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For Equilibrium between $\mathrm{Al}$ and $\mathrm{Al} 2 \mathrm{O} 3$ at $1273 \circ \mathrm{oC}$, a) Determine the standard enthalpy, entropy and Gibbs free energy of formation of Al2O3 at $298 \mathrm{~K}$ from Ellingham diagram. Compare the results in 1.a. b) Read the equilibrium oxygen pressure $\mathrm{pO} 2(\mathrm{~g}), \mathrm{pH} 2(\mathrm{~g}) / \mathrm{pH} 2 \mathrm{O}(\mathrm{g})$ ratio, and $\mathrm{pCO}(\mathrm{g}) / \mathrm{pCO} 2(\mathrm{~g})$ ratio from the Ellingham diagram using correct scales! c) Write chemical reactions for $\mathrm{Al}(\mathrm{l})$ used for these three different scales. d) What must be the gas mixture for all to protect Al(I) from oxidation? Use equilibrium constants!!
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Answer from Sia

Posted 3 months ago

Solution

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1 Answer

The standard enthalpy, entropy, and Gibbs free energy of formation of $\mathrm{Al}_2\mathrm{O}_3$ at 298 K can be determined from the Ellingham diagram by reading the slope and intercept at 298 K and using the Gibbs free energy equation.

Key Concept

The Ellingham diagram provides information about the temperature dependence of Gibbs free energy for the formation of compounds, from which standard enthalpy and entropy can be derived.

Explanation

The slope of the line on the Ellingham diagram corresponds to the entropy change, while the intercept at 298 K gives the Gibbs free energy change. The enthalpy change is calculated using the Gibbs free energy equation.

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2 Answer

The equilibrium oxygen pressure ratios can be read from the Ellingham diagram, and the corresponding chemical reactions for $\mathrm{Al}(l)$ can be written as follows:
1. $\mathrm{Al}(l) + \frac{3}{2}\mathrm{O}_2(g) \rightleftharpoons \mathrm{Al}_2\mathrm{O}_3(s)$
2. $\mathrm{Al}(l) + \frac{3}{2}\mathrm{H}_2\mathrm{O}(g) \rightleftharpoons \mathrm{Al}_2\mathrm{O}_3(s) + 3\mathrm{H}_2(g)$
3. $\mathrm{Al}(l) + 3\mathrm{CO}_2(g) \rightleftharpoons \mathrm{Al}_2\mathrm{O}_3(s) + 3\mathrm{CO}(g)$

Key Concept

The Ellingham diagram can be used to determine the equilibrium partial pressures of gases at a given temperature, which can then be used to write the corresponding chemical reactions.

Explanation

The equilibrium pressure ratios are determined from the diagram, and the reactions are written based on the stoichiometry of the reaction between $\mathrm{Al}(l)$ and the gas species.

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3 Answer

To protect $\mathrm{Al}(l)$ from oxidation, the gas mixture should have a high ratio of $\mathrm{H}_2$ to $\mathrm{H}_2\mathrm{O}$ and $\mathrm{CO}$ to $\mathrm{CO}_2$, maintaining a low oxygen potential and preventing the formation of $\mathrm{Al}_2\mathrm{O}_3$.

Key Concept

To protect a metal from oxidation, the gas mixture must have a lower oxygen potential than that required for the formation of the metal oxide.

Explanation

The gas mixture should favor the presence of reducing gases over oxidizing gases, which can be determined by the equilibrium constants of the relevant reactions.

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