Exam 3 - Equilibria Flashcards
Explain the Common Ion Effect
The presence of a common ion suppresses the ionization of a weak acid or weak base.
We have the initial concentration of the reactant but we now also have the initial concentration of a product. Polyprotic acids are an example of this.
Henderson Hasselbalch equation
pH= pKa + log( CB/WA)
Can only use if you can assume x is small
used to calculate the ratio of base to acid in order to reach a specified pH. Or to find pH when given amounts.
Buffer
A solution that contains a weak base and conjugate acid or a weak acid and conjugate base . Buffer solutions resist changes to pH.
What makes a “good” buffer?
WA/CB or WB/CA ratio is between 10-1-0.1
must be a lot of WA/CB or WB/CA
the pKa or pKb is near the target pH
Titration
The slow addition of one solution of a known concentration (called a titrant) to a known volume of another solution of unknown concentration until the reaction reaches neutralization, which is often indicated by a color change
Uses a balanced equation and mole to mole ratio
Three types of titraitons
Acid Base- unknown acid with known base
Redox- oxidizing agent with a reducing agent
Precipitation-reactants that form an insoluble product
Types of acid based titrations
Strong Acid, Strong Base
Weak Acid, Strong Base
Weak Base, Strong Acid
In this class we will NEVER titrate a weak acid and weak base. We want to force the equilibria to one side, both weak will never go to completion.
Indicator
A weak acid or base that changes color when ionized or de ionized.
Used to identify when we have reached the endpoint.
How to choose an indicator
Similar to choosing a buffer: we want an indicator with a pKa or pKb near the pH where the A/B ratio changes
How do you calculate the titration of a Strong Acid with a Strong Base?
Stoichiometric relationship
Get both molarities into moles (M x L). Subtract the larger mole from the smaller (which is the limiting agent), and convert back into Molarity (moles/total L). -log of that Molarity. Make sure you know if you are calculating pH or pOH due to larger concentration of H or OH
How do you calculate the titration of a weak acid with a strong base?
Stoichiometric and equilibrium relationship, has a buffer capacity
Get both molarities into moles (M x L). Subtract larger mole from the smaller (limiting agent). Convert back into Molarity (moles/ total L). Now you need an ICE table since a weak acid will not fully dissociate. Need a Ka/Kb to solve! If weak acid, switch to Kb after the half equivalence point.
Equivalence Point
where equal molar amounts of acid and base have been combined
“Amount of moles of titrant equals the amount of moles of unknown”
the point at which the acid has been neutralized completely by the base
Endpoint
The point at which the color of the indicator changes. The point at which we know all of the unknown has been reacted
Molar Solubility (M)
M= moles/Liter
The number of moles of solute in 1 liter of saturated solution (mol/L)
Solubility Product Constant (Ksp)
The equilibrium constant that indicates to what extent a slightly soluble ionic compound dissolves in water
the equilibrium of insoluble ionic compounds. (Ksp= [product][product], no reactant since its solid)
What determines the solubility of a product?
The Ksp.
Ksp < 1.0 x 10^ -5 is insoluble
Ksp from 1.0 x 10^ -5 to 1.0 x 10^ -3 is slightly soluble
K > 1.0 x 10^ -3 is soluble
Selective Precipitation
A technique of separating ions in an aqueous solution by using a reagent that precipitates one or more of the ions, while leaving other ions in solution
Complex Ion
Compounds that can form a series of covalent bons while still maintaining a charged state. Usually form when metal ions dissolve in water.
Very Large Keq: Kf
Ligand
Ions or neutral molecules that bond to a central metal atom or ion. Ligands act as Lewis bases (electron pair donors)
Polydentate Ligand
Ligand that contributes or accepts multiple lone pairs. Metal central atom, usually a transition metal. Ex: EDTA
Formation Constant (Kf)
The equilibrium constant that indicates to what extent complex-ion formation reactions occur
Calculate the pKa given a Kb
1.0 x 10^-14 / Kb = Ka.
-log(Ka)=pKa
What is the importance of the 1/2 equivalence point
It represents the point at which exactly half of the acid in the buffer solution has reacted with the titrant. The half equivalence point is relatively easy to determine because at the half equivalence point, the pKa of the acid is equal to the pH of the solution.
“where moles of WA = moles of CB or vice versa”—- because log(1)=0, pH=pKa
Label on a graph the endpoint and equivalence point
Endpoint is color change
Equivalence point is end of chemical reaction
Identify a weak acid/ strong base titration curve from a strong acid/weak base titration curve
In a weak acid-strong base titration, the pH is greater than 7 at the equivalence point. In a strong acid-weak base titration, the pH is less than 7 at the equivalence point
What is the ratio for mixing buffers?
WA/CB
WA/SB
SB/WA
WA/CB 1:1
WA/SB 2:1
WB/SA 2:1
Explain the graph of a strong acid strong base titration
Explain the graph of a weak acid strong base titration
Explain the graph of a strong acid weak base titration
What is the pH at the equivalence point of:
strong acid strong base
weak acid strong base
weak base strong acid
strong acid strong base: 7.00
weak acid strong base: above 7.00
weak base strong acid: below 7.00
What is Ksp? How does our ice table change
Ksp= solubility product.
Ksp= [prod] [ prod]
solid –> [aq] + [aq ] so ICE table is shifted, ignoring the reactant.
The smaller the Ksp the less soluble it is
If Q is less than or equal to Ksp, will precipitate form?
What if Q is greater than Ksp?
Q </= Ksp No, precipitate does not form
Q> Ksp = Yes, precipitate forms
What is Oxidation Reduction (Redox)
A transfer of electrons. Must take into account oxidation states and elections
An Oxidized substance has _______ electrons while a reduced substance has _____electrons
An Oxidized substance has ___Lost____ electrons while a reduced substance has __Gained___electrons
When an oxidation state INCREASES from reactants to products we say that the substance has been _______
When an oxidation state DECREASES from reactants to products we say that the substance has been _______
When an oxidation state increases from reactants to products we say that the substance has been __oxidized_____
When an oxidation state DECREASES from reactants to products we say that the substance has been __reduced_____
If a substance RELEASES electrons we would call that substance a _________ agent.
If a substance GAINS electrons we would call that substance a ________ agent.
If a substance released electrons we would call that substance a reducing agent.
If a substance gains electrons we would call that substance an oxidizing agent.
Label the reducing and oxidizing agents and find oxidations states
Zn(s) + 2HCl = ZnCl2 + H2
When balancing redox reactions, what two things must each step have before moving on?
Must be balanced by mass and by charge.
“free” Electrons should cancel
Steps to balancing a redox reaction
- Identify oxidation states
- Identify what is reducing step and oxidizing step.
- Balance in acidic solution : Balance by mass and by electrons, adding H+ and H2O as needed
- Balance in basic solution: change the H+ to H2O by adding OH- to that side, then add the same amount of OH- to other side, ensure balanced by mass and electrons
- make electrons equal in both half reactions. Add and cancel equations.
Voltaic cells (Galvanic cells)
thermodynamically spontaneous: have a negative delta G.
Ex: battery usage.
Electrolytic Cells
involve reactions that are thermodynamically nonspontaneous (positive delta G) but have been provided an extra “push” from an external source in order to proceed.
Ex: recharging batteries
Anode
Site of oxidation of the reducing agent.
Loses electrons
Cathode
Site of reduction of the oxidizing agent.
Gaines Electrons
Salt Bridge
Used to allow ions to migrate towards anodes and cathodes as chemical potential changes due to electron transfer.
Usually K+ and NO3-
Draw a voltaic cell transfer with all parts
Always indicate anode first, then salt bridge, then cathode.
ex: Fe/Fe+2 l KNO3 l MnO4-/Mn2+e
anode bridge cathode
Why do we need a salt bridge
because as the anode electrons are moving to the cathode they are decreasing the charge. When it becomes too negative, electrons do not want to continue in that direction (opposites attrack, lechatliers principle..) so the salt bridge keeps the balance to allow more electrons to move right.
Electrochemical Cell Potential
What is its unit of measure?
potential energy residing in an electrochemical reaction
Ex: water building up pressure behind a damn. Higher the damn, the higher the potential energy
Unit of measure is Voltage
Electrochemical Cell
The difference in electric potential between the cathode and the anode
Ecell= Ered + Eoxidation
Red Cat pleases An Ox
(Reduction/Cathode plus Anode/Oxidize)
Electromotive force (EMF)
a difference in potential that tends to give rise to an electric current.
Voltage vs Current
Voltage is the force pushing charges.
Current is the flow rate of charges.
Water example: Voltage is the size of pipe water is flowing through. Larger the current the more electrons are flower. The larger the voltage the more energy that current can have.
How to predict if a reaction is spontaneous
ΔG = −nFEc
n= moles of electrons transfered
F= Faradays constant 96,500 C/mol
Ec=electrochemical standard cell potential in volts
Since n and faradays are always +, the sign of Ec determined spont.
- Ec = + G, so non spont.
+ Ec = - g, so spont
Finding Ecell
Ecell= Ered + Eoxi
Use table 19.1 to find E of ox and red. recognize forward vs backward reactions and change sign as needed.
What is the Nernst Equation and why do we need it?
An equation relating the emf of a galvanic cell with the standard emf and the concentrations of reactants and products
How does changing Q effect Ecell? Use the Nernst equation to predict direction of shift
increase reactants pushes equation forward/spontaneous/ Increase Ecell to make negative G
Decrease reactants pushes equation in reverse, non spontaneous, lower Ecell positive G.
Increase products pushes to reverse, non spont, lower ECell positive G
Decrease products pushes forward, spontaneous, higher ecell, negative G
E> E standard when reactants are more than products
E< E standard when reactants are less than products
If Ecell is positive the reaction is ________
If Ecell is negative the reaction is ______
+ =spontaneous because it results in a - G
- = non spontaneous
What does it mean when Ecell = E^0cell?
when Ecell equals the Estandard cell we are at equilibrium
Florine, at the top right of the PT, is a strong what in terms of activity series and potential?
Florine is a strong oxidizing agent (steal electrons) because it has the largest EN. It will get reduced and has highest reduction potential (top of the list)
What equations are needed when asked to find K given a reaction? (Electrochem)
- Ecell = Ean+ Ecath
- G= -nF Ecell
- G= -RT lnKeq but manipulated
OR use manipulated Nernst :
10 ^ n*Ecell / 0.0592
What 3 things does the Nernst equation allow us to predict?
The electrochemical cell potential under any conditions
the direction the electrochemical cell will proceed when Prod > Reac
The direction the electrochemical cell will proceed when Prod < Reac
What are the two types of rechargeable batteries and the two types of non rechargeable batteries?
Rechargeable: Ni-Cd and Ni-Metal hybrid
Nonchargeable: Dry Cell and Lead Acid
What is electrolysis
A form of electrochemistry that requires an outside source of electrons to drive the redox reaction that otherwise would be nonspontaneous.
ex: H2O Ecell is -2.06. In order for this to be spontaneous (since - Ecell is +G and non spon) an oustide source must provide at least 2.06 V.
Voltaic vs electrolytic cells
Voltaic are spontaneous
Electrolytic are non spontaneous
label the following electrochemical cell notation:
Zn/Zn+2 // H+/H2
Zn –> Zn+2 + 2e- Oxidizing step/ Anode
// salt bridge
2H+ + 2e—> 2H Reduction step/Cathode
List of Strong Acids and Bases
Acids: HCl, HBr, HI, HNO3, HClO4, H2SO4
Bases: LiOH, NaOH, KOH, RbOH, Sr(OH)2, Ba(OH)2
