Thermochemistry and Thermodynamics

Introduction to Thermochemistry

Thermochemistry is the branch of chemistry concerned with the energy changes, particularly heat, that accompany chemical reactions. Thermodynamics more broadly studies energy transformations, including heat and work, and their movement between systems and surroundings.

• Energy: The capacity to do work or transfer heat. It cannot be created or destroyed (First Law of Thermodynamics).

• Work: Energy used to move an object against a force.

• Heat: Energy transferred due to temperature difference.

• Units: Joule (J), Calorie (cal), 1 cal = 4.184 J.

Forms of Energy

Energy exists in two main forms: kinetic (energy of motion) and potential (energy due to position).

• Kinetic Energy (Ek):

• Potential Energy: Energy stored due to position or configuration, such as in chemical bonds.

Electrostatic Potential Energy

Electrostatic potential energy is crucial in chemistry, as it relates to the interactions between charged particles (ions, electrons).

• Formula: , where is Coulomb's constant, and are charges, and is separation distance.

• Like charges repel, opposite charges attract.

Energy Transfer: System and Surroundings

Defining System and Surroundings

To analyze energy changes, chemists define a system (the part under study) and its surroundings (everything else).

• Open System: Exchanges both matter and energy with surroundings.

• Closed System: Exchanges energy but not matter.

• Isolated System: No exchange of matter or energy.

First Law of Thermodynamics

Internal Energy and Energy Changes

The First Law states that the total energy of the universe is constant. Internal energy () is the sum of all kinetic and potential energies within a system.

• Change in Internal Energy:

• Energy Transfer: (heat plus work)

• Sign conventions: and are positive if energy is gained by the system, negative if lost.

Exothermic and Endothermic Processes

• Exothermic: System releases heat (), e.g., combustion.

• Endothermic: System absorbs heat (), e.g., melting ice.

Enthalpy and Heats of Reaction

Definition and Calculation of Enthalpy

Enthalpy () is a state function used to describe heat flow at constant pressure. The change in enthalpy () is the heat absorbed or released during a reaction.

• Formula:

• Change in Enthalpy:

• Sign: (endothermic), (exothermic)

Standard Enthalpy of Formation

The standard enthalpy of formation () is the enthalpy change when one mole of a compound forms from its elements in their standard states (1 atm, 298 K).

• for elements in their standard state.

• Pay attention to the physical state (solid, liquid, gas) of substances.

Stoichiometry and Enthalpy Calculations

Stoichiometry in Thermochemistry

Enthalpy is an extensive property, so it depends on the amount of substance. Use stoichiometry to relate mass or moles to energy changes.

• Example: If 1 mol CH4 reacts, 890 kJ of heat is released.

• Always use balanced equations and correct states.

Hess's Law

Hess's Law states that the enthalpy change for a reaction is the sum of the enthalpy changes for individual steps. This allows calculation of for reactions using known values.

• Formula:

• Reverse reactions: Flip the sign of .

• Multiply/divide equations: Adjust accordingly.

Calorimetry: Measuring Heat Flow

Heat Capacity and Specific Heat

Calorimetry is the experimental measurement of heat flow. Heat capacity is the amount of heat required to raise the temperature of a substance by 1°C (or 1 K).

• Heat Capacity (C):

• Specific Heat Capacity (C_s): (per gram)

• Molar Heat Capacity: (per mole)

Calorimeters

Calorimeters are devices used to measure heat flow. There are two main types: constant-pressure (coffee-cup) and constant-volume (bomb) calorimeters.

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

• Bomb calorimeter: Used for combustion reactions at constant volume.

Calorimetry Calculations

• Heat absorbed by water:

• Heat of reaction: (for constant-pressure calorimetry)

• Bomb calorimeter:

Summary Table: Key Equations

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