1000 Questions Gibbs Free energy

Gibbs

Question 1

ΔHf of Cl2O7 is 75.73 kJ mol-1.

Give reasons why this compound would be unstable at room temperature.

Question 2

Calculate ΔH and ΔS  for the dissolution of NH4NO3 in water:

NH_4NO_{3(s)} + H_2O_{(l)} \rightarrow NH^+_{ 4(aq)} + NO^-_{3 (aq)}

Use the results of this calculation to determine the value of deltaGo for this reaction at 25o C, and explain why NH4NO3 spontaneously dissolves in water at room temperature.

Compound                        ΔHfo(KJ/mol)              So(J/mol.K)

NH4NO3(s)                                   -356.56                           151.08

NH4+(aq)                                     -132.51                           113.4

NO3–(aq)                                      -205.0                             146.4

Question 3

Calculate deltaH and deltaS  for the following reaction and decide in which direction each of these factors will drive the reaction.

N_{2(g)} + 3 H_{2(g)} \rightleftharpoons 2 NH_{3(g)}

Compound                        ΔHfo(KJ/mol)              So(J/mol.K)

N2(g)                                           0                                   191.61

H2(g)                                           0                                   130.68

NH3(g)                                     46.11                                192.45

Question 4

CaCl2 dissolves according to the following equation:

CaCl_{2(s)}\rightarrow Ca^2^+_{(aq)}+2Cl^-_{(aq)}

At what temperature will this dissolution take place?

ΔH of dissolution: -81.3 kJ/mol

ΔSo Ca2+: 53.1 J/mol.K, ΔSo CaCl2: 104.6 J/mol.K, ΔSo Cl–: 56.5 J/mol.K

Harder Question

Question 5

Predict which of the following reactions has a negative change in entropy.

 

  1. 2SO_{2(g)} + O_{2(g)} \rightarrow 2SO_{3(g)}
  2. MgO_{(s)} + CO_{2(g)} \rightarrow MgCO{3(s)}
  3. PCl{5(s)} \rightarrow PCl_{3(l)} + Cl_{2(g)}
  4. 2HgO_{(s)} \rightarrow 2Hg_{(l)} + O_{2 (g)}
  5. Ba^{2+}_{(aq)} + SO^{2-}_{4(aq)} \rightarrow BaSO_{4(s)}
  6. 2H_2O_{2(l)} \rightarrow 2 H_2O_{(l)} + O_{2(g)}

Question 6

For a certain process at 350 K, ΔG = -83.0 kJ/mol and ΔH = -59.7 kJ/mol.  Find the entropy change for this process.

Question 7

Cu2S reacts with S to produce CuS at 25°C.  The process is exothermic (ΔH˚ = -28.7 kJ/mol) with a decrease in disorder (ΔS˚ = -23.7 J/(mol•K)).  Determine whether this reaction is spontaneous at 25°C.

Cu_2S_{ (s)} + S_{ (s)} \rightarrow 2 CuS _{(s)}

Question 8

Hex-1-ene gas, C6H12, burns in oxygen to produce carbon dioxide and water vapour.

Use the data below to find the ΔG of the reaction and state whether the reaction is spontaneous or not.

Compound                        ΔHfo(KJ/mol)              So(J/mol.K)

O2(g)                                           0                                    205

C6H12(g)                                   -43                                   385

CO2(g)                                     -394                                   214

H2O(g)                                     -242                                  189

Question 9

The equation for the decomposition of calcium carbonate is given below.

CaCO_{3(s)} \rightarrow CaO_{(s)} + CO_{2(g)}

If ∆H for this reaction is +177 kJ mol-1 and ∆S is 161 J K-1 mol-1 find at what temperature this reaction becomes spontaneous.

Harder Question

Question 10

Calculate ΔG for the following reaction:

CS_{2(l)}+2O_{2(g)}\rightarrow CO_{2(g)}+2SO_{2(g)}

given the following data:

C_{(s)}+O_{2(g)}\rightarrow CO_{2(g)}\: \: \: \Delta G^0:-349.39kJ\: mol^-^1

S_{(s)}+O_{2(g)}\rightarrow SO_{2(g)}\: \: \: \Delta G^0:-300.13kJ\: mol^-^1

C_{(s)}+2S_{2(s)}\rightarrow CS_{2(l)}\: \: \: \Delta G^0:\: 67.1kJ\: mol^-^1

Hint: A Hess’s cycle can also be used with Gibb’s Free energy.