Hess's law and reaction enthalpy change | Chemistry | Khan Academy

<aside> 📎 Hess’ Law: The enthalpy change of a physical or chemical process depends only on the initial and final conditions of the process. Enthalpy change is independent of the pathway of the process and the number of intermediate steps in the process

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• Chemical changes in several steps
• Predicting $\triangle H$ using Hess’s Law
• Thermal stability

Enthalpies of Formation

• Standard molar enthalpy of formation
• When is the enthalpy of formation = zero?
• Writing formation equations
• Enthalpies of Formation & Hess’ Law

• Hess’ law states that the enthalpy change of a physical or chemical process depends only on the initial and final conditions
  • The enthalpy change of the overall process it eh sum of the enthalpy changes of its individual steps
• You can add any number of chemical equations to obtain an equation that you need, and the enthalpy change of the overall reaction is the sum of enthalpy changes of the individual reactions
• You can manipulate chemical equations to make them fit into another set of reactions by multiplying the equation by a constant or by reversing the equation
  • If you multiply by a constant, you must multiply the enthalpy change by that same constant
  • If you reverse an equation, you must change the sign of the enthalpy change
• The standard enthalpy of formation of a compound $(\triangle H \degree _f)$ is the enthalpy change for formation of 1 mol of the compound from its elements in their most stable state under standard conditions
• The enthalpy of formation of an element in its most stable state under standard conditions is arbitrarily set at zero

You can calculate the enthalpy change for any reaction by applying the formula: $\triangle H \degree r = \Sigma(n \triangle H \degree {f{products}}) - \Sigma(n \triangle H \degree {f_{reactants}})$