Chemistry 2014
Optional Thermochemistry Practice Problems
1. One of the components of polluted air is NO (g). It is formed in the high-temperature
environment of internal combustion engines when oxygen gas and nitrogen gas react to
form nitrogen monoxide. This reaction absorbs 180 Kj of heat. Write a complete
thermochemical equation. (Don’t forget your diatomics!)
A. N2 (g) + O2 (g) -----------> 2NO (g) ∆H = +180 kJ [Note! Thermochemical equations
include the state of matter for each reactant and product and the ∆H
B. Is this reaction endothermic or exothermic? Endothermic. It is absorbing heat.
2. Are the following processes exothermic or endothermic?
A. When solid KBr is dissolved in water, the solution gets colder. Endothermic (feels cold)
B. Natural gas (CH4) is burned in a furnace. Exothermic (feels hot—releasing heat)
C. When concentrated H2SO4 is added to water, the solution gets very hot. Exothermic
(feels hot—releasing heat)
D. Water is boiled in a tea kettle. Endothermic (heat going into systems)
3. The overall reaction in commercial heat packs can be represented as:
4Fe (s) + 3O2 (g) ------> 2Fe2O3 (s) ∆H = -1652 kJ
A.
How much heat is released when 4.00 mol of iron is reacted with excess oxygen?
There are 4 moles of iron for every 1,652 KJ released
4Fe (s) + 3O2 (g) ------> 2Fe2O3 (s) ∆H = -1652 kJ
B.
How much heat is released when 1.00 mol Fe2O3 is produced?
4Fe (s) + 3O2 (g) ------> 2Fe2O3 (s) ∆H = -1652 kJ
1 mole Fe2O3 X
C.
__1,652 KJ = -826 kJ
2 moles Fe2O3
How much heat is released when 1.00 gram of Fe is reacted with excess oxygen?
4Fe (s) + 3O2 (g) ------> 2Fe2O3 (s) ∆H = -1652 kJ
1 gram Fe X 1 mole Fe = .018 moles Fe
56 grams
26
O’Ryan
56
Fe
.018 moles Fe X
-1652 kJ = -7.43 kJ
4 Moles Fe