Standard heats of formation 求教

2013-10-15 3:51 pm

1. Use the following equations to calculate the standard heat of formation of magnesium nitrate in kilojoules

8Mg(s) + Mg(NO3)2(s) -> Mg3N2(s) + 6MgO(s) ΔH= -3884 KJ

Mg3N2(s) -> 3Mg(s) + N2(g) ΔH = +463 KJ

2MgO(s) -> 2Mg(s) + O2(g) ΔH = +1203KJ

2. Use bond dissociation energies from your text to calculate approximate ΔH for the combustion of ethanol : C2H5OH (l) + 3O2(g) -> 2CO2(g) + 3H2O(g)

3. Predict the sign of ΔS for the following processes :

a. Mixing of hydrogen gas with chlorine gas, ΔS = + or -

b. 2H2(g) + O2(g) -> 2H2O(g), ΔS = + or -

4. Predict whether reactions with given ΔH and ΔS are spontaneous or non spontaneous :

a. ΔH = -50 KJ and ΔS = -120 J/K at 300K

b. ΔH = +50KJ and ΔS = -120 J/K at 500K

回答 (1)

用戶4366

2013-10-16 5:27 am

✔ 最佳答案

1. ∆Hf Mg₃N₂ = -463 kJ/mole
∆Hf MgO = -1203/2 = -601.5 kJ/mole

8Mg(s) + Mg(NO3)2(s) -> Mg3N2(s) + 6MgO(s) ΔH= -3884 KJ

[∆Hf Mg₃N₂ + 6X∆Hf MgO] - [8X∆Hf Mg + ∆Hf Mg(NO₃)₂] = -3884 kJ

[ -463 + (6)(-601.5)] - [8X0 + ∆Hf Mg(NO₃)₂] = -3884 kJ

∆Hf Mg(NO₃)₂] = -188 kJ/mole

2. Use bond dissociation energies from your text to calculate approximate ΔH for the combustion of ethanol : C2H5OH (l) + 3O2(g) -> 2CO2(g) + 3H2O(g)

Sorry! I don't have your text.

3. Predict the sign of ΔS for the following processes :
a. Mixing of hydrogen gas with chlorine gas, ΔS = + or -
ΔS = +
b. 2H2(g) + O2(g) -> 2H2O(g), ΔS = + or -
ΔS = -

4. Predict whether reactions with given ΔH and ΔS are spontaneous or non spontaneous :
ΔG = ΔH - TΔ- TΔS
a. ΔH = -50 KJ and ΔS = -120 J/K at 300K
ΔG = -50 - (300)(-120) = 35950 kJ > 0 ==> non spontaneous
b. ΔH = +50KJ and ΔS = -120 J/K at 500K

ΔG = +50 - (500)(-120) = 60050 kJ > 0 ==> non spontaneous

2013-10-16 00:26:26 補充：
Sorry

ΔG = ΔH - TΔ- TΔS [ 打錯了 ]

ΔG = ΔH - TΔS ... [ 正確 ]

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