DASAR REAKSI ANORGANIK
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DASAR REAKSI ANORGANIK
Sayekti WahyuningsihJURUSAN KIMIA
Universitas Sebelas Maret w.Sayekti @yahoo.com
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Konsep energi ikatan dan struktur molekul
H(g) + H(g) H2 (g) H = -436 kJ/mol (entalpi pembentukan
H2)
H2 (g) H(g) + H(g) H = 436 kJ/mol ( energi ikat H2)
Prinsip dasar reaksi kimia
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Energi ikat
Energi yang dibutuhkan untuk menarik atom bebas dari pengaruh masing-masing atom terhadap atom lain, dan karena itu merupakan energi yang mengikat atom secara bersama-sama.
Perubahan entalpi pada dissosiasi molekul gas pada temperatur 278 K menjadi atom-atom gas ground state
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Bond energy vs mean interatomic distance
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Bond length vs bond strength
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Bagaimana korelasi antara bond length dengan bond energy?
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Bond energy
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Mengapa multiple bond memiliki energi ikat lebih besar?
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O-H bond formation
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Energi ikat kekuatan ikat dan kestabilan
N
F
F
F
O N O F
F
F
3(278) kJ/mol +222 kJ/mol= 1056kJ/mol
2(278) + 222 + 190 kJ/mol= 968 kJ/mol
Muatan berlawanan ?Elektron bebas ?
Ikatan kimia menjaga molekul-molekul, kristal, dan gas-gas diatomik untuk tetap bersama. Selain itu ikatan kimia juga menentukan struktur suatu zat.
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Pertemuan kedua
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Lewis Structure Counting electrons The total number of electrons represented in a Lewis structure is equal to the sum of
the numbers of valence electrons on each individual atom. Non-valence electrons are not represented in Lewis structures.
The octet rule states atoms with eight electrons in their valence shell will be stable, regardless of whether these electrons are bonding or nonbonding. The rule applies well to acidic compounds. The 18-Electron rule is operative on atoms from period 4, which have to achieve 18 electrons to their orbitals and achieve a stable configuration which has the same electron configuration as a Noble gas. Similarly from period 6, the atoms have to achieve 32 electrons to fill their orbitals.
Placing electrons Lewis structures for oxygen, fluorine, the hydrogen sulfate anion, and formamide Once the total number of available electrons has been determined, electrons must be
placed into the structure. They should be placed initially as lone pairs: one pair of dots for each pair of electrons available. Lone pairs should initially be placed on outer atoms (other than hydrogen) until each outer atom has eight electrons in bonding pairs and lone pairs; extra lone pairs may then be placed on the central atom. When in doubt, lone pairs should be placed on more electronegative atoms first.
Once all lone pairs are placed, atoms, especially the central atoms, may not have an octet of electrons. In this case, the atoms must form a double bond; a lone pair of electrons is
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Kecenderungan satu grup
energi ikat naik
Kecenderungan dalam satu periode
Thermodinamika energi ikat
C-C 347Si-Si 340Ge-GeSn-Sn
C-C 347N-N 160 O-O 146Energi ikat turun, panjang ikatan bertambah
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Pada ikatan rangkap, ikatan terdiri dari satu ikatan dan satu atau dua ikatan . Energi ikat dari ikatan rangkap merupakan hasil kontribusi dari ikatan dan ikatan .
Ikatan rangkap homonuclear
Energi ikat ikatan rangkap tergantung pada order ikatan, ukuran molekul, dan terdapatnya pasangan elektron bebas.
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Bond order
Orde ikatan (B.O.) =(jumlah elektron pada orbital bonding - jumlah elektron pada orbital non bonding) /2.
Bagaimana hubungan antara panjang ikatan, bond order dan energi ikat?
Spesies Bond order d, pm D, kJ/mol
H2+ 1/2 106 255
H2 1,0 74,1 432
Energi ikat H2+ =
255/2 = 216 kJ/mol(kenapa menjadi 255 kJ/mol?
Energi tolakan pasangan elektron=255-216 kJ/mol
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(8-3)/2 =2,5
Spesies Bond order d, pm D, kJ/mol
O2+ 2,5 112 -
O2 2,0 121 494
O2- 1,5 126 393
O22- 1,0 149 -
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Pada O2 estimasi kontribusi dari ikatan dan ikatan dapat dihitung dari persamaan:
E = Es (ds / dm)
E adalah energi dari kontribusi ikatan , Es adalah energi ikatan tunggal, ds adalah panjang ikatan tunggal, sedangkan dm adalah panjang ikatan rangkap.
E akan berharga maksimum jika tidak terdapat pasangan elektron bebas (group 14). Sedangkan jika terdapat pasangan elektron bebas (group 15, dan 16) akibat adanya tolakan pasangan elektron, harga E menjadi tidak maksimal.
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Energi ikat heteroatomDengan menggunakan pendekatan yang dikemukakan oleh Pauling pada pembentukan senyawa X-Z dari unsur X dan Z, energi ikat X-Z, E(X-Z), harus memenuhi dua ketentuan yaitu:
(1). E(X-Z) > [E(X-X) . E(Z-Z)]1/2, dan (2). E(X-Z) > [E(X-X) + E(Z-Z)]/2
[E(X-X) . E(Z-Z)]1/2 disebut sebagai rerata geometri [E(X-X) + E(Z-Z)]/2 disebut rerata aritmetika
Ikatan rangkap heteronuclear
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Struktur resonansi ionik memberikan kontribusi pada energi ikat heteroatom.
Energi resonansi ionik, , adalah sebesar: = E(X-Z) - [E(X-X) . E(Z-Z)]1/2 atau = E(X-Z) - [E(X-X) + E(Z-Z)] / 2
sehingga energi ikat heteroatom adalahE(X-Z) = + [E(X-X) . E(Z-Z)]1/2 atau E(X-Z) = + [E(X-X) + E(Z-Z)] / 21/2 akan berharga sama dengan yaitu
perbedaan elektronegatifitas antara X dan Z
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= a + 2bq asumsi : (X) = (Z)
a(X) + 2b(X).q(X) = a(Z) + 2b(Z).q(Z) q(X) = q(Z)
maka:a(X) + 2b(X).q(Z) = a(Z) + 2b(Z).q(Z)
q(Z) = [a(X) –a(Z)] / 2[b(X) + b(Z)]
metode estimasi karakter ionik
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Atom a B
K 2,42 1,92
Br 7,59 4,22
Muatan parsial K :
q(K) = [7,59-2,42] / [2(4,22 + 1,92)] = +0,421
Distribusi muatan parsial KBr adalan K
+0,421 Br-0,421
Karakter ionik KBr = 42,1%Karakter kovalen KBr = 57,9%
Contoh :
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Senyawa E(Cl-F), kJ/mol
ClF 251
ClF2174
ClF3152
Faktor- faktor yang mempengaruhi energi ikat heteroatom
1. Kombinasi faktor sterik dan faktor hibridisasi.
2. Ukuran molekul
E(H2O) > E(H2S) > E(H2Se) > E(H2Te)
3. Tolakan pasangan elektron, tolakan pasangan elektron dapat menyebabkan destabilisasi ikatan heteroatom.
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4. Efek resonansi ionik
Jika dilihat dari energi disosiasi, menurut pauling rata rata geometri dari energi ikat Cl-Cl dan F-F adalah :
(154 x 240 )1/2 = 192 kJ/mol
namun fakta dari energi ikat ClF > 192 kJ/mol ( ada tambahan sekitar 30%). Perbedaan sebesar (251-192) kJ/mol ( )merupakan tambahan energi dari efek resonansi ionik. Energi ikat yang sesungguhnya merupakan energi total dari kontribusi ikatan kovalen dan ikatan ionik.
Senyawa D, kJ/mol
F2154
Cl2240
ClF 251
E(X-Z) = + [E(X-X) . E(Z-Z)]1/2 251 kJ/mol = 59 kJ/mol + 192 kJ/mol
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Entalpi, H, adalah jumlah dari energi internal dan energi lainnya di dalam sistem.
H = E + PV (jika kerja lain PV saja)sedangkan
E = q - PV
sehingga pada tekanan tetap (reaksi kabanyakan di laboratorium)
H = ( q - PV ) + PV atau H = qp
Prinsip Entalpi dan kekuatan ikatan kimia
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PV = PVf - PVi
PV = P(Vf - Vi )
PV = nfRT - ni RT
PV = (nf - ni ) RT = nRT
Reaksi pada tekanan tetap : H E.Perubahan entalpi (H) dapat dihitung dari perbedaan harga entalpi dan dari perbedaan energi ikat antara reaktan produk maka:
H = [Hproduk -Hreaktan]
atau H = -[ Eikat produk -Eikat reaktan ]
Pada proses reaksi dengan disertai perubahan mol:
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H = -[ Eikat produk -Eikat reaktan ]
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ΔH1 = ΔH(A to B) Pathway 1 the most direct route with no intermediate stages,or ΔH1 = ΔH2 + ΔH3 pathway 2 involving one set of intermediates C, oror ΔH1 = ΔH4 + ΔH5 + ΔH6 pathway 3 involving two sets of intermediates D and Eetc. etc. - there is no limit to the complexity of the Hess's Law Cycle as long as A and B are constant.
Hess's Law Cycle
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Hubungan G, H, dan S
Entalpi H :H = E + PV (jika kerja lain PV saja)
Reaksi spontan didukung dari harga H negatif dan harga entropi S positif. Energi bebas Gibbs merupakan ukuran dari kespontanan reaksi yang besarannya tergantung pada harga H, T, dan S:
G = H - TS
Konsep entalpi, entropi dan energi bebas pada kespontanan reaksi
Jika H positif apakah reaksi selalu tidak spontan?Jika S negatif apakah reaksi selalu tidak spontan?
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