I-electrochemistry: ama-cell potential, i-free energy nesibalo se-Nernst

Ngaphakathi kweseli ye-galvanic: i-anode, i-cathode nebhuloho likasawoti

Iseli ye-galvanic (voltaic) iguqula ukusabela kwe-redox okuzenzakalelayo kube umsebenzi kagesi. Eselini ye-zinc–copper evamile, umucu we-zinc uhlala esixazululweni se-Zn²⁺ kanti umucu we-copper uhlala esixazululweni se-Cu²⁺. I-zinc yenziwa i-oxidation, Zn → Zn²⁺ + 2e⁻, futhi ama-electron ahamba ngocingo aye emucwini we-copper, lapho Cu²⁺ + 2e⁻ → Cu.

I-oxidation ihlala yenzeka ku-anode kanti i-reduction ku-cathode. Eselini ye-galvanic i-anode iyi-electrode engelihle, ngoba ikhulula ama-electron ocingweni. Ibhuloho likasawoti, elivame ukugcwaliswa ngesixazululo se-KNO₃, livumela ama-ayoni ukuthi ahambe phakathi kweziqukathi ezimbili ukuze kungabikho neyodwa enqwabelanisa ishaji ephelele. Ngaphandle kwalo, umsinga uma cishe ngaso leso sikhathi.

Ukubhalwa kweseli kubhala i-anode ngakwesobunxele ne-cathode ngakwesokudla: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s). Umugqa owodwa ukhombisa umngcele wesigaba kanti umugqa ophindwe kabili ukhombisa ibhuloho likasawoti. Uma isigamu sokusabela singenayo insimbi eqinile, i-electrode engasabeli efana ne-platinum ibhalwa ekugcineni kwangaphandle.

Ukufunda itafula lama-reduction potential ajwayelekile

Isigamu sokusabela ngasinye sithola i-reduction potential ejwayelekile, E°, elinganiswe ngokuqhathaniswa ne-standard hydrogen electrode, enikezwe u-0 V ncamashi. Izimo ezijwayelekile zingu-1 M ezintweni ezincibilikisiwe, 1 bar emagesini futhi, emathebulini amaningi, 25 °C. Konke okufakiwe kubhalwa njenge-reduction.

Uma u-E° emuhle kakhulu, ifomu le-oxidized lidonsa ama-electron ngamandla kakhulu. I-F₂ ku-+2.866 V iyi-oxidizing agent evamile eqine kakhulu kuSithasiselo L. I-Li⁺ ku-−3.04 V iyi-ayoni enzima kakhulu ukwenziwa i-reduction, ngakho insimbi ye-lithium iyi-reducing agent eqine kakhulu. Amanani asetshenziswe ngezansi avela kuSithasiselo L: Ag⁺/Ag +0.7996 V, Cu²⁺/Cu +0.337 V, Ni²⁺/Ni −0.257 V no-Zn²⁺/Zn −0.7618 V.

Isibonelo esisebenzile: u-E°cell weseli ye-zinc–copper

Sebenzisa u-E°cell = E°cathode − E°anode, womabili amanani athathwe njengoba ebhaliwe etafuleni le-reduction. I-copper ino-E° omuhle kakhulu, ngakho yenziwa i-reduction futhi iyi-cathode. E°cell = +0.337 V − (−0.7618 V) = +1.099 V, kubikwe ezindaweni ezintathu zedesimali ngoba u-0.337 unezintathu.

U-E°cell omuhle usho ukuthi ukusabela kuyazenzakalela njengoba kubhaliwe ngaphansi kwezimo ezijwayelekile. Ukube ubukhethe ama-electrode ngendlela engalungile, ubuzothola u-−1.099 V. Lokho kukutshela ukuthi ukusabela okuphambene yikona okuzenzakalelayo.

Ukuhlola okufanayo kusebenza kunoma yiliphi ibhangqa le-redox. Ingabe i-Fe³⁺ izokwenza i-oxidation ye-I⁻ ibe yi-I₂? Ngo-Fe³⁺/Fe²⁺ ku-+0.771 V njenge-cathode no-I₂/I⁻ ku-+0.5355 V njenge-anode, E°cell = +0.236 V, ngakho yebo. Ingabe i-Zn²⁺ izokwenza i-oxidation yensimbi ye-copper? E°cell = −0.7618 − 0.337 = −1.099 V, ngakho cha.

Isibonelo esisebenzile: kungani u-E° engalokothi aphindaphindwe nge-coefficient

Cabanga ngo-Ni(s) + 2 Ag⁺(aq) → Ni²⁺(aq) + 2 Ag(s). Ukulinganisa ama-electron, isigamu sokusabela se-silver kufanele siphindwe kabili. Kuyalingeka ukuphinda kabili no-E° waso, kodwa lokho kungaba iphutha. U-E° uyisakhiwo esiyi-intensive: ulinganisa amandla ngeyunithi ngayinye yeshaji, futhi ukuphinda kabili inani lokusabela kuphinda kabili kokubili amandla neshaji.

Ngakho E°cell = +0.7996 V − (−0.257 V) = +1.057 V, ngaphandle kwesici esingu-2. Inani lokusabela lingena kwenye indawo, ngo-n, inani lama-mole ama-electron adluliswayo, okulapha lingu-2.

Ukuxhumanisa u-E°cell no-ΔG° no-K

Umsebenzi kagesi omkhulu kakhulu iseli engawenza ulingana no-−ΔG, okunikeza ΔG° = −nFE°cell. Lapha u-F uyi-Faraday constant, 96,485 C nge-mole ngayinye yama-electron, futhi i-volt eyodwa iphindaphindwe nge-coulomb eyodwa iyi-joule eyodwa. Ngakho u-E° omuhle usho u-ΔG° omubi, futhi kokubili kusho ukuthi ukusabela kuyazenzakalela ngaphansi kwezimo ezijwayelekile.

Ngoba ΔG° = −RT ln K, i-cell potential iphinde inqume okungaguquki kokulingana. Ku-25 °C lokhu kuba log K = nE°cell / 0.0592 V, lapho u-0.0592 V eyi-2.303RT/F ku-298.15 K eyiswe ezinombolweni ezibalulekile ezintathu, njengoba i-OpenStax yenza.

Isibonelo esisebenzile. Ekusabeleni kwe-nickel–silver okungenhla, n = 2 no-E°cell = 1.057 V. ΔG° = −2 × 96,485 C/mol × 1.057 V = −2.04 × 10⁵ J, noma −204 kJ nge-mole ngayinye yokusabela njengoba kubhaliwe. log K = 2 × 1.057 / 0.0592 = 35.7, ngakho u-K ucishe abe ngu-5 × 10³⁵. Ukusabela kucishe kuphele ngokuphelele. Thatha u-K ngenombolo ebalulekile eyodwa: ushintsho luka-0.001 V ku-E° luhambisa u-log K cishe ngo-0.03.

Isibalo se-Nernst sokugxila okungajwayelekile

Amaseli angempela awavamile ukusebenza ku-1 M. Isibalo se-Nernst silungisa i-potential: E = E° − (0.0592 V / n) log Q ku-25 °C, lapho u-Q eyi-reaction quotient. Izinto eziqinile ziyashiywa ku-Q, njengoba nje zishiywa ku-K.

Isibonelo esisebenzile. Iseli ye-zinc–copper ino-[Zn²⁺] = 1.00 M no-[Cu²⁺] = 1.0 × 10⁻³ M. Ku-Zn + Cu²⁺ → Zn²⁺ + Cu, Q = [Zn²⁺]/[Cu²⁺] = 1.0 × 10³ no-n = 2. E = 1.099 − (0.0592/2) × 3.00 = 1.099 − 0.089 = 1.010 V. Ukuncisha i-cathode i-Cu²⁺ kwehlisa i-voltage, njengoba isimiso sika-Le Chatelier sibikezela.

Iseli yokugxila inesigamu sokusabela esifanayo nhlangothi zombili, ngakho E° = 0, futhi isebenza kuphela ngoba ukugxila kwehlukile. Kuma-electrode e-copper ku-Cu²⁺ engu-0.0010 M no-1.0 M, E = −(0.0592/2) log(0.0010/1.0) = +0.089 V. Uhlangothi oluhlanjululiwe yi-anode, ngoba i-oxidation lapho yenza i-Cu²⁺ eningi futhi ilinganise umehluko. Lapho u-E efika ku-zero, u-Q ulingana no-K futhi iseli selifile.

I-electrolysis nomthetho ka-Faraday

Iseli ye-electrolytic isebenzisa umthombo wamandla wangaphandle ukuqhuba ukusabela okungazenzakaleli, njengokunameka insimbi noma ukuhlukanisa amanzi. I-anode isengulapho i-oxidation yenzeka khona, kodwa manje iyi-electrode enhle. Inani lomkhiqizo lincike kuphela eshajini edlulayo: ishaji Q = umsinga I × isikhathi t, futhi ama-mole ama-electron = Q / F.

Isibonelo esisebenzile. Umsinga ongu-2.50 A ugeleza imizuzu engu-30.0 esixazululweni se-CuSO₄. Q = 2.50 A × 1800 s = 4500 C. Ama-mole ama-electron = 4500 / 96,485 = 0.0466 mol. I-Cu²⁺ ngayinye idinga ama-electron angu-2, ngakho kufakwa u-0.0233 mol we-Cu, okuthi ku-63.55 g/mol kube ngu-1.48 g.

Yenza ukubala uhlehla ukuthola isikhathi. Ukunameka u-1.00 g we-silver (Ag⁺ + e⁻ → Ag) kudinga ama-electron angu-1.00 / 107.87 = 0.00927 mol, okungu-894 C. Ku-0.500 A lokho kuthatha u-1789 s, cishe imizuzu engu-29.8.

Amaphutha ophawu nawokubala ezinkingeni zamaseli

Amaphutha amaningi ku-electrochemistry angamaphutha ophawu. Bhala isigamu sokusabela ngasinye, usibhale ilebula ye-oxidation noma ye-reduction, bese kuphela wenza izibalo.

• Ukuphendula uphawu lwe-potential ye-anode bese uphinde uyisuse, okubala ukuphendulwa kabili.
• Ukuphindaphinda u-E° nge-coefficient esetshenziswe ukulinganisa ama-electron.
• Ukudida ukuthi iyiphi i-electrode enhle. I-cathode inhle eselini ye-galvanic, kodwa i-anode inhle eselini ye-electrolytic.
• Ukusebenzisa u-n ongalungile ku-ΔG° = −nFE° noma esibalweni se-Nernst. U-n ungama-mole ama-electron adluliswayo ekusabeleni kukonke okulinganisiwe.
• Ukushiya u-ΔG° ngama-joule lapho impendulo idingeka ngama-kilojoule, noma okuphambene.
• Ukubhala u-Q ehlanekezelwe esibalweni se-Nernst. Imikhiqizo iya phezulu, njengakunoma iyiphi i-reaction quotient.
• Ukusebenzisa amaminithi esikhundleni samasekhondi ku-Q = It.