Enthalpy Changes: Quick Review
Enthalpy Changes
Enthalpies of Formation
Definition: The enthalpy change when 1 mole of a substance is formed from its elements in their standard states.
Standard State: 100 kPa (1 atm) and 298.15 K.
The standard enthalpy of formation of any element in its most stable form is zero. For example, \Delta Hf^0 (O2) = 0 and \Delta H_f^0 (C, graphite) = 0.
Enthalpies of Reactions
Definition: The heat change when molar quantities of reactants as specified by the chemical equation react to form products at standard conditions (298.15 K and 1 atm).
General Formula: \Delta H{rxn} = \Sigma n \Delta Hf \text{ (products)} - \Sigma n \Delta H_f \text{ (reactants)}
Enthalpy of reaction is the \Delta H that occurs for a reaction (\Delta H_{rxn}).
\Delta H{rxn} = \Delta H{\text{products}} – \Delta H_{\text{reactants}}
Enthalpies of Combustion
Definition: Amount of energy released or absorbed when 1 mole of substance is completely reacted with oxygen (kJ/mol). Always exothermic.
Standard conditions are 100 kPa and 298 K with all substances in their standard states.
Types of Enthalpies
\Delta H_{rxn} - Heat produced in a chemical reaction.
\Delta H_{comb} - Heat produced by a combustion reaction.
\Delta H_{neut} - Heat produced in a neutralization reaction.
\Delta H_{sol} - Heat produced when a substance dissolves.
\Delta H_{fus} - Heat produced when a substance melts.
\Delta H_{vap} - Heat produced when a substance vaporizes.
\Delta H_{sub} - Heat produced when a substance sublimes.
Enthalpy Changes
Enthalpy Changes
Enthalpies of Formation
Definition: The enthalpy change when 1 mole of a substance is formed from its elements in their standard states.
Standard State: Defined as 100 kPa (1 atm) and 298.15 K (25°C).
The standard enthalpy of formation of any element in its most stable form is zero. For example, \Delta Hf^0 (O2) = 0 and \Delta H_f^0 (C, graphite) = 0. This is because no energy is required to form an element from itself.
Enthalpies of Reactions
Definition: The heat change when molar quantities of reactants as specified by the chemical equation react to form products at standard conditions (298.15 K and 1 atm).
General Formula: \Delta H{rxn} = \Sigma n \Delta Hf \text{ (products)} - \Sigma n \Delta H_f \text{ (reactants)}, where n represents the stoichiometric coefficients in the balanced chemical equation.
Enthalpy of reaction is the \Delta H that occurs for a reaction (\Delta H{rxn}).- \Delta H{rxn} = \Delta H{\text{products}} – \Delta H{\text{reactants}}
Enthalpies of Combustion
Definition: Amount of energy released or absorbed when 1 mole of substance is completely reacted with oxygen (kJ/mol). Always exothermic, meaning energy is released, and \Delta H is negative.
Standard conditions are 100 kPa and 298 K with all substances in their standard states.
Types of Enthalpies
\Delta H_{rxn} - Heat produced in a chemical reaction.
\Delta H_{comb} - Heat produced by a combustion reaction.
\Delta H_{neut} - Heat produced in a neutralization reaction.
\Delta H_{sol} - Heat produced when a substance dissolves.
\Delta H_{fus} - Heat produced when a substance melts.
\Delta H_{vap} - Heat produced when a substance vaporizes.
\Delta H_{sub} - Heat produced when a substance sublimes.
Enthalpy Changes
Enthalpy of formation
Enthalpy of combustion
Enthalpy of neutralization
Enthalpy change of solution (\Delta H^0_{soln})
Enthalpy change of atomization
Enthalpy change of hydration
Enthalpy change of reaction (general term)
First/Second Ionization Energy (IE)
First/Second Electron Affinity (ea)
Enthalpy of Lattice Dissociation
Measuring Enthalpy Changes
Coffee-Cup Calorimeter: Used to measure \Delta H at constant pressure. It works best for solution-based reactions where heat transfer occurs within the solution.
Question Types
Solution + solution
Water + soluble salt
Metal + water: C(H2O) \times m(H2O) \times \Delta T (H_2O) = C(metal) \times m (metal) \times \Delta T (metal)
Standard enthalpy of solution is given
Enthalpy of combustion
Enthalpy of neutralization
Enthalpy change of solution (\Delta H^0_{soln})
Enthalpy change of atomization
Enthalpy change of hydration
Enthalpy change of reaction (general term)
First/Second Ionization Energy (IE)
First/Second Electron Affinity (ea)
Enthalpy of Lattice Dissociation
Measuring Enthalpy Changes
Coffee-Cup Calorimeter: Used to measure \Delta H at constant pressure.
Question Types
Solution + solution
Water + soluble salt
Metal + water: C(H2O) \times m(H2O) \times \Delta T (H_2O) = C(metal) \times m (metal) \times \Delta T (metal)
Standard enthalpy of solution is given