Chapter 12: Thermodynamics

Question 1

Which of the following describes a spontaneous reaction?
A) A reaction that occurs only when energy is supplied
B) A reaction that can happen without external energy being applied
C) A reaction that occurs quickly
D) A reaction that only happens in equilibrium

Answer 1

B) A reaction that can happen without external energy being applied

Question 2

What does the symbol ΔH represent in a chemical reaction?
A) Change in heat lost or gained at constant volume
B) Change in internal energy of the system
C) Change in enthalpy, or heat gained or lost at constant pressure
D) Change in entropy of the system

Answer 2

C) Change in enthalpy, or heat gained or lost at constant pressure

Question 3

Which of the following best describes a spontaneous reaction?
A) A reaction that requires continuous energy input to proceed
B) A reaction that happens without the need for activation energy
C) A reaction that only occurs under certain conditions like temperature and pressure
D) A reaction that occurs without the need for heat transfer

Answer 3

C) A reaction that only occurs under certain conditions like temperature and pressure

Question 4

Which of the following is true about the relationship between temperature and entropy?
A) As temperature decreases, entropy increases
B) The transition from gas to liquid has lower entropy than liquid to gas
C) Entropy is always higher in solid phase compared to gas phase
D) A gas to liquid phase change results in a decrease in entropy

Answer 4

D) A gas to liquid phase change results in a decrease in entropy

Question 5

What is true for a reaction in equilibrium regarding ΔS_universe?
A) ΔS_universe is negative
B) ΔS_universe is positive
C) ΔS_universe is zero
D) ΔS_universe is always greater than ΔS_sys

Answer 5

C) ΔS_universe is zero

Question 6

Which of the following is an example of a process where heat flows spontaneously from high to low temperature?
A) Ice melts at room temperature
B) Water boils at 100°C
C) Ice freezes at 0°C
D) Heat is added to a reaction system

Answer 6

A) Ice melts at room temperature

Question 7

How is a spontaneous reaction defined in terms of ΔS_universe?
A) ΔS_universe is negative
B) ΔS_universe is zero
C) ΔS_universe is positive
D) ΔS_universe is not related to spontaneity

Answer 7

C) ΔS_universe is positive

Question 8

What does a positive ΔH and negative ΔS indicate about a reaction’s spontaneity?
A) The reaction is spontaneous at high temperatures
B) The reaction is spontaneous at low temperatures
C) The reaction is always nonspontaneous
D) The reaction is in equilibrium

Answer 8

B) The reaction is spontaneous at low temperatures

Question 9

What is the main difference between spontaneous and nonspontaneous reactions?
A) Spontaneous reactions only occur at equilibrium
B) Nonspontaneous reactions require external energy to occur
C) Spontaneous reactions always have a positive ΔG
D) Nonspontaneous reactions never happen in nature

Answer 9

B) Nonspontaneous reactions require external energy to occur

Question 10

In which of the following reactions is the entropy (ΔS) likely to be positive?
A) Freezing of water
B) Condensation of steam
C) Melting of ice
D) Deposition of water vapor

Answer 10

C) Melting of ice

Question 11

What is the significance of the activation energy in a spontaneous reaction?
A) Activation energy is irrelevant in spontaneous reactions
B) Activation energy must always be zero for a reaction to be spontaneous
C) Activation energy is the energy required to start the reaction, regardless of spontaneity
D) Activation energy determines the temperature at which a reaction becomes spontaneous

Answer 11

C) Activation energy is the energy required to start the reaction, regardless of spontaneity

Question 12

If a reaction has a negative ΔH and a positive ΔS, what is its spontaneity at all temperatures?
A) Spontaneous
B) Nonspontaneous
C) At equilibrium
D) Depends on temperature

Answer 12

A) Spontaneous

Question 13

Which of the following statements about enthalpy (ΔH) is true?
A) ΔH is always negative in spontaneous reactions
B) ΔH is the heat lost or gained during a reaction
C) ΔH tells you whether a reaction will occur
ΔH is a measure of the chaos of a system

Answer 13

B) ΔH is the heat lost or gained during a reaction

Question 14

Which phase change has the highest increase in entropy (ΔS)?
A) Solid to liquid
B) Liquid to gas
C) Gas to liquid
D) Solid to gas

Answer 14

B) Liquid to gas

Question 15

If ΔG (Gibbs free energy) is negative, which of the following is true about the reaction?
A) The reaction is spontaneous
B) The reaction is nonspontaneous
C) The reaction is at equilibrium
D) The reaction requires activation energy to proceed

Answer 15

A) The reaction is spontaneous

Question 16

Which of the following represents a system in equilibrium?
A) ΔSuniverse>0
B) ΔH=0
C) ΔSuniverse=0
D) ΔG<0

Answer 16

C) ΔSuniverse=0

Question 17

What is the main factor in determining whether a reaction is spontaneous at a given temperature?
A) The temperature alone
B) ΔH and ΔS
C) The concentration of reactants
D) The presence of a catalyst

Answer 17

B) ΔH and ΔS

Question 18

What will the sign on ∆S be for the following reaction and why?

2 Mg (s) + O₂ (g) → 2 MgO (s)

a.) positive, because there is a solid as a product.

b.) positive, because there are more moles of reactant than product.

c.) positive, because it’s a synthesis reaction.

d.) negative, because there are more moles of gas on the reactant side than the product side.

e.) negative, because there are more moles of reactant than product

Answer 18

d.) negative, because there are more moles of gas on the reactant side than the product side.

Question 19

Which of the following are spontaneous processes?

1.) Ice melting at 1atm and 298 K (assume only ice is initially present).
2.) Heat flowing from a hot object to a cold object
3.) An iron bar rusting.

a.) 1 only
b.) 2 only
c.) 3 only
d.) 1 and 3 only
e.) 1, 2, and 3

Answer 19

e.) 1, 2, and 3

Question 20

Which of the following is true about all spontaneous processes?

a.) it releases energy as heat
b.) it will occur quickly
c.) it will continue on its own once begun
d.) it is never endothermic

Answer 20

c.) it will continue on its own once begun

Question 21

Which of the following has the smallest standard molar entropy, S° (298.15 K)?

a.) CaSO4(s)
b.) C (diamond)
c.) Xe(g)
d.) C60(s)
e.) Br2(l)

Answer 21

b.) C (diamond)

Perfect crystalline structures have lower entropy than all other states of matter because they are highly ordered. As molecules get larger, they have more entropy. Liquids by comparison have less order and have higher entropies than solids.

Question 22

Which of the following best describes the reaction?
2 H₂S(g) + 3 O₂(g) → 2 H₂O(g) + 2 SO₂(g)

a.) ΔS° has a large negative value
b.) ΔS° has a small negative value
c.) ΔS° has a large positive value
d.) ΔS° has a small positive value
e.) ΔS° is zero

Answer 22

b.) ΔS° has a small negative value

This reaction shows that there were five moles of gas on the reactant side and only 4 moles of gas on the product side. It has become slightly more ordered, which would be a small negative ΔS°.

Question 23

Which of the following reactions would have the most negative ∆S° value ?

a.) N2(g) + 3 Cl2(g) → 2 NCL3(g)
b.) CaSO4 * 7 H2O(s) → CaSO4(s) + 7 H2O(g)
c. K(s) + O2(g) → KO2(g)
d.) NH2(g) + HBr(g) → NH4Br(s)
e.) 2H2(g) + O2(g) → 2 H2O(g)

Answer 23

d.) NH2(g) + HBr(g) → NH4Br(s)

Question 24

Determine ∆S for the phase change of 1.45 moles of water from solid to liquid at 0 °C. (∆H = 6.01 kJ/mol)

ΔS= (n⋅ΔH)/T
​where n = moles

Answer 24

31.9 J/K

ΔS= (1.45 * 6010) / 273

Question 25

For the reaction 2 H₂(g) + O₂(g) → 2 H₂O(g) which equation would be used to determine the entropy change? a.) (2A + B) - 2C b.) C - (A + B) c.) 2C - (2A + B) d.) 2A + B + 2C e.) 2C - (A + B)

Answer 25

c.) 2C - (2A + B)

Question 26

Which of the following processes has a negative ∆S? a.) CaCO3(s) → CaO(s) + CO2(g) b.) H2O(g) → H2O(s) c.) CO2(s) → CO2(g) d.) C6H12O6(s) → C6H12O6(aq) e.) ∆S cannot be negative

Answer 26

b.) H2O(g) → H2O(s)

Question 27

In the statement 'all spontaneous changes result in an increase in entropy', the entropy change is _____. a.) ∆S(sys) b.) ∆S(surr) c.) ∆S(univ) d.) ∆S(rev)

Answer 27

c.) ∆S(univ)

Question 28

Which law states that the entropy of a perfect crystalline substance is zero at the absolute zero of temperature? a.) Zeroth law of thermodynamics b.) first law of thermodynamics c.) second law of thermodynamics d.) third law of thermodynamics e.) law of conservation of energy

Answer 28

d.) third law of thermodynamics

Question 29

The second law of thermodynamics states that _____ a.) no engine can be 100% efficient b.) transferring energy from a cold to a hot bath requires work c.) the entropy of the universe increases with a spontaneous process d.) all of these are true

Answer 29

d.) all of these are true

Question 30

Which of the following would have zero entropy? I. O₂(g) at 273 K II. amorphous SiCN(s) at 1 K III. KBr(s, perfect crystal) at 10 K IV. Li(s, perfect crystal) at 0 K a.) I only b.) III only c.) IV only d.) III and IV e.) I, II, and III

Answer 30

c.) IV only

Question 31

Calculate ∆S° for NH₃(g) + HCl(g) → NH₄Cl(s).

Answer 31

-284.9 J/mol*K start with finding the molar entropy of each compound (look it up) 94.6−(192.8+186.7) = -284.9

Question 32

For cyclohexane, the ∆H° of vaporization is 33.10 kJ/mol and the ∆S° of vaporization is 111.80 J/mol･K. At 1.00 atm and 201 K, what is the ∆G° of vaporization for cyclohexane, in kJ/mol?

Answer 32

10.63 kJ/mol use ΔG∘ =ΔH∘ −TΔS∘ 33.10kJ/mol - (.11180kJ/mol/K*201K) = 10.63 kJ/mol

Question 33

A reaction which is endothermic and has an overall increase in entropy is: a.) spontaneous only at high T b.) spontaneous only at low T c.) always spontaneous as written d.) always spontaneous in the reverse direction

Answer 33

a.) spontaneous only at high T

Question 34

If a reaction is exothermic (Δ𝐻<0) and has an increase in entropy (ΔS>0), what can be said about its spontaneity? A) The reaction is always non-spontaneous. B) The reaction is spontaneous only at high temperatures. C) The reaction is spontaneous only at low temperatures. D) The reaction is always spontaneous as written.

Answer 34

D) The reaction is always spontaneous as written. (Since both ΔH is negative and ΔS is positive, ΔG is always negative at all temperatures.)

Question 35

If a reaction is endothermic (ΔH>0) and has an increase in entropy (ΔS>0), when will it be spontaneous? A) Always spontaneous as written B) Spontaneous only at high temperatures C) Spontaneous only at low temperatures D) Always non-spontaneous

Answer 35

B) Spontaneous only at high temperatures (Since ΔH is positive, we need a large positive TΔS term to make ΔG negative, which happens at high T.)

Question 36

If a reaction is exothermic (ΔH<0) and has a decrease in entropy (ΔS<0), when will it be spontaneous? A) Always spontaneous as written B) Spontaneous only at high temperatures C) Spontaneous only at low temperatures D) Always non-spontaneous

Answer 36

C) Spontaneous only at low temperatures (Since ΔH is negative, the reaction is favored enthalpically. But since ΔS is negative, we need a low T so that TΔS does not overpower ΔH.)

Question 37

If a reaction is endothermic (ΔH>0) and has a decrease in entropy (ΔS<0), what can be said about its spontaneity? A) Always spontaneous as written B) Spontaneous only at high temperatures C) Spontaneous only at low temperatures D) Always non-spontaneous

Answer 37

D) Always non-spontaneous (Since both ΔH is positive and ΔS is negative, ΔG is always positive at all temperatures.)

Question 38

Which of the following statements about Gibbs free energy is true? A) If ΔG<0, the reaction is non-spontaneous. B) If ΔG>0, the reaction is spontaneous. C) If ΔG=0, the reaction is at equilibrium. D) If ΔG<0, the reaction is at equilibrium.

Answer 38

C) If ΔG=0, the reaction is at equilibrium.

Question 39

A reaction occurs at 300 K, and it is found that ΔH=50 kJ/mol and ΔS=200 J/mol·K. What can be said about its spontaneity? A) The reaction is spontaneous at this temperature. B) The reaction is non-spontaneous at this temperature. C) The reaction is always spontaneous as written. D) The reaction will never be spontaneous at any temperature.

Answer 39

A) The reaction is spontaneous at this temperature. (50)-(300*0.200) = -10 Since ΔG<0, the reaction is spontaneous at 300 K.

Question 40

If a reaction has ΔH=25 kJ/mol and ΔS=−50 J/mol·K, will the reaction become spontaneous? A) The reaction is always spontaneous. B) The reaction will never be spontaneous. C) The reaction will become spontaneous at very high temperatures. D) The reaction will become spontaneous at very low temperatures.

Answer 40

B) The reaction will never be spontaneous. T = ΔH/ΔS 25/-0.050 = -500K Since temperature cannot be negative, this reaction will never be spontaneous

Question 41

A reaction will be spontaneous at all temperatures if ______. a.) ΔH and ΔS are positive b.) ΔH and ΔS are negative c.) ΔH is positive and ΔS is negative d.) ΔH is negative and ΔS is positive

Answer 41

d.) ΔH is negative and ΔS is positive

Question 42

Which set of results applies to a reaction that is not spontaneous at 273K but is spontaneous at 400K? a.) ΔH < 0 and ΔS > 0 b.) ΔH > 0 and ΔS > 0 c.) ΔH < 0 and ΔS < 0 d.) ΔH > 0 and ΔS <0

Answer 42

b.) ΔH > 0 and ΔS > 0 If the reaction is spontaneous at high temperatures and nonspontaneous at low temperatures, this indicates that both the ∆H and ∆S are greater than 0.

Question 43

Consider the reaction: ICl(g) + Cl₂(g) → ICl₃(s). The ∆G(f)° of ICl(g) is -5.5 kJ/mol and the ∆G(f)° of ICl₃(s) is -22.59 kJ/mol What is the ∆G° of the reaction, in kJ/mol?

Answer 43

-17.09 kJ/mol products - reactants (-22.59 kJ/mol) - (-5.5 kJ/mol) = -17.09 kJ/mol