Thermochemistry pt 3

Multiple Choice

How does enthalpy (ΔH) change during these phase changes? See pages 389-390,

• Melting: negative ΔH; Freezing: positive ΔH

• No enthalpy change occurs during phase changes

• Melting: positive ΔH (absorbs heat); Freezing: negative ΔH (releases heat)

• Both melting and freezing have positive ΔH

Multiple Choice

Using the principles on page 390, if 6.01 kJ of energy is needed to melt 1 mole of ice, how much energy is released when 1 mole of water freezes?

• 3.01 kJ is released

• 6.01 kJ is released

• 12.02 kJ is released

• No energy is released

Multiple Choice

Based on the heating curve diagram on page 389, which process requires more energy for water?

• Evaporation (liquid → gas)

• Melting (solid → liquid)

• Both require the same energy

• Sublimation (solid → gas)

  
  

Multiple Choice

The enthalpy of fusion is the energy needed to melt 1 mole of solid to liquid. Why is enthalpy of fusion always positive? see page 390

• The temperature decreases during melting

• Energy is released

• Energy is absorbed

• The solid becomes less dense when melting

Multiple Choice

The enthalpy of vaporization is:​ ​ see pg 392

• The temperature at which liquids evaporate

• The energy needed to convert 1 mole of liquid to gas

• The energy released during freezing

• Only applicable to water

Multiple Choice

How is the enthalpy of condensation related to enthalpy of vaporization? pg 392

• enthalpy of condensation is always greater

• enthalpy of condensation= ½ enthalpy of vaporization

• They are unrelated

• enthalpy of vaporization = -enthalpy of condensation?

Multiple Choice

Which substance requires more energy to vaporize: fluorine gas (F₂) or hydrogen sulfide (H₂S)? pg 394

• Fluorine gas (F₂) – Its weaker forces make it easier to vaporize

• Hydrogen sulfide (H₂S) – Its polar bonds create stronger attractions between molecules

• Both need the same energy

• It depends on the container size

Multiple Choice

Which of these halogens needs the most energy to vaporize? pg 394

• Astatine (the largest halogen atom)

• Chlorine (medium-sized atom)

• Fluorine (the smallest halogen atom)

• All halogens vaporize equally easily

Multiple Choice

How is evaporation different from boiling? see page 395

• Evaporation happens when a few high-energy particles escape; boiling requires most particles to have enough energy

• Boiling produces bubbles; evaporation never does

• Evaporation only occurs in sunlight; boiling needs a stove

• Evaporation requires more energy than boiling

Multiple Choice

How can you calculate the heat absorbed or released when a substance changes state? pages 391 & 393

• Divide mass by temperature and add ∆H

• Multiply moles of the substance by its molar enthalpy of phase change (∆H)

• Use only the substance’s boiling point temperature

• Subtract ∆H of products from reactants