Units 6 and 9 Thermodynamics MCQ

The equation and standard cell potential for the decomposition of H2O2(aq) in acidic solution at 25°C is given above. The reduction half-reactions for the process are listed below. What is the standard reduction potential for the half-reaction represented above? A. -1.78 V B. -0.68 V C. +0.68 V D. +1.78 V

C. +0.68 V

In an insulated cup of negligible heat capacity, 50. g of water at 40.°C is mixed with 30. g of water at 20.°C. The final temperature of the mixture is closest to A. 22°C B. 27°C C. 30.°C D. 33°C E. 38°C

D. 33°C

3 C2H2(g) → C6H6(g) What is the standard enthalphy change ΔHo, for the reaction represented above? (ΔHof of C2H2(g) is 230 kJ mol-1; A. -607 kJ B. -147 kJ C. -19 kJ D. +19 kJ E. +773 kJ

A. -607 kJ

The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K Which of the following statements about the bonds in the reactants and products is most accurate? A. The sum of the bond enthalpies of the bonds in the reactant is greater than the sum of the bond enthalpies of the bonds in the products. B. The sum of the bond enthalpies of the bonds in the reactant is less than the sum of the bond enthalpies of the bonds in the products. C. The length of the bond between carbon and oxygen in CH3OH is shorter than the length of the bond between carbon and oxygen in CO. D. All of the bonds in the reactant and products are polar.

A. The sum of the bond enthalpies of the bonds in the reactant is greater than the sum of the bond enthalpies of the bonds in the products.

The following questions refer to the graph below, which shows the heating curve for methane, CH4. How much energy is required to melt 64 g of methane at 90 K? (The molar mass of methane is 16 g/mol.) A. 0.24 kJ B. 3.8 kJ C. 33 kJ D. 60. kJ

B. 3.8 kJ

2 H2O2(aq) → 2 H2O(l) + O2(g) ΔH° = −196 kJ/molrxn The decomposition of H2O2(aq) is represented by the equation above. A student monitored the decomposition of a 1.0 L sample of H2O2(aq) at a constant temperature of 300. K and recorded the concentration of H2O2 as a function of time. The results are given in the table below. Assume that the bond enthalpies of the oxygenhydrogen bonds in H2O are not significantly different from those in H2O2 . Based on the value of ΔH° of the reaction, which of the following could be the bond enthalpies (in kJ/mol) for the bonds broken and formed in the reaction? A. O-O in H2O2 300 O=O in O2 500 O-H3 500 B. O-O in H2O2 150 O=O in O2 500 O-H 500 C. O-O in H2O2 500 O=O in O2 300 O-H 150 D. O-O in H2O2 250 O=O in O2 300 O-H 150

B. O-O in H2O2 150 O=O in O2 500 O-H 500

Couldn't Do

Couldn't Do

The equilibrium constant for a reaction is greater than 1.0 at temperatures above 500K500K but less than 1.0 at temperatures below 500K. What can be concluded about the values of ΔH° and ΔS° for the reaction? (Assume that ΔH°and ΔS° are independent of temperature.) A. ΔH°>0Δand ΔS°>0 B. ΔH°>0 and ΔS°<0 C. ΔH°<0 and ΔS°>0 D. ΔH°<0 and ΔS°<0

A. ΔH°>0Δand ΔS°>0

H2(g) + Cl2(g) ⇄ 2 HCl(g) Kp = 2 × 1030 at 298 K HCl(g) can be synthesized from H2(g) and Cl2(g) as represented above. A student studying the kinetics of the reaction proposes the following mechanism Step 1: Cl2(g) → 2 Cl(g) (slow) ∆H° = 242 kJ/molrxn Step 2: H2(g) + Cl(g) → HCl(g) + H(g) (fast) ∆H° = 4 kJ/molrxn Step 3: H(g) + Cl(g) → HCl(g) (fast) ∆H° = -432 kJ/molrxn What is the value of the enthalpy change per mole of HCl(g) produced? A. −93 kJ B. −121 kJ C. −186 kJ D. −242 kJ

A. −93 kJ

The spontaneous reaction that occurs when the cell above operates is 2 Ag+ + Cd(s) → 2 Ag(s) + Cd2+ Which of the above occurs for the following circumstance? A 50-milliliter sample of a 2-molar Cd(NO3)2 solution is added to the left beaker. A. Voltage increases. B. Voltage decreases but remains above zero C. Voltage becomes zero and remains at zero D. No change in voltage occurs E. Direction of voltage change cannot be predicted without additional information

B. Voltage decreases but remains above zero

2 H2O2(aq) → 2 H2O(l) + O2(g) ΔH° = −196 kJ/molrxn The decomposition of H2O2(aq) is represented by the equation above. A student monitored the decomposition of a 1.0 L sample of H2O2(aq) at a constant temperature of 300. K and recorded the concentration of H2O2 as a function of time. The results are given in the table below. The reaction is thermodynamically favorable. The signs of ΔG° and ΔS° for the reaction are which of the following? A. ΔG° + ΔS° + B. ΔG° - ΔG° + (this is how it is in quiz) * C. ΔG° + ΔS° - D. ΔG° - ΔS° -

B. ΔG° - ΔG° +

BondApproximate Bond Enthalpy (kJ/mol) C-H 410 C−F 490 H−F 570 F−F 160 Based on the bond enthalpies given in the table above, which of the following is the best estimate of ΔH° for the reaction represented below? CH4+F2→CH3F+HFCH4+F2→CH3F+HF A. −650kJ/molrxn B. −490kJ/molrxn C. +490kJ/molrxn D. +650kJ/molrxn

B. −490kJ/molrxn

The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K What can be inferred about ∆S° for the reaction at 600 K? A. It must be positive, since the reaction is thermodynamically unfavorable at 600 K. BIt must be negative, since there are more moles of products than reactants. C. It must be positive, since ∆G° is negative and ∆H° is positive. D. It must be negative, since ∆G° is positive and ∆H° is positive.

C. It must be positive, since ∆G° is negative and ∆H° is positive.

CH4(g) + Cl(g) → CH3(g) + HCl(g) ΔH° = -14 kJ/molrxn NH3(g) + Cl(g) → NH2(g) + HCl(g) ΔH° = -36 kJ/molrxn H2O(g) + Cl(g) → OH(g) + HCl(g) ΔH° = + 40 kJ/molrxn Based on the data above, what can be concluded regarding the strength of the C-H, N-H, and O-H bonds in the molecules shown? A. The C-H bond is the strongest. B. The N-H bond is the strongest. C. The O-H bond is the strongest. D. Nothing can be concluded without knowing the strength of the H- Cl bond.

C. The O-H bond is the strongest.

CO(g) + 2 H2(g) ⇄ CH3OH(g) ΔH < 0 The synthesis of CH3OH(g) from CO(g) and H2(g) is represented by the equation above. The value of Kc for the reaction at 483 K is 14.5. Which of the following statements is true about bond energies in this reaction? A. The energy absorbed as the bonds in the reactants are broken is greater than the energy released as the bonds in the product are formed. B. The energy released as the bonds in the reactants are broken is greater than the energy absorbed as the bonds in the product are formed. C. The energy absorbed as the bonds in the reactants are broken is less than the energy released as the bonds in the product are formed. D. The energy released as the bonds in the reactants are broken is less than the energy absorbed as the bonds in the product are formed.

C. The energy absorbed as the bonds in the reactants are broken is less than the energy released as the bonds in the product are formed.

In an experiment, 30.0g30.0g of ethane and 30.0g30.0g of propanol are placed in separate reaction vessels. Each compound undergoes complete combustion with excess O2(g)O2(�). Which of the following best compares the quantity of heat released in each combustion reaction? A. q.ethane<q.propanol B. q.ethane=q.propanol C. q.ethane>q.propanol D. The quantities of heat released in the combustions of ethane and propanol cannot be compared without knowing the specific heat capacity of the compounds.

C. q.ethane>q.propanol

Reaction 1 : 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(l) ΔG ° = -1010 kJ/molrxn Reaction 2 : 2 NO2(g) → 2 NO(g) + O2(g) ΔG ° = 70 kJ/molrxn Reaction 3 : 4 NO2(g) + O2(g) + 2 H2O(l) → 4 HNO3(aq) ΔG ° = -170 kJ/molrxn Based on the values of ΔG° for the three reactions represented above, what is the value of ΔG° for the reaction represented below? 4 NH3(g) + 8 O2(g) → 4 HNO3(aq) + 4 H2O(l) A. -1040 kJ/molrxn B. -1110 kJ/molrxn C. -1250 kJ/molrxn D. -1320 kJ/molrxn

D. -1320 kJ/molrxn

Which of the following substances would be the best choice for filling the salt bridge in the galvanic cell shown above, and why? A. Distilled water, because it would avoid contaminating the solutions in the half-cells. B. 1.0MHCl(aq), because it would acidify the solutions and catalyze the overall cell reaction. C. 1.0MCH3OH(aq), because it would increase the pHpH of the solutions and catalyze the overall cell reaction. D. 1.0MKNO3(aq), because the mobile ions would allow charge to flow between the half-cells.

D. 1.0MKNO3(aq), because the mobile ions would allow charge to flow between the half-cells.

Data for H2OH2OSpecific heat capacity of liquid phase4.2J/(g⋅°C)4.2J/(g·°C)Heat of fusion330J/g330J/g A sample of ice at 0°C0°C is added to 100.g100.g of water at 33°C33°C. The mixture is stirred gently until the temperature of the water is 0°C0°C. All the remaining ice is quickly removed. Based on the information in the table above, the mass of ice that melted is closest to A. 0.13g 0.13g B. 0.42g 0.42g C. 1.3g 1.3g D. 42g

D. 42g

The spontaneous reaction that occurs when the cell above operates is 2 Ag+ + Cd(s) → 2 Ag(s) + Cd2+ Which of the above occurs for the following circumstance? The silver electrode is made larger. A. Voltage increases. B. Voltage decreases but remains above zero C. Voltage becomes zero and remains at zero D. No change in voltage occurs E. Direction of voltage change cannot be predicted without additional information

D. No change in voltage occurs

Diagram Which of the following best explains why the combustion reactions represented in the table are exothermic? A. The number of bonds in the reactant molecules is greater than the number of bonds in the product molecules. B. The number of bonds in the reactant molecules is less than the number of bonds in the product molecules. C. The energy required to break the bonds in the reactants is greater than the energy released in forming the bonds in the products. D. The energy required to break the bonds in the reactants is less than the energy released in forming the bonds in the products.

D. The energy required to break the bonds in the reactants is less than the energy released in forming the bonds in the products.

CH4(g)+2O2(g)→CO2(g)+2H2O(g) ΔH°<0 Which of the following statements must be true for the combustion of CH4(g)CH4(g), represented by the equation above? A. The bond dissociation energy between O atoms in O2O2 is greater than the sum of the bond dissociation energies of the two O−HO−H bonds in H2OH2O. B. More bonds are formed in the products than are broken in the reactants. C. The sum of the bond dissociation energies of the product molecules is equal to the sum of the bond dissociation energies of the reactant molecules. D. The sum of the bond dissociation energies of the product molecules is greater than the sum of the bond dissociation energies of the reactant molecules.

D. The sum of the bond dissociation energies of the product molecules is greater than the sum of the bond dissociation energies of the reactant molecules.

H2(g) + Cl2(g) ⇄ 2 HCl(g) Kp = 2 × 10^30 at 298 K HCl(g) can be synthesized from H2(g) and Cl2(g) as represented above. A student studying the kinetics of the reaction proposes the following mechanism Step 1: Cl2(g) → 2 Cl(g) (slow) ∆H° = 242 kJ/molrxn Step 2: H2(g) + Cl(g) → HCl(g) + H(g) (fast) ∆H° = 4 kJ/molrxn Step 3: H(g) + Cl(g) → HCl(g) (fast) ∆H° = -432 kJ/molrxn Which of the following statements identifies the greatest single reason that the value of Kp for the overall reaction at 298 K has such a large magnitude? A. The activation energy for step 1 of the mechanism is large and positive. B. The activation energy for step 2 of the mechanism is small and positive. C. The value of ΔS° for the overall reaction is small and positive. D. The value of ΔH° for the overall reaction is large and negative.

D. The value of ΔH° for the overall reaction is large and negative.

2H2O2(aq)→2H2O(l)+O2(g)ΔG°=−234kJ/molrxn The value of ΔG° for the reaction represented above implies that the decomposition of H2O2(aq) is thermodynamically favorable. However, H2O2(aq) is typically stable for up to a year stored in a dark bottle at 298K298K. The best explanation for this observation is that the decomposition reaction A. is only thermodynamically favorable in the presence of a catalyst B. occurs with an increase in entropy because O2(g) is a product C. is reversible and H2O2(aq) is produced almost as fast as it decomposes D. has a slow rate at 298K298K because the activation energy is relatively high

D. has a slow rate at 298K because the activation energy is relatively high

Mass of Solid 1.0 g Mass of Water 2000.0g ΔTwater+2.1°C A sample of a solid organic compound is completely combusted in a calorimeter. The heat generated by combustion is transferred to the water, causing the temperature of the water to increase. Based on the data in the table above, which of the following is the best estimate of the heat of combustion, ΔHcomb, of the organic solid? (The specific heat of water is 4.2J/(g⋅°C)4.2J/g⋅°C ) A. +18kJ/g+18kJ/g B. +8.4kJ/g+8.4kJ/g C. −8.4kJ/g−8.4kJ/g D. −18kJ/g

D. −18kJ/g