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 ( C H ) 6 6 produces carbon dioxide and water as the only products.

Write a balanced chemical equation for this reaction.

  • A C H ( ) + 6 O ( ) 6 C O ( ) + 3 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 2 C H ( ) + 9 O ( ) 1 2 C O ( ) + 6 H O ( ) 6 6 2 2 2 l g g g
  • D 2 C H ( ) + 1 5 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 ฮ” ๐ป and entropy change ฮ” ๐‘† for this reaction.

  • A ฮ” ๐ป is negative and ฮ” ๐‘† is positive.
  • B ฮ” ๐ป is positive and ฮ” ๐‘† is negative.
  • CBoth ฮ” ๐ป and ฮ” ๐‘† are positive.
  • DBoth ฮ” ๐ป and ฮ” ๐‘† are negative.
  • E ฮ” ๐ป is negative and the sign of ฮ” ๐‘† is zero.

Why is heating necessary for this reaction to take place?

  • ATo vaporize the liquid reactant
  • BTo produce a negative value of ฮ” ๐บ
  • CTo overcome the activation energy
  • DTo produce a positive value of ฮ” ๐‘†
  • ETo shift the position of equilibrium toward products

Q2:

Which of the following statements is false?

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

Q3:

Which of the following determines whether molecules react spontaneously?

  • AEnthalpy change is negative.
  • BKinetic energy is greater than activation energy.
  • CEntropy change is positive.
  • DProduct free energy is more negative than reactant free energy.
  • EFree energy change is greater in magnitude than activation energy.

Q4:

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

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

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
  • B2 only
  • C1 and 2
  • D2 and 3
  • E3 only

Q6:

Consider the following reaction for the decomposition of sodium bicarbonate: The ฮ” ๐ป โฆต r and ฮ” ๐‘† โฆต r for this reaction have values of 85.2 kJ/mol and 215 J/Kโ‹…mol, respectively. What is the minimum temperature required for an N a H C O ( ) 3 s sample to spontaneously decompose into the products shown above (under 1 bar pressure conditions)?

Q7:

For the process H O ( ) H O ( ) 2 2 l s , occurring at a temperature of โˆ’ 1 0 โˆ˜ C , the ฮ” ๐‘† m has a value of โˆ’ 2 0 . 5 4 J/Kโ‹…mol. Which of the following would reflect the values of ฮ” ๐ป m and ฮ” ๐บ m associated with the freezing of water at a temperature of โˆ’ 1 0 โˆ˜ C ?

  • A ฮ” ๐ป < 0 m ; ฮ” ๐บ > 0 m
  • B ฮ” ๐ป < 0 m ; ฮ” ๐บ = 0 m
  • C ฮ” ๐ป > 0 m ; ฮ” ๐บ = 0 m
  • D ฮ” ๐ป < 0 m ; ฮ” ๐บ < 0 m

Q8:

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

  • Aspontaneous at all temperatures
  • Bspontaneous only at high temperatures
  • Cnonspontaneous 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 ฮ” ๐ป r < 0 , ฮ” ๐‘† r < 0
  • B ฮ” ๐ป r < 0 , ฮ” ๐‘† r = 0
  • C ฮ” ๐ป r > 0 , ฮ” ๐‘† r > 0
  • D ฮ” ๐ป r > 0 , ฮ” ๐‘† r < 0
  • E ฮ” ๐ป r < 0 , ฮ” ๐‘† r > 0

Q10:

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

  • A ฮ” ๐ป < 0 r , ฮ” ๐‘† = 0 r
  • B ฮ” ๐ป > 0 r , ฮ” ๐‘† < 0 r
  • C ฮ” ๐ป < 0 r , ฮ” ๐‘† < 0 r
  • D ฮ” ๐ป < 0 r , ฮ” ๐‘† > 0 r
  • E ฮ” ๐ป > 0 r , ฮ” ๐‘† > 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 ฮ” ๐ป < 0 r , ฮ” ๐‘† > 0 r
  • B ฮ” ๐ป < 0 r , ฮ” ๐‘† < 0 r
  • C ฮ” ๐ป > 0 r , ฮ” ๐‘† < 0 r
  • D ฮ” ๐ป > 0 r , ฮ” ๐‘† > 0 r
  • E ฮ” ๐ป < 0 r , ฮ” ๐‘† = 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 ฮ” ๐ป < 0 r , ฮ” ๐‘† > 0 r
  • B ฮ” ๐ป > 0 r , ฮ” ๐‘† > 0 r
  • C ฮ” ๐ป > 0 r , ฮ” ๐‘† < 0 r
  • D ฮ” ๐ป < 0 r , ฮ” ๐‘† < 0 r
  • E ฮ” ๐ป < 0 r , ฮ” ๐‘† = 0 r

Q13:

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

  • Aspontaneous only at high temperatures
  • Bspontaneous only at low temperatures
  • Cspontaneous at all 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 high temperatures
  • Bspontaneous only at low temperatures
  • Cnonspontaneous at all 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 at all temperatures
  • Cspontaneous only at low temperatures
  • Dspontaneous only at high temperatures

Q16:

What are the values of Gibbs free energy, ๐บ , and the change in Gibbs free energy, ฮ” ๐บ , when a system reaches equilibrium?

  • ABoth ๐บ and ฮ” ๐บ are zero.
  • B ๐บ is maximized and ฮ” ๐บ is zero.
  • C ๐บ is zero and ฮ” ๐บ is minimized.
  • D ๐บ is minimized and ฮ” ๐บ is zero.
  • E ๐บ is zero and ฮ” ๐บ is maximized.

Q17:

The standard enthalpy change for a reaction, ฮ” ๐ป โฆต ๏Šจ ๏Šฏ ๏Šฎ , is 100 kJ/mol. The standard entropy change for the reaction is ฮ” ๐‘† โฆต ๏Šจ ๏Šฏ ๏Šฎ . It may be assumed that both quantities remain constant with varying temperature.

Calculate the minimum necessary value of ฮ” ๐‘† โฆต ๏Šจ ๏Šฏ ๏Šฎ for the reaction to be spontaneous at 298 K.

If ฮ” ๐‘† โฆต ๏Šจ ๏Šฏ ๏Šฎ = 2 0 0 J / K โ‹… m o l , 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 ๐‘† โฆต 2 9 8 ( J / K ยท m o l ) 166.38 33.15 109.20 41.60 160.98 110.50

Calculate, to 3 significant figures, the standard entropy change ฮ” ๐‘† โฆต 2 9 8 for this reaction, expressed per mole of zinc.

Calculate, to 3 significant figures, the maximum value of ฮ” ๐ป โฆต 2 9 8 , 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 ๐‘† โฆต 2 9 8 (J/Kโ‹…mol) 72.11 95.06

The standard enthalpy of fusion of sodium chloride, ฮ” ๐ป โฆต 2 9 8 , 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 0 7 4 K. Which of the following is not a potential explanation for the difference between the observed and calculated melting temperatures?

  • A ฮ” ๐ป > ฮ” ๐ป โฆต 1 0 7 4 โฆต 2 9 8 and ฮ” ๐‘† < ฮ” ๐‘† โฆต 1 0 7 4 โฆต 2 9 8
  • B ฮ” ๐ป > ฮ” ๐ป โฆต 1 0 7 4 โฆต 2 9 8 and ฮ” ๐‘† > ฮ” ๐‘† โฆต 1 0 7 4 โฆต 2 9 8
  • C ฮ” ๐ป < ฮ” ๐ป โฆต 1 0 7 4 โฆต 2 9 8 and ฮ” ๐‘† โ‰ˆ ฮ” ๐‘† โฆต 1 0 7 4 โฆต 2 9 8
  • D ฮ” ๐ป < ฮ” ๐ป โฆต 1 0 7 4 โฆต 2 9 8 and ฮ” ๐‘† < ฮ” ๐‘† โฆต 1 0 7 4 โฆต 2 9 8
  • E ฮ” ๐ป โ‰ˆ ฮ” ๐ป โฆต 1 0 7 4 โฆต 2 9 8 and ฮ” ๐‘† > ฮ” ๐‘† โฆต 1 0 7 4 โฆต 2 9 8

Q20:

Calcium carbonate decomposes into calcium oxide and carbon dioxide gas: The standard entropies and enthalpies of formation for calcium carbonate and other materials are shown in the table.

Material Standard Molar Entropy ๐‘† โฆต 2 9 8 (J/Kโ‹…mol) Standard Enthalpy of Formation ฮ” ๐ป โฆต f (kJ/mol)
C a C O ( ) 3 s 110.0 โˆ’ 1 2 2 0 . 0
C a S O ( ) 3 s 184.1 โˆ’ 1 7 5 2 . 7
C a O ( ) s 38.1 โˆ’ 6 3 4 . 9
C a ( O H ) ( ) 2 s 83.4 โˆ’ 9 8 5 . 2
C O ( ) g 197.7 โˆ’ 1 1 0 . 5
C O ( ) 2 g 213.8 โˆ’ 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 . 0 4 ร— 1 0 3 K
  • B 1 . 0 8 ร— 1 0 3 K
  • C 1 . 1 2 ร— 1 0 3 K
  • D 1 . 3 5 ร— 1 0 3 K
  • E 1 . 4 5 ร— 1 0 3 K

Calculate, to 1 significant figure, the equilibrium partial pressure of carbon dioxide when a sample of calcium carbonate is heated to 1 0 0 โˆ˜ C .

  • A 4 ร— 1 0 โˆ’ 2 0 bar
  • B 3 ร— 1 0 โˆ’ 1 9 bar
  • C 6 ร— 1 0 โˆ’ 3 0 bar
  • D 2 ร— 1 0 โˆ’ 1 8 bar
  • E 2 ร— 1 0 โˆ’ 2 7 bar

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