Hess's Law and Thermochemical Equations - SS2 Chemistry Lesson Note
Hess's Law and thermochemical equations are important concepts in thermodynamics that enable the calculation of enthalpy changes for chemical reactions. They provide a systematic approach to determining the overall enthalpy change of a reaction by combining enthalpy changes of other reactions.
Thermochemical Equations:
Thermochemical equations represent chemical reactions along with the corresponding enthalpy change (ΔH) expressed as a ΔH value. Thermochemical equations are typically written in the form:
A reactant + B reactant → C product + D product ΔH = ΔH (products) - ΔH (reactants)
Key Points about Thermochemical Equations:
● Enthalpy Change: The enthalpy change (ΔH) represents the heat energy exchanged during a chemical reaction. It can be exothermic (negative ΔH) or endothermic (positive ΔH).
● Stoichiometric Coefficients: The stoichiometric coefficients in a balanced thermochemical equation represent the relative amounts of reactants and products participating in the reaction.
Hess's Law:
Hess's Law states that the enthalpy change of a chemical reaction is independent of the reaction pathway and depends only on the initial and final states of the system. In other words, the enthalpy change is a state function.
Implications of Hess's Law:
● Enthalpy Conservation: Hess's Law ensures the conservation of energy, as the enthalpy change remains the same regardless of the individual steps taken to reach the final state.
● Enthalpy Cycle: Hess's Law allows the construction of enthalpy cycles, where a reaction can be broken down into a series of intermediate steps. The enthalpy changes of these steps can be combined to calculate the overall enthalpy change of the reaction.
Using Hess's Law to Calculate Enthalpy Changes:
Hess's Law provides a method for calculating the enthalpy change for a reaction using known enthalpy changes of other reactions. The steps involved are as follows:
● Identify Known Reactions: Look for reactions with known enthalpy changes that involve the same reactants or products as the target reaction.
● Manipulate Known Reactions: Multiply the known reactions by appropriate coefficients to match the stoichiometry of the target reaction.
● Combine Reactions: Add the manipulated reactions together, cancelling out any common species, to obtain the target reaction.
● Combine Enthalpy Changes: Add the enthalpy changes of the manipulated reactions to obtain the overall enthalpy change of the target reaction.
Applications of Hess's Law:
● Heat of Formation: Hess's Law allows the determination of the heat of formation of a compound by using known enthalpy changes of formation reactions.
● Indirect Measurements: Hess's Law enables the determination of enthalpy changes that are difficult to measure directly by combining known enthalpy changes of related reactions.
● Thermochemical Calculations: Hess's Law is used extensively in thermodynamic calculations, including the determination of enthalpy changes for combustion reactions, phase transitions, and other chemical processes.
Important Considerations:
● Reversing Reactions: Reversing a reaction changes the sign of the enthalpy change.
● Multiplying Reactions: Multiplying a reaction by a coefficient also multiplies the enthalpy change by the same coefficient.
Hess's Law and thermochemical equations provide a systematic approach to calculate enthalpy changes for chemical reactions. Thermochemical equations represent reactions along with their corresponding enthalpy changes. Hess's Law states that the enthalpy change is independent of the reaction pathway and can be calculated by combining known enthalpy changes of other reactions. The applications of Hess's Law include determining heat of formation, indirect measurements, and thermochemical calculations. Understanding Hess's Law and thermochemical equations allows for the calculation and prediction of enthalpy changes, facilitating the study of energy transformations in chemical reactions and other thermodynamic processes.