CHAPTER 2 THERMOCHEMISTRY

TOPYCS Definition of Thermochemistry and Enthalpy System and Environment Exothermyc and Endothermyc Reaction Graph of Exothermyc and Endothermyc Reaction Thermochemical Equation Qalorimeter The Hess Law Bonding of Energy

D. THERMOCHEMICAL EQUATION Definition What is definition of thermochemical equation?

Write down the termochemical equation : A thermochemical equation is an equivalent chemical equation with a value of the change in enthalpy. Examples: Write down the termochemical equation : In complete combustion for ethane gas the heat of 572 kJ is released. The change in enthalpy process of ice to become the water the heat of 14 kJ is required.

E. THE CHANGE IN STANDARD ENTHALPY Definition ♪ What is definition the change in standard enthalpy? - The change in standard enthalpy is the change in enthalpy is a chemical reaction measured at standard state (25 oC, 1 atm). It is symbolized by ΔHo.

♪ The kinds of the change in enthalpy are : 1. ΔH of : is the change in enthalpy of formation reaction for 1 mole compound from is elements. 2. ΔH od : is the change in enthalpy of dissociation reaction for 1 mole compound into its elements. 3. ΔH oc : is the change in enthalpy of combustion reaction for 1 mole compound.

Examples : Write down the thermochemical equation for the following statements : ΔH of NH4Cl(s) = -314,4 kJ/mole ΔH od NH3(g) = +45,9 kJ/mole ΔH oc CH3OH(l) = -638 kJ/mole In formation reaction 4,48 L SO3 gas the heat of 79,2 kJ is released. In dissociation reaction 56 grams NH4F(s) the heat of 185,6 kJ is required. (Ar N=14, H=1, F=19).

7. ΔH of CaCO3(s) = -207,8 kJ/mole 6. In complete combustion reaction 11,4 grams of octhane gas the heat of 547,1 kJ is released. (Ar C=12, H=1). 7. ΔH of CaCO3(s) = -207,8 kJ/mole 8. ΔH oc C2H5OH(l) = -1.380 kJ/mole.

F. CALORIMETER ♪ What is calorimeter? A calorimeter is an apparatus used to measure the change of the heat of a reaction. Heat capacity (C) : is the amount of heat needed to raise the temperature of a substance by 1 degree celcius. Specific heat (c) : is the amount of heat needed to raise the temperature of 1 gr of a substance by 1 degree celcius.

The equation for determine : Heat capacity : C = m . c Heat : q = C . Δt or q = m . c . Δt Where : C = heat capacity (J / oC) m = substance/solution mass (g) c = specific heat (J/g oC) q = heat (J) Δt = the change in temperature (oC or oK).

Δt = t2 – t1 where : t1 = t initial and t2 = t final For water, c = 1 J/g oC. Ezamples : Into a calorimeter fill in 250 grams of water at temperature 27 degree celcius. When the water is stirred, the temperature rise become to 35 degree celcius. a. Determine, what is heat of the water! b. Determine, what is the change enthalpy!

2. In a calorimeter is reacted 50 mL of HCl 0,1 M solution and 50 mL of NaOH 0,1 M solution. In the reaction the rise in temperature of 8 degree celcius occurs. If the calorimeter does not absorb heat and the specific heat of the solution is considered to be equal to that for water, that is c = 4,18 J/g oC, calculate the change in enthalpy for the following reaction: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) (Consider the density of solution is equal to 1 g/mL.

3. Untuk menaikkan suhu 100 gr air sebesa x oC, diperlukan energi 21 kJ. Tentukan nilai x (c air = 4,2 J/g oK). 4. Reaksi 100 mL larutan HCl 0,2 M dan 100 mL larutan NaOH 0,1 M, menyebabkan kenaikan suhu dari 27 oC menjadi 36,5 oC. Jika larutan di anggap sama dengan air, kalor jenis air = 4,2 J/g oC, massa jenis air = 1 g/mL, tentukan : kalor reaksi dan perubahan entalpi reaksi : HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l).

G. CALCULATING THE CHANGE IN ENTHALPY BASED ON THE ΔHof Formula : ΔHreaction = ΣΔHof products – ΣΔHof reactants Notes : for molecules F2, Cl2, Br2, I2, H2, O2, N2 the value of ΔHof = 0 (nol). Examples : Given that : ΔHof C3H8(g) = -24 kJ/mole ΔHof CO2(g) = -394 kJ/mole ΔHof H2O(l) = -286 kJ/mole Calculate the change in enthalpy for combustion reaction of propane!

and -286 kJ/mole. a. Determine ΔHoc C2H6(g). Exercise : 1. Given that formation heat of C2H6(g), CO2(g), and H2O(l) respectively -85 kJ/mole, -394 kJ/mole, and -286 kJ/mole. a. Determine ΔHoc C2H6(g). b. Calculate the heat of combustion reaction for 180 grams C2H6(g). 2. Please do questions number 2 page 149.

H. CALCULATING THE CHANGE IN ENTHALPY USING THE HESS’ LAW ♪ Explain what is the Hess’ Law! Examples : Question number 6 page 127. Question number 7 page 127. Exercise : Please do question number 20 page 146.

ENTHALPY BY DIAGRAM Examples : C(s) + 2H2(g) + O2(g) ΔH1 ΔH3 CH4(g) + 2O2(g) ΔH2 CO2(g) + 2H2O(l) Hubungan yang benar antara ΔH1, ΔH2, dan ΔH3 pada diagram entalpi tersebut adalah ….

I. MENGHITUNG ΔHr DARI DATA ENERGI IKATAN Rumus : ΣEireaktan (kiri) – ΣEiproduk(kanan) Keterangan : Ei = Energi ikatan rata-rata Examples : Tentukan entalpi reaksi pembakaran 15 gram etana, jika diketahui data energi ikatan rata-rata : C-C = 348 kJ/mol C-H = 413 kJ/mol O=O = 495 kJ/mol C=O = 799 kJ/mol O-H = 463 kJ/mol

Post test : 1. Tentukan kalor yg dibebaskan pada pembakaran 15 gram C3H7OH jika diketahui Ar C=12, H=1, O=16, dan energi ikatan (kJ/mol) : C-H = 413 O-H = 463 C-C = 348 C=O = 799 C-O = 358 O=O = 495. 2. Diketahui reaksi : C2H4 + X2 → C2H4X2 ; ΔH = -178 kJ Jika energi ikatan (kJ/mol) : C=C = 614 C-C = 348 C-H = 413 X-X = 186 Tentukan energi ikatan C-X.