Thermochemistry and Energy Changes

• Thermochemistry relates heat and motion

First Law of Thermodynamics

• Energy cannot be created or destroyed

Enthalpy

• Heat content at constant temperature

Exothermic

• System releases energy
• Temperature decreases
• Bond formation results in a negative value

Endothermic

• System absorbs energy
• Temperature increases
• Bond breaking results in a positive value

Overall Energy Change

• Reactants transform into products

Calculating Overall Energy Change

• Ensure the chemical reaction is balanced
• Count the number of bonds broken and formed, noting their corresponding bond energies
• Obtain the total energy input by summing the bond energies of the reactants
• Obtain the total energy released by summing the bond energies of the products
• Calculate the overall energy change by subtracting the BE products from the BE reactants
• Overall Energy Change (ΔΗ) = total energy input - total energy released = Σ BE Reactants – Σ BE Products
• Interpret the answer to determine if the reaction is endothermic or exothermic

Mass Changes and Enthalpy Changes

• If a reaction is reversed, the sign changes only
• If both sides of a reaction are multiplied by a factor, then H must change by the same factor
• Stoichiometry coefficients always refer to the moles

Thermochemical Equations & Stoichiometry

• Write down the given values and ratios

• Calculate the number of moles from the given mass using the molar mass ratio

• Calculate the ΔH by multiplying with the molar energy ratio

• Standard Mole is at neutral at 1 atm

• Reactants are in standard states

Standard Enthalpy of Reaction

• The heat of reaction is calculated for gaseous reaction
• Determine the standard heats of formation of the reactants and products
• Calculate the heat of reaction by subtracting the sum of standard heats of formation of products from the sum of standard heats of formation of reactants
• ΔH°rxn = Σ(ΔH°f products) – Σ(ΔH°f reactants)

Standard Enthalpy of Reaction using Hess' Law

• Calculate the standard enthalpy of reaction of acetylene from its elements
• Manipulate the sub-step processes so reactants and products align with the 1-step process by reversing sub-step processes, if necessary
• Manipulate the coefficients in the sub-step processes to be the same with the 1-step process.
• Cancel the substances found in both reactants and products
• Add the ΔH°rxn of all the sub-step processes to get the ΔH°rxn of the 1-step process

Spontaneous Reactions

• Spontaneous reactions occur under specific conditions and are mostly exothermic

Entropy

• A greater ordered system has small entropy
• Microstates are possible ways of distributing molecules
• Distribution: least is 1 and most is 3

Microstates and Entropy

• Entropy of a system relates to the natural logarithm of the number of microstates (W)
• S = k ln W
• S is entropy
• k is Boltzmann's constant = 1.38 × 10-23 J/K
• W is number of microstates
• The more microstates, the greater the entropy; the fewer microstates, the smaller the entropy

Second Law of Thermodynamics

• The entropy of the universe:
  • Increases in a spontaneous process, and ΔSuniv = ΔSsys + ΔSsurr > 0
  • Remains unchanged in an equilibrium process, ΔSuniv = ΔSsys + ΔSsurr = 0

Entropy Changes in the System

• Standard Entropy of Reaction (ΔS°rxn) can be calculated
  • ΔS°rxn = Σ(nΔS° products) – Σ(nΔS° reactants)
  • aA + bB → cC + dD, where a, b, c, d = stoichiometric coefficients
  • ΔS°rxn = [cΔS° (C) + dΔS° (D)] – [aΔS° (A) + bΔS° (B)]
  • ΔS°rxn > 0, leads to more disorder and is product-favored
  • ΔS°rxn < 0, leads to less disorder and is reactant-favored

Entropy Changes in the Surroundings

• Involve enthalpy change of the system (ΔH) and temperature (T) in Kelvin
• ΔSsurr = – ΔHsys / T

Gibbs Free Energy

• Gibbs free energy, denoted G, combines enthalpy and entropy into a single value
• The change in free energy, ΔG, equals the sum of the enthalpy plus the product of the temperature and entropy of the system
• ΔG can predict the direction of a chemical reaction under constant temperature and pressure

Gibbs Free Energy Definition

• The change in free energy, ΔG, equals the difference of the enthalpy and the product of the temperature and entropy of the system
• ΔG = ΔH – TΔS:
  • ΔG is the change in free energy
  • ΔH is the change in enthalpy
  • ΔS is the change in entropy
  • T is the temperature in Kelvin

Gibbs Free Energy Interpretation

• The sign of ΔG indicates the following:
• ΔG = ΔH – TΔS
  • ΔG > 0: nonspontaneous reaction
  • ΔG < 0: spontaneous reaction
  • ΔG = 0: at equilibrium

Gibbs Free Energy (Favorable Conditions)

• The signs of ΔH and ΔS are indicative of a reactions spontaneity
• ΔG = ΔH - TΔS
  • -ΔH and +ΔS: Spontaneous at ALL Temperatures
  • +ΔH and -ΔS: Nonspontaneous at ALL Temperatures
  • +ΔH and +ΔS: Spontaneous at HIGH Temperatures
  • -ΔH and -ΔS: Spontaneous at LOW Temperatures

Standard State Gibbs Free Energy of Formation

• Definition:
  • The partial pressure of any gas involved in the reaction is 1 atm
  • The concentrations of all aqueous solutions are 1 M
  • Measurements are generally taken at a temperature of 25° C (298 K)
  • The standard-state free energy of formation is the change in free energy that occurs when a compound is formed from its elements in their most thermodynamically stable states at standard-state conditions
  • ΔG°rxn = Σ(nΔG°f products) – Σ(nΔG°f reactants)

Calorimetry

• Measurement of heat exchange

Specific Heat

• The specific heat (s) of a substance is the amount of heat required to raise the temperature of one gram of the substance by one degree Celsius, measured in J/g·°C

Heat Capacity

• The heat capacity (C) of a substance is the amount of heat required to raise the temperature of a given quantity of the substance by one degree Celsius, measured in J/°C

Specific Heat & Heat Capacity

• The difference is that specific heat is an intensive property, does not depend on the amount of matter, whereas heat capacity is an extensive property and depends on the amount of matter
• Relationship: C = ms

Heat

• The specific heat, the amount of substance, and the change in the sample's temperature tells the amount of heat (q) that has been absorbed or released in a process
• q=msΔT
• C = ms, so q =CΔT
• ΔT is the temperature change, calculated as ΔT=Tfinal - Tinitial

Thermochemistry relates to heat and motion, governed by the first law of thermodynamics where energy is conserved. Enthalpy represents heat content at constant temperature. Exothermic reactions release energy, while endothermic reactions absorb energy changing reactants to products.