CHAPTER 2
THERMOCHEMISTRY
TOPYCS
A.
B.
C.
D.
E.
F.
G.
H.
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
1. Definition
 What is definition of thermochemical
equation?
A thermochemical equation is an equivalent
chemical equation with a value of the change in
enthalpy.
• Examples:
Write down the termochemical equation :
1. In complete combustion for ethane gas
the heat of 572 kJ is released.
2. The change in enthalpy process of ice to
become the water the heat of 14 kJ is
required.
E. THE CHANGE IN STANDARD
ENTHALPY
1. 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.
5. In dissociation reaction 56 grams
NH4F(s) the heat of 185,6 kJ is required.
(Ar N=14, H=1, F=19).
1.
2.
3.
4.
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 :
1. 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!
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 :
1. Question number 6 page 127.
2. 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.