Chemistry Exam 3
ΔGsys = ΔHsys - TΔSsys
The Gibbs free energy (G) combines the enthalpy and entropy of a system
How to calculate ΔG°
- Gibbs free energy - standard free energy of formation
Predicting the Sign of ΔS° rxn
- positive if the amount of gas increases - negative if the amount of gas decreases - likely to be positive if a new structure forms that has more freedom of motion
S° increases
- with increasing temperature - from solid to liquid to gas - as atomic (or molecular) size increases - with chemical complexity - when two or more gases mix
How do cations move through the salt bridge?
from the anode solution to the cathode solution
ΔG = ΔH - TΔS: Reaction becomes spontaneous as T decreases if
- ΔH < 0 and ΔS < 0 - ΔG becomes more negative as T decreases
ΔG = ΔH - TΔS: Reaction is spontaneous at all temperatures if
- ΔH < 0 and ΔS > 0 - ΔG < 0 for all temperatures
ΔG = ΔH - TΔS: Reaction is nonspontaneous at all temperatures if
- ΔH > 0 and ΔS < 0 - ΔG > 0 for all temperatures
ΔG = ΔH - TΔS: Reaction becomes spontaneous as T increases if
- ΔH > 0 and ΔS > 0 - ΔG becomes more negative as T increases
Spontaneous exothermic processes include
-freezing and condensation at low temperatures -combustion reactions -oxidation of iron and other metals
Spontaneous endothermic processes include
-melting and vaporization at higher temperatures -dissolving of most soluble salts
ΔG = -RTlnK
-only for standard conditions - a small change in ΔG causes a large change in K - as ΔG becomes more positive, K gets smaller -as ΔG becomes more negative, K gets bigger
Standard conditions
1 atm, 1 M, Q = 1
How do anion move through the salt bridge?
from the cathode solution to the anode solution
A voltaic cell prepared using aluminum and nickel has the following cell notation. Al(s) | Al3+(aq) || Ni2+(aq) | Ni(s) Which of the following reactions occurs at the anode? A. Al(s) → Al3+(aq) + 3e- B. Al3+(aq) + 3e → Al(s) C. Ni(s) → Ni2+(aq) + 2e- D. Ni2+(aq) + 2e- → Ni(s) E. None of these choices are correct.
A
Consider the a graph with a negative slope that shows ΔG° for a chemical process plotted against absolute temperature. From this plot, it is reasonable to conclude that A. ΔH° > 0, ΔS° > 0 B. ΔH° > 0, ΔS° < 0 C. ΔH° < 0, ΔS° > 0 D. ΔH° < 0, ΔS° < 0 E. None of these choices are correct
A
Consider the reaction CuO(s) + H2 (g) → Cu(s) + H2O(l) In this reaction, which substances are the oxidant (oxidizing agent) and reductant (reducing agent), respectively? A. CuO and H2 B. H2 and CuO C. CuO and Cu D. H2O and H2 E. None of these choices are correct
A
The line notation, Pt | H2 (g) | H+ (aq) || Cu2+(aq) | Cu(s), indicates that A. copper metal is a product of the cell reaction. B. hydrogen gas (H2 ) is a product of the cell reaction. C. Cu is the anode. D. Pt is the cathode. E. Cu2+ is the reducing agent.
A
Entropy (S)
A measure of disorder or randomness.
Voltaic cell (galvanic cell)
A type of electrochemical cell that uses a spontaneous redox reaction (ΔG < 0) to produce electrical energy (electricity)
Notation for a voltaic cell
Anode on left, cathode on right; the single line shows a phase boundary between the components of a half-cell; the double line shows that the half-cells are physically separated
Temperature's effect on vapor pressure
As temperature increases, the fraction of molecules with enough energy to enter the vapor phase increases, and the vapor pressure increases higher T → higher P
A metal with a body-centered cubic lattice will have ______ atom(s) per unit cell. A. 1 B. 2 C. 3 D. 4 E. 9
B
A voltaic cell prepared using aluminum and nickel has the following cell notation. Al(s) | Al3+(aq) || Ni2+(aq) | Ni(s) Which of the following represents the correctly balanced spontaneous reaction equation for the cell? A. Ni2+(aq) + Al(s) → Al3+(aq) + Ni(s) B. 3Ni2+(aq) + 2Al(s) → 2Al3+(aq) + 3Ni(s) C. Ni(s) + Al3+(aq) → Ni2+(aq) + Al(s) D. 3Ni(s) + 2Al3+(aq) → 3Ni2+(aq) + 2Al(s) E. None of these choices are correct.
B
For a chemical reaction to be spontaneous at all temperatures, which of the following conditions must be met? A. ΔS° > 0, ΔH° > 0 B. ΔS° > 0, ΔH° < 0 C. ΔS° < 0, ΔH° < 0 D. ΔS° < 0, ΔH° > 0 E. It is not possible for a reaction to be spontaneous at all temperatures
B
Which relationship or statement best describes ΔS° for the following reaction? Pb(s) + Cl2 (g) → PbCl2 (s) A. ΔS° ≈ 0 B. ΔS° < 0 C. ΔS° > 0 D. ΔS° = ΔH°/T E. More information is needed to make a reasonable prediction
B
A certain process has ΔSuniv > 0 at 25°C. What does one know about the process? A. It is exothermic., ΔH <0 B. It is endothermic. ΔH >0 C. It is spontaneous at 25°C D. It will move rapidly toward equilibrium. E. None of these choices are correct.
C
The line notation, Al(s) | Al3+(aq) || Co2+(aq) | Co(s), indicates that A. Co is the reducing agent. B. Co2+ ions are oxidized. C. Al is the reducing agent. D. Al3+ is the reducing agent. E. aluminum metal is the cathode.
C
Which of the following results in a decrease in the entropy of the system? A. O2 (g), 300 K → O2 (g), 400 K B. H2O(s), 0°C → H2O(l), 0°C C. N2 (g), 25°C → N2 (aq), 25°C D. NH3 (l), -34.5°C → NH3 (g), -34.5°C E. 2H2O2 (g) → 2H2O(g) + O2 (g)
C
Which species is the stronger oxidizing agent? Cl2 or O2?
Cl2
A voltaic cell is prepared using copper and silver. Its cell notation is shown below. Cu(s) | Cu2+(aq) || Ag+ (aq) | Ag(s) Which of the following processes occurs at the cathode? A. Cu(s) → Cu2+(aq) + 2e- B. Cu2+(aq) + 2e- → Cu(s) C. Ag(s) → Ag+ (aq) + e- D. Ag+ (aq) + e- → Ag(s) E. Cu(s) + 2Ag+ (aq) → Cu2+(aq) + 2Ag(s)
D
Which of the following statements about voltaic and electrolytic cells is correct? A. The electrons in the external wire flow from cathode to anode in both types of cell. B. Oxidation occurs at the cathode only in a voltaic cell. C. The free energy change, ΔG, is negative for an electrolytic cell. D. The cathode is labeled as positive (+) in a voltaic cell but negative (-) in an electrolytic cell. E. Reduction occurs at the anode in an electrolytic cell
D
q = mΔH
During a phase change, heat flow occurs at constant temperature, as the average distance between particles changes
inert electrode
Electrodes that do not react during electrolysis (often graphite)
first law of thermodynamics
Energy can be transferred and transformed, but it cannot be created or destroyed.
T o F: The sign of ΔH does, by itself, predict the direction of a spontaneous change
F
T or F: A nonspontaneous redox reaction (E° cell > 0) will occur between an oxidizing agent and any reducing agent that lies below it in the emf series (i.e., one that has a less positive value for E°)
F
T or F: A system with fewer microstates has higher entropy
F
T or F: All spontaneous endothermic processes result in a decrease in the freedom of motion of the particles in the system
F
T or F: At equilibrium, there is further net change, and ΔS° sys is balanced by ΔS° surr.
F
T or F: Entropy, like enthalpy, is a state function and is therefore dependent of the path taken between the final and initial states
F
T or F: Gas dissolved in solution or solid has more entropy than pure gas
F
T or F: Metals that can displace H2 from acid are metals that are weaker reducing agents than H2
F
T or F: The impact on the surroundings is smaller when the surroundings are at lower temperature, because there is a greater relative change in Ssurr.
F
T or F: if Q and K are very different, ΔG has a very small value (positive or negative). The reaction releases or absorbs a large amount of free energy
F
T or F: ΔG is the minimum work that must be done to a system to make a spontaneous process occur at constant T and P
F
Which species is the stronger reducing agent? Fe or Cu?
Fe
exothermic types of phase changes
Liquid to solid (freezing), -ΔHfus Gas to liquid(condensation), -ΔHvap Gas to solid (deposition), -ΔHsubl
The Anode is the electrode at which...
OXidation occurs
The CAThode is the electrode at which...
REDuction occurs
endothermic types of phase changes
Solid to liquid (melting/fusion), ΔHfus Liquid to gas (vaporization), ΔHvap Solid to gas( sublimation), ΔHsubl
Reaction Spontaneity of ΔG = ΔH - TΔS if: ΔH (negative) ΔS (positive) -TΔS (negative) ΔG (negative)
Spontaneous at all T
Reaction Spontaneity of ΔG = ΔH - TΔS if: ΔH (positive) ΔS (positive) -TΔS (negative) ΔG (positive or negative)
Spontaneous at higher T; nonspontaneous at lower T
Reaction Spontaneity of ΔG = ΔH - TΔS if: ΔH (negative) ΔS (negative) -TΔS (positive) ΔG (positive or negative)
Spontaneous at lower T; nonspontaneous at higher T
T or F: A change in the freedom of motion of particles in a system is a key factor affecting the direction of a spontaneous process
T
T or F: A decrease in the entropy of the system is outweighed by an increase in the entropy of the surroundings
T
T or F: A reaction at equilibrium (ΔGsys = 0) can no longer do any work
T
T or F: All spontaneous endothermic processes exhibit an increase in entropy
T
T or F: An exothermic reaction does not ensure spontaneity
T
T or F: Any metal that is lower in the list of electrode potentials (i.e., has a more negative E° value) will reduce the ion of a metal higher up the list
T
T or F: By convention, all standard electrode potentials refer to the half-reaction written as a reduction
T
T or F: E° values therefore reflect the ability of the reactant to act as an oxidizing agent
T
T or F: For a process to be spontaneous, a decrease in the entropy of the system must be offset by a larger increase in the entropy of the surroundings
T
T or F: Opening the stopcock increases the number of possible energy levels, which are closer together on average. More distributions of particles are possible.
T
T or F: S° for dissolved solid or liquid is usually greater than for pure substance
T
T or F: S° is largest for forms of an allotrope that allow for more freedom of motion
T
T or F: The free energy change (ΔG) is a measure of the spontaneity of a process and of the useful energy available from it
T
T or F: The higher the metal is in the list of half-cell potentials, the more positive its E° half-cell, and the weaker it is as a reducing agent.
T
T or F: The law of conservation of energy applies to all changes, and does not allow us to predict the direction of a spontaneous change
T
T or F: The oxidizing agent is the reactant from the half-reaction with the more positive E° half-cell.
T
T or F: The third law of thermodynamics permits us to measure the absolute values of the entropy for substances
T
T or F: When the stopcock opens, Wfinal, the systems has 2^n times as many microstates available as when it is onfied to the left flask (Wintital): Wfinal /Wintial = 2^n
T
T or F: the larger the number of possible microstates, the larger the number of ways in which a system can disperse its energy, larger entropy (S)
T
T or F: ΔG is the maximum useful work done by a system during a spontaneous process at constant T and P
T
spontaneous changes
Take place by themselves without any help and may be exothermic or endothermic
Third Law of Thermodynamics
The entropy of a pure, perfect, crystalline solid at 0 K is zero
S = klnW
The number of microstates (W) in a system is related to the entropy (S) of the system; where k (Boltzmann constant) = 1.38·10^-23 J/K
cell potential (Ecell)
The potential difference between the electrodes of an electrochemical cell when no current flows
standard cell potential (E°cell)
The potential of a cell measured with all components in their standard states and at a specific temperature with no current flowing
intermolecular forces' effect on vapor pressure
The weaker the intermolecular forces, the more easily particles enter the vapor phase, and the higher the vapor pressure. weaker forces → higher P
q = mcΔT
Within a phase, heat flow is accompanied by a change in temperature, since the average Ek of the particles changes
Liquid-Gas Equilibrium
[in a closed flask] rate of evaporation is equal to the rate of condensation
Body Centered Cubic (BCC) unit cell
a cell with atoms at the eight corners (1/8 atom) of a cube and at the center of the cell (1 atom); atoms/unit cell = 2 coordination #: 8
Face Centered Cubic (FCC) unit cell
a cell with atoms at the eight corners (1/8 atom) of a cube and half an atom (1/2 atom) on all six sides; atoms/unit cells = 4 coordination #: 12
phase diagram
a graph showing the conditions at which a substance exists as a solid, liquid, or vapor
salt bridge
a pathway constructed to allow the passage of ions from one side to another; electrical neutrality
phase
a physically distinct, homogeneous part of a system
second law of thermodynamics
all real processes occur spontaneously in the direction that increases the entropy of the universe
active electrode
an active component in its half-cell and is a reactant or product in the overall reaction
Clausius-Clapeyron equation
an equation that displays the exponential relationship between vapor pressure and temperature
intermolecular forces in a liquid
attractions are stronger because particles are touching, but they have enough kinetic energy to move randomly
intermolecular forces in a solid
attractions dominate the motion so much the particles are fixed in positions
ΔG < 0
spontaneous process
ΔG = ΔG + RTlnQ
calculating ΔG for any conditions
simple cubic unit cell
cubic unit cell in which lattice points are situated only at the corners (1/8 atom); atoms/unit cell = 1 coordination #: 6
microstate
each quantized energy state for a system of particles At a given set of conditions, each microstate has the same total energy as any other
surroundings
everything outside the system
standard entropy of reaction (ΔS° rxn)
is the entropy change that occurs when all reactants and products are in their standard states
boiling point
is the temperature at which the vapor pressure inside bubbles equals the external pressure
ΔHvap is always larger than ΔHfus because
it takes more energy to completely seperate the molecules from each other
which way will a reaction proceed if: Q > K
left
which way will a reaction proceed if: ΔG > 0
left
Cooking food on the mountain (5430 ft, in Boulder, Colorado) takes longer or shorter time?
longer
Oxidation involves the ________ , and reduction involves the ________ of electrons
loss; gain
ΔG > 0
non-spontaneous process
Ecell < 0
nonspontaneous
Reaction Spontaneity of ΔG = ΔH - TΔS if: ΔH (positive) ΔS (negative) -TΔS (positive) ΔG (positive)
nonspontaneous at all T
normal boiling point
observed at standard atmospheric pressure (1 atm) or 760 torr
reduction
one reactant gains electrons; oxidizing agent is reduced; ions to element
oxidation
one reactant loses electrons; reducing agent is oxidized; element to ions
intermolecular forces in a gas
potential energy (energy of attraction) is small compared to the kinetic energy (energy of motion)
Standard electrode potential of a half cell
potential of a given half-reaction when all components are in their standard states
ΔG = 0
process at equilibrium
inactive electrode
provides a surface for the reaction and completes the circuit, it does not participate actively in the overall reaction
which way will a reaction proceed if: Q < K
right
which way will a reaction proceed if: ΔG < 0
right
Ecell > 0
spontaneous
universe
system + surroundings
kinetic energy (Ek)
tends to disperse particles; energy of motion
potential energy (Ep)
tends to draw particles together; energy of attraction
The properties of each phase are determined by:
the balance between the potential (Ep ) and kinetic (Ek ) energy of the particles
as external pressure on a liquid increases
the boiling point increases
The cell potential (Ecell) of a voltaic cell depends on...
the difference in electrical potential between the two electrodes.
Anode
the electrode at which oxidation occurs; Positive electrode, written on the left
Cathode
the electrode at which reduction occurs; Negative electrode, written on the right
Standard Hydrogen Electrode (SHE)
the half-cell electrode that is normally chosen to have a potential of zero
The more negative the E° value...
the more readily the product will act as a reducing agent
The more positive the E° value...
the more readily the reactant will act as an oxidizing agent.
vapor pressure
the pressure exerted by the vapor on the liquid. The pressure increases until equilibrium is reached; at equilibrium the vapor pressure is constant
system
the substances involved in the chemical and physical changes under investigation
electrolytic cell
uses electrical energy to drive a nonspontaneous reaction (ΔG > 0)
cell potential can also be called
voltage of the cell or the electromotive force (emf)
What can you use to determine spontaneity?
ΔG and ΔSsys
