Question 1

Indicate which of the following is not an element in its standard state at 25   F1F1F1S1F1F1F10C and 1 atm.

Accepted Answer

A) O2(g) 
B) H2(g) 
C) Ne(g) 
D) N(g) 
E) C(s, graphite) 
A) O2(g) 
B) H2(g) 
C) Ne(g) 
D) N(g) 
E) C(s, graphite)

Question 2

Assuming that the charge of the ions remain constant in all cases, which of the following ion pairs has the greatest electrostatic potential energy (i.e., largest in magnitude)?

Accepted Answer

A)    
B)    
C)    
D)    
E)    
A)    
B)    
C)    
D)    
E)

Question 3

Use the following information to determine the standard enthalpy change when 1 mol of PbO(s) is formed from lead metal and oxygen gas. PbO(s)   F1F1F1S1F1F1F10C(graphite)   F1F1F1S1F1F1F10Pb(s)   F1F1F1S1F1F1F10CO(g) F1F1F1S1F1F1F10H   F1F1F1S1F1F1F10F1F1F1S1F1F1F10 107 kJ
2C(graphite)   F1F1F1S1F1F1F10O2(g)   F1F1F1S1F1F1F102CO(g) F1F1F1S1F1F1F10HF1F1F1S1F1F1F10 F1F1F1S1F1F1F10 F1F1F1S1F1F1F10222 kJ

Accepted Answer

A) (F1F1F1S1F1F1F10 436 kJ) 
B) (F1F1F1S1F1F1F10436 kJ) 
C) (F1F1F1S1F1F1F10218 kJ) 
D) (F1F1F1S1F1F1F10329 kJ) 
E) (F1F1F1S1F1F1F10115 kJ) 
A) (F1F1F1S1F1F1F10 436 kJ) 
B) (F1F1F1S1F1F1F10436 kJ) 
C) (F1F1F1S1F1F1F10218 kJ) 
D) (F1F1F1S1F1F1F10329 kJ) 
E) (F1F1F1S1F1F1F10115 kJ)

Question 4

In terms of the enthalpy of formation, which of the following compounds is the most unstable compared to its elements under standard conditions?

Accepted Answer

A) KCl(s),   435.9 kJ/mol 
B) PH3(g),   5.4 kJ/mol 
C) Fe3O4(s),   1,117.1 kJ/mol 
D) NiO(s),   239.7 kJ/mol 
E) ICl(g),   17.8 kJ/mol 
A) KCl(s),   435.9 kJ/mol 
B) PH3(g),   5.4 kJ/mol 
C) Fe3O4(s),   1,117.1 kJ/mol 
D) NiO(s),   239.7 kJ/mol 
E) ICl(g),   17.8 kJ/mol

Question 5

A 150 g piece of iron (CP F1F1F1S1F1F1F10 25.09 J/(mol F1F1F1S1F1F1F10   F1F1F1S1F1F1F10)) was heated to a temperature of 47   F1F1F1S1F1F1F10and then placed in contact with a 275 g piece of copper at 20   F1F1F1S1F1F1F10(CP F1F1F1S1F1F1F10 25.46 J/(mol F1F1F1S1F1F1F10   F1F1F1S1F1F1F10)). What was the final temperature of the two pieces of metal?

Accepted Answer

A) 25   F1F1F1S1F1F1F10 
B) 20   F1F1F1S1F1F1F10 
C) 30   F1F1F1S1F1F1F10 
D) 47   F1F1F1S1F1F1F10 
E) 33   F1F1F1S1F1F1F10 
A) 25   F1F1F1S1F1F1F10 
B) 20   F1F1F1S1F1F1F10 
C) 30   F1F1F1S1F1F1F10 
D) 47   F1F1F1S1F1F1F10 
E) 33   F1F1F1S1F1F1F10

Question 6

Using the following data for water, determine the final temperature when 100 g of ice at F1F1F1S1F1F1F1010   F1F1F1S1F1F1F10is heated with 350 kJ of energy. Boiling point
373 K
Melting point
273 K
Enthalpy of vaporization
2,260 J/g
Enthalpy of fusion
334 J/g
Specific heat capacity (solid)
2)11 J/(g F1F1F1S1F1F1F10 K)
Specific heat capacity (liquid)
4)18 J/(g F1F1F1S1F1F1F10 K)
Specific heat capacity (gas)
2)08 J/(g F1F1F1S1F1F1F10 K)

Accepted Answer

A) 309   F1F1F1S1F1F1F10 
B) 100   F1F1F1S1F1F1F10 
C) 382   F1F1F1S1F1F1F10 
D) 225   F1F1F1S1F1F1F10 
E) 325   F1F1F1S1F1F1F10 
A) 309   F1F1F1S1F1F1F10 
B) 100   F1F1F1S1F1F1F10 
C) 382   F1F1F1S1F1F1F10 
D) 225   F1F1F1S1F1F1F10 
E) 325   F1F1F1S1F1F1F10

Question 7

At a certain elevation, the boiling point of water is 98.5   . How much energy is needed to heat 35.0 mL of water to the boiling point at this elevation if the water initially was at 23.4F1F1F1S1F1F1F10C? (CP F1F1F1S1F1F1F10 75.38 J/(mol ·   ), d F1F1F1S1F1F1F10 1.00 g/mL)

Accepted Answer

A) 8.5 kJ 
B) 11.0 kJ 
C) 0.93 kJ 
D) 1.0 kJ 
E) 25.6 kJ 
A) 8.5 kJ 
B) 11.0 kJ 
C) 0.93 kJ 
D) 1.0 kJ 
E) 25.6 kJ

Question 8

A 15 g piece of iron (CP F1F1F1S1F1F1F10 25.09 J/(mol · F1F1F1S1F1F1F10C)) is heated to a temperature of 95F1F1F1S1F1F1F10C and placed into a bucket containing 4.5 gal of water (CP F1F1F1S1F1F1F10 75.38 J/(mol · F1F1F1S1F1F1F10C)) initially at 25F1F1F1S1F1F1F10C. Eventually, ________

Accepted Answer

A) the water will be warmer than the iron. 
B) the iron will be warmer than the water. 
C) the iron will be colder than the water. 
D) the iron and the water will be at the same temperature. 
E) the temperature will be the average of 95F1F1F1S1F1F1F10C and 25F1F1F1S1F1F1F10C. 
A) the water will be warmer than the iron. 
B) the iron will be warmer than the water. 
C) the iron will be colder than the water. 
D) the iron and the water will be at the same temperature. 
E) the temperature will be the average of 95F1F1F1S1F1F1F10C and 25F1F1F1S1F1F1F10C.

Question 9

Which statement A-D about a system and its surroundings is not correct?

Accepted Answer

A) A capped thermos bottle is an example of an isolated system. 
B) A metal box on a table is an example of a closed system. 
C) A cup of coffee is an example of an open system. 
D) No matter or energy can be exchanged between the surroundings and a closed system. 
E) Statements A-D all are correct. 
A) A capped thermos bottle is an example of an isolated system. 
B) A metal box on a table is an example of a closed system. 
C) A cup of coffee is an example of an open system. 
D) No matter or energy can be exchanged between the surroundings and a closed system. 
E) Statements A-D all are correct.

Question 10

Use the following information to determine the enthalpy for the reaction shown below. S(s) F1F1F1S1F1F1F10 O2(g) F1F1F1S1F1F1F10 SO2(g) F1F1F1S1F1F1F10HF1F1F1S1F1F1F10 F1F1F1S1F1F1F10 ?
S(s)   F1F1F1S1F1F1F10   O2(g)   F1F1F1S1F1F1F10SO3(g)   2SO2(g)   F1F1F1S1F1F1F10O2(g)   F1F1F1S1F1F1F102SO3(g)

Accepted Answer

A) (F1F1F1S1F1F1F10593 kJ) 
B) (F1F1F1S1F1F1F10593 kJ) 
C) (F1F1F1S1F1F1F10296 kJ) 
D) (F1F1F1S1F1F1F10296 kJ) 
E) (F1F1F1S1F1F1F10989 kJ) 
A) (F1F1F1S1F1F1F10593 kJ) 
B) (F1F1F1S1F1F1F10593 kJ) 
C) (F1F1F1S1F1F1F10296 kJ) 
D) (F1F1F1S1F1F1F10296 kJ) 
E) (F1F1F1S1F1F1F10989 kJ)

Question 11

For which reaction below does the enthalpy change under standard conditions correspond to a standard enthalpy of formation?

Accepted Answer

A) 2H2(g)   F1F1F1S1F1F1F10C(s)   F1F1F1S1F1F1F10CH4(g) 
B) CO2(g)   F1F1F1S1F1F1F10C(s)   F1F1F1S1F1F1F102CO(g) 
C) 2NO2(g)   F1F1F1S1F1F1F10N2O4(g) 
D) CO(g)   F1F1F1S1F1F1F10H2O(g)   F1F1F1S1F1F1F10CO2(g)   F1F1F1S1F1F1F10H2(g) 
E) CO2(g)   F1F1F1S1F1F1F10H2(g)   F1F1F1S1F1F1F10CO(g)   F1F1F1S1F1F1F10H2O(g) 
A) 2H2(g)   F1F1F1S1F1F1F10C(s)   F1F1F1S1F1F1F10CH4(g) 
B) CO2(g)   F1F1F1S1F1F1F10C(s)   F1F1F1S1F1F1F102CO(g) 
C) 2NO2(g)   F1F1F1S1F1F1F10N2O4(g) 
D) CO(g)   F1F1F1S1F1F1F10H2O(g)   F1F1F1S1F1F1F10CO2(g)   F1F1F1S1F1F1F10H2(g) 
E) CO2(g)   F1F1F1S1F1F1F10H2(g)   F1F1F1S1F1F1F10CO(g)   F1F1F1S1F1F1F10H2O(g)

Question 12

Which of the following fuels has the highest fuel value (kJ/g)?

Accepted Answer

A) natural gas, CH4(g), 16.0 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F10801 kJ/mol 
B) ethanol, CH3CH2OH(l), 46.1 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F101,368 kJ/mol 
C) heating oil, C16H34(l), 226 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F1010,800 kJ/mol 
D) coal, C135H96O9NS(l), 1906 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F1044,000 kJ/mol 
E) wood, C6H12O6(l), 180 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F102,540 kJ/mol 
A) natural gas, CH4(g), 16.0 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F10801 kJ/mol 
B) ethanol, CH3CH2OH(l), 46.1 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F101,368 kJ/mol 
C) heating oil, C16H34(l), 226 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F1010,800 kJ/mol 
D) coal, C135H96O9NS(l), 1906 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F1044,000 kJ/mol 
E) wood, C6H12O6(l), 180 g/mol,   F1F1F1S1F1F1F10Hcomb   F1F1F1S1F1F1F102,540 kJ/mol

Question 13

Steam in a cylinder is compressed by a piston exerting a constant pressure of 5 atm. The volume of the cylinder decreases by 15 L and simultaneously the steam is cooled, losing 105 kJ of energy as heat. How much energy, total, was gained or lost by the steam in this process? (101.3 J F1F1F1S1F1F1F10 1 L atm)

Accepted Answer

A) (F1F1F1S1F1F1F1030.0 kJ) 
B) (F1F1F1S1F1F1F1097.4 kJ) 
C) (F1F1F1S1F1F1F10113 kJ) 
D) (F1F1F1S1F1F1F10180.0 kJ) 
E) 113 kJ 
A) (F1F1F1S1F1F1F1030.0 kJ) 
B) (F1F1F1S1F1F1F1097.4 kJ) 
C) (F1F1F1S1F1F1F10113 kJ) 
D) (F1F1F1S1F1F1F10180.0 kJ) 
E) 113 kJ

Question 14

Using the data shown below for water, determine in the following heating curve what region the water will be in when 50 g of ice at F1F1F1S1F1F1F1025F1F1F1S1F1F1F10C is provided with 35 kJ of energy.   Boiling point
373 K
Melting point
273 K
Enthalpy of vaporization
2,260 J/g
Enthalpy of fusion
334 J/g
Specific heat capacity (solid)
2.11 J/(g F1F1F1S1F1F1F10 K)
Specific heat capacity (liquid)
4.18 J/(g F1F1F1S1F1F1F10 K)
Specific heat capacity (gas)
2.08 J/(g F1F1F1S1F1F1F10 K)

Accepted Answer

The answer of Using the data shown below for water,...

Question 15

To cool your 250 mL of coffee at 90F1F1F1S1F1F1F10C, you put a 100 g metal spoon in a glass of ice water to lower its temperature to 0F1F1F1S1F1F1F10C, and then you put the spoon in the coffee. After thermal equilibrium has been reached, what is the final temperature of your coffee? Assume energy is exchanged only between the spoon and the coffee. The heat capacity of the metal spoon is 80 J/F1F1F1S1F1F1F10C. Assume the specific heat and density of coffee are the same as that of water, 4.18 J/(g F1F1F1S1F1F1F10 F1F1F1S1F1F1F10C) and 1.00 g/mL.

Accepted Answer

The answer of To cool your 250 mL of coffee...

Question 16

Determine the standard enthalpy of formation for NO given the following information about the formation of NO2 under standard conditions, and   F1F1F1S1F1F1F10   (NO2)   F1F1F1S1F1F1F1033.2 kJ/mol. 2NO(g)   F1F1F1S1F1F1F10O2(g)   F1F1F1S1F1F1F102NO2(g) F1F1F1S1F1F1F10Hrxn   F1F1F1S1F1F1F10114.2 kJ

Accepted Answer

A) (90.3 kJ/mol) 
B) (F1F1F1S1F1F1F10   F1F1F1S1F1F1F1090.3 kJ/mol) 
C) (F1F1F1S1F1F1F10   F1F1F1S1F1F1F10147.4 kJ/mol) 
D) (F1F1F1S1F1F1F10   F1F1F1S1F1F1F10180.6 kJ/mol) 
E) (F1F1F1S1F1F1F10   F1F1F1S1F1F1F1073.7 kJ/mol) 
A) (90.3 kJ/mol) 
B) (F1F1F1S1F1F1F10   F1F1F1S1F1F1F1090.3 kJ/mol) 
C) (F1F1F1S1F1F1F10   F1F1F1S1F1F1F10147.4 kJ/mol) 
D) (F1F1F1S1F1F1F10   F1F1F1S1F1F1F10180.6 kJ/mol) 
E) (F1F1F1S1F1F1F10   F1F1F1S1F1F1F1073.7 kJ/mol)

Question 17

Napthalene is often used to determine the heat capacity of bomb calorimeters because it can be obtained in pure form and its energy of combustion is known very accurately (F1F1F1S1F1F1F1040.18 kJ/g). Determine the heat capacity of a calorimeter that had a temperature increase of 7.55F1F1F1S1F1F1F10C when 1.250 g of napthalene was used.

Accepted Answer

A) 6.65 kJ/F1F1F1S1F1F1F10C 
B) 5.32 kJ/F1F1F1S1F1F1F10C 
C) 2.13 kJ/F1F1F1S1F1F1F10C 
D) 8.36 kJ/F1F1F1S1F1F1F10C 
E) 10.8 kJ/F1F1F1S1F1F1F10C 
A) 6.65 kJ/F1F1F1S1F1F1F10C 
B) 5.32 kJ/F1F1F1S1F1F1F10C 
C) 2.13 kJ/F1F1F1S1F1F1F10C 
D) 8.36 kJ/F1F1F1S1F1F1F10C 
E) 10.8 kJ/F1F1F1S1F1F1F10C

Question 18

State the first law of thermodynamics. Describe two situations where F1F1F1S1F1F1F10Esys could be negative.

Accepted Answer

The first law of thermodynamics states t

Question 19

During a(n) ________ process, energy is transferred from the system to the surroundings.

Accepted Answer

A) exothermic 
B) endothermic 
C) thermodynamic 
D) thermochemical 
E) physical 
A) exothermic 
B) endothermic 
C) thermodynamic 
D) thermochemical 
E) physical

Indicate which of the following is not an element in its standard state at 25 C and 1 atm.

Assuming that the charge of the ions remain constant in all cases, which of the following ion pairs has the greatest electrostatic potential energy (i.e., largest in magnitude)?

Use the following information to determine the standard enthalpy change when 1 mol of PbO(s) is formed from lead metal and oxygen gas. PbO(s) C(graphite) Pb(s) CO(g) H  107 kJ 2C(graphite) O₂(g) 2CO(g) H  222 kJ

In terms of the enthalpy of formation, which of the following compounds is the most unstable compared to its elements under standard conditions?

A 150 g piece of iron (C_P  25.09 J/(mol  )) was heated to a temperature of 47 and then placed in contact with a 275 g piece of copper at 20 (C_P  25.46 J/(mol  )). What was the final temperature of the two pieces of metal?

At a certain elevation, the boiling point of water is 98.5 . How much energy is needed to heat 35.0 mL of water to the boiling point at this elevation if the water initially was at 23.4C? (C_P  75.38 J/(mol · ), d  1.00 g/mL)

A 15 g piece of iron (C_P  25.09 J/(mol · C)) is heated to a temperature of 95C and placed into a bucket containing 4.5 gal of water (C_P  75.38 J/(mol · C)) initially at 25C. Eventually, ________

Which statement A-D about a system and its surroundings is not correct?

Use the following information to determine the enthalpy for the reaction shown below. S(s)  O₂(g)  SO₂(g) H  ? S(s) O₂(g) SO₃(g) 2SO₂(g) O₂(g) 2SO₃(g)

For which reaction below does the enthalpy change under standard conditions correspond to a standard enthalpy of formation?

Which of the following fuels has the highest fuel value (kJ/g)?

Steam in a cylinder is compressed by a piston exerting a constant pressure of 5 atm. The volume of the cylinder decreases by 15 L and simultaneously the steam is cooled, losing 105 kJ of energy as heat. How much energy, total, was gained or lost by the steam in this process? (101.3 J  1 L atm)

Determine the standard enthalpy of formation for NO given the following information about the formation of NO₂ under standard conditions, and (NO₂) 33.2 kJ/mol. 2NO(g) O₂(g) 2NO₂(g) H_rxn 114.2 kJ

State the first law of thermodynamics. Describe two situations where E_sys could be negative.

During a(n) ________ process, energy is transferred from the system to the surroundings.

Matter and Energy: The Origin of the Universe

Atoms, Ions, and Molecules: Matter Starts Here

Stoichiometry: Mass, Formulas, and Reactions

Solution Chemistry: The Hydrosphere

Properties of Gases: The Air We Breathe

A Quantum Model of Atoms: Waves and Particles

Chemical Bonds: What Makes a Gas a Greenhouse Gas

Molecular Geometry: Shape Determines Function

Intermolecular Forces: The Uniqueness of Water

Solutions: Properties and Behavior

Solids: Crystals, Alloys, and Polymers

Chemical Kinetics: Reactions in the Atmosphere

Chemical Equilibrium: How Much Product Does a Reaction Really Make

Acidbase Equilibria: Proton Transfer in Biological Systems

Additional Aqueous Equilibria: Chemistry and the Oceans

Thermodynamics: Spontaneous and Nonspontaneous Reactions and Processes

Electrochemistry: The Quest for Clean Energy

Nuclear Chemistry: Applications to Energy and Medicine

Organic and Biological Molecules: the Compounds of Life

The Main Group Elements: Life and the Periodic Table

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Exam 5: Thermochemistry: Energy Changes in Reactions

Indicate which of the following is not an element in its standard state at 25 C and 1 atm.

Assuming that the charge of the ions remain constant in all cases, which of the following ion pairs has the greatest electrostatic potential energy (i.e., largest in magnitude)?

Use the following information to determine the standard enthalpy change when 1 mol of PbO(s) is formed from lead metal and oxygen gas. PbO(s) C(graphite) Pb(s) CO(g) H  107 kJ 2C(graphite) O2(g) 2CO(g) H  222 kJ

In terms of the enthalpy of formation, which of the following compounds is the most unstable compared to its elements under standard conditions?

A 150 g piece of iron (CP  25.09 J/(mol  )) was heated to a temperature of 47 and then placed in contact with a 275 g piece of copper at 20 (CP  25.46 J/(mol  )). What was the final temperature of the two pieces of metal?

At a certain elevation, the boiling point of water is 98.5 . How much energy is needed to heat 35.0 mL of water to the boiling point at this elevation if the water initially was at 23.4C? (CP  75.38 J/(mol · ), d  1.00 g/mL)

A 15 g piece of iron (CP  25.09 J/(mol · C)) is heated to a temperature of 95C and placed into a bucket containing 4.5 gal of water (CP  75.38 J/(mol · C)) initially at 25C. Eventually, ________

Which statement A-D about a system and its surroundings is not correct?

Use the following information to determine the enthalpy for the reaction shown below. S(s)  O2(g)  SO2(g) H  ? S(s) O2(g) SO3(g) 2SO2(g) O2(g) 2SO3(g)

For which reaction below does the enthalpy change under standard conditions correspond to a standard enthalpy of formation?

Which of the following fuels has the highest fuel value (kJ/g)?

Steam in a cylinder is compressed by a piston exerting a constant pressure of 5 atm. The volume of the cylinder decreases by 15 L and simultaneously the steam is cooled, losing 105 kJ of energy as heat. How much energy, total, was gained or lost by the steam in this process? (101.3 J  1 L atm)

Determine the standard enthalpy of formation for NO given the following information about the formation of NO2 under standard conditions, and (NO2) 33.2 kJ/mol. 2NO(g) O2(g) 2NO2(g) Hrxn 114.2 kJ

State the first law of thermodynamics. Describe two situations where Esys could be negative.

During a(n) ________ process, energy is transferred from the system to the surroundings.

Matter and Energy: The Origin of the Universe

Atoms, Ions, and Molecules: Matter Starts Here

Stoichiometry: Mass, Formulas, and Reactions

Solution Chemistry: The Hydrosphere

Properties of Gases: The Air We Breathe

A Quantum Model of Atoms: Waves and Particles

Chemical Bonds: What Makes a Gas a Greenhouse Gas

Molecular Geometry: Shape Determines Function

Intermolecular Forces: The Uniqueness of Water

Solutions: Properties and Behavior

Solids: Crystals, Alloys, and Polymers

Chemical Kinetics: Reactions in the Atmosphere

Chemical Equilibrium: How Much Product Does a Reaction Really Make

Acidbase Equilibria: Proton Transfer in Biological Systems

Additional Aqueous Equilibria: Chemistry and the Oceans

Thermodynamics: Spontaneous and Nonspontaneous Reactions and Processes

Electrochemistry: The Quest for Clean Energy

Nuclear Chemistry: Applications to Energy and Medicine

Organic and Biological Molecules: the Compounds of Life

The Main Group Elements: Life and the Periodic Table

Transition Metals: Biological and Medical Applications

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Use the following information to determine the standard enthalpy change when 1 mol of PbO(s) is formed from lead metal and oxygen gas. PbO(s) C(graphite) Pb(s) CO(g) H  107 kJ 2C(graphite) O₂(g) 2CO(g) H  222 kJ

A 150 g piece of iron (C_P  25.09 J/(mol  )) was heated to a temperature of 47 and then placed in contact with a 275 g piece of copper at 20 (C_P  25.46 J/(mol  )). What was the final temperature of the two pieces of metal?

At a certain elevation, the boiling point of water is 98.5 . How much energy is needed to heat 35.0 mL of water to the boiling point at this elevation if the water initially was at 23.4C? (C_P  75.38 J/(mol · ), d  1.00 g/mL)

A 15 g piece of iron (C_P  25.09 J/(mol · C)) is heated to a temperature of 95C and placed into a bucket containing 4.5 gal of water (C_P  75.38 J/(mol · C)) initially at 25C. Eventually, ________

Use the following information to determine the enthalpy for the reaction shown below. S(s)  O₂(g)  SO₂(g) H  ? S(s) O₂(g) SO₃(g) 2SO₂(g) O₂(g) 2SO₃(g)

Determine the standard enthalpy of formation for NO given the following information about the formation of NO₂ under standard conditions, and (NO₂) 33.2 kJ/mol. 2NO(g) O₂(g) 2NO₂(g) H_rxn 114.2 kJ

State the first law of thermodynamics. Describe two situations where E_sys could be negative.