Heat of Formation

Heat of Formation Heat of formation is a special type of synthesis reaction, Heat of Formation one in which we form one mole of a compound from its ΔHf elements. We are interested in how much energy is the amount of energy either absorbed or released during the reaction. ______or ______when 1 mole of the substance is Example: ______from its ______The formation reaction for liquid water is described by the following equation:

*Key items to note regarding this definition: The amount of energy released during this reaction, . one mole of the compound is 285.8 kJ, is referred to as the heat of formation and it formed . from its elements can be written as ΔHf, = -285.8 kJ.

You will find the heats of formation listed for a variety of compounds in the Table of Thermochemical Data.

The heat of formation for ______, such as H2(g), O2(g), Al(s), etc. is 0 kJ/mole.

Writing heat of formation reactions is important, please keep the following points in mind: * Balance the equation so that one mole of the compound is produced.

* Remember the diatomic molecules and write them correctly (H2, N2, O2, F2, Cl2, Br2, I2). - 2- + * The reactants must be elements, not polyatomic ions. (OH , CO3 , NH4 , etc.)

Comparison of similar reactions. Which one is a heat of formation reaction?

4 C(s) + 6 H2(l) + 1 O2(g)  2 C2H5OH(l) ΔH = -456 kJ/mol

2 C(s) + 3 H2(l) + ½ O2(g)  C2H5OH(l) ΔH = -228 kJ/mol

C2H5OH(l) 2 C(s) + 3 H2(l) + ½ O2(g) ΔH = 228 kJ/mol

PRACTICE 1) Write the heat of formation for each of the following, include the energy term as part of the reaction. a) C3H8(g)

b) NH4NO3 (s)

2) How much energy is produced when 6.7 kg of potassium chloride are formed from its elements?

3) What mass of sodium carbonate is required to release 56,000 kJ of energy when it forms from its elements?

4) Given the below reaction, what is the heat of formation for S2O5(g)?

4 S(g) + 5 O2(g) ---- 2 S2O5(g) + 600kJ

IN CLASS ASSIGNMENT: Heat of Formation 1. Write heats of formation reactions for each of the following compounds. Be sure to include the energy term with the equation, either as part of the equation or separately as ΔH. You will need to refer to a Table of Thermochemical Data. a) CO2 (g)

b) CuCl2 (g)

c) CuCl (g)

d) N2H4 (l)

e) NH4Cl (s)

o 2. The standard heat of formation, ΔH f, for sulfur dioxide (SO2) is -297 kJ/mol. How many kJ of energy are given

off when 25.0 g of SO2 (g) is produced from its elements?

3. The heat of reaction for the combustion of 1 mol of ethyl alcohol is -9.50 × 102 kJ: 2 C2H5OH (l) + 3 O2 (g) → 2 CO2 (g) + 3 H2O (l) + 9.5 × 10 kJ How much heat is produced when 11.5 g of alcohol is burned?

4. ΔH for the complete combustion of 1 mol of propane is -2.22 × 103 kJ:

C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (l) Calculate the heat of reaction for the combustion of 33.0 g of propane.