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, $\Delta H<em>f^0 (O</em>2) = 0$ 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: $\Delta H<em>{rxn} = \Sigma n \Delta H</em>f \text{ (products)} - \Sigma n \Delta H_f \text{ (reactants)}$

• Enthalpy of reaction is the $\Delta H$ that occurs for a reaction ($\Delta H_{rxn}$).

  • $\Delta H<em>{rxn} = \Delta H</em>{\text{products}} – \Delta H_{\text{reactants}}$

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, $\Delta H<em>f^0 (O</em>2) = 0$ 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: $\Delta H<em>{rxn} = \Sigma n \Delta H</em>f \text{ (products)} - \Sigma n \Delta H_f \text{ (reactants)}$, where n represents the stoichiometric coefficients in the balanced chemical equation.

• Enthalpy of reaction is the $\Delta H$ that occurs for a reaction ($\Delta H<em>{rxn}$).- $\Delta H</em>{rxn} = \Delta H<em>{\text{products}} – \Delta H</em>{\text{reactants}}$

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: $C(H<em>2O) \times m(H</em>2O) \times \Delta T (H_2O) = C(metal) \times m (metal) \times \Delta T (metal)$

• 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: $C(H<em>2O) \times m(H</em>2O) \times \Delta T (H_2O) = C(metal) \times m (metal) \times \Delta T (metal)$

• Standard enthalpy of solution is given