CHEM 112 - MIDTERM III Flashcards by Yasmine Saraf

When titrating a weak acid (HA) with a strong base (OH-) what is left in the solution & equivalence point?

left in the solution: H2O + A- (weak conjugate base)
-equivalence point: moles OH- added = moles HA in original solution

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When titrating a weak base (A-) with a strong acid (H+) what is left in the solution & equivalence point?

-left in the solution: H20 + HA (strong conjugate acid)
-equivalence point: moles H+ added = moles A- in original solution

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When we add a strong acid to the buffer, what happens?

Neutralization reaction occurs; acid is neutralized, consuming the conjugate base and producing conjugate acid –> [A-] decreases and [HA] increases

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When we add a strong base to the buffer, what happens?

[A-] increases and [HA] decreases

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Divide moles by total volume of solutions to find new concentrations of [HA] and [A-] AFTER neutralization is complete
Plug in equilibrium equations in Henderson-Hasselbalch equation to find pH

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Calculation of pH in the buffer region

Write out dissociation rxn (find out which reagent is being consumed, in a weak acid (first) and strong base, the base will be consumed)
Calculate the moles of HA (initial HA)
Calculate moles of OH- (initial OH-)
Use modified ICE table (using moles and subtract OH- from initial weak acid moles)
Use total volume of solutions to find new concentrations of the weak acid and conjugate base
Use H-H equation

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Calculation of pH at the equivalence point

Determine # of moles of OH- to get to the equivalence point
Determine the volume of OH- needed to get the moles calculated in (1)
Determine the new total volume
Determine the new concentration of the conjugate base (A-)
Complete a pH calculation for a weak base with the initial concentration determined in (4)

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Calculation of pH beyond equivalence point

Beyond the equivalence point, OH- is in excess. Calculate [OH-] by subtracting the initial number of moles of H3O+ from the number of moles of added OH- and by dividing by the current total volume
Calculate pOH and use this to find pH

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Between the initial pH and equivalence point, the solution becomes a _____.

buffer

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Halfway to the equivalence point the pH = ____

pKa

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In the buffer region, we use the stoichiometry and the _____ equation to calculate the pH

Henderson-Hachelbach

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At the equivalence point, the acid has been converted to conjugate base. We calculate pH by _____

Working an equilibrium problem for the ionization of water by the ion acting as a weak base

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Beyond the equivalence point, OH- is in excess, we calculate the pH by ____

Calculating [OH-] by calculating moles of OH- and subtracting from the acid (HA) and dividing the moles by the total volume. Find pOH and subtract from 14 for pH

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Salts are ____

ionic compounds dissolved in water to form aqueous solutions

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The extent of a dissociation describes the _____ of the ionic compound

solubility

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If an ion does NOT have a Ksp value, what does that mean?

They are fully soluble in solutions

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Salts that contain _____ are almost always soluble

Na+, K+, Cl-, and NO3-

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Most other salts are ____ to some degree

insoluble

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Ksp

Solubility product constant; measures how soluble a compound is

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If Ksp is very small then ____

the ion concentrations are very small and the compound is not very soluble

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Solubility products are usually highlighted by ____ arrows

double

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Molar solubility

the number of moles of solute per 1 L of a saturated solution (mol/L)

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Solubility

the number of grams of solute in 1 L of a saturated solution

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When Q is ____ than K, a precipitate will form

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If there are two possible precipitates that can form, the one with the ____ Ksp will precipitate first

smallest

If the concentrations of ions in solution of a slightly soluble salt (one with a Ksp) exceed the value of the ____ of that salt, a precipitate will form

molar solubility

Entropy (S)

The level of disorder in the system (ie. disordered bedroom - positive entropy because it takes energy to put your room back in order)

Entropy increases if the state of matter changes in the following directions:

Favoring free particle movement!! solid -> liquid solid -> gas liquid -> gas

Entropy also increases if the _____

number of gas particles increases (or level of disorder for particles increases)

Entropy increases if the ____ of a reaction increases

temperature

Entropy increases specifically with a _____ reaction

exothermic (releasing energy to surroundings)

Entropy decreases with a _____ reaction

endothermic (heat is taken from surroundings)

Enthalpy (Delta H)

Describes how much heat is exchanged with the surroundings

A spontaneous process is one that _____

occurs without outside intervention

Entropy of the universe

Entropy of the rxn + Entropy of the surroundings

Entropy of a rxn can be spontaneous if the ____ increases to a great extent

Delta S of the surroundings (ie. freezing water)

Gibbs Free Energy (Delta G)

Describes total energy change for the system

G < 0

spontaneous

G > 0

non-spontaneous

Delta G will move in the direction that is _____ to reach equilibrium

spontaneous (ie. the negative forward rxn vs. the positive reverse, etc.)

If the rxn is ALWAYS spontaneous, we can assume that entropy and enthalpy are ____

both negative!

If the rxn is ALWAYS non-spontaneous, we can assume that entropy and enthalpy are ___

both positive!

If the temperature of the surroundings increase, the rxn must be ____

exothermic

If the temperature of the surroundings decrease and absorb heat, the rxn must be _____.

endothermic

Changing from liquid to gas will result in a ____ enthalpy

positive (endothermic) bc it takes energy to vaporize

G Naut indicates the ____

Gibbs free energy change under STANDARD conditions (ie. 25 degrees Celsius)

Oxidation

When an atom/element LOSES electrons (LEO)

Reduction

When an atom/element GAINS electrons (GER)

Oxidizing agent

Oxidizes another substance (and itself is reduced)

Reducing agent

Reduces another substance (and itself is oxidized)

Procedure for Balancing Redox Reactions

1. Determine what is being oxidized and reduced 2. Write separate half reactions to balance 3. Balance atoms by adding H2O to balance O, balance H by adding H+ and balance the charge of each half reaction by adding electrons 4. Make the number of electrons in each half reaction the same to cancel them out of the entire reaction 5. Add the two half reactions together 6. (IF BASIC): add OH- to each side to neutralize H+

In a Galvanic Cell, electrons are produced at the _____

anode

Oxidation occurs at the _______ in a galvanic cell

anode

In a Galvanic Cell, electrons are accepted at the _____

cathode

Reduction occurs at the ____ in a galvanic cell

cathode

Salt Bridge

allows bystander cations and anions to move from one compartment to the other

Electrodes

the solid metal bars connected by the conductive wire of a battery (allows electrons to flow)

In the galvanic cell, electrons flow from the _____ to the ____

anode to cathode (ACE)

Cell diagram convention w/ single and double lines

(anode solid) l (anode ion) ll (cathode ion) ll (cathode solid) where the double line represents the salt-bridge separating the two half reactions

Standard cell potential (emf)

Ecell = E (cathode) - E (anode) ; the difference of the standard reduction potentials of the cathode and the anode

Standard reduction potentials are _____ prooperties

intensive properties; independent of the stoichiometry applied with balancing the redox reaction

The more positive Ecell value is the _____, which includes the ____ reaction

cathode (pawwwsitive); reduction

The reduction reactions have the electrons on the ____ side of the reaction

reactant

Diagonal Rule Of Ecell Half Reactions

A species on the left of a given half-cell reaction will oxidize the product of the second reaction The more positive the Ecell potential, will oxidize the other rxn's solid and will reduce itself

Ecell > 0, the redox reaction is _____

spontaneous (G < 0)

Ecell < 0, the redox reaction is _____

nonspontaneous (G > 0)