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.
The combustion of benzene produces carbon dioxide and water as the only products.
Write a balanced chemical equation for this reaction.
Predict the signs of the enthalpy change and entropy change for this reaction.
- A is negative and the sign of is zero.
- B is positive and is negative.
- CBoth and are positive.
- D is negative and is positive.
- EBoth and are negative.
Why is heating necessary for this reaction to take place?
- ATo produce a positive value of
- BTo produce a negative value of
- CTo overcome the activation energy
- DTo shift the position of equilibrium toward products
- ETo vaporize the liquid reactant
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.
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
Consider the following reaction for the decomposition of sodium bicarbonate. The and for this reaction have values of 85.2 kJ/mol and 215 J/K⋅mol respectively. What is the minimum temperature required for an sample to spontaneously decompose into the products shown above (under 1 bar pressure conditions)?
The standard enthalpy change for a reaction, , is 100 kJ/mol. The standard entropy change for the reaction is . It may be assumed that both quantities remain constant with varying temperature.
Calculate the minimum necessary value of for the reaction to be spontaneous at 298 K.
If , calculate the minimum temperature at which the reaction is spontaneous.
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.
|Standard Molar Entropy||166.38||33.15||109.20||41.60||160.98||110.50|
Calculate, to 3 significant figures, the standard entropy change for this reaction, expressed per mole of zinc.
Calculate, to 3 significant figures, the maximum value of , the standard molar enthalpy change per mole of zinc, at which this reaction would occur spontaneously at 298 K.
The standard entropies for two phases of sodium chloride are shown below:
|Standard Molar Entropy (J/K⋅mol)||72.11||95.06|
The standard enthalpy of fusion of sodium chloride, , 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 and
- B and
- C and
- D and
- E and
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 (J/K⋅mol)||Standard Enthalpy of Formation (kJ/mol)|
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 K
- B K
- C K
- D K
- E K
Calculate, to 1 significant figure, the equilibrium partial pressure of carbon dioxide when a sample of calcium carbonate is heated to .
- A bar
- B bar
- C bar
- D bar
- E bar