Chapter 5: Thermochemistry

Homework Assignment

All work and calculations must be shown

Question 1:

Provide a one sentence response to the following questions:

a. According to the first law of thermodynamics, what quantity is conserved?

b. What is meant by the internal energy of a system?

c. By what means can the internal energy of a closed system increase?

Question 2:

For the following processes, calculate the change in internal energy of the system and determine whether or not the process is exothermic or endothermic:

a. A balloon is cooled by removing 0.655 kJ of heat. Upon cooling the balloon shrinks, and the atmosphere does 382 J of work on the balloon.

b. A 100.0 g bar of gold is heated from 25 °C to 50 °C during which it absorbs 322 J of heat. Assume the volume of the gold bar remains constant.

Question 3:

How much work (in J) is involved in a chemical reaction if the volume decreases from 5.00 L to 1.26 L against a constant pressure of 0.857 atm?

Question 4:

Refer to the following balanced chemical equation:

a. Is the following reaction endothermic or exothermic?

b. Calculate the amount of heat transferred when 24.0 g of CH3OH (g) is decomposed by this reaction at constant pressure.

c. For a given sample of CH3OH, the enthalpy change during the reaction is 82.1 kJ. How many grams of CH4 (g) are produced?

d. How many kj of heat are released when 38.5 g of CH4 (g) reacts completely with O2(g) to form CH3OH(g) at constant pressure?

Question 5:

Refer to the following balanced chemical equation:

For this reaction calculate the H for the formation of:

a. Calculate the H for the formation of 1.36 mol of O2 (g)

b. Calculate the H for the formation of 10.4 g of KCl (Molar Mass of KCl = 74.55 g/mol)

Question 6:

Two solid objects, object A and object B, are placed in boiling water and allowed to come to the temperature of the boiling water. Each is then lifted out and placed in separate beakers containing 1000. g of water at 10.0 °C. Object A which weighs 10.0 g increased the water temperature by 3.50 °C; object B which weights 10.0 g increased the water temperature by 2.60 °C.

a. Without doing any calculations which object A or B has the larger specific heat? Justify your answer

b. Calculate the specific heat of object A.

c. Calculate the specific heat of object B.

d. Do the results form b and c support your claim from part A?

Question 7:

Refer to the balanced chemical equation:

a. When a 4.25 g sample of solid ammonium nitrate dissolves in 60.0 g of water in a calorimeter, the temperature drops from 22.0 °C to 16.9 °C. Calculate H in kJ/mol NH4NO3) for the solution process. Assume that the specific heat of the solution is the same as that of pure water.

b. Is this process endothermic or exothermic?

Question 8:

From the enthalpies of reaction:

Use Hess’s law to calculate H for the following reaction:

Question 9:

From the enthalpies of reaction:

Use Hess’s law to calculate H for the following reaction:

Question 10:

Using values from Appendix C, calculate the H° for the following reactions:

a. CaO (
s) + 2 HCl (
g) CaCl2 (
s) + H2O (
g)

b. 4 FeO (
s) + O2 (
g) 2 Fe2O3 (
s)