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1.2.2 Heat of Formation
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Standard Heat of Formation Δ H o f 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)
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Ex. the formation reaction for liquid water is described by the following equation: H 2 (g) + ½O 2 (g) → H 2 O(l) + 285.8 kJ The standard heat of formation is: 285.8 kJ. Since the reaction is exothermic: Δ H o f -285.8 kJ.
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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 H 2 (g), O 2 (g), Al(s), etc. is 0 kJ/mole. You'll find it useful to remember this.
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Look over the Heat of formation table See values in outline
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Keep the following points in mind: 1. Balance the equation so that one mole of the compound is produced. 2. Remember the diatomic (7) molecules and write them correctly (H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 ). 3. The reactants must be elements, not polyatomic ions. Examples of polyatomic ions are hydroxide, OH -, carbonate, CO 3 2-, and ammonium, NH 4 +.
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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?
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If you predicted 571.6 kJ of energy you're right! Our formation reaction tells us that 285.8 kJ of energy are released for every one mole of H 2 O. This can be written mathematically as: 285.8 kJ/mol H 2 O×2 mol H 2 O =571.6 kJ Our new equation looks like this: 2 H2(g) + O2(g) → 2 H2O(l) + 571.6 kJ
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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. CO 2 (g), CuCl 2 (g), CuCl (g), N 2 H 4 (l), NH 4 Cl (s).
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C (s) + O 2 (g) → CO 2 (g) + 393.5 kJ orC (s) + O 2 (g) → CO 2 (g) Δ H = -393.5 kJ Cu (s) + Cl 2 (g) → CuCl 2 (s) + 220.1 kJ or Cu (s) + Cl 2 (g) → CuCl 2 (s) Δ H = -220.1 kJ Cu (s) + ½ Cl 2 (g) → CuCl 2 (s) + 137.2kJ or Cu (s) + ½ Cl 2 (g) → CuCl 2 (s) Δ H = -137.2 kJ N 2 (g) + 2H 2 (g) + 50.6 kJ → N 2 H 4 (l) or N 2 (g) + 2H 2 (g) → N 2 H 4 (l) Δ H = +50.6 kJ ½N 2 (g) + 2H 2 (g) + ½Cl 2 (g) → NH 4 Cl (s)+ 314.4 kJ or ½N 2 (g) + 2H 2 (g) + ½Cl 2 (g) → NH 4 Cl (s) Δ H = -314.4 kJ
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The standard heat of formation, Δ H o f, for sulfur dioxide (SO 2 ) is -297 kJ/mol. How many kJ of energy are given off when 25.0 g of SO 2 (g) is produced from its elements?
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Step 1: Calculate moles SO2 a. Find the molar mass of SO2 = 32.1 + 2(16.0) = 64.1 g/mol b. moles SO2 = Step 2: Determine kJ for 0.390 mol We know from the question that 297 kJ of energy is released for 1 mole of SO 2 Determine how much energy will be released for 0.390 mol of SO 2 :
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The heat of reaction for the combustion of 1 mol of ethyl alcohol is -9.50 × 10 2 kJ: C 2 H 5 OH (l) + 3 O 2 (g) → 2 CO 2 (g) + 3 H 2 O (l) + 9.5 × 10 2 kJ How much heat is produced when 11.5 g of alcohol is burned?
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First determine how many moles of ethyl alcohol are being combusted. You need to begin by finding the molar mass of C 2 H 5 OH, which is 46.0 g/mol. moles C 2 H 5 OH = From our balanced equation we see that 9.50 × 10 2 kJ of energy are released for every 1 mole of C 2 H 5 OH. Now we determine how much energy will be released for 0.250 mol:
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Δ H for the complete combustion of 1 mol of propane is -2.22 × 10 3 kJ: C 3 H 8 (g) + 5 O 2 (g) → 3 CO 2 (g) + 4 H 2 O (l) Calculate the heat of reaction for the combustion of 33.0 g of propane.
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4. determine moles of propane actually used. The molar mass of C 3 H 8 is: moles C 3 H 8 = From our balanced equation we see that 2.22 × 10 3 kJ of energy are released for every 1 mole of C 3 H 8. Now determine how much energy will be released for 0.750 mol:
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Assignment- practice problems
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