Chimica inorganica - le soluzioni

Le soluzioni

Miscele omogenee di una o più sostanze: il soluto e il solvente.

Tipologie di soluzioni

• Soluzioni reali - I soluti hanno interazioni intermolecolari.
• Soluzioni ideali - I soluti hanno interazioni solo con il solvente.
• Soluzione satura - Situazione di equilibrio dinamico in cui soluto, sciolto e indissolto continuano a scambiare al limite della capacità di un soluto di sciogliersi in una data massa di solvente.

Solubilità

La solubilità di una sostanza è la concentrazione di una soluzione satura, dipende da:

• Solvente
• Temperatura
• Pressione (per gas)

I composti ionici tendono a sciogliersi nei solventi polari. I composti non ionici tendono a sciogliersi in solventi non polari.

X⁺Y^- + H₂O ⇌ X⁺ + Y^-

Dissoluzione dovuta alla forte interazione degli ioni con le molecole di H₂O. La dissoluzione di composti poco polari è regolata dall'entropia.

Entalpia di dissoluzione

Entalpia di dissoluzione = somma dell'entalpia reticolare e dell'entalpia di idratazione.

ΔH_s = ΔH_R + ΔH_H

• Na⁺ + Cl^- (s) → NaCl(s) ΔH_R < 0
• Na⁺ + Cl^- + H₂O → Na⁺(aq) + Cl^-(aq) ΔH_H ≪ 0

ΔH_s può essere positivo o negativo (sono comunque numeri piccoli). ΔH_s < 0 significa processo esotermico mentre ΔH_s > 0 significa processo endotermico. ΔS (entropia) è sempre positiva.

Concentrazione molare

Concentrazione molare (M) = molarità [X] = ^{mol X}/_{V soluzione} = ^mol/_L

Unità di concentrazione

• Molalità (m) = mol soluto / kg solvente
• Concentrazione in peso = massa soluto / V soluzione
• Frazione molare X = mol₁ / (mol₁ + mol₂ + mol₃ + … + mol_n), Non ha unità di misura
• Percentuale
  • w/w = grammi soluto / 100 g soluzione
  • v/v = ml soluto / 100 ml soluzione

Esercizi

1. 1 mg di proteina PM = 50 kDa sciolta in 1.5 ml di soluzione tampone
1. Concentrazione peso/volume
2. Concentrazione molare
3. Concentrazione molare se 1 ml di soluzione finale viene portato a 25 ml

a. Concentrazione in peso = 1 mg / 1.5 ml = 0.067 mg/ml = 0.067 g/L

M = 0.067 g/L / 50000 g/mol = 1.3 x 10^-6 mol/L = 1.3 μM

c. Mol prelevate = 1.3 μM. 1 ml = 1.3 x 10^-6 mol/L x 10^-3 L = 1.3 x 10^-9 mol

M = 1.3 x 10^-9 mol / 0.025 L = 5.3 x 10^-8 mol/L

L'emoglobina è formata da 4 subunità

PM_sub = 15 kDa, PM Hb = 60 kDa, M = 3 x 10^-7 mol/L

1. Concentrazione molare subunità
2. Concentrazione in peso di Hb
3. Concentrazione in peso subunità

a. [M_sub = 4.3 x 10^-5] M = 1.2 µM

b. Peso/V Hb = [Hb] PM_Hb = 3 x 10^-7 mol/L x 60⁰⁰⁰ mol = 1.8 x 10^-2 g/L

c. Peso/V_sub = [Sub.] PM_sub = 1.2 x 10^-6 mol/L x 15⁰⁰⁰ mol = 1.8 x 10^-2 g/L

35 g di NaCl in 100 ml di soluzione (35% p/V)

M_NaCl? M_NaCl = mol/V = 35 g/58.5 mol g/L= 6 mol/9.1 L

30 g CH₃OH + 70 ml H₂O d = 0.98 g/L d_H2O = 1 g/ml % p/V CH₃OH?

Massa totale = 30 + 70 = 100 g V = 100 g/0.98 g/L = 1.02 L = 102 ml

% p/V = 30 g/102 ml x 100 = 29.4 g/100 ml

Butanolo C₄H₁₀O % m/V = 1.02

M C₄H₁₀O = ? M = peso x 10 x 10.2 mol/L 100 ml PM = 76 = 0.13

HNO₃ % p/p = 46%

PM_HNO3 = 63 g/mol d = 1.3 g/ml Per ipotesi 100 g di soluzione

M = ? 46 g/mol = = 0.73 mol 63 g/mol 100 g

V = = 76.9 ml 1.3 g/ml M = 0.73 mol = 9.49 mol/L 0.0769 L

CH₃COOH soluzione 1 M PM = 60 g/mol d = 1.1 g/ml % p/p = ?

Per ipotesi 100 g di soluzione V = 100 g = 0.0909 L 1.1 g/ml mol_CH3COOH = 0.0909 L x 1 mol/L = 0.0909 mol

g_CH3COOH = 0.0909 mol x 60 g/mol = 5.145 g

% p/p = 5.145 g = 5.145% 100 g

200 g di H₂O + 70 g solfato di alluminio idrato Al₂(SO₄)₃·2H₂O + 20 g glucosio + 5 g H₂SO₄

Al₂(SO₄)₃·2H₂O → 2Al³⁺ + 3SO₄^2- + 2H₂O

H₂SO₄ → 2H⁺ + SO₄^2-

Specie: H₂O, Al³⁺, SO₄^2-, C₆H₁₂O₆, H⁺

mol H₂O = 200 g / 18 g/mol = 11.1 mol

mol Al₂(SO₄)₃·2H₂O = 70 g / 378 g/mol = 0.19 mol

mol C₆H₁₂O₆ = 20 g / 180 g/mol = 0.11 mol

mol H₂SO₄ = 5 g / 98 g/mol = 0.051 mol

X_H2O = 11.1 mol + (0.19·2) mol / (11.1 + (0.19·2) + (0.19·2) + (0.19·3) + 0.051 + 0.11 + (0.051·2)) mol

mol H₂O   mol Al³⁺   mol SO₄^2-   mol C₆H₁₂O₆   mol H⁺

Legge di Henry

La solubilità di un gas in un liquido è proporzionale alla sua pressione parziale.

S = k_H P

k_H è la costante di Henry, dipende da:

• Gas
• Solvente
• Temperatura

Esempio: Cl₂ gassoso sciolto in cloroformio mmolare, k_HCl₂ = 0.15 atm (inventato), PCl₂ = 25 torr

SCl₂ = 0.15 atm            25 torr                 760 torr                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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