Modified Chapter 6 (Enthalpy Change) 2024 PDF

Chapter 6 Thermochemistry: Enthalpy Change (ΔH) Thermochemistry: Is the study of heat that is released or absorbed during chemical reaction, phase change or other chemical activity. Enthalpy (H): Amount of energy released or absorbed during chemical reaction. –ve exothermic and +ve endothermic. H...

Chapter 6 Thermochemistry: Enthalpy Change (ΔH) Thermochemistry: Is the study of heat that is released or absorbed during chemical reaction, phase change or other chemical activity. Enthalpy (H): Amount of energy released or absorbed during chemical reaction. –ve exothermic and +ve endothermic. Hess’s law : The heat change in a chemical reaction is the same regardless of the number of stages in which the reaction is effected.  What is Enthalpy Change (ΔH)? Enthalpy (H) is the total heat content of a system. Enthalpy change (ΔH) is the difference in enthalpy between the products and reactants during a chemical or physical process at constant pressure ΔH = H_products - H_reactants Types of Enthalpy Change: - Exothermic (ΔH < 0): Releases heat to surroundings. - Endothermic (ΔH > 0): Absorbs heat from surroundings.  Units of ΔH SI Unit: Joules (J) or kilojoules (kJ) Common Use in Chemistry: kJ/mol Example: 1- Combustion of methane( exothermic reaction) CH4 + 2O2 → CO2 + 2H2O ΔH = -890 kJ/mol 2- Decomposition of calcium carbonate ( endothermic) CaCO₃ → CaO + CO₂ ΔH = +178 kJ/mol 6 Examples of Thermochemical Equations and Calculations Understanding Heat in Chemical Reactions  Example 2: Endothermic Reaction Decomposition of Calcium Carbonate: CaCO₃ → CaO + CO₂ ΔH = +178 kJ/mol Meaning: 178 kJ of heat is absorbed when 1 mole of CaCO₃ decomposes. Example Calculate the Heat Absorbed for 50 g of CaCO₃ - Molar mass of CaCO₃ = 100 g/mol - Moles = 50 / 100 = 0.5 mol - Heat absorbed = 0.5mole × 178KJ/1mole = 89 kJ  Example 1: Exothermic Reaction Combustion of Methane: CH₄ + 2O₂ → CO₂ + 2H₂O ΔH = -890 kJ/mol Meaning: 890 kJ of heat is released when 1 mole of methane combusts. Example: Calculate the Heat Released for 2 Moles of Methane - Heat released = 2mole × -890 KJ/1mole= -1780 kJ Example 3: Neutralization Reaction Reaction of HCl and NaOH: HCl + NaOH → NaCl + H₂O ΔH = -57 kJ/mol Meaning: 57 kJ of heat is released when 1 mole of HCl reacts. Example Calculate the Heat Released for 0.1 mol of HCl ? Heat released = 0.1mole × -57KJ/1mole = -5.7 kJ  Hess's Law Concept Hess's Law states: - The total enthalpy change of a reaction is the same regardless of the pathway taken, as long as the initial and final conditions are the same. example Find ΔH For The reaction A →C ΔH= ΔH3 Given The following data A →B ΔH= ΔH1 B→C ΔH= ΔH2 Add the two reactions-------------------------------- A→C ΔH3= ΔH1 + ΔH2  Example: Formation of CO₂ Reaction: C (s) + O₂ (g) → CO₂ (g) Find ΔH for the reaction Given Data: 1. C (s) + ½O₂ (g) → CO (g) ΔH₁ = -110.5 kJ 2. CO (g) + ½O₂ (g) → CO₂ (g) ΔH₂ = -283.0 kJ -------------------------------------------------- C (s) + O₂ (g) → CO₂ (g) ΔH = -110.5 kJ+ -283.0 kJ ΔH = -393.5 kJ Example Reverse equation one and add its answer to equation 2 will gives the needed reaction 13 Example: Given the following two reactions: C(s) + O2(g) → CO2(g) ∆H = -393.5 kJ 2Fe(s) + 3/2 O2 (g) → Fe2O3(s) ∆H = -824.2 kJ Calculate the enthalpy change for 2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2(g) A) -467.9 kJ B) -430.7 kJ C) 430.7 kJ D) 467.9 kJ E) -2828.9