Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved Thermochemical Equations Thermochemical Equations

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations In the equation describing the exothermic reaction of calcium oxide and water, the enthalpy change can be considered a product. CaO(s) + H 2 O(l) → Ca(OH) 2 (s) kJ Calcium oxide is one of the components of cement.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations CaO(s) + H 2 O(l) → Ca(OH) 2 (s) kJ A chemical equation that includes the enthalpy change is called a thermochemical equation.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations The heat of reaction is the enthalpy change for the chemical equation exactly as it is written. Heats of reaction are reported as ΔH. The physical state of the reactants and products must also be given. The standard conditions are that the reaction is carried out at kPa (1 atm) and 25°C. Heats of Reaction

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations CaO(s) + H 2 O(l) → Ca(OH) 2 (s) Each mole of calcium oxide and water that reacts to form calcium hydroxide produces 65.2 kJ of heat. ΔH = –65.2 kJ In exothermic processes, the chemical potential energy of the reactants is higher than the chemical potential energy of the products. Heats of Reaction

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations 2NaHCO 3 (s) + 85 kJ → Na 2 CO 3 (s) + H 2 O(l) + CO 2 (g) Baking soda (sodium bicarbonate) decomposes when it is heated. This process is endothermic. Heats of Reaction The carbon dioxide released in the reaction causes muffins to rise while baking.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations 2NaHCO 3 (s) + 85 kJ → Na 2 CO 3 (s) + H 2 O(l) + CO 2 (g) Heats of Reaction Remember that ΔH is positive for endothermic reactions. Therefore, you can write the reaction as follows: 2NaHCO 3 (s) → Na 2 CO 3 (s) + H 2 O(l) + CO 2 (g)ΔH = 85 kJ

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations The amount of heat released or absorbed during a reaction depends on the number of moles of the reactant involved. The decomposition of 2 mol of sodium bicarbonate requires 85 kJ of heat. Therefore, the decomposition of 4 mol of the same substance would require twice as much heat, or 170 kJ. Heats of Reaction

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations Heats of Reaction To see why the physical state of the reactants and products must be stated, compare the following two equations. difference = 44.0 kJ H 2 O(l) → H 2 (g) + O 2 (g)ΔH = kJ 1 2 H 2 O(g) → H 2 (g) + O 2 (g)ΔH = kJ 1 2

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations Heats of Reaction To see why the physical state of the reactants and products must be stated, compare the following two equations. The vaporization of 1 mol of liquid water to water vapor at 25°C requires 44.0 kJ of heat. H 2 O(l) → H 2 O(g) ΔH = 44.0 kJ difference = 44.0 kJ H 2 O(l) → H 2 (g) + O 2 (g)ΔH = kJ 1 2 H 2 O(g) → H 2 (g) + O 2 (g)ΔH = kJ 1 2

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculate the amount of heat (in kJ) required to decompose 2.24 mol NaHCO 3 (s). Using the Heat of Reaction to Calculate Enthalpy Change 2NaHCO 3 (s) + 85 kJ → Na 2 CO 3 (s) + H 2 O(l) CO 2 (g)

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Analyze List the knowns and the unknown. 1 KNOWNS UNKNOWN amount of NaHCO 3 (s) that decomposes = 2.24 mol ΔH = 85 kJ for 2 mol NaHCO 3 ΔH = ? kJ for 2.24 mol NaHCO 3 Use the thermochemical equation to write a conversion factor relating kJ of heat and moles of NaHCO 3. Then use the conversion factor to determine ΔH for 2.24 mol NaHCO 3.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Write the conversion factor relating kJ of heat and moles of NaHCO 3. Calculate Solve for the unknown. 2 The thermochemical equation indicates that 85 kJ are needed to decompose 2 mol NaHCO 3 (s). 85 kJ 2 mol NaHCO 3 (s)

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Using dimensional analysis, solve for ΔH. Calculate Solve for the unknown kJ 2 mol NaHCO 3 (s) ΔH = 2.24 mol NaHCO 3 (s)  = 95 kJ

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The 85 kJ in the thermochemical equation refers to the decomposition of 2 mol NaHCO 3 (s). Therefore, the decomposition of 2.24 mol should absorb more heat than 85 kJ. The answer of 95 kJ is consistent with this estimate. Evaluate Does the result make sense? 3

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations Heats of Combustion The heat of combustion is the heat of reaction for the complete burning of one mole of a substance.

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations Heats of Combustion Small amounts of natural gas within crude oil are burned off at oil refineries. This is an exothermic reaction. Burning 1 mol of methane releases 890 kJ of heat. The heat of combustion (ΔH) for this reaction is –890 kJ per mole of methane burned. CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O(l) kJ

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Like other heats of reaction, heats of combustion are reported as the enthalpy changes when the reactions are carried out at kPa and 25°C. Heats of Combustion at 25°C SubstanceFormulaΔH (kJ/mol) HydrogenH2(g)H2(g) –286 Carbon C(s, graphite) –394 MethaneCH 4 (g) –890 AcetyleneC2H2(g)C2H2(g)–1300 EthanolC 2 H 6 O(l)–1368 PropaneC3H8(g)C3H8(g)–2220 GlucoseC 6 H 12 O 6 (s)–2808 OctaneC 8 H 18 (l)–5471 SucroseC 12 H 22 O 11 (s)–5645

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following thermochemical equations represents an endothermic reaction? A.C graphite (s) + 2 kJ C diamond (s) B.2H 2 (g) + O 2 (g) 2H 2 O kJ

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following thermochemical equations represents an endothermic reaction? A.C graphite (s) + 2 kJ C diamond (s) B.2H 2 (g) + O 2 (g) 2H 2 O kJ

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts & Key Equation The value of ΔH of a reaction can be determined by measuring the heat flow of the reaction at a constant pressure. In a chemical equation, the enthalpy change for the reaction can be written as either a reaction or a product. q sys = ΔH = –q surr = –m  C  ΔT

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms calorimetry: the precise measurement of heat flow out of a system for chemical and physical processes calorimeter: an insulated device used to measure the absorption or release of heat in chemical or physical processes enthalpy (H): the heat content of a system at constant pressure

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms thermochemical equation: a chemical equation that includes the enthalpy change heat of reaction: the enthalpy change for a chemical equation exactly as it is written heat of combustion: the heat of reaction for the complete burning of one mole of a substance