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I'm not an expert teacher or lecturer of chemistry. I was only a student from SMA NEGERI 15 SURABAYA who had been one of the Bronze Medalist Participants of Olimpiade Sains Nasional X (2011) of Chemistry In Manado, North Sulawesi, 11 - 16 September 2011 and graduated in 2012. Now, I'm studying at Universitas Airlangga in Surabaya, Indonesia. I do love chemistry and I would like to help them who had difficulties in studying chemistry. That's why, please understand me if you found some misconcepts in my entries. Suggestions are always necessary in order to develop this blog. And I'm sorry because my English isn't so well.

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The Documents

Monday, June 20, 2011

Persamaan Nërst

Melanjutkan pembahasan kita tentang termodinamika kimia, energi bebas gibbs juga dapat menentukan potensial sel suatu reaksi elektrokimia apabila reaksi tersebut tidak berlangsung dalam keadaan tidak standar (25oC, 1M, 1atm).

Dari persamaan berikut:

∆G = ∆H - T∆S

∆H = ∆U + P ∆V

∆U = Q – W

atau:

∆U = Q + W - P ∆V

(W menerima usaha)

Apabila digabungkan maka:

∆G = ∆H - T∆S

∆G = ∆U + P ∆V - T∆S

∆G = Q + W - P ∆V + P ∆V - T∆S

∆G = Q + W - T∆S

Untuk Q, ∆G dalam keadaan standar bernilai 0:

∆G = ∆HSistem - T∆SSistem

∆H = T∆S

Pada tekanan tetap, ∆H = Q

Maka Q = T∆S

Sehingga:

∆G = T∆S + W - T∆S

∆G = W

Untuk reaksi reversible sebagaimana dalam sel elektrokimia:

∆G = W = - n F ESel

Perlu anda ingat bahwa ∆G hanya berlaku untuk reaksi reversible saja, kenyataannya terdapat sejumlah besar arus listrik yang bergerak melalui sel.

Sehingga ∆G = W maksimal.

Dan hubungan ∆G terhadap kesetimbangan:

∆G = ∆Gϴ + R T ln Qeq

Sehingga:

- n F ESel = - n F EϴSel + R T ln Qeq

Dibagi dengan - n F sehingga Persamaan Nërst dinyatakan dalam:

Karena kita tahu nilai R (8,314) dan F (96450) pada suhu 25oC, sehingga, Persamaan Nërst dapat dinyatakan sebagai:


Keterangan:

Anda dapat membulatkan konstanta tersebut menjadi 0,05916.

Kenapa suhu saya tulis tebal? karena persamaan terakhir berlaku pada suhu 25oC saja. Apabila suhu dinyatakan dalam T, maka:


Soal:

Diketahui sel zink-cuprum adalah:

Zn | Zn2+ (0,05M) || Cu2+ (1,00 M) | Cu

Dengan:

Ered Zn2+ / Zn = - 0,76 V

Ered Cu2+ / Cu = + 0,34 V

Hitung Esel sel volta tersebut! (Jawaban: 1,14 Volt)
(Soal OSP)

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