CHE 025

Enthalpy

100

At 25 ^oC, ΔH_freezing is -56.12 kJ/mol, and ΔH_deposition is -403.3 kJ/mol. What is ΔH_condesation?

-347.2 kJ/mol

200

If the ΔH of the following reaction is -24.3 kJ/mol, how much heat is released if you react 27.3 g of LiOH with excess H₂SO₃?

H₂SO₃ + 2LiOH → Li₂SO₃ + 2H₂O

-13.8 kJ

300

What is the ΔH of the following reaction? Is the reaction endothermic or exothermic?

CS₂ + 3O₂ → CO₂ + 2SO₂

ΔH_f, CS₂ = -107.2 kJ/mol

ΔH_f, CO₂ = -393.5 kJ/mol

ΔH_f, SO₂ = -296.1 kJ/mol

-878.5 kJ/mol

Exothermic

400

2CuO(s) + 4HCl(g) → 2CuCl(s) + Cl₂(g) + 2H₂O(g)

Calculate the change in enthalpy of the previous redox reaction between CuO and HCl using the following reactions:

(1) CuO(s) + H₂(g) → Cu(s) + H₂O(g), ΔH = -85 kJ

(2) 2Cu(s) + Cl₂(g) → 2CuCl(s), ΔH = -274 kJ

(3) H₂(g) + Cl₂(g) → 2HCl(g), ΔH = -184 kJ

ΔH = -76 kJ

500

A 2.53 g sample of NaCN is reacted with 100.0 g of H₂SO₄ in a container open to the atmosphere according to the following unbalanced reaction.

H₂SO₄ (aq) + NaCN (aq) → HCN (aq) + Na₂SO₄ (aq)

If the temperature of the solution increases from 24.0 ^oC to 37.3 ^oC, and the C_s of the solution is 2.34 J/g*C, what is the ΔH of the reaction?

-123 kJ/mol

Entropy

100

Place the noble gasses in order of decreasing entropy.

Og > Rn > Xe > Kr > Ar > Ne > He

200

Predict the sign of the entropy change for the reaction below. Briefly explain your reasoning.

CaCO₃ (s) → CaO (s) + CO₂ (g)

Entropy change is positive, because there are more moles of products than reactants, or because there is a gas in the products, but not the reactants.

300

What is the ΔS_rxn of the following reaction? Is the reaction favored thermodynamically?

Mg (s) + 2HCl (aq) → MgCl₂(aq) + H₂(g)

S^o Mg (s) = 32.7 J/mol*K

S^o HCl (aq) = -92.3 J/mol*K

S^o H₂ (g) = 130.5 J/mol*K

S^o MgCl₂ (aq) = -191.8 J/mol*K

ΔS_rxn = 90.6 J/mol*K

Favored because ΔS > 0

400

What is the value of ΔS_universe in the following reaction at 28.4 ^oC? Is the reaction spontaneous?

FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl

ΔH_rxn = -237 kJ/mol

ΔS_rxn = -766 J/mol*K

19.9 J

Spontaneous

500

C₄H₈(g) + 6O₂(g) → 4CO₂(g) + 4H₂O(l), ΔS = ?

Calculate the entropy for the combustion reaction of butane using the following reactions conducted at 25 ^oC:

(1) 2H₂(g) + O₂(g) → 2H₂O(g), S^o = -571 J/C

(2) C₄H₈(g) + H₂(g) → C₄H₁₀(g), S^o = -126 J/C

(3) 2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(l),    S^o = -5754 J/C

-2718 J/mol*^oC

Gibb's Free Energy

100

What is the difference between a spontaneous and non-spontaneous reaction?

A spontaneous reaction requires no outside energy to start.

200

What must the signs of ΔH and ΔS be if a reaction is spontaneous at all temperatures?

ΔH must be negative.

ΔS must be positive.

300

What is the value of ΔG of the following reaction at 50 ^oC? Is the reaction spontaneous at this temperature?

SiCl₄ + 4H₂O → H₄SiO₄ + 4HCl

ΔH = -206.6 kJ/mol

ΔS = -137.9 J/mol*K

ΔG = -162.0 kJ/mol

Spontaneous

400

Below what temperature does the following reaction become nonspontaneous?

2HNO₃(aq) + NO(g) → 3NO₂(g) + H₂O(l)

ΔS = 287.5 J/mol*K

ΔH = 136.5 kJ/mol

474.8 K or 201.6 ^oC

500

What is the standard free energy change of the following unbalanced chemical reaction at 25 ^oC?

Na₂CO₃ + HCl → NaCl + H₂O + CO₂

G^o(Na₂CO₃) = -127.6 kJ/mol

G^o(HCl) = 13.5 kJ/mol

G^o(NaCl) = -52.4 kJ/mol

G^o(H₂O) = 42.6 kJ/mol

G^o(CO₂) = -35.8 kJ/mol

2.60 kJ/mol