Anj Heat Review 2025

The arrangement of particles is most ordered in a sample of

• NaCl(aq)

• NaCl(l)

• NaCl(g)

• NaCl(s)

Answer explanation

The arrangement of particles is most ordered in NaCl(s) because it is a solid, where particles are tightly packed in a fixed structure. In contrast, NaCl(aq) and NaCl(g) have more disorder due to the presence of liquid and gas states.

Which form of energy is associated with the random motion of the particles in a sample of water?

• chemical energy

• electrical energy

• nuclear energy

• thermal energy

Answer explanation

Thermal energy is the energy associated with the random motion of particles in a substance. In water, this motion contributes to its temperature, making thermal energy the correct choice.

4. Object A at 40°C and object B at 80°C are placed in contact with each other. Which statement describes the heat flow between the objects?

• A) Heat flows from object A to object B.

• B) Heat flows from object B to object A.

• C) Heat flows in both directions between the objects.

• D) No heat flow occurs between the objects.

Answer explanation

Heat always flows from the hotter object to the cooler one. Since object B is at 80°C and object A is at 40°C, heat flows from object B to object A, making option B the correct choice.

The temperature 30. K expressed in degrees Celsius is

• 243°C

• –243°C

• 303°C

• –303°C

Answer explanation

To convert Kelvin to Celsius, subtract 273.15. Thus, 30 K - 273.15 = -243.15°C, which rounds to -243°C. Therefore, the correct answer is –243°C.

At STP, a sample of which element has the highest entropy?

• Na(s)

• Hg(l)

• Br₂(l)

• F₂(g)

Answer explanation

At STP, gases have higher entropy than liquids and solids due to greater molecular motion. F₂(g) is a gas, while Na(s), Hg(l), and Br₂(l) are solid or liquid, making F₂(g) the element with the highest entropy.

Given the diagram representing a heating curve for a substance: During which time interval is the average kinetic energy of the particles of the substance constant while the potential energy of the particles increases?

• A) AC

• B) BC

• C) CD

• D) DF

Answer explanation

During interval BC, the substance is undergoing a phase change (melting), where the average kinetic energy remains constant while the potential energy increases as the particles move apart.

Which physical changes are endothermic?

• melting and freezing

• melting and evaporating

• condensation and sublimation

• condensation and deposition

Answer explanation

Melting and evaporating are endothermic processes because they absorb heat from the surroundings. In contrast, freezing and condensation release heat, while sublimation and deposition involve heat transfer in different ways.

How many Joules of heat energy are released when 50. grams of water are cooled from 70.°C to 60.°C?

• 42 J

• 210 J

• 2100 J

• 4200 J

Answer explanation

To calculate the heat energy released, use the formula Q = mcΔT. Here, m = 50 g, c = 4.18 J/g°C, and ΔT = 70°C - 60°C = 10°C. Thus, Q = 50 * 4.18 * 10 = 2090 J, which rounds to 210 J.

What is the minimum number of kiloJoules needed to change 40.0 grams of water at 100C to steam at the same temperature and pressure?

• 1,810 kJ

• 90.4 kJ

• 2.26 kJ

• .400 kJ

Answer explanation

To convert 40.0 grams of water at 100°C to steam, we use the heat of vaporization, which is approximately 40.7 kJ/mol. For 40.0 g (0.044 mol), the energy required is 1,810 kJ, making 1,810 kJ the correct answer.

Show a numerical setup for calculating the quantity of heat in joules required to completely vaporize 102.3 grams of H2O(l) at 100.°C and 1.0 atm.

• Q = m × ΔHvap

• Q = m × c × ΔT

• Q = m × ΔHfus

• Q = m × c × ΔHvap

Answer explanation

To calculate the heat required to vaporize water, use Q = m × ΔHvap, where m is the mass and ΔHvap is the heat of vaporization. This formula is appropriate since we are vaporizing liquid water at its boiling point.