International Baccalaureate Chemistry. Topic 5 - Energetics. Thermochemistry. *. Thermochemistry is the study of energy changes associated with chemical reactions. Studies the amount of energy in a chemical reaction Energy is measured in Joules (J)
Topic 5 - Energetics
Energy an object possesses by virtue of its position or chemical composition.
KE = mv2
Energy an object possesses by virtue of its motion.
This quantity, H, is called the enthalpy of reaction, or the heat of reaction.
The change in enthalpy, H, is the enthalpy of the products minus the enthalpy of the reactants:
H = Hproducts−Hreactants
Since we cannot know the exact enthalpy of the reactants and products, we measure H through calorimetry, the measurement of heat flow.
Specific heat =
mass temperature change
m THeat Capacity and Specific Heat
Specific heat, then, is
q = mC T
By carrying out a reaction in aqueous solution in a simple calorimeter such as this one, one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter.
Because the specific heat for water is well known (4.184 J/g K), we can measure H for the reaction with this equation:
q = mC T
Solution:Dq(Cold) = -Dq (hot) mCDT= -mCDT
Let T = final temperature
(50 g) (4.184 J g-1oC-1)(T- 20oC) = -(80 g) (4.184 J g-1oC-1)(T-60oC)
(50 g)(T- 20oC) = -(80 g)(T-60oC)
50T -1000 = – 80T + 4800
130T = 5800
T = 44.6 oC
Reactions can be carried out in a sealed “bomb,” such as this one, and measure the heat absorbed by the water.
Use Fig. 5.5
Hess’s law states that “If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.”
Because H is a state function, the total enthalpy change depends only on the initial state of the reactants and the final state of the products.
Standard enthalpies of formation, Hθf, are measured under standard conditions (25°C and 1.00 atm pressure).
C(s) + O2(g)→ CO2(g) -393.5
CO(g) + ½O2(g) → CO2(g -283
C(s)+ ½O2(g) → CO(g) ????
2C(s) + H2(g) → C2H2(g)
C2H2(g) +5/2O2(g) → 2CO2(g) + H2O(l) -1299.6
C(s) + O2(g) → CO2(g) -393.5
H2(g) + ½O2(g) → H2O(l) -285.9
NO(g) + O3(g) → NO2(g) + O2(g) -198.9
O3(g) → 3/2 O2(g) -142.3
O2(g) → 2O(g) +495.0
NO(g) + O(g) → NO2(g)
2B(s) + 3H2(g) → B2H6(g)
2B(s) + 3/2O2(g) → B2O3 -1273
B2H6(g) + 3O2(g) → B2O3(s) + 3H2O(g) -2035
H2(g) + ½O2(g) → H2O(l) -286
H2O(l) → H2O(g) +44
HCl(g) → H(g) + Cl(g)
note: Cl2 and H2 are not formed.
Bonds Broken (kJ mol-1)Bonds Formed(kJ mol-1)
C=C 612 C-C 348
4(C-H) 4(412) 6(C-H) 6(412)
Total: 2696 kJ 2820 kJ
ΔH = 2696 - 2820 = -125 kJ mol-1
C2H6(g) + 7/2 O2(g) → 2CO2(g) + 3H2O(g)
Bonds Broken (kJ mol-1)Bonds Formed (kJ mol-1)
6 C-H 6(413) = 2478 4 C=O 4(745) = 2980
7/2 O=O 7/2(495) = 1732.5 6 O-H 6(467) = 2802
1 C-C 1(347) = 347
Total = 4557.5 Total = 5782
ΔH = 4557.5 – 5782 = -1224.5 kJ mol-1