TERMOKIMIA. STANDART KOMPETENSI; 2. Memahami perubahan energi dalam kimia dan cara pengukuran. ENTALPI DAN PERUBAHANNYA

TERMOKIMIA STANDART KOMPETENSI; 2. Memahami perubahan energi dalam kimia dan cara pengukuran. ENTALPI DAN PERUBAHANNYA KOMPETENSI DASAR; 2.1. Mendeskripsikan perubahan entalpi suatu reaksi, reaksi eksoterm dan reaksi endoterm. A. PERUBAHAN ENTALPI Entalpi adalah total jumlah energi yang terkandung oleh suatu zat. Entalpi suatu zat diberi simbol "H". HA menyatakan jumlah total energi yang terkandung zat A. Entalpi suatu zat tidak dapat diukur. Yang dapat diukur adalah perubahannya. Perubahan entalpi diberi simbol "∆H". Perubahan entalpi reaksi kimia didefinisikan selisih entalpi hasil reaksi terhadap entalpi pereaksi. Atau, ∆Hreaksi = Hhasil reaksi - Hpereaksi Agar lebih jelasnya, lakukan percobaan berikut. -

Sediakan 50 ml aquades (H2O) dan 5 gram CaO. Ukur suhu aquades dan catat. Masukkan 5 gram CaO ke dalam aquades. Ukur suhunya dan catat. (lihat gambat di bawah). termometer

termometer

5 gram CaO

50 ml H2O tawal = .... 0C

50 ml H2O takhir = ... 0C

Berdasarkan percobaan di atas, tentukan; a. Manakah sistemnya? Jawab. ......................................................................................................................................... b. Manakah lingkungannya? Jawab. ......................................................................................................................................... c. Hpereaksi = ....................................... Hhasil reaksi = .......................................... ∆Hreaksi = Hhasil reaksi - Hpereaksi = ................... - ....................... 1

d. e.

Reaksi di atas termasuk reaksi melepaskan kalor atau membutuhkan kalor? Jawab. ......................................................................................................................................... Bagaimanakah kesimpulan yang dapat kamu ambil tentang harga perubahan entalpi? Jawab. .........................................................................................................................................

B. REAKSI EKSOTERM DAN REAKSI ENDOTERM -

REAKSI EKSOTERM Reaksi kimia yang melepas kalor dari sistem ke lingkungannya. Setelah kalor dilepaskan, suhu sistem menjadi turun. Hpereaksi > Hhasil reaksi. Harga ∆Hreaksi < 0 atau ∆Hreaksi bertanda negatip (-).

-

REAKSI ENDOTERM Reaksi kimia yang menyerap kalor dari lingkungannya. Setelah kalor diserap, suhu sistem menjadi naik. Hpereaksi < Hhasil reaksi. Harga ∆Hreaksi > 0 atau ∆Hreaksi bertanda positip (+). Agar lebih jelasnya, lakukan percobaan berikut. termometer

termometer

10 ml HCl 2 M

Mg 2 sendok spatula

10 ml HCl 2 M tawal = .... 0C takhir = ... 0C

tawal = .... 0C takhir = ... 0C termometer

termometer

10 ml H2O

tawal = .... 0C takhir = ... 0C

10 ml H2O + Ba(OH) 2 sendok spatula tawal = .... 0C takhir = ... 0C

2

1. 2. 3. 4. 5. 6. 7. 8.

Pertanyaan. Manakah dari keempat reaksi di atas yang tergolong reaksi eksterm? Jawab. ......................................................................................................................................... Manakah dari keempat reaksi di atas yang tergolong reaksi endoterm? Jawab. ......................................................................................................................................... Bagaimanakah entalpi sistem reaksi eksoterm? Jawab. ......................................................................................................................................... Bagaimanakah entalpi sistem reaksi endoterm? Jawab. ......................................................................................................................................... Bagaimanakah harga ∆H sistem reaksi eksoterm? Jawab. ......................................................................................................................................... Bagaimanakah harga ∆H sistem reaksi endoterm? Jawab. ......................................................................................................................................... Buatkan diagram tingkat energi reaksi eksoterm! Jawab. ......................................................................................................................................... Buatkan diagram tingkat energi reaksi endoterm! Jawab. .........................................................................................................................................

PENENTUAN ∆H REAKSI KOMPETENSI DASAR; 2.1. Menentukan ∆H reaksi berdasarkan eksperimen, hukum Hess, data perubahan entalpi pembentukan standart, dan data energi ikatan. A. PERSAMAAN TERMOKIMIA Persamaan termokimia adalah persamaan reaksi yang mengikutsertakan perubahan kalor. Contoh. Jika 1 mol gas H2 dicampur dengan ½ mol gas O2 menghasilkan 1 mol cair H2O, ternyata dibebaskan kalor sebanyak 286 kJ/mol pada suhu 298 0K. Persamaan termokimianya; H2(g) + ½ O2(g) → H2O(l)

∆H = -286/kJ/mol.

Untuk kondisi standar, yaitu tekanan 1 atm dan suhu 25 0C, maka persamaan termokimia contoh di atas adalah; H2(g) + ½ O2(g) → H2O(l)

∆H0 = -286/kJ/mol.

3

B. JENIS KALOR REAKSI/PERUBAHAN ENTALPI Isilah titik-titik pada tabel di bawah berdasarkan nama jenis kalor reaksinya. No 1 2 3 4 5

Reaksi C(s) + 2H2(g) → CH4(g) NO(g) → ½ N2(g) + ½ O2(g) CH4(g) + O2(g) → CO2(g) + 2H2O(l) HCl(g) + H2O(l) → HCl(aq) HCl(aq) + NaOH(aq) → NaCl(aq) + H2(l)

Nama Kalor Reaksi ............................................. ............................................. ............................................. ............................................. .............................................

LATIHAN 1. 1. Berikanlah pengertian setiap simbol berikut dan berikan masing-masing dua contoh! a. ∆ H 0f b. ∆ H 0d c. ∆ H 0c Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... 2. Diketahui; ½ N2(g) + ½ O2(g) → NO(g) ∆H = +90,3 kJ/mol. Tentukan; a. perubahan entalpi pembentukan 5 mol gas NO, b. perubahan entalpi penguraian 6 gram gas NO, dan c. perubahan entalpi pembakaran 14 gram gas N2. Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... .................. 3.

Tuliskan persamaan termokimia untuk setiap pernyataan berikut! a. Entalpi pembakaran gas propana sebesar -2219,6 kJ/mol. b. Entalpi pembentukan metana -75,4 kJ/mol. c. Entalpi penguraian H2SO4 +815 kJ/mol. Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... 4

C. PENENTUAN HARGA ∆H Harga ∆H ditentukan dengan menggunakan kalorimeter. Besarnya ∆H ditentukan dengan menggunakan rumus; Q = m.c.∆t Q c m ∆t

= = = =

besarnya kalor yang dibebaskan/diperlukan kalor jenis larutan massa zat suhu akhir - suhu awal = t2 - t1

Untuk lebih jelasnya, lakukan percobaan seperti berikut. 1. termometer termometer

2.

3. 4.

5.

(I)

(II)

50 ml NaOH 1 M tNaOH = .... 0C

50 ml HCl 1M tHCl = ... 0C

Hitung suhu awal dengan rumus; Jawab. t +t Suhu awal = NaOH HCl = ... 2 Campurkan larutan tabung I dan tabung II. Catat suhunya. Jawab. Suhu akhir = ....... Tentukan m dan ∆t! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... Hitung Q dengan c = 4,18 J/gram.0C! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... 5

6.

7.

8.

Hitung mol NaOH dan mol HCl yang bereaksi! Hitung pula mol H2O yang terbentuk! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... Berilah nama untuk jenis reaksinya! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... Hitung kalor yang dibebaskan pada penetralan 1 mol H2O! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... .....................................................................................................................................................

LATIHAN 2. 1. Campuran 0,2 gram C2H5OH dengan 250 gram air dalam kalorimeter menaikan suhu dari 0 0C menjadi 35 0C. Hitunglah perubahan entalpi reaksinya! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... 2.

Satu gram NaOH dilarutkan kedalam 50 ml aquades. Dicatat suhu mula-mulanya 25 0C dan suhu akhirnya 45 0C. Jika massa NaOH diabaikan, maka hitunglah ∆H pelarutan NaOH per mol! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... .....................................................................................................................................................

D. PENENTUAN HARGA ∆H BERDASARKAN HUKUM HESS Hukum Hess menyatakan bahwa besarnya ∆H reaksi tidak bergantung pada jalannya reaksi tetapi bergantung pada keadaan awal reaksi dan akhir reaksi. ∆Hr = ∆H1 + ∆H2 + ∆H3 + ... 6

Contoh. C(s) + ½ O2(g) → CO(g) CO(g) + ½ O2(g) → CO2(g)

∆H = -110,52 kJ/mol ...................... (1) ∆H = -283,00 kJ/mol ...................... (2)

Hasil penjumlahan (1) dan (2) diperoleh; C(s) + ½ O2(g) → CO(g) ∆H = -110,52 kJ/mol CO(g) + ½ O2(g) → CO2(g) ∆H = -283,00 kJ/mol ---------------------------------------------------------------- + C(s) + O2(g) → CO2(g) ∆H = 393,52 kJ/mol Proses di atas, juga dapat dinyatakan kedalam diagram berikut. C + ½ O2 C + ½ O2

∆H2 ∆H1

CO

atau

∆H1

∆H2 CO + ½ O2

+ ½ O2

CO2 ∆H3

∆H3

CO2 E. EKSPERIMEN HUKUM HESS I. termometer termometer

2 gram NaOH

50 ml H2O tawal = .... 0C

50 ml H2O takhir = ... 0C

Q

= m.c.∆t = ........... = ........... mol NaOH = ........................................................................................................................... ∆H1 =................................................................................................................................ Persamaan reaksi; ................................................................................................................... II. 7

(1)

(2)

50 ml NaOH 1 M tNaOH = .... 0C

50 ml HCl 1M tHCl = ... 0C

termometer termometer

(campuran (1) dan (2)) termometer

50 ml NaOH 1 M + 50 ml HCl 1 M takhir = ... 0C

t NaOH + t HCl = ................................. 2 takhir = ........................ Q = m.c.∆t = ........... = ........... mol HCl = ........................................................................................................................... mol NaOH = ........................................................................................................................... ∆H2 =................................................................................................................................. Persamaan reaksi; ...................................................................................................................

tawal =

III. termometer termometer

2 gram NaOH

100 ml HCl 0,5 M tawal = .... 0C Q

=

100 ml HCl 0,5 M takhir = ... 0C

m.c.∆t 8

= ........... = ........... mol HCl = ........................................................................................................................... mol NaOH = ........................................................................................................................... ∆H3 =................................................................................................................................ Persamaan reaksi; ................................................................................................................... Buatlah diagram energi dari percobaan (I), (II), dan (III)! Tentukan pula kesimpulannya! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... . Dari hukum Hess, disimpulkan bahwa; DH r =

å DH

0 f hasil reaksi

- å DH 0f pereaksi

Untuk molekul diatomik (N2, O2, dsb), DH 0f = 0 kJ/mol. Untuk unsur bebas (Al, Fe, Zn, Cu, dsb), DH 0f = 0 kJ/mol. CONTOH SOAL. Diketahui; DH 0f CS2 = +88 kJ/mol DH 0f CO2 = -394 kJ/mol DH 0f SO2 = -297 kJ/mol Hitunglah ∆H reaksi ; CS2(s) + 2O2(g) → CO2(g) + 2SO2(g)! Jawab. DH r = å DH 0f hasil reaksi - å DH 0f pereaksi = (DH 0f CO 2 + 2 DH 0f SO 2 ) - (DH 0f CS2 + 2 ΔH 0f O 2 ) = (-394 + 2(-297)) - (88 - 2.0) = - 988 - 88 = - 1076 kJ/mol. LATIHAN 3. 1. ∆H pembakaran glukosa dan ∆H pembakaran etanol masing-masing -282 kJ/mol dan -1380 kJ/mol. Hitunglah ∆H reaksi; C6H12O6 → 2C2H5OH + 2CO2! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... 9

..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... 2.

Jika diketahui ∆Hf SO2 = -297 kJ/mol dan ∆Hc SO2 = -99 kJ/mol, maka hitung ∆H reaksi; 2S(s) + 3O2(g) → 2SO3 ! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... .....................................................................................................................................................

DH 0f H2O(l) = -285,5 kJ/mol DH 0f CaO(g) = -634,5 kJ/mol CaO(g) + H2O(l) → Ca(OH)2(s) ∆H = -64 kJ/mol Hitunglah DH 0f Ca(OH)2(s) ! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... F. PENENTUAN ∆H REAKSI DENGAN MENGGUNAKAN ENERGI IKATAN (EI) Energi ikatan adalah energi yang diperlukan untuk memutuskan 1 mol ikatan dalam senyawa menjadi atom-atom dalam keadaan gas. Energi ikatan dapat digunakan untuk menentukan ∆H reaksi kimia. Rumus yang digunakan adalah; 3.

Diketahui;

DH r = å EIpereaksi - å EIhasil reaksi

CONTOH SOAL. Diketahui; EIH-H = 436 kJ/mol EIBr-Br = 193,9 kJ/mol EIH-Br = 366 kJ/mol Hitunglah ∆H reaksi; ½ H2(g) + ½ Br(g) → HBr(g)! Jawab. DH r = å EIpereaksi - å EIhasil reaksi = (½EIH-H + ½EIBr-Br) - EIH-Br = (½.436 + ½.193,9) - 366 = 314,8 - 366 = -51,2 kJ/mol. 10

LATIHAN 4. 1. Diketahui;

Energi ikatan C = C = 607 kJ/mol Energi ikatan C - C = 348 kJ/mol Energi ikatan C - H = 413 kJ/mol Energi ikatan H - H = 436 kJ/mol Hitung kalor yang dibebaskan pada reaksi 7 gram gas etena dengan gas hidrogen! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... .....................................................................................................................................................

2.

Pembakaran 1 mol gas metana dibebaskan energi sebesar 820 kJ. Diketahui; Energi ikatan O = O = 498 kJ/mol Energi ikatan C = O = 749 kJ/mol Energi ikatan C - H = 381 kJ/mol Hitung energi ikatan H - O! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... .....................................................................................................................................................

3.

Diketahui;

Energi ikatan H - Cl = 431 kJ/mol Energi ikatan Cl - Cl = 242 kJ/mol Energi ikatan H - H = 436 kJ/mol Hitung kalor yang diperlukan untuk menguraikan 14,6 gram HCl! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... 11

G. BAHAN BAKAR Reaksi pembakaran pada setiap pembakaran bahan bakar selalu menghasilkan kalor. Nilai kalor adalah jumlah kalor yang dibebaskan dari setiap pembakaran bahan bakar. Berikut beberapa nilai kalor bahan bakar. Nilai Kalor (kkal/gram) 11 2,5 - 4,5 4 9 10

Bahan Bakar Bensin Kayu Karbohidrat Lemak Minyak tanah

Energi yang dibebaskan pada pembakaran sempurna jauh lebih besar dibandingkan energi yang dibebaskan pada pembakaran tidak sempurna. Bahan bakar + O2 → CO2(g) + H2O(g) + E1 ............ (pembakaran sempurna) Bahan bakar + O2 → CO(g) + H2O(g) + E2 ............ (pembakaran tidak sempurna) E1 > E2 LATIHAN 5. 1. Berikan contoh entalpi pembakaran dalam kehidupan sehari-hari! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... 2.

Sebutkan beberapa faktor yang berpengaruh terjadinya pembakaran! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... .....................................................................................................................................................

3.

Berikan 5 contoh bahan bakar! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... 12

4.

Sebutkan kerugian-kerugian yang diakibatkan pembakaran tidak sempurna pada kendaraan bermotor! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... .....................................................................................................................................................

5.

Tuliskan reaksi pembuatan glukosa pada tumbuhan! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... .....................................................................................................................................................

6.

DH 0f C6H12O6 = 1256 kJ/mol DH 0f CO2 = -394 kJ/mol DH 0f H2O = 1256 kJ/mol Mr C6H12O6 = 180. Hitung energi yang dihasilkan pada pemkaran 18 gram glukosa dalam tubuh manusia! Jawab. ......................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... ..................................................................................................................................................... Diketahui;

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