Entropy, enthalpy and Gibbs free energy

Which reaction has the greatest decrease in entropy?

• 2H₂ (g) +O₂(g) => 2H₂O(l)

• H₂O(g) => H₂O(l)

• 2H₂ (g) +O₂(g) => 2H₂O(g)

• H₂O(g) => H₂O(s)

For which of these processes is the value of ΔH expected to be negative?

1. The temperature increases when calcium chloride dissolves in water.

2. Steam condenses to liquid water

3. Water boils

4. Dry ice sublimates

• 1 and 2 only

• 3 and 4 only

• 2 and 3 only

• 1 only

What factors determine the spontaneity of a reaction?

• enthalpy only

• entropy only

• enthalpy and entropy

• enthalpy, entropy and temperature

When liquid condenses, what are the signs for Δ H and Δ S ?

• Δ H = + Δ S = +

• Δ H = - Δ S = -

• Δ H = + Δ S = -

• Δ H = - Δ S = +

What signs of Δ H and Δ S will always yield a reaction that is non-spontaneous ? (Think of the equation for ΔG°!!)

• Δ H = + Δ S = +

• Δ H = - Δ S = -

• Δ H = - Δ S = +

• Δ H = + Δ S = -

Predict the signs of Δ H , Δ S and Δ G for the reaction:

Mg(s) → Mg(g)

• Δ H= - , Δ S = + and Δ G = - (all temps)

• Δ H= + , Δ S = + and Δ G = - (At all temps)

• Δ H= + , Δ S = + and Δ G = - (At low temps)

• Δ H= + , Δ S = + and Δ G = - (At high temps)

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

Calculate ΔH° for the combustion of methane.

CH₄ (g) + 2 O₂ (g) → CO₂ (g) + 2 H₂O (g)

Given the enthalpies of formation below.

CH₄ (g): -50.8 kJ/mol

O₂ (g): 0 kJ/mol

CO₂ (g): -394.4 kJ/mol

H₂O (g): -228.57 kJ/mol

• -801kJ/mol

• +801kJ/mol

• +572kJ/mol

• -572kJ/mol

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

A reaction has a negative ΔH and a positive ΔS. Which of the following is true?

• It will be spontaneous at all temperatures.

• It will be nonspontaneous at all temperatures.

• It will be spontaneous at low temperatures.

• It will be spontaneous at high temperatures.

The formation ½ A2 + 2 B2 + C --> CAB4 has an enthalpy of formation of -104 kJ and a change in entropy of -60.8 J/K at 30 °C. What is the free energy and spontaneity of the reaction?

• -85.6 kJ, spontaneous

• -18.3 kJ, not spontaneous

• +18.3 kJ, spontaneous

• +85.6 kJ, not spontaneous

At what temperature will a reaction become spontaneous is the change in enthalpy is -192 KJ/mol and the change in entropy is -562 J/mol?

• Above 341.6 K

• Above 34 K

• below 34 K

• below 341.6 K