D - Ions Flashcards

1
Q

What is the equilibrium expression for reaction

aA + bB → cC + dD?

What are the units of K?

A

K = ([C]c[D]d)/([A]a[B]b)

Units of K are variable, but for the expression K has no units

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2
Q

What is the expression for reaction quotient Q for reaction

aA + bB → cC + dD

and how is it different to K?

A

Q = ([A]a[B]b)/([C]c[D]d)

Q gives the status of the reaction by mass balance.

Q can have many values whereas K only has one.

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3
Q

What happens when Q < K, Q > K and Q = K?

A

Q < K → the reaction will favour the products

ΔG = negative

A + B → C + D

Q > K → the reaction will favour the reactants

ΔG = positive

A + B ← C + D

Q = K → the system is at equilibrium

ΔG = 0

A + B ⇌ C + D

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4
Q

What is the Arrhenius definition of acids and bases?

A

Acid → increases the concentration of H3O+

Base → increases the concentration of OH-

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5
Q

How are pKa and pKb related to the strength of the acid or base?

A

Low pKa = equilibrium lies to the right = strong acid

Low pKb = equilibrium lies to the right = strong base

pKa and pKb are the pH at the equilibrium point; the lower the value of pKa/b, the sooner the species will dissociate

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6
Q

What are the equations for Ka and Kb?

A

HA(aq) + H2O(l) → A-(aq) + H3O+(aq)

Ka = ([A-][H3O+])/[HA]

BOH(aq) → B+(aq) + OH-(aq)

Kb = ([B+][OH-])/[BOH]

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7
Q

What is a buffer?

A

Buffers resist change in pH by providing a source and sink for protons.

The solution contains roughly equal amounts of a weak acid and its conjugate base.

The pH will be roughly maintained when small amounts of acid or base are added.

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8
Q

What are the steps involved in designing a specific buffer?

A

1 → Choose an acid + conjugate base that has its pKa ≈ pH needed

2 → Calculate the ratio of HA : A- using the Henderson-Hasselbalch equation.

3 → Check with pH meter and adjust if necessary.

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9
Q

What happens when a sparingly soluble salt dissolves in water, and what is the equilibrium expression?

A

An equilibrium is set up between the undissolved salt and the dissociated ions:

MX(s) → M+(aq) + X-(aq)

Ksp = [M+][X-]

(The reactant is not included because it is presumed to have a concentration of 1M)

MX2(s) →​ M+(aq) + 2X-(aq)

Ksp = [M+][X-]2

Ksp → solubility product

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10
Q

What happens when a sparingly soluble electrolyte is dissolved in one of its ions?

A

The solubility decreases a lot.

Eg. AgCl dissolving in 0.1M NaCl

As the concentration of Cl- is a lot higher than it would be on dissolving in water, the concentration of Ag+ must be lower in order to keep the value of Ksp constant.

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11
Q

What is the difference between voltage and current?

A

Voltage → the driving force of a reaction; gives the direction of a reaction

Current → gives the quantity of electricity

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12
Q

What is the generic form of an electrochemical cell (no Pt)?

A

(Red)(s) | (Ox)(l) ‖​ (Ox)(l) |​ (Red)(s)

Left(Red) + Right(Ox) → Left(Ox) + Right(Red)

‖ = salt bridge

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13
Q

What is the generic form of an electrochemical cell with Pt, and why is Pt used?

A

Pt | (Red)(l) , (Ox)(l) ‖​ (Ox)(l) ,​ (Red)(l) | Pt

Left(Red) + Right(Ox) → Left(Ox) + Right(Red)

The Pt provides a solid surface for the reaction to occur on

‖ = salt bridge

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14
Q

How are electrode potentials measured?

A

They are all measured against a standard Hydrogen electrode, which is used as a reference because it has a voltage of 0V.

Electrode reaction: H+(aq) + e- → ½H2(g)

The H2 gas must be at standard pressure (1 atm) and the acid at standard concentration (1M).

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15
Q

Why might testing electrodes with biological applications involve using 2 standard electrodes?

A

The standard Hydrogen electrode will have a very low pH due to the concentration of H+ required.

There may be another electrode with the same reaction as the standard H2 electrode but at pH 7.0.

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16
Q

What is the equation for calculating the cell electromotive force (EMF)?

A

CELL = EºRIGHT - EºLEFT

or

CELL = EºRED - EºOX

17
Q

What is the equation for calculating Gibb’s free energy from Eº?

A

ΔGº = - n F Eº

n = number of electrons

F = Faraday constant (96485.332 C mol-1)

Eº = standard redox potention (V)

18
Q

How can cell measurements at non-standard conditions be corrected to standard conditions?

A

Nernst equation:

E = Eº - [RT/nF] ln Q

where

E = measured EMF

Q = reaction quotient

[RT/F] = 0.02569V at 25ºC

19
Q

What is an ion-selective electrode?

A

It has an electrode potential dependent on one specific ion in an external solution, which develops a junction potential with the internal solution also containing that ion.

20
Q

What is a glass electrode?

A

Aka a pH probe.

It is sensitive to H+ ions.

Usually consists of an Ag/AgCl electrode dipping in HCl solution, which is contained in a tube with a thin-walled glass bulb at the end.

This bulb is made of special conducting glass that allows diffusion of H+ ions and, therefore, allows a junction potential to develop across its walls.

21
Q

What is the difference between a galvanic cell and an electrolytic cell?

A

Galvanic cell → produces electricity. The reaction will proceed until equilibrium.

Electrolytic cell → A non-spontaneous reaction is driven by an external source of current.

22
Q

What is the Nernst equation?

A

E = Eº - (RT/nF) ln Q

23
Q

How do you convert between Amperes and moles of electrons?

A

Amperes x time = Coulombs

96495 Coulombs = 1 Faraday

1 Faraday = 1 mole of electrons