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Exam 20: Thermodynamics: Entropy, free Energy, and the Direction of Chemical Reactions

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Question 41

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Which of the following results in a decrease in the entropy of the system?

A) O₂(g) ,300 K $\to$ O₂(g) ,400 K
B) H₂O(s) ,0°C $\to$ H₂O(l) ,0°C
C) N₂(g) ,25°C $\to$ N₂(aq) ,25°C
D) NH₃(l) ,-34.5°C $\to$ NH₃(g) ,-34.5°C
E) 2H₂O₂(g) $\to$ 2H₂O(g) + O₂(g)

F) A) and B)
G) A) and C)

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Question 42

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Which of the following is true for pure oxygen gas,O₂(g) at 25°C?

A) ( $\Delta$ H° _f > 0)
B) ( $\Delta$ H° _f < 0)
C) ( $\Delta$ G° _f > 0)
D) ( $\Delta$ G° _f < 0)
E) (S° > 0)

F) A) and E)
G) None of the above

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Question 43

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For a chemical reaction to be spontaneous only at low temperatures,which of the following conditions must be met?

A chemical reaction has $\Delta$ H° = 42.8 kJ and $\Delta$ S° = 92.5 J/K,at 25°C.Calculate the temperature at which $\Delta$ G° = 0.State any approximation involved in your calculation.

When a sky diver free-falls through the air,the process is

You are given pure samples of ammonia,NH₃(g) ,and nitrogen trifluoride,NF₃(g) .What prediction would you make concerning their standard molar entropies at 298 K?

For a chemical reaction to be spontaneous only at high temperatures,which of the following conditions must be met?

For a given reaction,a change in the pressure may result in a change in the sign of $\Delta$ G.

State the second and third laws of thermodynamics.

Calculate $\Delta$ G° for the combustion of propane. C₃H₈(g) + 5O₂(g) $\to$ 3CO₂(g) + 4H₂O(g)

Use the given data at 298 K to calculate $\Delta$ G° for the reaction 2Cl₂(g) + SO₂(g) $\to$ SOCl₂(g) + Cl₂O(g)

a.Explain what is meant by a spontaneous process. b.Is a spontaneous process necessarily a rapid one? Explain,and provide a real reaction as an example to illustrate your answer.

Hydrogen sulfide decomposes according to the following reaction 2H₂S(g) $\to$ 2H₂(g) + S₂(g) For this reaction at 298K $\Delta$ S° = 78.1 J/K, $\Delta$ H° = 169.4 kJ,and $\Delta$ G° = 146.1 kJ.What is the value of $\Delta$ G° at 900 K?

Which of the following values is based on the Third Law of Thermodynamics?

The temperature at which the following process reaches equilibrium at 1.0 atm is the normal melting point for phosphoric acid. H₃PO₄(s) H₃PO₄(l) Use the following thermodynamic data at 298 K to determine this temperature.

For the reaction of xenon and fluorine gases to form solid XeF₄, $\Delta$ H° = -251 kJ and $\Delta$ G° = -121 kJ at 25°C.Calculate $\Delta$ S° for the reaction.

For what signs of $\Delta$ H and $\Delta$ S will a process a.be spontaneous at high temperatures but not at low temperatures? b.not be spontaneous at any temperatures?

Nitric oxide reacts with chlorine to form NOCl.The data refer to 298 K. 2NO(g) + Cl₂(g) $\to$ 2NOCl(g) What is the value of $\Delta$ G° for this reaction at 550 K?

In some spontaneous processes,the entropy of the surroundings decreases.

Consider the figure below which shows $\Delta$ G° for a chemical process plotted against absolute temperature.From this plot,it is reasonable to conclude that: