CHEM 102 Exam 1

The 3rd law of thermodynamics says that entropy of a pure, perfect crystal is ___ at absolute zero in Kelvins

zero

Rotation of the molecule about an axis or rotation about sigma bonds.

Rotational

entropy

(S) measurement of randomness in a system by the ratio of heat delivered and the temperature at which it is delivered, delta s =(qrev/t) during isothermic process

The Haber process for the production of ammonia involves the equilibrium N2 (g) + 3H2 (g) --> 2NH3 (g) Assume that ΔH° and ΔS° for this reaction do not change with temperature. (a) Predict the direction in which ΔG° for the reaction changes with increasing temperature. (b) Calculate ΔG° at 25 °C and 500 °C.

(a) .We expect ΔS° for this reaction to be negative because the number of molecules of gas is smaller in the products. Because ΔS° is negative, -TΔS° is positive and increases with increasing temperature. As a result, ΔG° becomes less negative (or more positive) with increasing temperature. Thus, the driving force for the production of NH3 becomes smaller with increasing temperature. b) At T = 25 °C = 298 K, we have -33.3 kJ At T = 500 °C = 773 K, we have 61 kJ

Predict whether each process is spontaneous as described, spontaneous in the reverse direction, or in equilibrium: (a) Water at 40 °C gets hotter when a piece of metal heated to 150 °C is added. (b) Water at room temperature decomposes into H2(g) and O2(g). (c) Benzene vapor, C6H6(g), at a pressure of 1 atm condenses to liquid benzene at the normal boiling point of benzene, 80.1 °C.

(a) This process is spontaneous. Whenever two objects at different temperatures are brought into contact, heat is transferred from the hotter object to the colder one. The final temperature, after the metal and water achieve the same temperature , will be somewhere between the initial temperatures of the metal and the water. (b) this process is not spontaneous. Rather, the reverse process—the reaction of H2 and O2 to form H2O—is spontaneous. (c) The normal boiling point is the temperature at which a vapor at 1 atm is in equilibrium with its liquid. Thus, this is an equilibrium situation. If the temperature were below 80.1 °C, condensation would be spontaneous.

(a) Using standard enthalpies of formation and standard entropies in Appendix C, calculate ΔH° and ΔS° at 298 K for the reaction 2 SO2(g) + O2(g) -->2 SO3(g). (b) Use your values from part (a) to estimate ΔG° at 400 K.

(a) ΔH° = -196.6 kJ, ΔS° = -189.6 J/K; (b) ΔG° = -120.8 kJ

What is the difference between a state and a microstate of a system? --A thermodynamic state is a single possible arrangement of the positions and kinetic energies of the molecules. --A thermodynamic state is a set of conditions, usually temperature and pressure, that defines the properties of a bulk material. --A microstate is a snapshot of positions and speeds at a particular instant. --A thermodynamic state is a set of conditions, usually temperature, volume and number of moles, that defines the properties of a bulk material. --A microstate is a single possibility for all the positions and kinetic energies of all the molecules in a sample.

--A thermodynamic state is a set of conditions, usually temperature and pressure, that defines the properties of a bulk material. --A microstate is a snapshot of positions and speeds at a particular instant. --A microstate is a single possibility for all the positions and kinetic energies of all the molecules in a sample.

The normal boiling point of ethanol, C2H5OH, is 78.3 °C, and its molar enthalpy of vaporization is 38.56 kJ/mol. What is the change in entropy in the system when 68.3 g of C2H5OH(g) at 1 atm condenses to liquid at the normal boiling point?

-163 J/K

Consider the vaporization of liquid water to steam at a pressure of 1 atm. At what temperature are the two phases in equilibrium?

100 ∘C

Calculate the standard free-energy change for the formation of NO(g) from N2(g) and O2(g) at 298 K: N2(g) + O2(g) --> 2 NO(g) given that ΔH° = 180.7 kJ and ΔS° = 24.7 J/K. Is the reaction spontaneous under these conditions?

173.3 kJ Because its positive, the reaction is not spontaneous

Use data in Appendix C to estimate the normal boiling point, in K, for elemental bromine, Br2(l)

330 K

During a certain reversible process, the surroundings undergo an entropy change, ΔSsurr = - 75 J/K . What is the entropy change of the system for this process?

75 J/K

What are the criteria for spontaneity a) in terms of entropy and b) in terms of free energy? A. the entropy of the universe increase and the free energy of a system decreases at constant temperature B. the entropy of the system decreases and the free energy of the universe increases at constant temperature C. the entropy of the universe decreases and the free energy of a system increases at constant temperature D. the entropy of the system increases and the free energy of the universe decreases at constant temperature

A

As a system goes from state A to state B, its entropy decreases. What can you say about the number of microstates corresponding to each state? A has less microstates than B. A has more microstates than B. A has the same amount of microstates B.

A has more microstates than B.

Which of these statements is true? All spontaneous reactions have a positive entropy change. All spontaneous reactions have a negative entropy change. All spontaneous reactions have a negative free energy change. All spontaneous reactions have a positive free energy change. All spontaneous reactions have a negative enthalpy change.

All spontaneous reactions have a negative free energy change.

If the process is exothermic, does the entropy of the surround 1) always increase 2) always decrease or 3) sometimes increases and sometimes decreases, depending on the process?

Always increase

The normal boiling point of benzene is 80 C. At 100 C and 1 atm, which term is greater in magnitude for the vaporization of benzene? A. ΔH = T ΔS B. ΔH < T ΔS C. ΔH > T ΔS D. Cannot determine without additional information

B

What kind of motion can a molecule undergo that a single atom cannot? A. molecules and single atoms experience the same type of motion B. a molecule can vibrate and rotate; a single atom undergoes neither C. a molecule can undergo translational motion and rotate; a single atom undergoes neither D. a molecule can vibrate and undergo translational motion; a single atom undergoes neither

B

The rusting of iron is spontaneous and decreases the entropy if the system (iron and oxygen) What can be concluded about the entropy change of the surrounding? A. We need to know if the change involves a closed or open system to make a conclusion. B. The entropy of the surroundings must increase by the same amount as the entropy decrease of the system. C. The entropy of the surroundings must increase by a greater amount than the entropy decrease of the system. D. The entropy of the surroundings must decrease by a smaller amount than the entropy decrease of the system.

C

Select each process that produces an increase in the entropy of the system: CaO(s) + CO2(g) → CaCO3(s) HCl(g) + NH3(g) → NH4Cl(s) CO2(s) → CO2(g) 2 SO2(g) + O2(g) → 2 SO3(g)

CO2(s) → CO2(g)

If you are told that the entropy of a system is zero, what do you know about the system? A. The system is a substance at the triple point. B. The system is pure liquid at 0 K (absolute zero). C. The contents of the system are at their standard states. D. They system must be a perfect crystal at 0 K (absolute zero).

D

Why are the spontaneous processes shown sometimes said to be "downhill" in free energy? A. The "downhill" in free energy leads to a transition state. B. An equilibrium is always achieved when both enthalpy and entropy decrease in magnitude, that is "downhill". C. Free-energy difference between a product and a reactant is always negative, that is "downhill". D. An equilibrium reaches a state of minimum free energy, which is lower than the free energies of the reactants and products, in a "downhill" direction.

D

Entropy (S) is a state function, so the change in entropy is given by Sfinal ___ Sintial

minus (-)

First Law of Thermodynamics

Energy is neither created nor destroyed, but may be converted from one form to another or transferred from a system to the surroundings or vice versa.

G= Gibbs free energy, H= enthalpy, S=entropy, K= temperature in Kelvins, what is true?

G= H-TS

Gibbs free energy

G=delta H-TdeltaS

Entropy increases when 4 things occur..

Gases are formed from liquids and solids; Liquids or solutions are formed from solids; The number of gas molecules increases; The number of moles increases.

Which of the following statements is true? The free-energy change for a reaction is independent of temperature. If Q = 0, the system is at equilibrium. If Q>1, ΔG>ΔG∘. The larger the Q, the larger the ΔG∘. If a reaction is spontaneous under standard conditions, it is spontaneous under all conditions.

If Q>1, ΔG>ΔG∘

You have a system made up of only water, with the container and everything beyond being the surrounding. Consider a process in which the water is first evaporated and then condensed back into its original container, is this two-step necessarily reversible?

No, because there is no evidence that the surroundings are also restored to their original state.

At 1 atm pressure, CO2(s) sublimes at -78 °C. Is this process spontaneous at -100 °C and 1 atm pressure?

No, the reverse process is spontaneous at this temperature.

If a process is nonspontaneous, does that mean the process can occur under any circumstance?

No. Nonspontaneous processes can occur with some continuous external assistance.

Does the entropy of the surroundings increase for spontaneous processes?

Not necessarily.

Are the processes that move a system toward equilibrium spontaneous or nonspontaneous?

Spontaneous

The number of microstates and, therefore, the entropy, tends to increase with increases in

Temperature, Volume, and The number of independently moving molecules (freedom of motion) S(g) > S(l) > S(s)

Third Law of Thermodynamics

The entropy of a pure crystalline substance at absolute zero is 0; It is not possible to reach absolute zero in any finite number of processes

If a reaction is exothermic and its entropy change is positive, which statement is true? The reaction is spontaneous only at higher temperatures. The reaction is spontaneous at all temperatures. The reaction is spontaneous only at lower temperatures. The reaction is nonspontaneous at all temperatures.

The reaction is spontaneous at all temperatures.

What can be said about the favorability of the overall reaction? This reaction is thermodynamically favorable. This reaction is thermodynamically neutral. This reaction is thermodynamically unfavorable. There is not enough information to determine thermodynamic favorableness.

This reaction is thermodynamically favorable.

Movement of the entire molecule from one place to another.

Translational

Periodic motion of atoms within a molecule.

Vibrational

entropy decreases when

a gas becomes a liquid

For a reaction, ΔH is +137 kj/mol and ΔS is +120 J/K mol. This reaction is

spontaneous only at high temperatures

Predict whether ΔS is postive or negative for each process, assuming each occurs at constant ΔS temperature: a) H2O(l) --> H2O (g) b) Ag (aq) + Cl (aq) --> AgCl (s) c) 4Fe (s) + 3O2 (g) --> 2Fe2O3 (s) d) N2 (g) +O2 (g) --> 2NO (g)

a) + b) - c) - d) unpredictable

(a) Write the chemical equation that defines the normal boiling point of liquid carbon tetrachloride, CCl4(l). (b) What is the value of ΔG° for the equilibrium in part (a)? (c) Use data from Appendix C and Equation 19.12 to estimate the normal boiling point of CCl4.

a)CCl4 (l) --> CCl4 (g) P= 1 atm b)ΔG = 0 c) 95.0 J/K

Consider the vaporization of liquid water to steam at a pressure of 1 atm. In what temperature range is it a spontaneous process? below 100 ∘C above 100 ∘C below 0 ∘C above 0 ∘C at no temperature

above 100 ∘C

Consider the vaporization of liquid water to steam at a pressure of 1 atm. In what temperature range is it a nonspontaneous process? below 100 ∘C above 100 ∘C below 0 ∘C above 0 ∘C at no temperature

below 100 ∘C

Irreversible processes

cannot be undone by exactly reversing the change to the system; Spontaneous processes are irreversible.

Calculate the standard free-energy change for the formation of NO(g) from N2(g) and O2(g) at 298 K: N2(g) + O2(g) --> 2 NO(g) given that ΔH° = 180.7 kJ and ΔS° = 24.7 J/K. Is the reaction spontaneous under these conditions?

delta G= 173.3 kJ; reaction is nonspontaneous

since entropy is a state function.. delta S =

delta S= Sfinal - Sinital

the change in entropy on the molecular scale (equation)

delta s= k ln wfinal/wintial ;

Consider the vaporization of liquid water to steam at a pressure of 1 atm. Is this process endothermic or exothermic? endothermic exothermic

endothermic

-TΔS is

entropy

Δ H is

ethalpy

Standard entropies tend to _____ with _____ molar mass; which means

increase;increasing ; Larger and more complex molecules have greater entropies.

Generally, when a solid is dissolved in a solvent, entropy ____.

increases

How does the entropy of the system change when equal volumes of ethanol and water are mixed to form a solution? increases decreases stays the same

increases

How does the entropy of the system change when the temperature of the system increases? increases decreases stays the same

increases

How does the entropy of the system change when the volume of a gas increases? increases decreases stays the same

increases

The second law of thermodynamics says that the entropy of the universe ___ whenever a spontaneous process occurs

increases

All spontaneous processes are ___

irreversible

If delta G is _____, the reaction is spontaneous. If delta G is _____, the system is at equilibrium. If delta G is _____, the reaction is nonspontaneous

negative, 0, positive

If a process is nonspontaneous, does that mean the process cannot occur under any circumstances? yes no

no

If enthalpy change is positive and entropy change is negative, the reaction is

non-spontaneous at all temperatures

Δ H= + ΔS= - -TΔS= + Δ G= +

nonspontaeous at all temperatures

Spontaneous processes

processes that can proceed without any outside intervention; only occur in one direction; can be spontaneous at one temperature and nonspontaneous at another

Entropy (S) is a measurement of the ___ of a system

randomness (disorder)

When a molecule spins like a top it undergoes ____ motion

rotational

All irreversible processes are not necessarily ___

spontaneous

If enthalpy change is negative and entropy change is positive, the reaction is

spontaneous at all temperatures

Δ H= - Δ S= + -TΔS= - Δ G= -

spontaneous at all temperatures

ΔH= + Δ S= + -TΔS= - Δ G= + or -

spontaneous at high temperatures; nonspontaneous at low temperatures

What can be said about an exothermic reaction with a negative entropy change? spontaneous at all temperatures. spontaneous at high temperatures. spontaneous at low temperatures. spontaneous in the reverse direction at all temperatures. nonspontaneous in either direction at all temperatures.

spontaneous at low temperatures.

ΔH= - Δ S= - -TΔS= + Δ G= + or -

spontaneous at low temperatures; nonspontaneous at high temperatures

What can be said about an endothermic reaction with a negative entropy change? spontaneous at all temperatures. spontaneous at high temperatures. spontaneous at low temperatures. spontaneous in the reverse direction at all temperatures. nonspontaneous in either direction at all temperatures.

spontaneous in the reverse direction at all temperatures.

The second law of thermodynamics

the entropy of the universe increases for spontaneous processes (delta universe > 0)and the entropy of the universe does not change for reversible processes (delta universe = 0)

Spontaneity can depend on temperature. At T > 0 ∘C, ice melts spontaneously to liquid water. At T < 0 ∘C, the reverse process, water freezing to ice, is spontaneous. At T = 0 ∘C the two states are in equilibrium. In which direction is this process exothermic? the freezing of liquid water to ice the melting of ice to liquid water

the freezing of liquid water to ice

microstate

the motions of a sample of molecules at a particular instant in time.

reversible process

the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process.

An automobile being driven from W to E is experiencing ___ motion

translational

Changes in a molecule's bond length or bond angles are examples of ___ motion

vibrational

At equilibrium, the value of free energy change (delta G) is

zero.

Calculate ΔG° for a reaction for which ΔH° = 24.6 kJ and ΔS° = 132 J/K at 298 K. Is the reaction spontaneous under these conditions?

ΔG° = -14.7 kJ; the reaction is spontaneous

free energy at equilibrium

ΔG° = -RT ln K

ΔG° and ΔS° are found how

ΔG°= E n ΔG°f (products) - E m ΔG°i (reactants), ΔS°= E n ΔS°(products) - E m ΔS°(reactants) cC + dD - aA - bB

In a particular spontaneous process the entropy of the system decreases. What can you conclude about the sign and magnitude of ΔSsurr? --ΔSsurr is negative and greater than the magnitude of the decrease in ΔSsys. --ΔSsurr is negative and less than the magnitude of the decrease in ΔSsys. --ΔSsurr is positive and greater than the magnitude of the decrease inΔSsys. --ΔSsurr is positive and less than the magnitude of the decrease in ΔSsys.

ΔSsurr is positive and greater than the magnitude of the decrease in ΔSsys.

In a particular spontaneous process, the number of microstates available to the system decreases. What can you conclude about the sign of ΔSsurr?

ΔSsurr is positive.

Predict the sign of ΔSsys for each of the following processes. Calcium phosphate precipitates upon mixing Ca(NO3)2(aq) and (NH4)3PO4(aq).

ΔSsys<0

Predict the sign of ΔSsys for each of the following processes. Gaseous CO reacts with gaseous H2to form liquid methanol, CH3OH.

ΔSsys<0

Predict the sign of ΔSsys for each of the following processes. Molten gold solidifies.

ΔSsys<0

Predict the sign of ΔSsys for each of the following processes. Gaseous Cl2 in the stratosphere to form gaseous Cl atoms.

ΔSsys>0