Entropy

When solid sodium hydrogencarbonate is added to dilute hydrochloric acid, the mixture starts fizzing and the temperature drops.

NaHCO₃(s) + HCl(aq) → NaCl(aq) + CO₂(g) + H₂O(l)

Which statement about this reaction is not true?

• ΔH is positive.

• ΔS_surroundings is positive.

• ΔS_system is positive.

• ΔS_total is positive.

Butane has a higher standard molar entropy than 2-methylpropane at 298 K and 1 atm, when both compounds are gases.

The best explanation for this fact is that butane has

• a higher boiling temperature.

• a more positive standard molar enthalpy change of formation.

• fewer ways of distributing energy quanta.

• more ways of distributing energy quanta.

Nitrogen reacts with hydrogen to form ammonia.

 

N₂(g) + 3H₂(g)  ---> 2NH₃(g)      ΔH = −92.2 kJ mol⁻¹

 

What is the value of ΔS^Θ surroundings, in J mol⁻¹ K⁻¹, for this reaction at 25°C?

• −3688

• −309.4

• +309.4

• +3688

Ethanol burns in excess oxygen to produce carbon dioxide and water.

 

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

 

The standard molar entropies of the reactants and products at 298 K are given in the table. The value of  for this reaction, in J mol⁻¹ K⁻¹, is

• −138

• −82

• +82

• +138

Potassium nitrate is very soluble in water:

KNO₃(s) + aq ---> K⁺(aq) + NO₃^-(aq) ΔH^Θ = +34.9 kJ mol^-1

The solubility of potassium nitrate increases rapidly with temperature.
The best explanation for this is

• A

• B

• C

• D

What are the signs of the entropy changes at 273 K when water freezes?

• A

• B

• C

• D

When a gas jar containing pure oxygen is inverted over a gas jar containing pure nitrogen, the gases mix spontaneously. What is the best explanation for this?

• Oxygen is denser than nitrogen.

• The standard molar entropy of oxygen (102.5 J K^–1 mol^–1) is greater than that of nitrogen (95.8 J K^–1 mol^–1).

• The mixing of the gases decreases the energy of the system.

• The mixing of the gases increases the entropy of the system.

The molar entropy of a perfect crystal is zero

• in a vacuum.

     

• at absolute zero, 0 K.

• in its standard state at 1 atmosphere and 298 K.

• at the 'triple point' when the gas, liquid and solid states of a substance are in equilibrium.

The oxidation of sulfur dioxide is a reaction in the manufacture of sulfuric acid:

2SO₂(g) + O₂(g) ----> 2SO₃(g) ΔH^Θ = -198kJ mol^-1

What are the signs of the entropy change of the system (ΔS_system) and the entropy change of the surroundings (ΔS_surroundings) for this reaction?

• A

• B

• C

• D

When heated at 500°C, magnesium carbonate decomposes in an endothermic reaction.

MgCO₃(s) ---> MgO(s) + CO₂(g)

What are the signs of the entropy changes at 500°C?

• A

• B

• C

• D