Kemi

Små kemi opgaver

11. januar kl. 11:49 af Lise123Lise - Niveau: A-niveau

Hej jeg har nogle problemmer med følgende opgaver:

1) Use standard entropies and heats of formation to calculate ΔG°f for
    a) cadmium(II) chloride (s).

2) It has been proposed that wood alcohol, CH3OH, a relatively inexpensive fuel to produce, be decomposed to produce methane. Methane is a natural gas commonly used for heating homes. Is the decomposition if wood alcohol to methane and oxygen thermodynamically feasible at 25°C and 1 atm?

3) Discuss the effect of temperature change of the spontaneity of the following reactions at 1 atm:
    a) Al2O3 (s) + 2Fe (s) ------> 2Al (s) + Fe2O3 (s)
                       ΔH°=851,4 kJ;     ΔS°=38,5 J/K

      At what temperature does ΔG° become zero for each reactions. Explain the significance of your answers.


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Svar #1
11. januar kl. 13:21 af mathon

\small \begin{array}{llllll} \textbf{1)}\\&& CdCl_2\,_{\textit{(s)}}\textup{:}\\&&& &\Delta {H_f}^\Theta =-391.5\;\frac{kJ}{mol}\qquad \Delta {S_f}^\Theta =0.1153\;\frac{kJ}{mol\cdot K}\\\\&&& &\Delta {G_f}^\Theta =-391.5\;\frac{kJ}{mol}-\left (298.15\;K \right )\cdot \left (0.1153\;\frac{kJ}{mol\cdot K} \right )=-425.877\;\frac{kJ}{mol} \end{array}


Svar #2
11. januar kl. 13:38 af Lise123Lise

Tak! Er det også muligt at få lidt hjælp til de 2 andre opgaver? (:


Svar #3
11. januar kl. 18:29 af Lise123Lise

Eller bare give et hint til hvilke formler der skal benyttes?


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Svar #4
12. januar kl. 11:25 af mathon

\small \begin{array}{llllll} \textbf{2)}\\&& 2\,CH_3OH\,_{\textit{(l)}}\;\longrightarrow\;2\,CH_4\,_{\textit{(g)}}\;+\;O_2\,_{\textit{(g)}}\;\\\\&&\qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \begin{array}{|l|c|c|} \hline \, \, \, \, \textup{stof}&\Delta {H_m}^\Theta &{S_m}^\Theta\\ \hline CH_3OH&-201\;\frac{kJ}{mol}&0.240\;\frac{kJ}{mol\cdot K}\\ \hline CH_4&-75\;\frac{kJ}{mol}&0.186\;\frac{kJ}{mol\cdot K}\\ \hline O_2&0&0\\ \hline \end{array}\\\\ & &\Delta {G_f}^\Theta =2\cdot \left ( -75\;\frac{kJ}{mol} \right )+2\cdot\left ( 298.15\;K \right ) \left ( 0.186\;\frac{kJ}{mol\cdot K} \right )-\left ( 2\cdot \left ( -201\;\frac{kJ}{mol} \right )+2\cdot\left ( 298.15\;K \right ) \left ( 0.240\;\frac{kJ}{mol\cdot K} \right) \right )=\\\\&&\; \; \; \; \; \; \; \; \; \; \; \; \; -85.720\;\frac{kJ}{mol}\\ \textup{hvoraf}\\ \textup{ses, at}&&\textup{ovenst\aa ende reaktion forl\o ber spontant ved 25}\degree C\textup{ og 1 atm.} \end{array}


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Svar #5
12. januar kl. 12:10 af mathon

\small \begin{array}{llllll} \textbf{3)}\\& \textbf{a)}\\&& Al_2O_3 \,_{\textit{(s)}}\;+\; 2\,Fe\,_{\textit{(s)}}\; \longrightarrow\; 2\,Al\,_{\textit{(s)}} + Fe_2O_3\,_{\textit{(s)}}\\\\\\&& \Delta {H_m}^\Theta =851.4 \frac{kJ}{mol}\qquad {S_m}^\Theta =0.0385\; \frac{kJ}{mol\cdot K}\qquad \textup{er forkerte}\\\\\textup{da}&& \Delta {G_m}^\Theta = \Delta {H_m}^\Theta-T\cdot {S_m}^\Theta>0\\\\&& \end{array}


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