Chemistry - Calculating Standard Enthalpy Changes Using Standard Heats of Formation

Reference Table: Standard Heats of Formation (in kJ/mol)

C3H8(g): -103.8

CO2(g): -393.5

H2O(l): -285.8

H2O(g): -241.8

CH4(g): -74.8

NH3(g): -46.1

NO(g): +90.2

Fe2O3(s): -824.2

Al2O3(s): -1675.7

CaO(s): -635.1

CaCO3(s): -1206.9

C2H5OH(l): -277.7

HCl(g): -92.3

Note: Elements in their standard state (such as O2, Fe, and H2) have a heat of formation of 0 kJ/mol.

Formula: Enthalpy of reaction = (Sum of heats of formation of products) - (Sum of heats of formation of reactants) 

Practice Problems

1. Propane Combustion: Calculate the enthalpy change for the combustion of 1 mole of propane gas:

   C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(l)

2. Methane Combustion: Calculate the enthalpy change for the combustion of methane:

   CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)

3. Thermite Reaction: Determine the heat released when aluminum reacts with iron(III) oxide:

   2Al(s) + Fe2O3(s) -> Al2O3(s) + 2Fe(s)

4. Ammonia Oxidation: Calculate the enthalpy change for the following reaction:

   4NH3(g) + 5O2(g) -> 4NO(g) + 6H2O(g)

5. Decomposition of Limestone: How much heat is required (or released) during the decomposition of calcium carbonate?

   CaCO3(s) -> CaO(s) + CO2(g)

6. Ethanol Combustion: Calculate the enthalpy change for the combustion of liquid ethanol:

   C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l)

7. Synthesis of Hydrogen Chloride: Calculate the enthalpy change for:

   H2(g) + Cl2(g) -> 2HCl(g)

8. Rusting of Iron: Calculate the enthalpy change for the formation of rust:

   4Fe(s) + 3O2(g) -> 2Fe2O3(s)

9. Formation of Water: Using the table, what is the enthalpy change for the reaction of hydrogen and oxygen gas to produce liquid water?

   2H2(g) + O2(g) -> 2H2O(l)

10. Reverse Reaction: If the enthalpy change for a reaction (A + B -> C) is -150 kJ, what is the enthalpy change for the reverse reaction (C -> A + B)?

Answer Key

1. -2219.9 kJ

2. -802.3 kJ

3. -851.5 kJ

4. -905.2 kJ

5. +178.3 kJ

6. -1366.7 kJ

7. -184.6 kJ

8. -1648.4 kJ

9. -571.6 kJ

10. +150 kJ