Unit 8 Review: Thermochemistry

A person with a body temperature of 37°C holds a cup of hot water in their hand. The temperature of the cup is 44°C. The air temperature is 22°C. Which phrase describes a heat transfer that occurs in this system?

• from the air to the cup

• from the hand to the cup

• from the air to the hand

• from the cup to the hand

Answer explanation

Heat transfers from the cup (44°C) to the hand (37°C) because heat moves from warmer to cooler objects. Thus, the correct phrase is 'from the cup to the hand'.

Tags

NGSS.HS-PS3-4

Heat flows from an object at a temperature of 20.°C to an object at a temperature of

• 15°C

• 25°C

• 35°C

• 45°C

Answer explanation

Heat flows from a higher temperature to a lower temperature. Since 20°C is higher than 15°C, heat will flow from the 20°C object to the 15°C object, making 15°C the correct choice.

Tags

NGSS.HS-PS3-4

The average kinetic energy of the particles in a sample of matter is expressed as

• density

• volume

• pressure

• temperature

Answer explanation

The average kinetic energy of particles in a sample of matter is directly related to its temperature. Higher temperatures indicate greater kinetic energy, making temperature the correct choice.

Which temperature represents the highest average kinetic energy of the particles in a sample of matter (use the formula for temperature conversions)?

• 298 K

• 267 K

• 27°C

• 12°C

Answer explanation

To compare average kinetic energy, convert temperatures to Kelvin. 27°C = 300 K, 298 K = 298 K, 267 K = 267 K, and 12°C = 285 K. 27°C (300 K) has the highest average kinetic energy.

Tags

NGSS.MS-PS1-4

NGSS.MS-PS3-4

The temperature of a sample of matter is a measure of the

• average kinetic energy of its particles

• average potential energy of its particles

• total kinetic energy of its particles

• total potential energy of its particles

Answer explanation

The temperature of a sample reflects the average kinetic energy of its particles, indicating how fast they are moving. Higher temperatures mean higher average kinetic energy.

Tags

NGSS.HS-PS3-2

NGSS.HS-PS3-4

What is the temperature, in degrees Celsius, of a sample of matter at 35 K?

• –238°C

• –308°C

• 35°C

• 308°C

Answer explanation

To convert Kelvin to Celsius, use the formula: °C = K - 273.15. For 35 K, the calculation is 35 - 273.15 = -238.15°C, which rounds to -238°C. Thus, the correct answer is –238°C.

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 from the Kelvin temperature. Thus, 30 K - 273.15 = -243.15°C, which rounds to -243°C. Therefore, the correct answer is –243°C.

During which two processes does a substance release energy?

• freezing and condensation

• freezing and melting

• evaporation and condensation

• evaporation and melting

Answer explanation

A substance releases energy during freezing (liquid to solid) and condensation (gas to liquid) as it transitions to a more ordered state, thus releasing latent heat.

Tags

NGSS.MS-PS1-4

NGSS.MS-PS3-4

Which equation represents an exothermic reaction at 298 K? (Hint: Use Table I)

• N₂(g) + O₂(g) → 2NO(g)

• C(s) + O₂(g) → CO₂(g)

• KNO₃(s) → K⁺(aq) + NO₃⁻(aq)

• NH₄Cl(s) → NH₄⁺(aq) + Cl⁻(aq)

Answer explanation

The reaction C(s) + O₂(g) → CO₂(g) is exothermic because it is the only equation with a -ΔH.

Tags

NGSS.HS-PS1-4

Given the balanced equation representing a reaction: CH₄(g) + 2O₂(g) → 2H₂O(g) + CO₂(g) + heat. Which statement is true about energy in this reaction?

• The reaction is exothermic because it releases heat.

• The reaction is exothermic because it absorbs heat.

• The reaction is endothermic because it releases heat.

• The reaction is endothermic because it absorbs heat.

Answer explanation

The reaction is exothermic because it releases heat, indicating that energy is released to the surroundings during the reaction, which is characteristic of exothermic processes.

Tags

NGSS.HS-PS1-4