Thermochemistry and Calorimetry Concepts

Introduction to Thermodynamics

• This is a course on thermodynamics (CHEM 217) at the University of Ghana, Department of Chemistry.

System, Boundary, Surroundings

• A system is the part of the universe under study.
• Surroundings comprise the rest of the universe.
• The real or imaginary surface separating the system from its surroundings is the boundary.

Types of Thermodynamic Systems

• Isolated system: Cannot transfer matter or energy with the surroundings.
• Closed system: Can transfer energy (heat, work, radiation) but not matter.
• Open system: Can transfer both energy and matter.

Homogeneous and Heterogeneous Systems

• Homogeneous system: Uniform throughout, consisting of a single phase (e.g., pure solid, liquid, or gas).
• Heterogeneous system: Consists of two or more physically distinct phases (e.g., ice in contact with water).
• A phase is a homogeneous, physically distinct, and mechanically separable portion of a system.

Intensive and Extensive Properties

• Intensive properties: Do not depend on the amount of matter (e.g., pressure, temperature, density, concentration).
• Extensive properties: Depend on the amount of matter (e.g., volume, number of moles, enthalpy, entropy, Gibbs' free energy).

State of a System

• A thermodynamic system is in a state when all its properties are fixed.
• Fundamental macroscopic properties determining a system's state: pressure (P), temperature (T), volume (V), mass, and composition.

Thermodynamics

• Study of energy flow in physical or chemical transformations.
• First law of thermodynamics: Energy of the universe is constant (conservation of energy).

Thermochemistry

• Study of heat changes associated with chemical reactions.

Work and Energy

• Energy is the capacity to do work.
• Internal energy (U or E) is the total energy stored in a system (cannot be measured absolutely).
• Work is the transfer of energy to a system by a process equivalent to raising or lowering a weight. (work done on a system is positive, work done by a system is negative).
• When energy is transferred to a system by doing work, with no other change occurring, ∆U = w.

The First Law of Thermodynamics

• The total internal energy of an isolated system is constant.
• In practice, we only measure the change in internal energy.
• ΔE = Efinal - Einitial = Eproducts - Ereactants .
• ΔEsystem + ΔEsurroundings = 0

Work

• Work required to move an object against an opposing force is calculated by multiplying the opposing force by the distance moved against it.
• Work = opposing force x distance moved
• Unit is the joule (J), where 1 J = 1 kg.m²/s².
• Types of work associated with chemical processes are work done by a gas through expansion or compression and motion of a car.

Deriving the Equation for Work

• Work done by a gas expanding against an external pressure is w = -PΔV.
• Work done on a gas (compression) is +PΔV.
• Change in volume (ΔV) is the product of the area of the piston times the distance it moves.

Calculating the work of reversible isothermal expansion of a gas

• The work done in a reversible isothermal expansion is w = -nRTln(Vfinal/Vinitial).

Examples (various calculations)

• Various examples are provided demonstrating the application of the equations mentioned.

Enthalpy / Heat

• Enthalpy (H): Energy transferred due to a temperature difference.
• Energy flows from a high-temperature region to a low-temperature region.
• Heat (q): Energy transfer as a result of a temperature difference. Heat transfer into a system is positive. Heat transfer out of a system is negative.
• When no other changes except heat transfer occur. ΔΕ =q

Measuring Heat; Calorimetry

• Science of measuring heat using observations of temperature change
• Calorimeter is a device for determining heat associated with a chemical reaction.

Heat Capacity

• Heat capacity (C): The energy required to raise the temperature of a substance by one unit.
• Units: J/°C or J/K
• Specific heat capacity: Energy required to raise the temperature of one gram by one degree Celsius,
• Molr heat capacity : Energy required to raise the temperature of one mole of a substance by one degree Celsius
• Heat capacities of metals differ from that of water.

Constant-Pressure Calorimetry

• Measuring enthalpy changes of reactions in solution.
• At constant pressure, the heat of reaction, q, equals enthalpy change, ∆H: qp=∆H.

Constant-Volume Calorimetry

• Measuring enthalpy changes in reactions that occur in a rigid or fixed-volume container.
• For reactions occurring at constant volume q = ΔE
• Bomb calorimeters are devices used to measure these reactions.

Heat Transfers at Constant Pressure

• Enthalpy (H) is a state function.
• Change in enthalpy (ΔH) is equal to heat gained or released at constant pressure.
• Every chemical reaction involves energy changes.

Determining a reaction enthalpy from experimental data

• Sample problems are provided demonstrating procedures and formulas to determine enthalpy values when certain parameters are given. (using coffee-cup calorimetry)

Standard Enthalpies of Formation

• Standard reaction enthalpy per mole of formula units of a substance created from its elements in the most stable form.
• Elements' standard enthalpy of formation is zero.

Combining Reaction Enthalpies: Hess' Law

• The overall enthalpy change of a reaction is the sum of the enthalpy changes of the steps into which the reaction can be divided.
• Enthalpy and reaction enthalpy values are independent of the path.

Characteristics of Enthalpy Changes and Hess Law

• Key characteristics and rules to perform operations using Hess Law are discussed.

Using Hess's Law

• Procedures are shown demonstrating how to utilize Hess' law to determine enthalpy values of specific reactions.

Bond Dissociation Enthalpy/Energy

• Enthalpy change for breaking one mole of a bond under standard conditions (gaseous state).
• Bond enthalpy values differ across environments or compounds.

Bond Enthalpy/Energy

• Average of bond dissociation enthalpies for a specific bond.
• Provides estimations for enthalpy changes.

Estimation of B.E. from ΔH and ΔH_atm

• Procedure for estimating bond energy from known enthalpy values using Hess' Law.

Example Problems (various calculations)

• Various example calculations are given, demonstrating procedures and formulas for determining enthalpy values.

Entropy and Disorder

• Entropy (S) is a measure of randomness or disorder.
• Entropy of an isolated system increases in a spontaneous process.
• Entropy is a state function.
• Entropy changes (ΔS) are the change in degree of disorder ΔS = S_final - S_initial.

Calculating the change in entropy due to an increase in temperature

• Formulas are provided for calculating the entropy change due to temperature change in constant volume.

Calculating the change in entropy when an ideal gas expands isothermally

• How to calculate the entropy change when an ideal gas expands isothermally.

Calculating Entropy Changes When Both Temperature and Volume Change

• How to determine entropy change when both temperature and volume changes occur.

Standard Molar Entropies

• Calculate the standard molar entropies for a material.

Calculating the standard reaction entropy

• Determine the overall reaction entropy.

Global Changes in Entropy

• How to determine the total entropy change.

The Overall Change in Entropy

• Calculate the overall change in entropy.

Calculating the Total Entropy Change for the Expansion of an Ideal Gas

• A procedure for calculating total entropy change when an ideal gas expands.

Assessing Spontaneity

• A procedure using enthalpy and entropy to assess whether a reaction is spontaneous under standard conditions.

Equilibrium

• Equilibrium is the state where there's no net change in the composition of the reaction mixture.

Focusing on the System

• Using Gibbs Free Energy to determine spontaneity for a reaction.

Summary of factors that affect spontaneity

• Summary of factors, such as enthalpy, entropy, and temperature that affect spontaneity.

Deciding Whether a Process is Spontaneous

• Procedure for determining the Gibbs free energy change for a reaction and concluding spontaneity based on the ΔG value.

Gibbs Free Energy of Reaction

• The Gibbs Free energy of reaction is the standard Gibbs free energy of reaction per mole for the formation of a compound formed from its elements in their most stable form. (G°).

Calculating a Standard Gibbs Free Energy of Formation from Enthalpy and Entropy Data

• How to calculate standard Gibbs free energy of formation.

The Dependence of Free Energy on Pressure

• How pressure affects the free energy of a chemical reaction (introducing the reaction quotient Q).

Calculating Equilibrium Constant

• How to calculate the equilibrium constant given a standard Gibbs Free Energy changes.

Criteria for Spontaneity in a Chemical Reaction

• Summary of spontaneous processes, equilibrium, and Non-spontaneous processes and their associated conditions.

This quiz tests your understanding of key concepts in thermochemistry and calorimetry, including ionic reactions, enthalpy changes, and the second law of thermodynamics. Questions cover the role of heat transfer, the significance of entropy changes, and the use of bomb calorimeters. Prepare to deepen your grasp of crucial principles in energy chemistry!