Hess's Law Lecture Notes PDF

Mark Xavier Bailon Hess’s Law Learning Apply stoichiometric calculations to Objectives: thermochemical equations; Apply Hess’s law to determine the heat of the overall reaction of a system; and Determine the heat of reaction from...

Mark Xavier Bailon Hess’s Law Learning Apply stoichiometric calculations to Objectives: thermochemical equations; Apply Hess’s law to determine the heat of the overall reaction of a system; and Determine the heat of reaction from standard enthalpies of formation of reactants and products. Thermochemical Equation A thermochemical equation is a chemical equation that includes the enthalpy change of the reaction CH4(g) + 2 O2(g) → CO2(g) + 2 H2O (l) ∆𝑯rxn = −802.4 kJ/mol Important Things to Note: 1. The heat of reaction depends on the physical state of reactants and products. H2O(s) → H2O(l) ∆𝑯rxn = 6.01 kJ/mol H2O(g) → H2O(l) ∆𝑯rxn = − 44.0 kJ/mol 2. The heat of reaction is an extensive property 2H2O(s) → 2H2O(l) ∆𝑯rxn = 12.02 kJ/mol 1/2H2O(g) → 1/2H2O(l) ∆𝑯rxn = − 22.0 kJ/mol 3. The magnitude of the heat of reaction of a reverse process is equal to the forward but opposite in sign. H2O(l) → H2O(s) ∆𝑯rxn = − 6.01 kJ/mol Hess’s Law If a chemical reaction is carried in two or more steps, the overall heat of of heat reaction is the sum of heat of reactions for the individual steps. summation Regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. This additivity property of the heat of reaction stem from the fact that enthalpy is a state function. Example 1: Solve the overall heat of reaction in converting a mole of solid water to gas in from a two-step process described by the following thermochemical equations: Overall rxn: H2O(s) → H2O (g) ∆𝑯rxn = ? Step 1: H2O(s) → H2O (l) ∆𝑯rxn = 6.01 kJ/mol Step 2: H2O(g) → H2O(l) ∆𝑯rxn = − 44.0 kJ/mol Example 2: The standard enthalpies of combustion of C(graphite), H2(g) and C3H8(g) are –393.5, –285.8, and –2219.9 kJ mol–1, respectively. Use these values to compute for the standard enthalpy change for the reaction: Standard enthalpy of formation Change that occurs in the formation of one mole of the substance in the standard state from the reference forms of the elements in their standard states. The reference forms of the elements in all but a few cases are the most stable forms of the elements at one bar and the given temperature. The degree symbol denotes that the enthalpy change is a standard enthalpy change, and the subscript f signifies that the reaction is one in which a substance is formed from its elements. The standard enthalpy of formation is 0 for a pure element in its reference form. Standard Molar Enthalpies of Formation Example 3 Calculate the standard enthalpy of reaction for the decomposition of sodium bicarbonate, a minor reaction that occurs when baking soda is used in baking. Standard Enthalpies of Reaction Example 4 Calculate the standard enthalpy of combustion of ethane, C2H6(g), a component of natural gas. Reactions involving ions in solution Example 5 Thank You!