Thermochemistry

Enthalpy and Heat Transfer

Thermochemistry is the study of energy changes, particularly heat, that accompany chemical reactions. The enthalpy (H) of a system is a measure of its total heat content. The change in enthalpy, ΔH, represents the heat transferred during a chemical reaction at constant pressure.

• Endothermic reactions absorb heat from the surroundings (ΔH > 0).

• Exothermic reactions release heat to the surroundings (ΔH < 0).

• The SI unit for heat is the Joule (J); the calorie (cal) is a non-SI unit, where 1 cal = 4.18 J.

Enthalpy Changes in Chemical Reactions

The enthalpy change for a reaction can be represented as:

• For combustion of methane:

• For the reverse reaction:

Calorimetry

Calorimetry is the measurement of heat flow in chemical reactions. Two common types of calorimeters are used:

• Coffee-cup calorimeter: Used for reactions in solution at constant pressure.

• Bomb calorimeter: Used for measuring heat of combustion at constant volume.

Heat Capacity

• Molar heat capacity (Cm): Heat required to raise the temperature of 1 mole of a substance by 1 K.

• Specific heat capacity (Cs): Heat required to raise the temperature of 1 gram of a substance by 1 K.

• Formula: , where q is heat absorbed, m is mass, and ΔT is temperature change.

Specific Heats of Substances

The specific heat values of various substances are important for calorimetry calculations.

Example Calculation

To calculate the heat absorbed by a sample:

• Given: 35.2 g iron, temperature increases from 23.7°C to 29.5°C, Cs = 0.450 J/g·K.

• ΔT = 29.5°C - 23.7°C = 5.8 K

• q = m × Cs × ΔT = 35.2 g × 0.450 J/g·K × 5.8 K = 91.8 J

Hess's Law

Hess's Law states that the enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, regardless of the pathway taken. This allows calculation of ΔH for reactions that are difficult to measure directly.

• Basic rule:

Example: Combustion of Carbon

• Given reactions: (1) (2) Find ΔH for: (3)

Example: Formation of Acetylene

• Given reactions and enthalpy changes, use Hess's Law to calculate ΔH for .

Enthalpies of Formation

The standard enthalpy of formation (ΔHf°) is the enthalpy change when one mole of a compound is formed from its elements in their standard states at 1 atm and 298 K.

• ΔHf° of the most stable form of an element is zero (e.g., C(graphite), O2(g)).

• ΔHf° is not zero for less stable forms or compounds.

Calculating Enthalpy Change Using Enthalpies of Formation

• Enthalpy change for a reaction:

• Example: Combustion of ethanol Use tabulated ΔHf° values to calculate ΔH°rxn.

Formation Reactions

• If elements in their standard state form 1 mole of a compound in its standard state, then ΔH = ΔHf°.

• Example:

Summary Table: Standard Enthalpies of Formation

Key Points

• Enthalpy changes can be calculated using calorimetry, Hess's Law, or standard enthalpies of formation.

• Specific heat and molar heat capacity are essential for measuring heat changes in substances.

• Formation reactions and tabulated ΔHf° values are used to determine reaction enthalpies.

Additional info: Academic context was added to clarify definitions, formulas, and examples for completeness and exam preparation.