Phase Change

The enthalpy of combustion is the enthalpy change

• when one mole of the pure compound is formed from its elements under standard conditions

• when 1 mole of pure substance is completely burned in excess oxygen, under the given temperature and pressure of the reaction

• when 1 mole of H⁺ (aq) ions from acid reacts with 1 mole of OH^- (aq) ions from an alkali to form 1 mole of water under the stated condition of the experiment

• the magnitude of the enthalpy change of a chemical reaction depends only on the difference in the enthalpy content of the products and the reactants, and does not on how the reaction is completed

Which of the following equations represents the enthalpy change of combustion?

• 2C₄H₁₀(g) +13O₂(g) → 8CO₂ (g) + 10H₂0(g)

• C₄H₁₀(g) +13/2O₂(g) → 4CO₂ (g) + 5H₂0(g)

• 3C₄H₁₀ + 9/2O₂ → 4CO (s)+ 5H₂O

• C₄H₁₀ + H₂ → 4C(s) + 5H₂O(g)

Condensation is ...

• a change of state from a gas to a liquid

• a change of state from a liquid to a solid

• vaporization that occurs throughout the liquid rapidly

Freezing is ...

• a change of state from a solid to a liquid

• a change of state from a liquid to a solid

• a change of state from a gas to a liquid

Which phase change is exothermic?

• H₂O(s) → H₂O(l)

• H₂O(l) → H₂O(s)

• H₂O(s) → H₂O(g)

• H₂O(l) → H₂O(g)

Which phase change is endothermic?

• gas → solid

• gas → liquid

• liquid → solid

• liquid → gas