Spontaneous Reactions

Thermodynamics is the branch of physical science concerned with heat and its transformations to and from other forms of energy. 

Spontaneous & Nonspontaneous Reactions

Concept: Understanding Spontaneity

7m
Video Transcript

Hey guys, in this new video, we're going to take a look at thermal chemical processes. We're going to say that the section of physical science concerned with the transformation of heat and other forms of energy from one type to another is called thermodynamics. What we're going to say here is when it comes to a chemical reaction, just realize, sometimes they occur, other times they don't. It's really based on the conditions at the moment that a chemical reaction can or won't happen. What we're going to say is this chapter is concerned with one keyword – spontaneous. Is a reaction spontaneous or is a reaction non-spontaneous? The deeper into this chapter we get into, we’re going to learn that there are certain variables such as delta G, delta S and even k. All of these together in some way help to determine if a reaction is spontaneous or not.
When we say spontaneous, just remember, we're going to say a reaction that requires no outside energy source is classified as a natural process. Think about it. Let's think of a boulder, a huge rock, rolling down a hill. We're going to say that huge boulder doesn't require any type of energy to roll down that hill. It's using its own momentum in order to do that. We're going to say that since it doesn't require any type of energy for it to happen, then it's a spontaneous reaction. On the flip side, let's say we have a reaction where we have to continuously feed it energy in order for it to occur. If you're supplying a continuous amount of energy to something, we're going to classify it as a non-spontaneous reaction. We're going to say non-spontaneous reactions are unnatural. They constantly need energy for them to occur. Let's say we want to run our car. Our car cannot spontaneously run itself without any type of energy. We have to give it a battery, we have to give it gasoline. Without these sources of energy, the car can’t ignite and start off and move on its own. Just remember, the movement of a car will be classified as a non-spontaneous reaction.
Just remember, spontaneity deals with thermodynamics. We're shifting our reaction from one side to another in a balanced equation. But remember, spontaneous has nothing to do with kinetics. Kinetics deals with speed. A spontaneous reaction could take a minute to occur, it could take a million years to occur. The fact that it occurs without the need of outside energy makes it a spontaneous reaction. The length of time it takes for it to happen has nothing to do with if it’s spontaneous or not. Knowing this, let's take a look at the first example.
Here it says: Which of the following statements is not true? We're going to say the reverse of a spontaneous reaction is always non-spontaneous. Just remember, we're talking about thermodynamics here. Thermodynamics deals with shifting of reaction, so we go in both directions. We can go in the forward direction or we can go in the reverse direction. Just remember, if you're spontaneous in one direction, then you would be non-spontaneous in the reverse. The first one is true. Next, we're going to say a spontaneous reaction always moves towards equilibrium. Again, we're talking about thermodynamics. Thermodynamics deals with equilibrium. We're going to say if a reaction is spontaneous, it wants to get to equilibrium because when you’re at equilibrium, you’re balanced. Everything is copacetic. Everything is good. We're going to say here a spontaneous reaction does move towards equilibrium.
Next, a highly spontaneous reaction can occur at a fast or a slow rate. We're going to say, like we said before, spontaneity has nothing to do with kinetics. A spontaneous reaction could take a minute to happen or it could take a million years to happen. The fact that it can happen makes it a spontaneous reaction. Spontaneous reactions occur. This leaves us with our last choice here. Here it says that it’s possible to create a non-spontaneous reaction. This might seem weird but non-spontaneous reactions are again unnatural. They do not occur. We can supply constant energy to something, but once you take away that energy source, the reaction is going to cease. It's going to stop. It's not going to continue. When we say it is not possible to create a non-spontaneous reaction, what we're saying here is I can't create something and then take away the energy and expect it to continue to happen. That's what this sentence is really saying. So, D would be false. We cannot create a non-spontaneous reaction. Once I have a non-spontaneous, for it to continue to occur, I need to constantly feed it energy. Once I take away that energy, it no longer happens. I really can't create a non-spontaneous reaction because you cannot supply energy forever towards a reaction. You don't have enough sources to do that.
Seeing how we answer these questions, I want you guys to attempt to do the next one on your own. Just remember, thinking about it in your head in order to answer the question. Does this happen in real life? If it happens in real life, it's most likely a spontaneous reaction. If it doesn't quite make sense, “This shouldn't happen. My car shouldn't just start on its own and drive itself home without the use of a battery or gasoline”, then you're going to say that’s a non-spontaneous reaction. Again, natural processes occur. Natural processes are spontaneous. Unnatural processes don't occur in real life. They don't make any sense for them to happen. Those are non-spontaneous. Use that type of logic in order to solve this practice question.

A reaction that requires no outside energy source is classified as a natural process and is spontaneous. 

A reaction that requires a continuous energy source to happen is classified as an unnatural process and is nonspontaneous. 

Problem: Which of the following statements is/are true?

a) The rusting of iron by oxygen is a non-spontaneous reaction.

b) The addition of a catalyst to a reaction increases spontaneity.

c) The movement of heat from a cold object to a hot object is a non-spontaneous reaction.

d) The diffusion of perfume molecules from one side of a room to the other is a non-spontaneous reaction.

e) None of the above. 

5m

Spontaneous Reactions Additional Practice Problems

Thermodynamic data are given below (at T = 25. °C) and may be of use in doing this problem.

Phosphoric acid (H3PO4) may be prepared by adding tetraphosphorus decaoxide (P4O10) to water (H2O), by the process

                P 4O10(s) + 6 H2O(I) → 4 H3PO4(s)

a) What are ΔS°rxn and ΔG°rxn for the below reaction, at T = 25.0 °C?

b) Give the correctly balanced formation reaction for H 3PO4(s).

Watch Solution

For a chemical reaction to be spontaneous for standard conditions which of the following must be true?

a) ΔH°rxn > 0

b) ΔS°rxn > 0

c) ΔG°rxn > 0

d) Both b and c

e) None of the above

Watch Solution

Which statement best describes the following endothermic reaction?

C2H6(g) → C2H4(g) + H2(g)

This reaction is ________.

a. spontaneous at all temperatures

b. spontaneous only at a high temperature

c. spontaneous only at a low temperature

d. nonspontaneous at all temperatures

e. more information is needed to accurately describe the spontaneity of the reaction

 

Watch Solution

Which of the following is always positive when a spontaneous process occurs?

a. ΔSsystem

b. ΔSsurroundings

c. ΔSuniverse

d. ΔHuniverse

e. ΔHsurroundings

Watch Solution

A reaction that is spontaneous as written _________.

a. is very rapid

b. will proceed without outside intervention

c. is also spontaneous in the reverse direction

d. has an equilibrium position that lies far to the left

e. is very slow

 

Watch Solution

Assuming the following ΔH°rxn and ΔS°rxn values are independent of temperature, which of reactions I, II, and III is/are spontaneous at 400°C?

a) I

b) II

c) III

d) I and II

e) II and III

Watch Solution

Solid ammonium nitrate is very soluble in water. When it dissolves, the solution becomes very cold. Based on this information alone, what are the most likely signs of ΔH ̊, ΔS ̊ and ΔG ̊?

a) +, +, ‐

b) +, +, +

c) +, ‐, ‐

d) ‐, ‐, ‐

e) ‐, +, ‐

Watch Solution

For the reaction

2 C(s) + 2 H2(g) → C2H4(g) 

∆H°r = +52.3 kJ · mol−1 and ∆S°r =−53.07 J · K−1· mol−1 at 298 K. This reaction will be spontaneous at

1. no temperature. 

2. temperatures below 985 K.

3. temperatures above 985 K.

4. temperatures below 1015 K.

5. all temperatures.

 

Watch Solution

For the reaction

2 SO3(g) → 2 SO2(g) + O2(g)

∆Hr = +198 kJ·mol−1 at 298 K. Which statement is true for this reaction?

1. The reaction will not be spontaneous at any temperature.

2. The reaction will not be spontaneous at high temperatures.

3. ∆Gr will be negative at high temperatures. 

4. ∆Gr will be positive at high temperatures.

5. The reaction is driven by the enthalpy.

Watch Solution

Which of the following is true of a general thermodynamic state function?

1. The change of the value of a state function is independent of the path of a process.

2. The value of a state function does NOT change with a change in temperature of a process.

3. The change in the value of a state function is always negative for a spontaneous reaction.

4. The value of the state function remains constant.

5. The change in the value of the state function is always positive for endothermic processes.

Watch Solution

Ethane is C2H6. Consider the reaction below: 

C2H6(g) ⇋ 2 Cgraphite(s) + 3 H2(g) 

∆Grxn = 7.86 kJ · mol−1 The decomposition of ethane is (spontaneous/non-spontaneous) and therefore ethane is (stable/unstable).

1. spontaneous, unstable

2. spontaneous, stable

3. non-spontaneous, unstable

4. non-spontaneous, stable

 

Watch Solution

For the reaction

2 SO3(g) → 2 SO2(g) + O2(g)

∆Hr = +198 kJ · mol−1 and ∆Sr = 190 J · K−1 · mol−1 at 298 K. The forward reaction will be spontaneous at

1. temperatures above 1042 K. 

2. temperatures below 1042 K.

3. all temperatures.

4. no temperature.

5. temperatures above 1315 K.

Watch Solution

The law states that a substance that is perfectly crystaline at 0 K has an entropy of zero. This law is called

1. the second law of thermodynamics.

2. the first law of thermodynamics.

3. None of these

4. the third law of thermodynamics.

Watch Solution

You observe that carbon dioxide sublimes. Which of the following statements about the signs of this process is/are true?

I) Work (w) is positive.

II) Heat (q) is negative.

III) Change in Gibbs free energy (∆G) is positive.

IV) Change in entropy (∆S) is positive.

1. I and II

2. II and III

3. I, II and III

4. I only

5. IV only 

6. III and IV

Watch Solution

Consider the following sets. Which one indicates a  NON-spontaneous reaction.

A. ΔH° = -35 kJ; ΔS° = 40 J/K; and the temperature is 80°C.

B. ΔH° = -19 kJ; ΔS° = -60 J/K; and the temperature is 10°C.

C. ΔH° = -15 kJ; ΔS° = -40 J/K; and the temperature is 150°C.

D. ΔH° = 25kJ; ΔS° = 70 J/K; and the temperature is 125°C.

E. ΔH° = 20 kJ; ΔS° = 75 J/K; and the temperature is 80°C.

Watch Solution

For the reaction A2(g) + B(s) → A2B(g)

ΔH° > 0 and ΔS° > 0

Which of the following statements is true?

A. The reaction is spontaneous at low temperatures.

B. The reaction is spontaneous at all temperatures.

C. ΔH° becomes more positive as temperature increases.

D. The reaction is spontaneous at high temperatures.

E. The reaction is non-spontaneous at all temperatures.

Watch Solution

Which of the following is true regarding a non-spontaneous process:

I. The process will occur with an outside influence

II. The process will occur given sufficient time

III. The process will never occur

A. only I

B. only II

C. both I and II

D. only III

E. None of the statements

Watch Solution

If Q ˃ K, what must be true regarding ΔG?

A. it is positive

B. it equals zero

C. it is negative

Watch Solution

The following reaction is endothermic:

2 H2O (l) → 2 H2(g) + O2(g)

Which of the following answers correctly describes the spontaneity:

A. This reaction is always spontaneous

B. This reaction is only spontaneous at low temperatures

C. This reaction is only spontaneous at high temperatures

D. This reaction is never spontaneous

 

Watch Solution

Given that the normal freezing point of ammonia is -78°C. Predict the signs of ΔH, ΔS and ΔG for ammonia when it freezes at -80°C and 1 atm.

NH3(l) → NH3(s)
          ΔH        ΔS       ΔG
A.         +         -         +
B.         -         -         -
C.         -         -         0
D.         -         +         -
E.        +         +         0

 

 

Watch Solution

For the reaction given below, ΔH° = -1516kJ and ΔS° = -432.8J/K at 25°C

SiH4(g) + 2O2(g) →  SiO2(g) + 2H2O(l) 

The reaction is spontaneous:
a. only below a certain temperature
b. only above a certain temperature
c. at all temperatures
d. at no temperatures
e. cannot tell from the information available.

 

 

Watch Solution

Which of the following processes WILL NOT be spontaneous at 1000 K?

A)        ΔH = +40.2kJ/mole      ΔS =  -41.8 J/mole K

B)        ΔH = +40.2kJ/mole      ΔS =  +41.8 J/mole K

C)        ΔH = -41.8 kJ/mole      ΔS =  +40.2 J/mole K

D)        ΔH = +41.8 kJ/mole      ΔS =  -40.2 J/mole K

E)         both A and D

Watch Solution

Consider a 1.00 L solution of 0.150 M HCN (Ka = 4.9 * 10−10); Assume carbon is the central atom. 

Indicate whether each statement is true or false.  

 

A. There are more HCN molecules in the solution than CN ions 

B. The acid reaction (corresponding to Ka) would have a ΔG° < 0 

C. Adding 0.150 moles of a strong base to this solution would make a buffer 

D. HCN is polar and has a linear shape 

E. The best Lewis structure would have a single bond and triple bond on C

F. HCN would exhibit hydrogen bonds with water 

Watch Solution

Consider a reaction that has a positive ΔH and a positive ΔS. Which of the following statements is TRUE?

A) This reaction will be non-spontaneous only at high temperatures.

B) This reaction will be non-spontaneous at all temperatures.

C) This reaction will be spontaneous at all temperatures.

D) This reaction will be spontaneous only at high temperatures.

E) It is not possible to determine without more information.

 

Watch Solution

Which of the following statements is TRUE?

A) Entropy is not a state function.

B) Endothermic processes are never spontaneous.

C) Endothermic processes decrease the entropy of the surroundings, at constant T and P.

D) Exothermic processes are always spontaneous.

E) None of the above are true.

Watch Solution

For the chemical reacion, A → B, choose the combination that is correct.

A) ΔG < 0; Q > K

B) ΔG < 0; Q < K

C) ΔG < 0; Q = K

D) ΔG° < 0; Q = K

E) ΔG° < 0; Q < K

 

Watch Solution

If a process has a ΔH = +48 kJ/mol and a ΔS = -130 J/mol K at what T will it become spontaneous?

A) 298 K

B) 273 K

C) 369 K

D) 642 K

E) Never

 

Watch Solution

Sodium carbonate can be made by heating sodium bicarbonate:

2 NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g).

Given that ΔH° = 128.9 kJ/mol and ΔG° = 33.1 kJ/mol at 25°C, above what minimum temperature will the reaction become spontaneous under standard-state conditions?

A) 0.4 K

B) 3.9 K

C) 321 K

D) 401 K

E) 525 K

Watch Solution

The thermodynamic condition for a spontaneous process at constant T and P is

A) ΔS > 0.

B) ΔS < 0.

C) ΔG < 0.

D) ΔG > 0.

E) ΔG° = 0.

Watch Solution

The reaction rates of many spontaneous reactions are actually very slow. Which of these statements is the best explanation for this observation?

A) Kp for the reaction is less than one.

B) The activation energy of the reaction is large.

C) ΔG° for the reaction is positive.

D) Such reactions are endothermic.

E) The entropy change is negative. 

Watch Solution

A spontaneous endothermic reaction always

A) causes the surrroundings to get colder.

B) bursts into flame.

C) requires a spark to initiate it.

D) releases heat to the surrroundings.

E) results in an increase in ΔSsurr

Watch Solution

Which of the following is an example of a nonspontaneous process?

A) ice melting at room temperature

B) sodium metal reacting violently with water

C) rusting of iron at room temperature

D) a ball rolling downhill

E) water freezing at room temperature

Watch Solution

When barium hydroxide is dissolved in water, the temperature of the solution increases. Which of the following statements regarding this dissolution process is TRUE?

(a) The reaction is spontaneous at all temperatures.

(b) The reaction is driven only by entropy.

(c) The reaction is driven only by enthalpy.

(d) The reaction is spontaneous only at low temperatures.

(e) The reaction is spontaneous only at high temperatures.

Watch Solution

Which of the following statements is TRUE?

(a) An exothermic process will always be spontaneous.

(b) A process in which the entropy of the system increases will always be spontaneous.

(c) An endothermic process can never be spontaneous.

(d) A process in which the entropy of the surroundings increases will always be spontaneous.

(e) An exothermic process that is accompanied by an increase in the entropy of the system will always be spontaneous.

Watch Solution

Consider the following facts: water freezes spontaneously at -5 °C and 1 atm, and ice has a more ordered structure than liquid water. Explain how a spontaneous process can lead to a decrease in entropy.

Watch Solution

The plot of change in free energy versus temperature (T) shown below corresponds to a process that is enthalpically ________ (favored, disfavored) and entropically _______ (favored, disfavored). Explain.

Watch Solution

Which reaction below is most likely spontaneous at any temperature? 

a) CH3CH2OH (g) → CH3CH2OH (l)

b) 2 H2 (g) + O2 (g) → 2 H2O (g)

c) C6H6O6 (s) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l)

d) CO2 (s) → CO2 (g)

e) None of the above

Watch Solution

Which of the following relationships must be true of any reaction at equilibrium?

a) ∆Suniv = 0

b) ∆G ̊ = 0

c) ∆S ̊ = 0

d) Both a and b

e) All of the above

Watch Solution

Which of the following statements are true?

I. In a spontaneous process, the path between reactants and products is reversible.

II. The reaction of nitrogen atoms to form N2 molecules at 25° C and 1 atm is spontaneous.

III. The melting of a solid is spontaneous above its melting point but nonspontaneous below its melting point.

IV. The condensation of a liquid will be reversible if it occurs at any temperature other than the boiling point at a specified temperature.

V. When a piece of metal is heated to 150° C and added to water, the fact that the water gets hotter is an example of a spontaneous process. 

 

a) I, II, III

b) II, III, IV

c) II, III, V

d) III, IV, V

e) I, III, V

 

Watch Solution

The normal freezing point of ammonia is -78°C. Predict the signs of ∆H, ∆S, and ∆G for ammonia when it freezes at -80°C and 1 atm: NH3(l) → NH 3(s) 

Watch Solution

The equilibrium constant for the reaction

NH4Cl(s) → NH3(g) + HCl(g)

is 1.1 × 10−16 at 25°C. If the equilibrium constant is 6.5 × 10 −2 at 300°C, which statement is correct?

a) This is an enthalpy-driven reaction.

b) The reaction is spontaneous at high temperatures.

c) The reaction is spontaneous at all temperatures.

d) The reaction is exothermic.

e) The reaction is not spontaneous at any temperature.

Watch Solution

What factor ultimately determines whether any physical process, including a chemical reaction, is spontaneous or not?

a) the change in entropy of the components involved in the process

b) the change in enthalpy of the components involved in the process

c) the change in enthalpy of the universe

d) the change in entropy of the universe

e) the change in entropy of the world outside the process

f) the change in enthalpy of the world outside the process

Watch Solution

Which of the following is true regarding a non-spontaneous process:

            I.    The process will occur with an outside influence

            II.   The process will occur given sufficient time

            III.  The process will never occur

A.  only I         B.  only II       C.  both I and II          D.  only III      E.  None of the statements

Watch Solution

The following reaction is endothermic:

            2 H2O (l) → 2 H2 (g) + O2 (g)

Which of the following answers correctly describes the spontaneity:

           

A.  This reaction is always spontaneous

            B.  This reaction is only spontaneous at low temperatures

            C.  This reaction is only spontaneous at high temperatures

            D.  This reaction is never spontaneous

Watch Solution

For a reaction in which ΔH = 125 kJ/mol and ΔS = 325 J/K•mol, determine the temperature in Celsius above which the reaction is spontaneous.

A.  385 °C             B.  273 °C       C.  112 °C       D.  405 °C       E.  25 °C

 

Watch Solution

Given the following reaction: A(g) ⇌ B(g), the forward reaction is spontaneous under standard conditions. Which of the following statements must be TRUE?

I.                The reverse reaction is nonspontaneous under standard conditions.

II.              A(g) will be completely converted to B(g) if sufficient time is allowed.

III.             A(g) will be completely converted to B(g) rapidly.

 

A) none of these

B) I and I

C) I, II, and III

D) I

E) II and III

Watch Solution

Calculate ΔG° for the reaction NH4NO3(s) → NH4+(aq) + NO3(aq) to determine whether the reaction is spontaneous or not. Temperature: 298.15 K and ΔS°: 108.7 J/K•mol

Compound          ΔHf° (kJ/mol)

NH4NO3(s)          −365.56

NH4+(aq)             −132.51

NO3(aq)             −205.0

A.  4.4 J , nonspontaneous

B.  – 4.4 kJ, spontaneous

C.  28.5 kJ spontaneous

D.  −28.5 J nonspontaneous

E.  5.6 kJ nonspontaneous

 

Watch Solution

When can Q > K but ΔG = 0? 

a) When the reaction is going forwards after being equilibrium

b) When the reaction is going backward towards equilibrium 

c) For an every endothermic reaction when Q = K 

d) When the reaction is going backward after being at equilibrium 

e) never

Watch Solution

The Three Laws of Thermodynamics are as follows:

1. The total ____________ of the universe is constant.

2. The total ___________ of the universe increases for a spontaneous process.

3. The ____________ of a perfectly ordered crystal at 0 K is 0.

The answers in order are

(a) energy, energy, energy      

(b) energy, entropy, energy      

(c) entropy, energy, energy

(d) entropy, energy, entropy    

(e) energy, entropy, entropy

Watch Solution

Physical processes, including chemical reactions, tend to go towards ________ entropy and _________ energy. 

The answers in order are

(a) lower, higher      

(b) higher, lower      

(c) lower, lower      

(d) higher, higher

Watch Solution

A spontaneous chemical reaction is FAVORED by which of the following?

a) increasing energy and increasing entropy

b) lowering energy and increasing entropy

c) increasing energy and decreasing entropy

d) lowering energy and decreasing entropy

Watch Solution

Which is the best estimate for the boiling point of benzene (°C) given that ΔH° of vaporization is 31 kJ/mol and ΔS° of vaporization is 90 J/mol K?

a.  25

b.  45

c.  65

d.  15

e.  5

Watch Solution

Why does the solubility of NaCl only increase slowly with water temperature?

A. ΔSsol > 0 but small in magnitude

B. ΔSsol < 0 but small in magnitude

C. ΔHsol > 0 but small in magnitude

D. ΔHsol < 0 but small in magnitude

E. it has a very large lattice energy

Watch Solution

Consider the reaction of N2O5 at 25°C for which the following data are relevant:

2 N2O5 (g)      ⇌       4 NO 2 (g) + O2 (g)

What is the ΔH° for the reaction?

A. 110.02 kJ

B. 21.86 kJ

C. -21.86 kJ

D. 155.20 kJ

E. -155.20 kJ

Watch Solution

As O2 (l) is cooled at 1 atm, it freezes at 54.5 K to form Solid I. At a lower temperature, Solid I rearranges to Solid II, which has a different crystal structure. Thermal measurements show that ΔH for the I→II phase transition = -743.1 J/mol, and ΔS for the same transition = -17.0 J/K mol. At what temperature are Solids I and II in equilibrium?

A. 2.06 K

B. 43.7 K

C. 31.5 K

D. 53.4 K

E. They can never be in equilibrium because they are both solids.

Watch Solution

For the reaction A + B ⇌ C + D, ΔH°= +40 kJ and ΔS°= +50 J/K. Therefore, the reaction at standard conditions is

A. spontaneous at temperatures less than 10 K.

B. spontaneous at temperatures greater than 800 K.

C. spontaneous only at temperatures between 10 K and 800 K.

D. spontaneous at all temperatures.

E. Nonspontaneous at all temperatures.

Watch Solution

A mixture of hydrogen and chlorine remains unreacted until it is exposed to ultraviolet light from a burning magnesium strip. Then the following reaction occurs very rapidly:

H2 + Cl2      →       2 HCl       ΔG = - 45.54 kJ

                                              ΔH = - 44.12 kJ

                                              ΔS = - 4.76 J/k

Select the statement below which BEST explains this behavior.

A. The reactants are thermodynamically more stable than the products.

B. The reaction has a small equilibrium constant.

C. The ultraviolet light raises the temperature of the system and makes the reaction more favorable.

D. The negative value for ΔS slows down the reaction.

E. The reaction is spontaneous by thermodynamics, but the reactants are kinetically stable.

Watch Solution

Consider the equation carefully, and think about the sign of ∆S for the reaction it describes.

NH4Br (s) → NH3 (g) + HBr (g) ∆H = +188.3 kJ.

Which response describes the thermodynamic spontaneity of the reaction?

1. The reaction is spontaneous only at relatively high temperatures.

2. All responses are correct

3. The reaction is spontaneous at all temperatures.

4. The reaction is not spontaneous at any temperatures.

5. The reaction is spontaneous only at relatively low temperatures.

Watch Solution

The sublimation of solid carbon dioxide is a spontaneous process. Predict the sign (+, -, or 0) of ∆Gº r , ∆Hº r , and ∆Sº r , respectively.

1. -, -, -

2. 0, +, +

3. -, 0, +

4. -, +, -

5. -, +, + 

Watch Solution

Which of the following is always positive when a spontaneous process occurs?

a. ΔSsystem

b. ΔSsurroundings

c. ΔSuniverse

d. ΔHuniverse

e. ΔHsurroundings

 

 

Watch Solution

Which is NOT a consequence of the second law of thermodynamics?

1. Heat spontaneously flows from a warmer body to a colder body.

2. When a book falls, “ordered” kinetic energy is transformed into random heat energy

3. Ice melts above OºC.

4. During any naturally occurring process, the energy of the universe remains constant. 

Watch Solution

Which is a state function (mark all that apply)?

a) flame heating

b) enthalpy

c) entropy

d) electrical work

e) none of these

Watch Solution

What is true for the following reaction under standard condition?

C2H6 (g) → C2H4 (g) + H2 (g)

ΔH° is 137 kJ and ΔS° is 120 J/K

a) spontaneous at all temperatures

b) spontaneous only at high temperature

c) spontaneous only at low temperature

d) not spontaneous at all temperatures

e) cannot be determined

Watch Solution

Which statement below is true about the standard Gibbs energy -33.3kJ at 25°C for the reaction.

N2 (g) + 3 H2 (g) → 2 NH3 (g)                      ΔG° = -33.3kJ

a. ΔG° corresponds to 1 atm of N 2 reaction with 3 atm of H2 to form 2 atm of NH 3.

b. ΔG° corresponds to 1 atm of N 2 reaction with 1 atm of H2 to form 2 atm of NH  3.

c. ΔG° corresponds to 1 atm of each N 2, H2, and NH3 reacting to 100% completion.

d. ΔG° corresponds to 1 atm of each N 2, H2, and NH3 reacting until equilibrium is established.

e. None of the above are true.

Watch Solution

Which reaction below is most likely spontaneous at any temperature?

a. CH3CH2OH (g) → CH3CH2OH (I)

b. 2 H2 (g) + O2 (g) → 2 H2O (g)

c. C6H6O6 (s) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (I)

d. CO2 (s) → CO2 (g)

e. None of the above

Watch Solution

A reaction has a standard entropy change of +32.83 J/K and a standard enthalpy of +8.10 kJ. At what temperature is the reaction spontaneous? Assume the enthalpy and entropy of reaction are independent of temperature.

a) Spontaneous when T > 246.7 K

b) Spontaneous when T < 246.7 K

c) Spontaneous when T > 25 K

d) The reaction is non-spontaneous at all temperatures

e) The reaction spontaneous at all times

Watch Solution

The compound benzoic acid, C6H5COOH, melts and 122.4°C. If the enthalpy of fusion of this compound is 17.3 kJ/mol, what is the entropy of the fusion.

a) 31.3 J/(mol•K)

b) 34.1 J/(mol•K)

c) 38.0 J/(mol•K)

d) 40.2 J/(mol•K)

e) 43.8 J/(mol•K)

Watch Solution

Which statement best describes spontaneous reactions?

a) Spontaneous reactions occur very quickly

b) Spontaneous reactions can proceed without any external input

c) The reverse of a spontaneous reaction is also spontaneous reaction

d) All spontaneous reactions are reversible reactions

e) Most spontaneous reactions become less spontaneous at high temperatures

Watch Solution

The following reaction is exothermic.

                             N(g) + 3 H(g) → 2 NH(g)

This means the reaction

  1. will be spontaneous in the forward direction at low temperature.
  2. is not spontaneous in the forward direction at any temperature.
  3. will be spontaneous in the forward direction at high temperature.
  4. is spontaneous in the forward direction at all temperatures.
Watch Solution

Which of the following is required for a spontaneous reaction:

 

A. The entropy of the universe increases.

B. The enthalpy of the universe increases.

C. The Free Energy of the universe increases.

D. The temperature of the universe increases. 

Watch Solution

When ammonium ion and nitrate ion combine they form ammonium nitrate, NH4NO3. During this combination reaction the container feels cold to the touch. What are the signs for ∆H, ∆S, ∆G (at high T), and ∆G (low T)? 

 

 

Watch Solution

The reaction between elemental nitrogen and elemental hydrogen, shown below, is most kinetically and thermodynamically favorable. 

       N2 (g) + 3 H2 (g)   →    2 NH3 (g) 

Based on this information, which of the following energy diagrams best describes this reaction? 

Watch Solution

For a certain chemical reaction,ΔH = 128 kJ/mol and ΔS = 64 J/ (mol × K) . What is the minimum temperature at which this reaction will be spontaneous?

 

A. 2 K                    B. 2000 K                     C. 2000 °C                     D. 2 °C

Watch Solution

If a reaction is exothermic but experiences a decrease in its entropy, under what conditions is the reaction most likely to be spontaneous?

 

A. Low Temperature

B. High Temperature

C. Low Pressure

D. High Pressure

Watch Solution

Consider the conversion of a substance from solid to liquid.

Solid ⇌ Liquid 

At one atmosphere pressure and at the melting point of the substance, ________ . 

Watch Solution

Above what temperature does the following reaction become spontaneous?

FeO (s) + CO (g) CO2 (g) + Fe (g)                             ∆H = – 11.0 kJ ∆S = – 17.4 J/K

 

A. 191K

B. 632K

C. 298K

D. Not enough information to determine

E. Always spontaneous

Watch Solution

Which of the following statements is TRUE?

 

a. State functions are not dependent on the path taken to reach a particular state.

b. The change in the internal energy of a system can be determined by using constant volume calorimetry.

c. Energy is neither created nor destroyed.

d. The enthalpy of a reaction can be determined by using constant pressure calorimetry.

e. All of the above statements are true. 

Watch Solution