Thermodynamics 1.2.2 Heat of Formation.

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Presentation transcript:

Thermodynamics 1.2.2 Heat of Formation

2.2 Heat of Formation Standard Heat of Formation ΔHof the amount of energy gained or lost when 1 mole of the substance is formed from its elements under standard conditions (25°C, 1 atm = 101.3 kPa)

Standard Heat of Formation Ex. the formation reaction for liquid water is described by the following equation: H2 (g) + ½O2 (g) → H2O(l) + 285.8 kJ The standard heat of formation is: 285.8 kJ. Since the reaction is exothermic: ΔHof -285.8 kJ.

A heat of formation is a type of reaction where one mole of the compound forms from its elements The heat of formation for pure elements, such as H2(g), O2(g), Al(s), etc. is 0 kJ/mole. You'll find it useful to remember this.

Writing Heat of Formation Reactions Keep the following points in mind: Balance the equation so that one mole of the compound is produced. Remember the diatomic (7) molecules and write them correctly (H2, N2, O2, F2, Cl2, Br2, I2). The reactants must be elements, not polyatomic ions. Examples of polyatomic ions are hydroxide, OH-, carbonate, CO32-, and ammonium, NH4+.

Review H2(g) + ½O2(g) → H2O(l) + 285 kJ. If 285.8 kJ of energy are released during the formation of one mole of H2O(l), how much energy do you imagine would be released if two moles of water were produced?

Practice problems 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 Δ H. You will need to refer to a Table of Thermochemical Data. CO2 (g), CuCl2 (g), CuCl (g), N2H4 (l), NH4Cl (s).

Example The standard heat of formation, ΔHof, 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?

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

Example 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.