Worksheet: Spontaneous Reactions

In this worksheet, we will practice using the second law of thermodynamics to predict the spontaneity of physical processes and chemical reactions.

Q1:

The combustion of benzene (CH)66 produces carbon dioxide and water as the only products.

Write a balanced chemical equation for this reaction.

  • A 2 C H ( ) + 1 5 O ( ) 1 2 C O ( ) + 6 H O ( ) 6 6 2 2 2 l g g g
  • B C H ( ) + 8 O ( ) 6 C O ( ) + 3 H O ( ) 6 6 2 2 2 l g g g
  • C C H ( ) + 6 O ( ) 6 C O ( ) + 3 H O ( ) 6 6 2 2 2 l g g g
  • D 2 C H ( ) + 9 O ( ) 1 2 C O ( ) + 6 H O ( ) 6 6 2 2 2 l g g g
  • E 3 C H ( ) + 1 5 O ( ) 1 8 C O ( ) + 9 H O ( ) 6 6 2 2 2 l g g g

Predict the signs of the enthalpy change Δ𝐻 and entropy change Δ𝑆 for this reaction.

  • A Δ 𝐻 is negative and the sign of Δ𝑆 is zero.
  • B Δ 𝐻 is positive and Δ𝑆 is negative.
  • CBoth Δ𝐻 and Δ𝑆 are positive.
  • D Δ 𝐻 is negative and Δ𝑆 is positive.
  • EBoth Δ𝐻 and Δ𝑆 are negative.

Why is heating necessary for this reaction to take place?

  • ATo produce a positive value of Δ𝑆
  • BTo produce a negative value of Δ𝐺
  • CTo overcome the activation energy
  • DTo shift the position of equilibrium toward products
  • ETo vaporize the liquid reactant

Q2:

Which of the following statements is false?

  • AIn a spontaneous process, the enthalpy of the system must decrease.
  • BIn a spontaneous process, the entropy of the universe must increase.
  • CA spontaneous process may become nonspontaneous above a threshold temperature.
  • DIn a spontaneous process, the Gibbs free energy of the system must decrease.
  • EA spontaneous process occurs without a continual input of energy from an external source.

Q3:

Which of the following determines whether molecules react spontaneously?

  • AFree energy change being greater in magnitude than activation energy
  • BProduct-free energy being more negative than reactant-free energy
  • CEntropy change being positive
  • DEnthalpy change being negative
  • EKinetic energy being greater than activation energy

Q4:

Why is the conversion of diamond to graphite not observed at room temperature and pressure?

  • AThe activation energy for the process is too high.
  • BThe melting point of diamond is too high.
  • CThe process involves a decrease in entropy.
  • DThe process is nonspontaneous.
  • EThe process is endothermic.

Q5:

Which of the following processes are spontaneous?

  1. Water freezing above its freezing point
  2. Combustion of gasoline
  3. Iron rusting in a humid atmosphere
  • A1 and 3
  • B1 and 2
  • C3 only
  • D2 and 3
  • E2 only

Q6:

Consider the following reaction for the decomposition of sodium bicarbonate. 2NaHCO()NaCO()+CO()+HO()32322ssgl The Δ𝐻⦵r and Δ𝑆⦵r for this reaction have values of 85.2 kJ/mol and 215 J/K⋅mol respectively. What is the minimum temperature required for an NaHCO()3s sample to spontaneously decompose into the products shown above (under 1 bar pressure conditions)?

Q7:

For the process HO()HO()22ls, occurring at a temperature of −10∘C, the Δ𝑆m has a value of −20.54 J/K⋅mol. Which of the following would reflect the values of Δ𝐻m and Δ𝐺m associated with the freezing of water at a temperature of −10∘C?

  • A Δ 𝐻 < 0 m , Δ 𝐺 = 0 m
  • B Δ 𝐻 < 0 m , Δ 𝐺 > 0 m
  • C Δ 𝐻 < 0 m , Δ 𝐺 < 0 m
  • D Δ 𝐻 > 0 m , Δ 𝐺 = 0 m

Q8:

For a given reaction, the enthalpy and entropy changes are negative. The reaction is .

  • Anonspontaneous at all temperatures
  • Bspontaneous only at high temperatures
  • Cspontaneous at all temperatures
  • Dspontaneous only at low temperatures

Q9:

A given reaction is non-spontaneous at all temperatures. Which of the following best accounts for this behavior?

  • A Δ 𝐻 > 0 , Δ 𝑆 < 0 r r
  • B Δ 𝐻 < 0 , Δ 𝑆 = 0 r r
  • C Δ 𝐻 < 0 , Δ 𝑆 < 0 r r
  • D Δ 𝐻 > 0 , Δ 𝑆 > 0 r r
  • E Δ 𝐻 < 0 , Δ 𝑆 > 0 r r

Q10:

A given reaction is spontaneous at all temperatures. Which of the following best accounts for this behavior?

  • A Δ 𝐻 > 0 r , Δ 𝑆 < 0 r
  • B Δ 𝐻 < 0 r , Δ 𝑆 > 0 r
  • C Δ 𝐻 > 0 r , Δ 𝑆 > 0 r
  • D Δ 𝐻 < 0 r , Δ 𝑆 < 0 r
  • E Δ 𝐻 < 0 r , Δ 𝑆 = 0 r

Q11:

A given reaction is spontaneous at high temperatures but not at low temperatures. Which of the following best accounts for this behavior?

  • A Δ 𝐻 < 0 r , Δ 𝑆 > 0 r
  • B Δ 𝐻 < 0 r , Δ 𝑆 = 0 r
  • C Δ 𝐻 > 0 r , Δ 𝑆 < 0 r
  • D Δ 𝐻 < 0 r , Δ 𝑆 < 0 r
  • E Δ 𝐻 > 0 r , Δ 𝑆 > 0 r

Q12:

A given reaction is spontaneous at low temperatures but not at high temperatures. Which of the following best accounts for this behavior?

  • A Δ 𝐻 < 0 r , Δ 𝑆 > 0 r
  • B Δ 𝐻 > 0 r , Δ 𝑆 > 0 r
  • C Δ 𝐻 < 0 r , Δ 𝑆 = 0 r
  • D Δ 𝐻 < 0 r , Δ 𝑆 < 0 r
  • E Δ 𝐻 > 0 r , Δ 𝑆 < 0 r

Q13:

For a given reaction, the enthalpy change is positive and the entropy change is negative. The reaction is .

  • Aspontaneous at all temperatures
  • Bspontaneous only at high temperatures
  • Cspontaneous only at low temperatures
  • Dnonspontaneous at all temperatures

Q14:

For a given reaction, the enthalpy change is negative and the entropy change is positive. The reaction is .

  • Aspontaneous only at low temperatures
  • Bnonspontaneous at all temperatures
  • Cspontaneous only at high temperatures
  • Dspontaneous at all temperatures

Q15:

For a given reaction, the enthalpy and entropy changes are positive. The reaction is .

  • Anonspontaneous at all temperatures
  • Bspontaneous only at high temperatures
  • Cspontaneous only at low temperatures
  • Dspontaneous at all temperatures

Q16:

What are the values of Gibbs free energy, 𝐺, and the change in Gibbs free energy, Δ𝐺, when a system reaches equilibrium?

  • ABoth 𝐺 and Δ𝐺 are zero.
  • B 𝐺 is maximized and Δ𝐺 is zero.
  • C 𝐺 is zero and Δ𝐺 is minimized.
  • D 𝐺 is zero and Δ𝐺 is maximized.
  • E 𝐺 is minimized and Δ𝐺 is zero.

Q17:

The standard enthalpy change for a reaction, Δ𝐻⦵, is 100 kJ/mol. The standard entropy change for the reaction is Δ𝑆⦵. It may be assumed that both quantities remain constant with varying temperature.

Calculate the minimum necessary value of Δ𝑆⦵ for the reaction to be spontaneous at 298 K.

If Δ𝑆=200/⋅⦵JKmol, calculate the minimum temperature at which the reaction is spontaneous.

Q18:

Zinc reacts with solid copper sulfate to form copper and zinc sulfate. The standard entropies of these and other materials are shown in the table.

Material C u ( ) g C u ( ) s C u S O ( ) 4 s Z n ( ) s Z n ( ) g Z n S O ( ) 4 s
Standard Molar Entropy 𝑆⦵(J/K·mol) 166.38 33.15 109.20 41.60 160.98 110.50

Calculate, to 3 significant figures, the standard entropy change Δ𝑆⦵ for this reaction, expressed per mole of zinc.

Calculate, to 3 significant figures, the maximum value of Δ𝐻⦵, the standard molar enthalpy change per mole of zinc, at which this reaction would occur spontaneously at 298 K.

Q19:

The standard entropies for two phases of sodium chloride are shown below:

Phase N a C l ( ) s N a C l ( ) l
Standard Molar Entropy 𝑆⦵ (J/K⋅mol) 72.11 95.06

The standard enthalpy of fusion of sodium chloride, Δ𝐻⦵, is 27.95 kJ/mol.

Assuming that these thermodynamic parameters do not vary with temperature, estimate the melting point of sodium chloride to the nearest degree Celsius.

The observed melting point of sodium chloride is 1,074 K. Which of the following is not a potential explanation for the difference between the observed and calculated melting temperatures?

  • A Δ 𝐻 ≈ Δ 𝐻 ⦵      ⦵    and Δ𝑆>Δ𝑆⦵⦵
  • B Δ 𝐻 > Δ 𝐻 ⦵      ⦵    and Δ𝑆<Δ𝑆⦵⦵
  • C Δ 𝐻 < Δ 𝐻 ⦵      ⦵    and Δ𝑆≈Δ𝑆⦵⦵
  • D Δ 𝐻 < Δ 𝐻 ⦵      ⦵    and Δ𝑆<Δ𝑆⦵⦵
  • E Δ 𝐻 > Δ 𝐻 ⦵      ⦵    and Δ𝑆>Δ𝑆⦵⦵

Q20:

Calcium carbonate decomposes into calcium oxide and carbon dioxide gas: CaCO()CaO()+CO()32ssg. The standard entropies and enthalpies of formation for calcium carbonate and other materials are shown in the table.

Material Standard Molar Entropy 𝑆⦵ (J/K⋅mol) Standard Enthalpy of Formation Δ𝐻⦵f (kJ/mol)
C a C O ( ) 3 s 110.0 − 1 , 2 2 0 . 0
C a S O ( ) 3 s 184.1 − 1 , 7 5 2 . 7
C a O ( ) s 38.1 − 6 3 4 . 9
C a ( O H ) ( ) 2 s 83.4 − 9 8 5 . 2
C O ( ) g 197.7 − 1 1 0 . 5
C O ( ) 2 g 213.8 − 3 9 3 . 5

All thermodynamic parameters are measured under a standard pressure of 1.000 bar. It may be assumed that the parameters do not vary significantly with temperature.

A sample of calcium carbonate is stored in an atmosphere of carbon dioxide at standard pressure. Calculate, to 3 significant figures, the minimum temperature in kelvin at which the sample would spontaneously decompose.

  • A 1 . 1 2 × 1 0  K
  • B 1 . 0 8 × 1 0  K
  • C 1 . 3 5 × 1 0  K
  • D 1 . 4 5 × 1 0  K
  • E 1 . 0 4 × 1 0  K

Calculate, to 1 significant figure, the equilibrium partial pressure of carbon dioxide when a sample of calcium carbonate is heated to 100∘C.

  • A 2 × 1 0    bar
  • B 4 × 1 0    bar
  • C 3 × 1 0    bar
  • D 6 × 1 0    bar
  • E 2 × 1 0    bar

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