CHAPTER 6 - Thermochemistry
Prof. Kweyete
1. Calculate the total change in internal energy, E, of a
system when 400 J heat is applied to expanding O2 (g) and
the gas does 350 J of work on its surroundings.
ΔE = q + w = 400J – 350J = 50J
2. Calculate the kinetic energy in J, calories, and kJ of a
particle (mass = 9.11x10-28 g) at 6.00x105 m/s.
E = (1/2)mv2 = (1/2)(9.11 x 10-31kg)(6.0 x 105 m/s)2 = 1.64 x 10-19J
= (1.64 x 10-19J) ÷ 4.184J/cal = 3.93 x 10-20 calories
= 1.64 x 10-22kJ
3. Given the following data:
a) 1/2 Br2 (l)  Br (g)
H0 = 111.75 kJ
b) 1/2 Cl2 (g)  Cl (g)
H0 = 121.38 kJ
c) 1/2 Br2 (l) + 1/2 Cl2 (g)  BrCl (g) H0 = 14.7 kJ
determine H0rx for reaction; Br (g) + Cl (g)  BrCl (g)
Reverse a,=> ΔHº = -111.75 kJ
Reverse b, => ΔHº = -121.38 kJ
Add to c. => ΔHº = 14.7 kJ
ΔH = -218.4 kJ
4. Calculate H0 in kJ for reaction;
CuO(s)+ CO (g)  Cu(s) + CO2 (g)
given the following thermochemical data :
(a) 2CO(g) + O2 (g)  2CO2(g)
H0 = - 566.1 kJ
(b) 2Cu(s) + O2 (g)  2CuO(s)
H0 = - 310.5 kJ
Multiply (a) by ½ => ΔHº = -566.1 kJ/2
= -283.05 kJ
Reverse (b) & multiply by ½ => ΔHº = 310.5 kJ/2 = 155.25 kJ
ΔHº
= -128 kJ
5. Consider the following reactions (a) & (b) :
Calculate Hf0 for H2O2 (l) ; H2 (g) + O2 (g)  H2O2 (l)
a) H2O2 (l)  H2O (l) + 1/2 O2 (l) H0 = -98.3 kJ
b) H2 (g) + 1/2 O2 (g)  H2O (l)
H0 = -285.8 kJ
Reverse (a) => ΔHº = 98.3 kJ
Add to (b) => ΔHº = -285.8 kJ
ΔHº = -187.5 kJ
6. A 1.0 g sample of propane, C3H8, was burned in calorimeter.
The temperature rose from 28.5 0C to 32.0 0C and heat of
combustion 10.5 kJ/g. Calculate the heat capacity of the
calorimeter apparatus in kJ/0C .
Equation for bomb calorimeter: ΔH = CbombΔT
10.5 kJ/g = Cbomb(32.0 ºC – 28.5 ºC)
Cbomb = 3.00 kJ/ºC
7. What is the resulting temperature when 35.0 g of water at 75
0
C is mixed with 15.0 g of water at 10 0C? (Heat capacity (Cp) of
water = 4.184 J/g. 0C)
Equation:
m1CpΔT1 = m2CpΔT2
(35g)(Cp)(75-Tf) = (15g)(Cp)(Tf – 10)
Tf = 55.5 ºC
8. A 20.0 g piece of a metal with specific heat of 0.080 cal/g.0C
at 68 0C dropped into 15.0 ml water in a calorimeter at 23 0C.
Calculate the final equilibrium temperature of the mixture.
Equation: m1CmetalΔT1 = m2CpΔT2
(20g)(0.08cal/g)(68 -Tf) = (15g)(1.0 cal/gºC)(Tf – 23)
Tf = 27.34 ºC
9. When 72 g of a metal at 97.00C is added to 100 g of water at
25.0 0C, the final temperature is 45.5 0C. What is the heat capacity
per grams of the metal? (Heat capacity of H2O = 4.184 J/g. 0C)
Equation: m1CmetalΔT1 = m2CpΔT2
(72g)(Cmetal)(97 – 45.5) = (100g)(4.184J/gºC)(45.5 – 25)
Cmetal = 2.31 J/gºC
10. The heat capacity of lead is 0.13 J/g.0C. How many joules of
heat would be required to raise the temperature of 15.0 g of lead
from 22 0C to 38 0C?
Equation : ΔH = mCmetalΔT
= (15g)(0.13J/gºC)(38 – 22)
= 31.2 J