{"cells": [{"cell_type": "markdown", "metadata": {"tags": ["module-ltg"]}, "source": ["# Stability Diagram\n", "[Low-Temperature Geochemistry](module-ltg) \n", "```{index} Stability Diagram\n", "```\n", "\n", "Stability diagrams (dominance diagrams) are graphical representations of equilibria between minerals and aqueous solutions. They are useful in predicting what will happen when aqueous solutions of given composition interact with specific minerals. \n", "\n", "Stability diagrams can be built from thermodynamic data on $\\Delta G$ of various reactions, $\\Delta G^\\circ = -RT \\ln(K)$, and expressions of thermodynamic equilibrium constants ($K$). Let's recall Problem 1 in Practical 3 of Low-Temperature Geochemistry."]}, {"cell_type": "code", "execution_count": 1, "metadata": {"tags": ["hide-input"]}, "outputs": [], "source": ["# import relevant modules\n", "\n", "%matplotlib inline\n", "import numpy as np\n", "import matplotlib.pyplot as plt\n", "import pandas as pd\n", "from IPython.display import display"]}, {"cell_type": "markdown", "metadata": {}, "source": ["Consider the weathering of the mineral albite (a Na-plagioclase feldspar)\n", "\n", "$$2NaAlSi_{3}O_{8}(s)+2H^{+}(aq)+9H_{2}O(l)=Al_{2}Si_{2}O_{5}(OH)_{4}(s)+2Na^{+}(aq)+4H_{4}SiO_{4}(aq)$$\n", "\n", "a) Write the reaction quotient for this reaction ($Q=\u2026$).\n", "\n", "$$Q=\\frac{a_{Al_{2}Si_{2}O_{5}(OH)_{4}} \\cdot a_{Na^+}^2 \\cdot a_{H_{4}SiO_{4}}^4}{a_{NaAlSi_{3}O_{8}}^2 \\cdot a_{H^{+}}^2 \\cdot a_{H_{2}O}^9}$$\n", "\n", "b) Now assume this reaction is at equilibrium, write the equilibrium constant for this reaction ($K_{eq}=\u2026$).\n", "\n", "At equilibrium, $Q=K_{eq}$, so it\u2019s the same:\n", "\n", "$$Q=K_{eq}=\\frac{a_{Al_{2}Si_{2}O_{5}(OH)_{4}} \\cdot a_{Na^+}^2 \\cdot a_{H_{4}SiO_{4}}^4}{a_{NaAlSi_{3}O_{8}}^2 \\cdot a_{H^{+}}^2 \\cdot a_{H_{2}O}^9}$$\n", "\n", "c) Assume that you know the activities of each species ($a_i$), how would you simplify your answer in part a or b? Remember that for pure solids and liquids $a_j=1$.\n", "\n", "The key is to note that the activity for pure liquids and pure solids are 1, so we can simplify the expression as follows:\n", "\n", "$$K_{eq}=\\frac{1 \\cdot a_{Na^+}^2 \\cdot a_{H_{4}SiO_{4}}^4}{1 \\cdot a_{H^{+}}^2 \\cdot 1}=\\frac{a_{Na^+}^2 \\cdot a_{H_{4}SiO_{4}}^4}{a_{H^{+}}^2}$$\n", "\n", "d) The answer to part c should be\n", "\n", "$$K_{eq}=a_{H_{4}SiO_{4}}^4 \\cdot \\frac{a_{Na^+}^2}{a_{H^{+}}^2}$$\n", "\n", "Now take the logarithm on both side:\n", "\n", "$$\\log (K_{eq})=\\log \\left(a_{H_{4}SiO_{4}}^4 \\cdot \\frac{a_{Na^+}^2}{a_{H^{+}}^2}\\right)$$\n", "\n", "Simplify this expression by evaluating the logs:\n", "\n", "$$\\log (K_{eq})=4\\log (a_{H_{4}SiO_{4}}) + 2\\log \\left(\\frac{a_{Na^+}}{a_{H^{+}}}\\right)$$\n", "\n", "Rearrange to get:\n", "\n", "$$\\log \\left(\\frac{a_{Na^+}}{a_{H^{+}}}\\right) = -2\\log (a_{H_{4}SiO_{4}}) + \\frac{1}{2}\\log (K_{eq})$$\n", "\n", "Note this could be the equation of a straight line: $y=mx+b$, with slope $m=-2$, $y=\\ln\\left(\\frac{a_{Na^+}}{a_{H^{+}}}\\right)$, $x=\\log (a_{H_{4}SiO_{4}})$ and the y-intercept $b=\\frac{1}{2}\\log (K_{eq})$ If you were to plot this on a log-log plot, the slope is known, ($m=-2$), so all there is to define, is the intercept, meaning that one needs to evaluate $K_{eq}$.\n", "\n", "(i) Evaluate $K_{eq}$ by applying Hess\u2019 law to calculate the Gibbs free energy using the data in Table 1 (taken from P. Ryan\u2019s book, p. 278) and then applying this to the expression $\\Delta G_r^\\circ = -RT \\ln(K_{eq})$ and solving for $K_{eq}$."]}, {"cell_type": "code", "execution_count": 4, "metadata": {}, "outputs": [{"name": "stdout", "output_type": "stream", "text": ["Table 1: Thermodynamic data from P. Ryan's book, p278.\n"]}, {"data": {"text/html": ["
Chemical formula of compound or species | Name | $G_{f}^{o}$ |
---|---|---|
$NaAlSi_{3}O_{8}$ | \n", "Albite (low albite) | \n", "-3712 | \n", "
$Al_{2}Si_{2}O_{5}(OH)_{4}$ | \n", "Kaolinite | \n", "-3778 | \n", "
$Na_{0.33}Al_{2}(Si_{3.67}Al_{0.33})O_{10}(OH)_{2}$ | \n", "Na-beidellite | \n", "-5354 | \n", "
$Al(OH)_3$ | \n", "Gibbsite | \n", "-1144 | \n", "
$H^+$ | \n", "Hydrogen ion | \n", "0 | \n", "
$H_{2}O(l)$ | \n", "Water | \n", "-237.2 | \n", "
$Na^{+}(aq)$ | \n", "Sodium ion | \n", "-261.7 | \n", "
$H_{4}SiO_{4}$ | \n", "Dissolved silica | \n", "-1316 | \n", "