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15240672380's Question
Chemistry
Posted 8 months ago

Calculate the enthalpy and entropy of chemical reaction at 1000K. Leave calculations in the integral form after putting cp values. Organize them as we do it in the class. 4/3Al(l)+O2( g)2/3Al2O3( g)4 / 3 \mathrm{Al}(\mathrm{l})+\mathrm{O} 2(\mathrm{~g}) \boxtimes 2 / 3 \mathrm{Al} 2 \mathrm{O} 3(\mathrm{~g}) at 1273 K1273 \mathrm{~K} DATA: H\boxtimes \mathrm{H} Al (s『l)=10500 joules at 932 K932 \mathrm{~K}, Standard molar enthalpy formation and entropy at 298K: HofAl2O=\triangle \mathrm{HofAl} 2 \mathrm{O}= -1674000 joules, SoAl(s)=28.3 J/K, So Al2O3=59.6 J/K, SoO2=205 J/K
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Answer from Sia
Posted 8 months ago
Solution
1
Write the balanced chemical equation: The balanced chemical equation for the reaction given is 4/3Al(s)+O2(g)2/3Al2O3(g)4/3 \mathrm{Al(s)} + \mathrm{O}_2(g) \rightarrow 2/3 \mathrm{Al}_2\mathrm{O}_3(g)
2
Calculate the standard enthalpy change for the reaction at 298 K: The standard enthalpy change for the reaction (ΔHrxn\Delta H^\circ_{\text{rxn}}) at 298 K can be calculated using the standard enthalpies of formation (ΔHf\Delta H^\circ_{\text{f}}) of the products and reactants. The formula is ΔHrxn=(ΔHf, products)(ΔHf, reactants)\Delta H^\circ_{\text{rxn}} = \sum (\Delta H^\circ_{\text{f, products}}) - \sum (\Delta H^\circ_{\text{f, reactants}}). For the given reaction, ΔHrxn=23ΔHf, Al2O343ΔHf, AlΔHf, O2\Delta H^\circ_{\text{rxn}} = \frac{2}{3} \Delta H^\circ_{\text{f, Al}_2\mathrm{O}_3} - \frac{4}{3} \Delta H^\circ_{\text{f, Al}} - \Delta H^\circ_{\text{f, O}_2}. Since ΔHf, O2\Delta H^\circ_{\text{f, O}_2} is zero for elements in their standard state, we have ΔHrxn=23(1674000 J)43(0 J)\Delta H^\circ_{\text{rxn}} = \frac{2}{3}(-1674000 \text{ J}) - \frac{4}{3}(0 \text{ J})
3
Calculate the standard entropy change for the reaction at 298 K: The standard entropy change for the reaction (ΔSrxn\Delta S^\circ_{\text{rxn}}) at 298 K can be calculated using the standard molar entropies (SS^\circ) of the products and reactants. The formula is ΔSrxn=(Sproducts)(Sreactants)\Delta S^\circ_{\text{rxn}} = \sum (S^\circ_{\text{products}}) - \sum (S^\circ_{\text{reactants}}). For the given reaction, ΔSrxn=23SAl2O343SAlSO2\Delta S^\circ_{\text{rxn}} = \frac{2}{3} S^\circ_{\text{Al}_2\mathrm{O}_3} - \frac{4}{3} S^\circ_{\text{Al}} - S^\circ_{\text{O}_2}. Plugging in the values, we get ΔSrxn=23(59.6 J/K)43(28.3 J/K)205 J/K\Delta S^\circ_{\text{rxn}} = \frac{2}{3}(59.6 \text{ J/K}) - \frac{4}{3}(28.3 \text{ J/K}) - 205 \text{ J/K}
4
Adjust enthalpy and entropy to 1000 K: To adjust the enthalpy and entropy to 1000 K, we need to integrate the heat capacities (CpC_p) from 298 K to 1000 K. The change in enthalpy (ΔH\Delta H) and entropy (ΔS\Delta S) can be calculated using the formulas ΔH=ΔHrxn+2981000CpdT\Delta H = \Delta H^\circ_{\text{rxn}} + \int_{298}^{1000} C_p \, dT and ΔS=ΔSrxn+2981000CpTdT\Delta S = \Delta S^\circ_{\text{rxn}} + \int_{298}^{1000} \frac{C_p}{T} \, dT. Since the heat capacities are not provided, we leave the integrals in their general form
Answer
The standard enthalpy change for the reaction at 298 K is ΔHrxn=23(1674000 J)=1116000 J\Delta H^\circ_{\text{rxn}} = \frac{2}{3}(-1674000 \text{ J}) = -1116000 \text{ J}. The standard entropy change for the reaction at 298 K is ΔSrxn=23(59.6 J/K)43(28.3 J/K)205 J/K=149.1 J/K\Delta S^\circ_{\text{rxn}} = \frac{2}{3}(59.6 \text{ J/K}) - \frac{4}{3}(28.3 \text{ J/K}) - 205 \text{ J/K} = -149.1 \text{ J/K}. The changes in enthalpy and entropy at 1000 K cannot be calculated without the heat capacities and are left in integral form.
Key Concept
The standard enthalpy and entropy changes of a reaction at a given temperature can be calculated using the standard enthalpies of formation and standard molar entropies of the reactants and products.
Explanation
The enthalpy change at a different temperature requires the integration of heat capacities over the temperature range, and the entropy change also requires integration of heat capacities divided by temperature. Without specific CpC_p values, these calculations remain in integral form.

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