1. Lesson 4 – Representing Enthalpy Changes

2. Method One THERMOCHEMICAL EQUATIONS WITH ENERGY TERMS THERMOCHEMICAL EQUATIONS WITH ENERGY TERMS a balanced chemical equation that includes the heat transferred to or from the surroundings a balanced chemical equation that includes the heat transferred to or from the surroundings Ex. H 2 (g) + ½O 2 (g)  H 2 O (l) + 285.8 kJ MgCO 3 (s) + 117.3 kJ  MgO (s) + CO 2 (g) To write a thermochemical equation, you must know To write a thermochemical equation, you must know –the balanced chemical equation –the molar heat of reaction for one substance –if the reaction is endothermic or exothermic

3. Sample Problem 1 From a calorimetry experiment, a student determines that the specific heat of solution of KBrO 3 (s) is an endothermic 0.25 kJ/g. Calculate the molar heat of solution and write the thermochemical equation.

4. Sample Problem 2 The combustion of magnesium releases 24.7 kJ per gram of magnesium. Calculate the molar heat of combustion of magnesium and write the thermochemical equation.

5. Sample Problem 3 Use the following thermochemical equation to calculate the molar heat of combustion of hexane. 2C 6 H 14 (l) + 19O 2 (g)  12CO 2 (g) + 14H 2 O (g) + 7086kJ

6. Method Two THERMOCHEMICAL EQUATIONS WITH ΔH VALUES The enthalpy of reaction is indicated by a separate expression beside the chemical equation The enthalpy of reaction is indicated by a separate expression beside the chemical equation »Remember: exothermic reactions have -ΔH endothermic reactions have +ΔH Examples: H 2 (g) + ½O 2 (g)  H 2 O (l)  H o rxn = -285.8 kJ MgCO 3 (s)  MgO (s) + CO 2 (g)  H o rxn = 117.3 kJ

7. Method Three MOLAR ENTHALPY OF REACTION the quantity of heat transferred in a reaction per mole of a specified substance the quantity of heat transferred in a reaction per mole of a specified substance  H r (kJ/mol)  H r (kJ/mol) to calculate the molar heat of reaction: to calculate the molar heat of reaction:  H r = specific heat of reaction x molar mass standard molar enthalpy of reaction (  H r o ): molar enthalpy that is determined at SATP standard molar enthalpy of reaction (  H r o ): molar enthalpy that is determined at SATP

8. Sample Problem 4 Ethane is the second largest component of natural gas. If its specific heat of combustion is -51.85 kJ/g, what is the molar heat of combustion of ethane?

9. Method Four POTENTIAL ENERGY DIAGRAM a graphical representation of the energy transferred during a physical or chemical change a graphical representation of the energy transferred during a physical or chemical change exothermic reactions: products are lower in energy than reactants (energy lost to surroundings) exothermic reactions: products are lower in energy than reactants (energy lost to surroundings) endothermic reactions: products are higher in energy than reactants (energy absorbed from surroundings) endothermic reactions: products are higher in energy than reactants (energy absorbed from surroundings)