Enthalpy change (Delta H) is the heat exchanged by a system at constant pressure, typically measured in kJ/mol.

Exothermic reactions release heat to the surroundings, resulting in a negative Delta H.

By convention, the standard enthalpy of formation (Delta H_f^0) of an element in its standard state (e.g., O2(g), C(s)) is zero.

Hess’s law allows the calculation of Delta H by summing enthalpy changes of intermediate steps, regardless of the reaction path.

A negative Delta H (-393.5 kJ/mol) indicates an exothermic reaction, releasing heat to the surroundings.

Bond energy is the energy needed to break one mole of a specific bond in the gas phase, typically positive.

Decomposition of water requires energy to break O-H bonds, making it endothermic with a positive Delta H.

Delta H^0 = sum Delta H_f^0 (products) - sum Delta H_f^0 (reactants) uses standard enthalpies of formation.

Neutralization of strong acid and base produces water, with a standard enthalpy of about -57 kJ/mol due to H2O formation.

Melting requires energy to break intermolecular forces, making it endothermic with a positive Delta H.

The reverse reaction (decomposition) has the opposite Delta H. For 1 mole: Delta H = -(-572)/2 = +286 kJ/mol.

Work is a path function, depending on the process, unlike enthalpy, internal energy, and temperature.

For 2 moles, Delta H = 2 × (-890) = -1780 kJ, as combustion is exothermic.

Enthalpy (Delta H) is defined as the heat change at constant pressure, typically using a calorimeter.

Formation of H2O has a highly negative Delta H (-286 kJ/mol per mole of H2O) due to strong O-H bonds formed, larger than CO2’s -393.5 kJ/mol.

Delta H = (sum of bond energies of reactants) - (sum of bond energies of products), as breaking bonds absorbs energy and forming bonds releases it.

In exothermic reactions, products have lower enthalpy than reactants, releasing heat (Delta H < 0).

The standard state is typically 25°C (298 K) and 1 atm pressure for gases, liquids, or solids in their most stable form.

The standard enthalpy of formation is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states.

Combustion reactions, like ethanol burning, are exothermic, releasing heat, so Delta H is negative.