperature (

T

𝑇

), and reaction quotient (

Q

𝑄

). It determines cell potential (

E

𝐸

), predicts ion behavior across membranes, and enables calculation of equilibrium constants

electrolytic cell

An electrochemical cell where electrolysis takes place and the electric current must be supplied by an external power slurfe

How to find the equillbriuk constant from Eknaughtcell

ln(K)= nFEcell/RT

Components of a electrolytic cell

2 electrodes share the same compartment

One electrolyte

Usually leads to depisiting on the electrode or dissolving

requires external energy source

what is required of the external energy source for a electrolytic cell

The external energy must generate a potwntial difference greater than the potential difference of the reaction in the spontaneous direction

Often requires overpotential

Overpotential

The additional potential difference that must be applied beyond the cell potential to cause appreciable electrolysis: extra potential must be supplied on top of the minimum to actually drive the nonspontaneous electrolysis

How to determine which species is reduced and which is oxidizied in a electrolytic cell

The one that is oxidizied will require the least external potential

The one that is required the most external potential will be reduced

Average rate of a reaction equation

the change in the concentration of a reactant/product / time interval of change

relationship between average rate of consumption and average rate of production

average rate of consumption= - average rate of production (assuming same molar stoich)

average rate relationship for this reaction; h2O→2H+0

-2 (average rate of consumption of h2o)= rate of production of H

unique average reaction rate

A unique rate ensures that no matter which species you use, you get the same reaction rate by dividing by the stoichiometric coefficient.; divide by stoichiometric coefficient and multiply by negative sign

reaction rate always refers to

the instantaneous rate; the slope of the tangent of a graph of concentration against time

as the reaction proceeds, the instantaneous rate of reaction _____

decreases

rate constant (kr)

-the same for a reaction at anypoint in time; rate/[A]^order

rate law

kr[reactant]^reaction order

expression for the instantaneous reaction rate at any stage of the reaction

what does it mean for the reaction if it is 1st, 2nd and 0th order

1st order reaction; rate of the reaction is proportional to the 1st power of the concentration; doubling the concentration would double the rate

2nd order reaction; doubling the concentration would quadruple the rate

0th order reaction; reaction for which rate is independent from the concentration of the reactant; as long as reactant is present the rate or kr, is the same; the rate imeediately dips to almost zero when reactant is gone

overall reaction order

value of the overall reaction order determines the units of kr

what the units for the rate law where the overall reaction order is;

0, 1, 2, 3 (in terms of concentration (moles/liter))

when the rate is reported in PP (kpa/s)

concentration

0; mol L-1 s-1

1; s-1

2; L mol-1s-1

3;L2 mol-2 s-1

pressure;

0; kpaxs-1

1; s-1

2; kpa-1 s-1

3;kpa-2 s-1

how to physically increase rate in a reaction

for solutions; increase the cocnentration

for hetergenous phase reactions; increase the surface area

integrated rate law

gives the concentration of the reactants or products at any time (t) after the start of the reaction; unlike rate law that is depdendent on intital)

rate law is the measurement of the reaction at what point in time

the intital/starting concentration

what does the graph look like for the plot of concentration vs. time for 0th, 1st and 2nd order reactions for integrated rate laws; what is the integrated rate law in the form of to prove that a reaction is wihtin that order to gve a linear line on the plot

0th; linear line; rate is constant since concentration independent, the differecnce in concentration is proportional to the time for when the reaction is in progress; the slope is kr

1st; exponential decay; the slope is kr and taking the natural log (ln) of concentrations linearlizes the graph

2nd; also exponential decay, but in comparison to the first ordedr, the concentration of the reactant decreaes rapidly at first and then changes more slowly as the rxn goe sto compoletion; the slope if kr but the concentrations are inverse

integrated rate law is easier to do when

there is only one species representing the rate law

pseudo first order reaction

apply when you want to the integrated rate law for a ratelaw with two species, make the species with the very high concentration as kr’=kr (of the actual rxn) [large conc species'] so that the new rate law that you can integrate becomes; kr’[B]

half life

the time needed for the concentration of ANY intital amount to drop to half

half life for 1st order reactions

half life only depends on the rate constant (not intial conc), so no matter the starting concentration, the time it takes to be the half of that intital amount is ALWAYS THE SAME

where 0th and 2nd order depends on intitial concentration

how to find the concentration remaning after ‘n’ half lifes for a 1st order reaction

(1/2)^n x [Aintial]

molecularity in elementary reactions

shows how in writing out elementary reactions/steps, stoichiometric coefficients are not used; only individual molecules are depicted; unimolecular, bimolecular, tetramolecular

what is the nuance about tetramolecularity

tertramolecularity is the least likely elementary reaction due to low probability of successful collision

what is the order of elementary step rate laws

the order is the same as the number of molecules of that species present as a reactant

what does it mean when an elementary step is regarded to have fast equillbirum (DIFFERENT FROMFAST)

-when writing out the elementary step you must include both the forward and backward reaction and its associated rate laws

-this is an indication that you should use prequillbrium method to get rid of intermediates

methods for removing intermediates from rate law; when do you use what

steady state approximation; a reaction labeled as just fast single sided arrow; no reverse reaqction needed, use net rate of formation equation and equate that to 0

preqeuillbrium; when a reaction is labeled as fast equillbrium with double sided arrow and you also write the reverse reaction

short cuts for determining the overall rate law when looking at elementary steps

-if rate determining step is indicated; labeled as slow, rate determining,

-if there is only one elementary step; use the rate law derived from that elementary step directly

equation for net rate of formation and net rate of consumption; when do you use this; what to not forget to do when writing this out

net rate of formation; sum of rates in its formation-sum of rates in consumption (not the same as the reverse reaction) (consumption is where the intermediate shows up as the reactant)

net rate of consumtpion; sum of rates in its consump - sum of rates in its formation

-you only use it to eliminate out the intermediate in the elementary step that produces the product of the overall reaction

-DO NOT FORGET TO MULTIPLY EACH RATE LAW OF CONSUMP OR FORM BY THE STOCHIOMETRIC COEFFICIENT SEEN ON THE INTERMEDIATE

what is the effect of temp on rate constant

rate constant increases with temperature

the rate contstant increases more dramamtically for reactions with a higher activation energy

arrhenius plot; what does it mean when the line has a steep/ nonsteep slope

plots lnKr vs. 1/T

steeper slope; the higher the activatione energy

collision theory

molecules collide with eachother if 2 molecules collide less than a certain KE they simply bounce off of eachother with no reaction

if they reach Emin, then new bonds are formed

Collision frequency

number of collisions in a given time interval divided by the length of the interval (collisions per second); what is collision frequency between molecule A and B dependent on?

collision cross section; tells us that the greater the area that a molecule presents as a target the higher frequency

mean relative speed; mean speed at which the molecules approach eachother in a gas

According to collision theory the rate constant is dependent on

• collision freq

• collion orientation

• activation energy

• temp

what does A indicate in the eqaution for k=Ae^-Ea/RT

A is the frequency factor of how often molecules collide

Why does the equillbrium constant for a reaction increases and decrease depending on temp, what relationship shows this

K=kr/kr’

-higher the Ea, the more kr increases as there is an increase in temp

-think about it in terms of what k would be as an increase in temp occurs for an exothermic vs, endothermic reaction (in the forward direction)

what is collision frequency theory only applicable to

gases

steric requirement and steric factor (P)

a reactions depednence on relative orientation taken into acount by the value of the steric factor (P)

the more complex the species, the higher the value of the steric factor

transition state theory (what is it applicable to)

-accounts for reactions in both solutions and gases

-states the transition from an activated complex to the formation of a transition state complex

activated complex and transition state complex

activated comlex; two reactants that are ready to react form, atoms are rearranging, old bonds weaken and new bonds partially form

transition statel the highest energy point of the reaction

Catalyst

a substance that inc the rate of reaction with out being consumed in the reaction by providing a alternative pathway with a smaller energy barrier

what are all the factors that increase the rate of reaction/ rate constant

Factor	Changes Rate?	Changes kr?
Temperature	Yes	Yes
Catalyst	Yes	Yes
Concentration	Yes	No
Pressure (gases)	Yes	No
Surface area	Yes	No
Nature of reactants	Yes	Yes

how to find the equillbrium constant of an overall reaction from elementary steps

use K=kr/kr’ and multiply; Koverall= K1K2 etc.

do you include h2o (l) in the rate law

no, you do not include solvents

when do you still include intermediates in the rate law

• when the substitution one of the intermediates in the main rate law includes a intermediate

Arrhenius equation problem comparing two temperatures; what does the relationship between k2 and k1 are usually indicated as

ln(k2/​k1​​)=Ea​​/R(1/T2​​−1/T1​​)

relationship between activation energy for forward and reverse reaction to determine if the reaction is endothermic or exothermic

ΔH=Ea​(forward)−Ea​(reverse)

delta H>0 endothermic

delta H<0 exothermic

what does a catalyst look like on the reaction

appears as a reactant and is produced on the other side while the intermediate is produced and then consumed

what does the addition of a catalyst do to rate of reaction, K, activation energy

Quantity	Effect of catalyst
Forward rate	↑ increases
Reverse rate	↑ increases
Activation energy (forward)	↓ decreases
Activation energy (reverse)	↓ decreases
Equilibrium constant KKK	unchanged
ΔH	unchanged

what is the equation to determine the relationship between the rate of reaction constant of a catalyzed and uncatalyzed reaction

kcat/kuncat = e^(Ecat-Euncat)/RT

dont forget the negative sign in Ea

in arrhenius

Ecell and E knaught cell both do not change even if the coefficients for the reaction change

d

what is the E knaught of a SHE cell awlays

0